Natural Selection

[Dak]

one improvement step improvement ahead improvement of improvement you improvement.

 

hick! t'was me sisters 21st today

[lucaspa]

Then give me a definition of "useful" that doesn't entail the definition of "do better" (or v.v.) A syllogism is a logical chain, as you put it, which admits of no possible exceptions. Empirical data, of course, always do.

 

I fail to see what H-W has to do with it. Are you suggesting, perhaps, that as all alleles at a locus are not in H-W equilibrium, selection must have occurred?

 

1. You need to go back and tell us why you need a definition of "useful" that doesn't entail the defintion of "do better". What we have from Darwin is " But if variations useful to any organic being do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life;"

 

From Merriam-Webster:

 

"1 : capable of being put to use; especially : serviceable for an end or purpose <useful tools>

2 : of a valuable or productive kind "

 

Now, where is "do better"?

 

2. Hardy-Weinberg gives you a starting point to determine what will happen if there is NOTHING acting on the population. That give us an objective place to determine if something is happening to the population. As you implied, the effect does not have to be natural selection. It can be genetic drift. BUT, there are mathematical tests to distinguish between the two. So, instead of having a judgement call of "do better", H-D serves as the objective scorekeeper. Once we identify alleles/traits that are increasing, eliminate genetic drift as the cause, then we can use reverse engineering to determine why they are doing better.

 

Sorry, but that is the way I and my mathematically inclined colleagues use the term "in general".

 

You still haven't provided a source for how your "colleagues" use the term. If they truly do, then they have written it down somewhere. All I'm asking you is to provide a source. Instead, you keep telling me to take your word for it. But that is the issue: are you stating the principle correctly. To decide that, we can't take your word.

[

quote]x is not true in general means that x is not always true. x not-true = x false.

 

If x is not always true, then x not-true is NOT = x false. For instance, to say "the statement 'the earth is flat' is false" is not to say " 'the earth is flat' is not always true" Because that implies that the earth can sometimes be flat.

 

How about this, as a slightly less jargonistic compromise?

 

The statement that x is true is not true in general, therefore the statement that x is true in general is false i.e. the statement that x is false in general is true? Any better?

 

Nope. Because 'x is true' is not the same statement as 'x is true in general'. You have added the phrase "in general" and, thus, changed the statement.

[lucaspa]

Yup, we all agreed that as a definition of evolution.

 

No, we don't. In particular several very prominent evolutionary biologists do not agree that "change in allele frequency" is evolution. It is incomplete.

 

" 'Evolution' implies change with continuity, usually with a directional component. Biological evolution is best defined as change in the diversity and adaptation of populations of organisms." pg. 47

 

"No Darwinian I know questions the fact that the processes of organic evolution are consistent with the laws of the physical sciences, but it makes no sense to say that biological evolution has been "reduced" to physical laws. Biological evolution is the result of specific processes that impinge on specific systems, the explanation of which is meaningful only at the level of complexity of those processes and those systems. And the classical theory of evolution has not been reduced to a "molecular theory of evolution," an assertion based on such reductionist definitions of evolution as "a change in gene frequencies in natural populations." This reductionist definition omits the crucial aspects of evolution: changes in diversity and adaptation. (Once I gave a lump of sugar to a racoon in a zoo. He ran with it to his water basin and washed it vigorously until there was nothing left of it. No complex system should be taken apart to the extent that nothing of significance is left.)" Ernst Mayr, Evolution, Scientific American 239: 47-55, Sept. 1978.

 

A complete definition of evolution is given by Futuyma in Evolutionary Biology:

 

"Thus, evolution, in a broad sense is descent with modification, and often with diversification. Many kinds of systems are evolutionary ... In all such systems there are populations, or groups, of entities; there is variation in one or more characteristics among the members of the population; there is HEREDITARY SIMILARITY between parent and offspring entities; and over the course of generations there may be changes in the proportions of individuals with different characteristics within populations. This process consitutes descent with modification. Populations may become subdivided so that several populations are derived from a COMMON ANCESTRAL POPULATION. If different changes in the proportions of variant individuals transpire in te several populations,the populations DIVERGE, OR DIVERSIFY. ... All these properties of an evolutionary process pertain to populations of organisms, in which there is hereditary transmission of characteristics (based on genes, composed of DNA or, in a few cases, RNA), variation owing to mutation, and sorting of variation by several kinds of processes. Chief among these sorting processes are CHANCE (random variation in the survival or reproduction of different variants), and natural selection (consistent, nonrandom differences among variants in their rates of survival or reproduction). It is natural selection that causes adaptation -- improvement in function. Thus biological (or organic) evolution is change in the properties of populations of organisms , or groups of such populations, over the course of generations. ... Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportions of different forms of a gene within a population, such as the alleles that determine the different human blood types, to the alterations that led from the earliest organisms to dinosaurs, bees, snapdragons, and humans." Douglas Futuyma, Evolutionary Biology, (1999) pg 4. [emphasis in original]

[lucaspa]

Ah yes, that was it. I do not like, on purely theoretical grounds, the bland assumption that only selection drives evolution.

 

Please expand on this statement. What do you mean by "drives evolutoin"?

 

There are the hyperselectionists -- such as Dawkins -- that make the statement that natural selection accounts for all traits in all species. The pluralists -- such as Gould and Lewontin -- argued against this. Some traits are not direct products of natural selection -- such as male nipples and the spurs on the ankle bones in pandas.

 

Everyone agrees that natural selection is the only process that gives you the designsin organisms.

 

But I use it, I don't think too abusively, to refer to any situation that restricts interbreeding. Not all of these, of course, are geographic.

 

Not a good idea to invent your own definitions in science. Especially when you are discussing with non-scientists.

 

"Allopatry

The condition of two populations of the same species being separated by a geographic barrier that prevents them from interbreeding." http://www.cartage.org.lb/en/themes/Reference/dictionary/Biologie/A/354.html

 

Allopatry is specifically associated with geographical isolation. As you noted, you can have restrictions on interbreeding among populations in the same geographic area. In fact, Mayr lists the types of processes that result in reproductive isolation. However, each of these has different terms (such as ethnological isolation), so using "allopatry" in the general sense you do is actually confusing.

 

"Classification of Isolating Mechanisms

1. Premating or prezygotic mechanisms: Mechanisms that prevent interspecific matings.

(a) Potential mates are prevented from meeting (seasonal and habitat isolation)

(b) Behavioral incompatibilities prevent mating (ethological isolation)

© Copulation attempted but no transfer of sperm takes place (mechanical isolation)

 

2. Postmating or postzygotic mechanisms:

Mechanisms that reduce full success of interspecific crosses

(a) Sperm transfer takes place but egg not fertilized (gametic incompatibility)

(b) Egg fertilized but zygote dies (zygotic mortality)

© Zygote develops into an F1 hybrid of reduced viability (hybrid viability)

(d) F1 hybrid is fully viable but partially or completely sterile, or produces deficient F2 (hybrid sterility)"

Ernst Mayr, What Evolution Is pg 171

[lucaspa]

Well, it can contribute or even drive speciation, fixing different genes in different populations so that, say, pattern changes enough that they no longer recognize each other as viable mates, or changing various genes that result in hybrid embryos being inviable.

 

I've seen several papers that refute the idea of neutral speciation. Instead, reproductive isolation is the result of natural selection for reproductive isolation, not chance.

 

1. M Nei and J Zhang, Evolution: molecular origin of species. Science 282: 1428-1429, Nov. 20, 1998. Primary article is: CT Ting, SC Tsaur, ML We, and CE Wu, A rapidly evolving homeobox at the site of a hybrid sterility gene. Science 282: 1501-1504, Nov. 20, 1998. As the title implies, has found the genes that actually change during reproductive isolation.

5. Rice, W. R. 1985. Disruptive selection on habitat preference and the evolution of reproductive isolation: an exploratory experiment. Evolution. 39:645-646.

6. Ringo, J., D. Wood, R. Rockwell, and H. Dowse. 1989. An experiment testing two hypotheses of speciation. The American Naturalist. 126:642-661.

7. Schluter, D. and L. M. Nagel. 1995. Parallel speciation by natural selection. American Naturalist. 146:292-301.

1. G Kilias, SN Alahiotis, and M Pelecanos. A multifactorial genetic investigation of speciation theory using drosophila melanogaster Evolution 34:730-737, 1980.

3. KF Koopman, Natural selection for reproductive isolation between Drosophila pseudobscura and Drosophila persimilis. Evolution 4: 135-148, 1950.

1. Speciation in action Science 72:700-701, 1996

 

Well, that depends. I'd argue that some traits *can* be defined, a priori, as advantageous. A good example would be more potent venom for a snake; it kills the prey faster, thereby reducing the risk of injury to the snake or the risk of loss of prey, but doesn't actually cost any more to make, as it's just a differently-shaped protien (same amino-acid number would mean same metabolic cost to manufacture a given quantity).

 

However, the argument is first based on the hypothesis that venom itself is advantageous. And this, in turn, is a posteriori. It's established empirically that the cost of producing venom is offset by the benefits of venom killing the prey. Once you have venom as advantageous by post hoc analysis, then you can make the a priori argument about more potent venom.

 

Also, there can be a cost in manufacturing or obtaining the amino acids. Thus, changing one amino acid for another is not necessarily neutral.

 

Eventually, there is not going to be any selection pressure for more potent venom. After the venom reaches a point where it kills the prey reliably quickly enough that the prey doesn't get away, then any increases on this would not be selective (in and of itself). This more potent venom would be selectively neutral.

 

However, note that if more potent venom is linked to an increase in the size of the snake, then the more potent venom kills larger animals and again confers a selective advantage.

[Xerxes]

Please expand on this statement. What do you mean by "drives evolutoin"?

OK, "drives" is not the best word. I merely meant that selection is not the only known mechanism by which evolution (aka change in allele frequency over time) can occur.

 

 

Everyone agrees that natural selection is the only process that gives you the designs in organisms.

Yikes! What on earth can you mean by designs? Assuming you're not an adherent of ID, let me try and guess.

 

"An organism is designed to fit its environment perfectly by natural selection" Something along those lines, perhaps? If this a wrong summary of your position, please say. Yes, it is true in general that natural selection operates to match phenotype to environment as closely as possible. This would be an asymmtote if environments were stable, which they are not in general.

 

But allele frequecies change regardless of this "pressure"

 

 

 

Not a good idea to invent your own definitions in science. Especially when you are discussing with non-scientists.

True, but as I said in an earlier post, if I a) admit I am slightly abusing terminology and b) am willing to explain what I mean, I cannot see the problem. Terminology is, after all, entirely abtitrary.

 

"Allopatry

The condition of two populations of the same species being separated by a geographic barrier that prevents them from interbreeding."

Of course that is the historical definition. But surely you can see that that any barrier that prevents sub-populations from interbreeding leads to precisely the same outcome, so there is no need for multiple definitions.

 

Oh by the way, I note that Mayr's classification uniformly uses (or pre-supposes) some form of isolation. Personally I do not see a functional distinction between them.

 

P.S. I note you have several other posts prior to the one that I just responded to. Let me get to them.

[Xerxes]

"1 : capable of being put to use; especially : serviceable for an end or purpose <useful tools>

2 : of a valuable or productive kind "

 

Now, where is "do better"?

OK, I refuse to argue the finer points of semantics. If you cannot see "useful" embedded if either of these definitions, then fine. I can. But it really isn't substanttive.

 

2. Hardy-Weinberg gives you a starting point to determine what will happen if there is NOTHING acting on the population. That give us an objective place to determine if something is happening to the population.

Good - H-W can be "reversed" in the sense that if their equilibrium doesn't apply, then something else must be going on. But what?

As you implied, the effect does not have to be natural selection. It can be genetic drift. BUT, there are mathematical tests to distinguish between the two.

Yay, not only drift! H-W is a theorem in population genetics, all sorts of effects can lead to deviation form H-W equilbrium. I invite you to show me the "mathematical tests" that distinguish the various population effects that lead to lack of equilibrium in this sense. I cannot bring any to mind, maybe I'm being slow here.

 

 

 

Nope. Because 'x is true' is not the same statement as 'x is true in general'. You have added the phrase "in general" and, thus, changed the statement.

Look, the statement "sqrt(x) is an integer" is true when x = 4. This statement is false for almost all other integer values of x. So the statement cannot be generalized, and I say that sqrt(x) = integer is false in general.

 

Do we really care about this issue? Are we trying to score points (I hope not). I cannot understand the problem (but if you do have one, I am quite willing to use different terminology, it's a trivial thing).

[Xerxes]

No, we don't. In particular several very prominent evolutionary biologists do not agree that "change in allele frequency" is evolution. It is incomplete.

No, this is not right, whoever states it.

 

" 'Evolution' implies change with continuity, usually with a directional component.

Directional?????? I have no idea what that means. Pray tell.

 

This reductionist definition omits the crucial aspects of evolution: changes in diversity and adaptation.

I have no idea what Mayr thinks he means by "changes in diversity". Do you? Maybe change in allele frequency? Hm. Adaptation is, I remind you once again, is an a posteriori judgement.

 

 

 

there is variation in one or more characteristics among the members of the population;

Good heavens!

there is HEREDITARY SIMILARITY between parent and offspring entities;

And what the **** does hereditary similarity mean? It is meaningless.

and over the course of generations there may be changes in the proportions of individuals with different characteristics within populations.

Hey! We call it the change in allele frequence over time!

It is natural selection that causes adaptation -- improvement in function.

Ha! Every time I see that "I" word I want to throw up.

 

Sorry, but I seriously doubt your source here.

[lucaspa]

OK, "drives" is not the best word. I merely meant that selection is not the only known mechanism by which evolution (aka change in allele frequency over time) can occur.

 

Natural selection is by far the overwhelmingly predominant mechanism for changing allele frequency. Of course, you did note the objections to equating change in allele frequency to evolution, didn't you? Changing allele frequency is part of evolution, but evolution is a lot more than just changing allele frequency.

 

The other "major" way to change the frequency of alleles in population is chance. But, as you know from the equations, unless the population size is very small, chance alone has very poor odds of fixing an allele in a population and it works very slowly, taking millions of generations to either fix or eliminate an allele from a population. This is what allows population geneticists to determine whether changes in a population are due to genetic drift (chance) or natural selection.

 

Yikes! What on earth can you mean by designs? Assuming you're not an adherent of ID, let me try and guess.

 

I mean just what everyone else means by designs: "to devise for a specific function or end". Notice that you have "designed ... by natural selection". The presumption with ID is that "design" always has the attached prepositional phrase "by an intelligent entity". Thus, their argument is that if you see something that has a specific function or end, then it was manufactured (designed) by an intelligent entity.

 

Darwin's genius was in discovering an unintelligent process that also "devised for a specific function or end". Darwin agreed with Paley that living organisms exhibit traits that have a specific function or end. He just found another means -- a "secondary cause" -- to make them. Natural selection is an algorithm for producing designs: follow the steps and design is guaranteed. You can find this whole argument done in much greater detail in Daniel Dennett's Darwin's Dangerous Idea.

 

Evolutionists have been tiptoeing around the issue of "design" for a long time. Dawkins used the term "designoid". I simply agree with Dennett that we ought to face the problem head-on. Let's challenge that hidden prepositional phrase used by creationists "by an intelligent entity" and realize that designs can arise by sources other than an intelligent entity.

 

But allele frequecies change regardless of this "pressure"

 

Please provide the equations and examples to show this. Hardy-Weinberg states (and shows mathematically) that, in populations meeting certain characteristics, allele frequencies are constant.

 

Futuyma discusses this on pages 236-263 of Evolutionary Biology.

 

He starts off by stating:

"The Hardy-Weinberg principle is the foundation on which almost all of the theory of population genetics of sexually reproducing organisms -- which is to say, most of the genetic theory of evolution, rests. Its importance cannot be overemphasized. We will encounter it repeatedly in the theory of natural selection and other causes of evolution."

 

"An important consequence of the Hardy-Weinberg principle is that no matter what the past history of a population may have been, a single generation of random mating yields the Hardy-Weinberg genotype frequencies."

 

After one generation of random mating, gene frequencies will be p^2:2pq:q^2.

 

Now, Hardy-Weinberg has several assumptions.

 

1. Mating is random. So sexual selection will skew that. (However, several studies have shown that sexual selection is a subset of natural selection, since selection is based on fitness characteristics of the mates.)

 

2. The population is very large. So, if there is a finite population, pure random chance could alter the gene freqencies in the next generation. This is random genetic drift.

 

3. Genes are not added from outside the population. Immigrants may carry different gene frequencies, and this is gene flow or migration.

 

4. Genes do not change from one allelic state to another. Such alteration is termed mutation.

 

5. All individuals have equal probability of survival and of reproduction. Natural selection upsets this.

 

So, Futuyma concludes (with italics):

"Inasmuch as nonrandom mating, chance, gene flow, mutation, and selection can alter the frequencies of alleles and genotypes,these are the major factors of evolutionary change within populations." pg 237

 

Now, Evolutionary Biology is the major textbook for college level evolutionary biology courses. If you want to argue with a textbook, you are going to have to provide considerable amounts of data to do so -- since the textbook already contains considerable amounts of data to back it.

 

True, but as I said in an earlier post, if I a) admit I am slightly abusing terminology and b) am willing to explain what I mean, I cannot see the problem. Terminology is, after all, entirely abtitrary.

 

Terminology is NOT "entirely" arbitrary. It is related to observations. I submit that your definition of "slightly" is different from the standard usage. I can't see a reason to alter the terminology. The problem is that your alterations lead to confusion and are unnecessary.

 

Of course that is the historical definition. But surely you can see that that any barrier that prevents sub-populations from interbreeding leads to precisely the same outcome, so there is no need for multiple definitions.

 

Yes, there is, because the generic term we are talking about is reproductive isolation. What are the processes that result in reproductive isolation? Allopatry -- geographical isolation -- is one of them. As you note, there are others. So calling ALL of them "allopatry" simply confuses the issue. If you want to talk about the generic topic, then say "reproductive isolation"

 

Oh by the way, I note that Mayr's classification uniformly uses (or pre-supposes) some form of isolation. Personally I do not see a functional distinction between them.

 

Mayr's classification is the different ways that reproductive isolation can occur. DUH. So he is describing the processes that result in reproductive isolation. You are trying to substitute "allopatry" for "reproductive isolation". Again, since we already have a perfectly good term -- reproductive isolation -- your substitution only leads to confusion.

 

If you mean "reproductive isolation" -- and you do -- just say so.

[lucaspa]

No, this is not right, whoever states it.

 

Then you must show why Mayr's argument is not right. Simple denial doesn't cut it. Mayr gave the reasons that "change in allele frequencies" is an inadeqate definition: "This reductionist definition omits the crucial aspects of evolution: changes in diversity and adaptation. "

 

A change in allele frequency does not tell you why populations split and we have cladogenosis. Change in allele frequency only gives us anagenesis.

 

Also, changes in allele frequency does not tell us why we see adaptations (designs) in organisms. A change in allele frequency could simply leave the organism either: unchanged in adapation or less well adapted.

 

Sorry, but I seriously doubt your source here.

 

You did see the source, didn't you? Douglas Futuyma. IF you are really into evolutionary biology (which I'm beginning to seriously question), you must know who Futuyma is. Here is the PubMed search page for him:

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?CMD=search&DB=pubmed

 

Here's the Amazon.com search of his books: http://www.amazon.com/s/ref=nb_ss_b/002-9368283-7697633?url=search-alias%3Dstripbooks&field-keywords=Douglas+Futuyma&Go.x=8&Go.y=8

 

Here's his faculty page: http://life.bio.sunysb.edu/ee/futuyma.html

 

You might want to note this from that page:

 

"the President of the Society for the Study of Evolution and the American Society of Naturalists, and the editor of Evolution. He is the author of the successful textbook Evolutionary Biology."

 

I drew the definition from that "successful textbook". "Successful" here means that evolutionary biologists use that book as textbook in their classes.

 

So, if you are going to argue against the President of the Society for the Study of Evolution and the editor of the journal Evolution about the defintion of evolution, be prepared to back your claims with lots of data.

 

Of course, you could also try to argue against the National Academy of Science:

 

"Evolution in its broadest sense explains that what we see today is different from what existed in the past. Galaxies, stars, the solar system, and Earth have changed through time, and so has life on Earth.

"Biological evolution concerns changes in living things during the history of life on Earth. It explains that living things share common ancestors. Over time, biological processes such as natural selection give rise to new species. Darwin called this process "descent with modification," which remains a good definition of biological evolution today." Appendix and Frequently Asked Questions Science and Creationism, A View from the NAS, the section "What is Evolution?" pg 27

 

Notice that they avoided "changes in allele frequencies" in their defintion.

 

Or you can try another other successful college-level introductory evolutionary biology textbook:

 

"Most of the processes described in this book concern change between generation within a population of a species, and it is this kind of change we shall call evolution. When the members of a population breed and produce the next generation, we can imagine a lineage of populations, made up of a series of populations through time. Each population is ancestral to the descendant population in the next generation: a lineage is an "ancestor-descendant" series of populations. Evolution, then, consists of change between between generations within a population lineage. Darwin defined evolution as "descent with modification," where the word "descent" refers to the way evolutionary modification takes place in a series of populations that are descended from one another." Mark Ridley, Evolution, 2nd Edition (1996) pg 4.

 

"1. Evolution means descent with modification, or the change in the form, physiology, and behavior of organisms over many generations of time. The evolutionary changes of living things occur in a diverging, tree-like pattern of lineages." pg 19.

 

Directional?????? I have no idea what that means. Pray tell.

 

How can you be an evolutionary geneticist and not know? The equations for frequency change under positive selection show gene frequency changing in only one direction -- toward fixation. That's "directional"

 

I have no idea what Mayr thinks he means by "changes in diversity". Do you? Maybe change in allele frequency? Hm. Adaptation is, I remind you once again, is an a posteriori judgement.

 

It's Futuyma. You should read more carefully. And, if you would have read the entire definition before you started nit-picking, you would have found this:

 

"Populations may become subdivided so that several populations are derived from a COMMON ANCESTRAL POPULATION. If different changes in the proportions of variant individuals transpire in the several populations,the populations DIVERGE, OR DIVERSIFY. "

 

So, it is clear the "changes in diversity" means that the descendent populations are different from each other.

 

And I cited a couple of papers to you to show that adaptation is not always posterior. Sometimes it has been predicted ahead of time what would be adaptive. You seemed to either have not recognized what the data showed or just chose to ignore it. The second is not good science.

 

Good heavens! And what the **** does hereditary similarity mean? It is meaningless.

 

Oh, please. "hereditary similarity" simply means that offspring are similar to their parents. It is the same thing as Darwin saying: "from the strong principle of inheritance they will will tend to produce offspring similarly characterized. "

 

How can you possibly be a geneticist and not know that offspring are similar to their parents?

 

Now, why the "Good heavens" in reference to variation. Darwin and many others have already demonstrated variation among individuals. As a geneticist, you should be very aware that recombination and mutation mean that, altho offspring are similar to their parents, they are not identical. Nor are siblings (other than identical twins) identical to each other.

 

Hey! We call it the change in allele frequence over time!

 

Yes. No one ever said this didn't happen. We have just said that it is not a complete or sufficient definition of evolution. Back to Mayr. If you confine the definition of evolution to "changes in allele frequency" then you omit critical and necessary parts of evolution. You do know what "omits" means, don't you?

 

Ha! Every time I see that "I" word I want to throw up.

 

Go ahead and throw up. Means nothing. Within the limited scope that Futuyma is using it, "improves" is accurate. Here Futuyma is not using it as a long-term goal or that some traits are absolutely "good" or "better", but in the context that adaptations show an improvement in the ability to use nylon as a food source, or the improvement in the ability to avoid predators by puffing up to a large size, or the ability to survive in an acidic environment, or an improvement in the ability to circumvent prey being immune to venom, or improve the ability to live on mine tailings with high levels of toxic heavy metals:

 

 

1. Birth of a unique enzyme from an alternative reading frame of the pre-existed, internally repetitious coding sequence", Ohno, S, Proc. Natl Acad. Sci. USA 81:2421-2425, 1984.

39. C. Zimmer, How the pufferfish got its puff, Discover, Sept. 1997, pp 30-31 3. D. Grady, Quick-change pathogens gain an evolutionary advantage.Science, vol.274: 1081, 1996 (November 15). The primary research articleis JE LeClerc, B Li, WL Payne, TA Cebula, High mutation frequencies among Eschericia coli and Salmonella pathogens. Science, 274: 1208-1211, 1996 (Nov.15).

7. S Grenard, Is rattlesnake venom evolving? Natural History 109:44-49, July/Aug 2000.

4. Macnair, M. R. 1981. Tolerance of higher plants to toxic materials.In: J. A. Bishop and L. M. Cook (eds.). Genetic consequences of man made change. Pp.177-297. Academic Press, New York.

5. Toxic Tailings and Tolerant Grass by RE Cook in Natural History, vol90(3): 28-38, 1981

[Xerxes]

Natural selection is by far the overwhelmingly predominant mechanism for changing allele frequency.

I doubt this is true. I gave examples earlier of neutral alleles at isozyme and DNA polymorphism loci. Their appearance in a population over time is in no way related to selection (natural selection, that is)

Changing allele frequency is part of evolution, but evolution is a lot more than just changing allele frequency.

In what sense?

 

The other "major" way to change the frequency of alleles in population is chance. But, as you know from the equations, unless the population size is very small, chance alone has very poor odds of fixing an allele in a population and it works very slowly, taking millions of generations to either fix or eliminate an allele from a population.

Chance has ...poor odds? I don't follow. Anyway, as above, we know from evidence that loads and loads of neutral mutations are fixed in effectively H-W populations. How to account for it?

I mean just what everyone else means by designs: "to devise for a specific function or end". Notice that you have "designed ... by natural selection".

Having claimed the right to use my own (defined) terminology, I should be slow to criticize here. But you are getting yourself into a sematic quicksand.

 

Say I am an architect: my job is to "design" buildings. The first building I put up falls down the next day. Was it fit for purpose? No. Was it designed? Yes, but poorly, but it was still designed! Conversely, if I am not an architect, I might well be able, by pure chance i.e. without design, to erect a buildilng that lasts for centuaries.

 

 

 

So, Futuyma concludes (with italics):

"Inasmuch as nonrandom mating, chance, gene flow, mutation, and selection can alter the frequencies of alleles and genotypes,these are the major factors of evolutionary change within populations." pg 237

I fully endorse this. Once again, I merely note that selection is only one of many mechanisms by which allele frequencies change over time. This, all along, has been my only, very minor, point.

 

You are trying to substitute "allopatry" for "reproductive isolation". Again, since we already have a perfectly good term -- reproductive isolation -- your substitution only leads to confusion.

 

If you mean "reproductive isolation" -- and you do -- just say so.

Look, if it means so much to you, or anybody else, I am happy to do that. I am not bogged down with particular terminogies, I am quite flexible here. It's no big deal.

[lucaspa]

Good - H-W can be "reversed" in the sense that if their equilibrium doesn't apply, then something else must be going on. But what? Yay, not only drift! H-W is a theorem in population genetics, all sorts of effects can lead to deviation form H-W equilbrium. I invite you to show me the "mathematical tests" that distinguish the various population effects that lead to lack of equilibrium in this sense. I cannot bring any to mind, maybe I'm being slow here.

 

I suggest chapter 13 in Douglas Futuyma's Evolutionary Biology. Pay particular attention pages 371-373. Futuyma also cites this book as giving a comprehensive treatment:

Endler, JA Natual selection in the Wild Princeton University Press, 1986

 

8. Gilad Y, Rosenberg S, Przeworski M, Lancet D, Skorecki K. Proc Natl Acad Sci U S A. 2002 Jan 22;99(2):862-7. Evidence for positive selection and population structure at the human MAO-A gene. http://www.pnas.org/cgi/reprint/99/2/862

 

"Data Analysis. We calculated three summaries of diversity levels: Watterson’s W (9), based on the number of segregating sites in the sample; (10), the average number of pairwise differences in the sample; and H, a summary that gives more weight to high frequency-derived variants (11). Under the standard neutral model of a random-mating population of constant size, all three summaries estimate the population mutation parameter 3N(for X-linked loci), where N is the diploid long-term inbreeding effective population size, and is the mutation rate per generation. To test whether the frequency spectrum of mutations

conformed to the expectations of this standard neutral model, we calculated the value of three test statistics: Tajima’s D (12), which considers the difference between and W, Fay and Wu’s H test (11), which considers the difference between and H, and the HKA (Hudson–Kreitman–Aguade) test (13), which tests whether levels of polymorphism are consistent with levels of divergence, as expected under the neutral model, by comparison

with one or more reference loci."

 

That's 3 different tests. The equations didn't come thru well on copy and paste, but the article is free and you can get your own PDF copy to read.

 

Look, the statement "sqrt(x) is an integer" is true when x = 4. This statement is false for almost all other integer values of x. So the statement cannot be generalized, and I say that sqrt(x) = integer is false in general.

 

You still didn't provide a source of a colleague using the statement. You have seemed to have changed the terminology. We were talking about "true in general" but now you have "false in general". Apples and oranges. This does make more sense and I have seen this phrase used in my math classes and textbooks. It means that the equation is false for most values of x but that there are some values for which it is valid.

 

BUT, this isn't the original we were discussing.

 

It appears that what you are trying to do is extend what you do in mathematics outside of it. Let's do this in science:

 

What you have here is the statement "sqrt(x) is an integer". That's really a hypothesis. It implies that the statement is true for ALL values of "x". As you have shown by testing, most values of "x" do not yield an integer. Therefore, as stated, the hypothesis is false. What happens in science, as opposed to what you are doing here, is to modify the hypothesis. We can probably come up with an equation (other than using integer^2) to describe the sequence 4, 9, 16, 25, 36 etc. Maybe not. I'm too restricted by time to try. But if all else failed, we could say "sqrt(x) is an integer only when x = integer^2". That's a supported hypothesis (altho somewhat circular and uninteresting). But it's more how science works rather than say "generally true".

[Xerxes]

We were talking about "true in general" but now you have "false in general". Apples and oranges. This does make more sense and I have seen this phrase used in my math classes and textbooks. It means that the equation is false for most values of x but that there are some values for which it is valid.

Here is my original comment, the one to which you took objection.

 

The mistake, in my opnion, is that made by people like Des Morris, who went that one step too far: if it exists today, it must have been selected for, which we now know to be false in general

 

BUT, this isn't the original we were discussing.

 

It appears that what you are trying to do is extend what you do in mathematics outside of it.

To which I can only say, if it is not true in mathematics/logic, it's not true in "science"

[lucaspa]

I doubt this is true. I gave examples earlier of neutral alleles at isozyme and DNA polymorphism loci. Their appearance in a population over time is in no way related to selection (natural selection, that is) In what sense? Chance has ...poor odds? I don't follow

 

Apples and oranges. What you are talking about now is mutations when you say "their appearance". Yes, alleles "appear" by mutation -- this is the "variation" in natural selection. But, after they appear, we are talking about changes in their frequency -- the proportion of individuals that have the genes.

 

Now, for evoution -- changes in populations -- to happen, alleles must become FIXED -- become the sole allele in the population. The relative strength of genetic drift to natural selection is very low. Especially if the effective population size is greater than about 10.

 

You need the equations on page 393 of Futuyma. If s = 0 the probability of fixation is P = 1/(2N). Whereas if s >0, then the probability is P=2s(1-e^-4Ns) if s is small and P = 2s if s is larger.

 

So, let's take an effective population of 1000. If s = 0 (neutral), then P = 1/2000 or 0.0005. However, if s = 0.01 (a "large" selection coefficient) then P = 0.02. This is 40 times more probable than if s = 0. Now, if s<0, then P = 0.00004 by using the equation that combines probability of fixation by selection and genetic drift.

 

Anyway, as above, we know from evidence that loads and loads of neutral mutations are fixed in effectively H-W populations. How to account for it?

 

Because the data you have is not fixation! What the data shows is that there are many alleles present in populations. But fixation means that only one of those alleles would be present. You aren't really an evolutionary geneticist, are you? The average time to fixation of a newly arisen neutral allele that actually does become fixed is 4N generations for a diploid population.

 

So, if N = 1000, that is 4,000 generations! So what we are seeing with the data you posted is simply a snapshot in time. If we are dealing with humans and a generation time of 24 years, that is 100,000 years to fixation. Of course, most wild populations are much larger than 1,000. That increases the time a lot.

 

Basically, gene frequencies are essentially unchanged from generation to generation under genetic drift in populations greater than N = 200.

 

Having claimed the right to use my own (defined) terminology, I should be slow to criticize here. But you are getting yourself into a sematic quicksand.

 

Say I am an architect: my job is to "design" buildings. The first building I put up falls down the next day. Was it fit for purpose? No. Was it designed?

 

Actually, the answer is "yes" for both questions. The building was fit for a purpose -- (housing people) -- just badly fit. IOW, it was a bad design. What you have here is 2 criteria -- fit for the purpose of housing human beings and longevity. You are trying to conclude that longevity has to be maintained in order to for the building to be "designed". Does not follow.

 

Yes, but poorly, but it was still designed! Conversely, if I am not an architect, I might well be able, by pure chance i.e. without design, to erect a buildilng that lasts for centuaries.

 

See? You realize it was designed, just poorly.

 

No, your building would still be design. Not chance. After all, you too are being an architect. What you are saying is that you have no formal training in it. But you are designing a building (and manufacturing it). However, what you are saying is that you came up with a more long-lasting design than the professional architect -- but did so without the formal training on what you should be doing.

 

Now, Gould has stated that one of the ways we can recognize that evolution happened instead of ID is that so many of the designs in nature are indeed poor ones. They are buildings that last for a day but not longer. Of course, they don't have to be better, but an intelligence (especially a supernatural one) could be reasonably expected to come up with only good designs.

 

Gould is restating, in fact, an argument used by 19th century theologians to reject Special Creation (ID).

 

I fully endorse this. Once again, I merely note that selection is only one of many mechanisms by which allele frequencies change over time. This, all along, has been my only, very minor, point.

 

I'm glad you "endorse" the obvious. I'm sure that Futuyma is so relieved to have your endorsement.

 

What people are saying is:

1) Selection is the only one of these processes that produces adaptations (designs).

2) That chance, gene flow, and mutations are relatively minor ones of altering frequencies. Mutations, by their very nature, are initially confined to a very small proportion of the population.

3) Sexual selection (nonrandom mating), it turns out, is usually an extension of natural selection.

 

I am happy to do that. I am not bogged down with particular terminogies, I am quite flexible here. It's no big deal.

 

Excellent. It will greatly facilitate communication and discussion to use the standard terms whenever possible. Thank you.

[Xerxes]

 

Now, for evoution -- changes in populations -- to happen, alleles must become FIXED -- become the sole allele in the population.

No, this is not the definition of fixation.

 

You need the equations on page 393 of Futuyma. If s = 0 the probability of fixation is P = 1/(2N). Whereas if s >0, then the probability is P=2s(1-e^-4Ns) if s is small and P = 2s if s is larger.

If I don't know what s, N and P are, how can I comment on what follows? Define terms.

 

But fixation means that only one of those alleles would be present.

As above, this is not what fixation means!

You aren't really an evolutionary geneticist, are you?

Ah. Then I'm done with you my friend. Have a nice day.

[Dak]

Now, for evoution -- changes in populations -- to happen, alleles must become FIXED -- become the sole allele in the population.

 

that's not true. the design of a species isn't an absolute thing: it contains many 'usually this' or 'most often, but not allways, that'.

 

if an advantageous allele has increased so that it is predominant (but not exclusive), than that species has evolved.

 

or: a change in frequency from 0.2 to 0.9 is a change in frequency, and thus evolution; it is not neccesary for the frequency to become 1 before it is evolution.

 

Xerxes: why do you oppose the use of 'improve' so much; the non-random adaptation and increase in suitability to the environment from what are essentially random changes is, imo, the most significant aspect of evolution? i'm not really getting your point about a posteriori?

[CharonY]

If I don't know what s, N and P are, how can I comment on what follows? Define terms.

Out of memory:

P = probability (cough); in this case of fixation

N = population number/size

s = selection coefficient (that is whether the allele confers selective advantages/disadvantager or not/being neutral).

 

And just shortly, fixation with regards to an allele indeed means a frequency increase to one. Maybe there is a confusion with internal equilibria frequencies?

 

In addition, for instance for bacterial population the models are quite different(as offsprings are clones). Ok and there are about a gazillion models trying to estimate fixation probabilities and times in dependence on about the same amount of paramteres, but that's another discussion...

[lucaspa]

that's not true. the design of a species isn't an absolute thing: it contains many 'usually this' or 'most often, but not allways, that'.

 

if an advantageous allele has increased so that it is predominant (but not exclusive), than that species has evolved.or: a change in frequency from 0.2 to 0.9 is a change in frequency, and thus evolution; it is not neccesary for the frequency to become 1 before it is evolution.

 

No. Think of the sickle cell gene in African populations. The frequency of that gene is different from other human populations, but we don't say that H. sapiens has "evolved" because of it. Nor do you say that those African populations have evolved while the rest of the species has not.

 

For every trait or allele, you can make a bell-shaped distribution curve where you plot trait or allele on the x-axis and number of individuals on the y-axis.

 

During evolution, the bell-shaped curve shifts either left or right. In order to do that, some alleles must be lost (at the tail at the back end of the shift) and other alleles must be fixed (at the middle or front of the curve shift). As the curve continues to shift over generations, eventually the new curve will not overlap the old one. THEN we know evolution has occurred. You can see the bell shaped curves depicted and the divergence of them as new species are formed in Futuyma or

1. ME Heliberg, DP Balch, K Roy, Climate-driven range expansion and morphological evolution in a marine gastropod. Science 292: 1707-1710, June1, 2001

2. Kellogg DE and Hays JD Microevolutionary patterns in Late Cenozoic Radiolara. Paleobiology 1: 150-160, 1975.

 

Changing frequencies of an allele within a population is not really evolution, which is another reason why the "change in allele frequencies" is not a definition of evolution. Your example -- which would be equivalent to the peppered moth -- is an example of natural selection at work. But natural selection, by itself, is not evolution either.

 

In order to have common ancestry, and descent with modification, you must have fixation of some alleles and loss of others.

[lucaspa]

No, this is not the definition of fixation.

 

Simple denial without any supporting evidence. If fixation is not when the allele becomes present in every member of the population, then what is it? And please post your source.

 

If I don't know what s, N and P are, how can I comment on what follows?

 

Well, you can drop the pretense of being an evolutionary geneticist. Why did you adopt the pretense to begin with? To try to give your posts some "authority"? Anyone who has done even casual reading in population genetics and evolutionary genetics must know what the terms mean. And, yes, I did identify them. Go back and read carefully. Or Charon was nice enough to give them to you.

 

As above, this is not what fixation means! Ah. Then I'm done with you my friend. Have a nice day.

 

Simple denial without providing any information on what fixation is. Then using this as an excuse to bail on the discussion. Sorry, but I've seen the tactic before. It's not the most graceful way of admitting you were in error, but it is one way.

 

FYI: "In the following, we study the simplest possible question: what is the probability that a single mutant generates a lineage that takes over the entire population? This fixation probability determines the rate of evolution. "

http://www.univie.ac.at/virtuallabs/Moran/

[Dak]

No. Think of the sickle cell gene in African populations. The frequency of that gene is different from other human populations, but we don't say that H. sapiens has "evolved" because of it.

 

tis true, but also redundant.

 

your saying that a local change in allele frequency to less than 1 does not equate to a species-wide evolution

 

i agree, and never said otherwize

 

Nor do you say that those African populations have evolved while the rest of the species has not.

 

yes i do.

 

we could say that the local human population has adapted, which, imo, is evolution, albeit local evolution. what the rest of humanity descides to spend it's time doing does not change this.

 

to say that any shift in allele frequency is not evolution, but the last, tiny, step where the less-common allele drops out of the population is evolution seems entirely arbritrary to me.

 

for an over-simplistic example: imagine a population of animals. each gene has two alleles: c and g (crap and good).

 

for each gene, the freqency is c:0.99 g:0.01

 

ages later, the frequency is c:0.1 g:0.9

 

has that species not evolved? is an average member of the species initially posessed mainly of crap alleles, whilst, ages later, is posessed mainly of good alleles, hence the average suitability of the members of the species inproves over time (alternately: the basic description of the species changes over time) without alleles becoming exclusive.

 

anyhoo... with only one allele form, evolution can't happen: hence, evolution must happen with more than one allele present. to say that the evolution 'hasn't happened' untill it has 'become complete by making an allele exclusive' (which is what it seems you're saying) is, tbh, rather bazarr

[lucaspa]

tis true, but also redundant.

 

your saying that a local change in allele frequency to less than 1 does not equate to a species-wide evolution

 

i agree, and never said otherwize

 

These aren't the original claims:

 

Lucas: Now, for evolution -- changes in populations -- to happen, alleles must become FIXED -- become the sole allele in the population.

 

Dak: if an advantageous allele has increased so that it is predominant (but not exclusive), than that species has evolved.

 

or: a change in frequency from 0.2 to 0.9 is a change in frequency, and thus evolution; it is not neccesary for the frequency to become 1 before it is evolution.

 

You seem to have 3 simultaneous claims:

 

1. Evolution is a shift in gene frequencies without fixation. IOW, fixation is not necessary for the process to be included in evolution.

 

2. A shift in gene frequencies is tied to a change in designs, and that is tied to whether a "species has evolved".

 

Do you see the difference? In 1 you are saying that a shift in gene frequencies is part of evolution. In 2 you say a shift in gene frequencies = a species has "evolved". Those 2 statements are not necessarily saying the same thing.

 

3. You say that a local change in gene frequency is not "species wide evolution" but you do say that a change in gene frequency means the population has "evolved" and use the term specifically for Africans and the sickle-cell allele.

 

Let's start from the top. #1 is correct and my original claim was poorly worded. It does look like I'm saying that shifting allele frequencies within a population is not part of evolution.

 

"Biological evolution may be slight ...; it embraces everything from slight changes in the proportions of different forms of a gene within a population, such as the alleles that determine the different human blood types" Futuyma op cit, pg 4

 

Instead, what I meant to say was that loss of alleles and fixation of alleles is an inevitable result of evolution -- common ancestry:

 

"The geneology of the genes in the present population is said to coalesce to a single common ancestor [individual]. Because that ancestor represents one of the several original alleles, the population, descended entirely from that ancestor, must eventually become monomorphic: one or the other of the original alleles becomes fixed (reaches a frequency of 1.00)" D. Futuyma, Evolutionary Biology, pg 299

 

Of course, that means that the other is lost.

 

So, let me rephrase: When evolution -- changes in populations -- happens, some alleles must become fixed and other alleles must become lost.

 

#2. When we say "evolved", we most often mean complete changes in a population such that it is different from the original "Biology To develop (a characteristic) by evolutionary processes. " http://www.ask.com/reference/dictionary/ahdict/34692/evolve

 

Changes in allele frequency most often don't fit this usage. For instance, the allele frequency in the peppered moths changed twice, but you could always find both light and dark moths in the population. When we commonly think of "evolved", it means that all the moths are now a new color.

 

IOW, the bell-shaped curve of properties/traits has to shift so that there is no overlap with the preceding curve Just shifting gene frequencies won't do that. The curves will still overlap and represent simply variations of one population. You need to drop some alleles entirely and fix others so that the whole population is different.

 

Also remember that changing traits in a population is not (most often) a case of one allele, but an accumulation of many alleles.

 

You maintain that populations rarely have one allele at a locus. That is correct because mutations add alleles to the population. However, remember accumulation. Let's say there is Locus 1 that has alleles A and B. B has a higher selection coefficient than A. Eventually B will become fixed. But in the meantime 2 new alleles appear: C and D. C is worse than B and D but D is better than B (and C). So D becomes fixed. In that process alleles E and F also appear. E is worse than D and F, while F is better than D. So F is fixed.

 

So, where you had a population that was A and B, you now have a population that is either all F or still has some D in it. Completely different set of alleles.

[CharonY]

Well, one probably has to remember that evolution is a gradual event and, what is even worse, the definition of evolution is not as sharp throughout biology than one might want to (slightly similar to the species concept).

A situation in which allele(s) became fixed clearly defines a state that is different from the starting population, so one can say that it has evolved. On the other hand evolution is the process that leads to the changes in a population. As such changes in allele frequencies even if fixation has not happened yet is still evolution. Fixation itself is a rather rare event and mainly occurs in highly inbred populations and is mostly used as a quasi-steady state model (at least in my understanding).

[lucaspa]

Well, one probably has to remember that evolution is a gradual event and, what is even worse, the definition of evolution is not as sharp throughout biology than one might want to (slightly similar to the species concept).

 

I would say the definition is pretty "sharp". Futuyma's definition (echoed by other evolutionary biologists) is sharp. Not short, but sharp. What gets us in trouble is 1) the reductionist definition of change in allele frequencies and 2) the colloquial use of "evolution" or "evolve" to mean a permament change in the population.

 

The definition of "species" is imprecise precisely because evolution is true. With a gradual and continuous transformation of a population from one species to another, there is no way to make a precise definition of species. Some population, somewhere, is always going to fit in the gray area while evolving from one species to another.

[CharonY]

Well Futuyma's concise quote regarding evolution is:

"In the broadest sense, evolution is merely change, and so is all-pervasive; galaxies, languages, and political systems all evolve. Biological evolution ... is change in the properties of populations of organisms that transcend the lifetime of a single individual. The ontogeny of an individual is not considered evolution; individual organisms do not evolve. The changes in populations that are considered evolutionary are those that are inheritable via the genetic material from one generation to the next. Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportion of different alleles within a population (such as those determining blood types) to the successive alterations that led from the earliest protoorganism to snails, bees, giraffes, and dandelions."

 

I copied that from somewhere else because frankly, I cannot find my copy atm.

This quote (especially the last sentence) implies that biological evolution happens within a continuum including also simple alterations of allele frequency (with fixation being the endpoint of such a process). One has to keep in mind that "changes in allele frequency" is not the definition of evolution per se, but it can serve as a minimal definition.

 

Regarding species while what you said has some truth in it, you have consider that there are inherent problems with the definition proposed by Mayr. Take prokaryotes for instance. I assure you, you will have problems defining a species concept that suits them. Moreover, evolution and species concepts are to some point intertwined. You need somewhat closed populations in order to observe evolution and in turn evolution is the reason why different populations exist in the first place. So a fuzziness regarding species will also have an impact on the definition of evolution. I'd be glad if you can prove me wrong but so far I did not ran about anything like sharp, universal definitions for both concepts...

 

Actually how did we come to this. Oh yeah, wheter evolution is only happening when fixation occurs. right.

[lucaspa]

I copied that from somewhere else because frankly, I cannot find my copy atm.

 

You got it from me! I'm the one that posted it.

 

One has to keep in mind that "changes in allele frequency" is not the definition of evolution per se, but it can serve as a minimal definition.

 

No, it can't. Because it leaves out many of the most important things: such as speciation. Evolution includes "changes in allele frequency", but can't be "changes in allele frequency". The NAS uses "descent with modification" as the minimal definition of evolution:

 

Appendix and Frequently Asked Questions Science and Creationism, A View from the NAS, the section "What is Evolution?" says:

 

"Evolution in its broadest sense explains that what we see today is different from what existed in the past. Galaxies, stars, the solar system, and Earth have changed through time, and so has life on Earth.

"Biological evolution concerns changes in living things during the history of life on Earth. It explains that living things share common ancestors. Over time, biological processes such as natural selection give rise to new species. Darwin called this process "descent with modification," which remains a good definition of biological evolution today." pg 27

 

I'd be glad if you can prove me wrong but so far I did not ran about anything like sharp, universal definitions for both concepts...

 

You never made any statement that Futuyma's definition of evolution was not "sharp".

 

The NAS definition for evolution of "descent with modification" is both sharp and universal.

 

Of course, I am the one that agreed that a precise definition of species was impossible -- because evolution is true.

 

And yes, biological species concept applies only to sexually reproducing species. If you are looking at fossils, you use the morphological species concept. If prokaryotes, then it's the genetic species concept.

 

All of them are "fuzzy", because transformation of one species to another is simply not sharp and definitive.

Regarding species while what you said has some truth in it, you have consider that there are inherent problems with the definition proposed by Mayr.

 

Moreover, evolution and species concepts are to some point intertwined.

 

Remember, there were species concepts long before evolution was thought of. Linneaus was a creationist, after all. And the contemporaries of Darwin that classified and named new species were adherents of Special Creation.

 

Now there is an intertwining because evolution is recognized as true. Thus we now have "phylogenetic species concept" and even an "evolutionary species concept".

 

So a fuzziness regarding species will also have an impact on the definition of evolution.

 

The fuzziness of species had an impact in realizing evolution existed in the first place, but not on the definition -- see at end of post.

 

Actually how did we come to this. Oh yeah, wheter evolution is only happening when fixation occurs. right.

 

Which I had already decided I misspoke when I said that. Look post #71.

 

"Nor shall I discuss here the various definitions which have been given of of the term species. No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distanct act of creation. The term 'variety' is almost equally difficult to define; but here community of descent is almost universally implied, though it can rarely be proved." Charles Darwin, Origin of the Species, 6th edition, pg 58

 

"Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgement and wide experience seems the only guide to follow. We must, however, in many cases, decide by a majority of naturalists, for few well-marked and wll-known varieties can be named which have not been ranked as species by at least some competent judge." pg 62

 

"Some few naturalists maintain that animals never present varieties; but then these same naturalists rank the lightest difference as of specific value; and when the same indetical form is met with in two distant countries, or in two geological formaions, they believe that two distinct species are hidden under the same dress. The term species thus comes to be a mere useless abstraction, implying and assuming a separate act of creation. It is certain that many forms, considered by highly-competent judges to be varieties, resemble species so completely in character, that they have been thus ranked by other highly-competent judeges. But to discuss whether they ought to be called species or varieties, before any definition of these terms has been generally accepted, is vainly to beat the air." pg 64

 

Darwin describes the work of De Candolle, who had done an exhaustive study of oak species and varieties.

"De Candolle then goes on to say that he gives the rank of species to the forms that differ by characters never varying on the same tree, and never found connected by intermediate states. After this discussion, the result of so much labour, he emphatically remarks: 'They are mistaken, who repeat that the greater part of our species are clearly limited, and that the doubtful species are in a feeble minority. This seems to gbe true, so long as a genus is imperfectly known, and its species were founded upon a few specimens, that is to say, were provisional. Just as we come to know them better, imtermediate forms flow in, and doubts as to specific limits augment.'" pg 65

 

"Finally, De Candolle admits that out of the 300 species, which will be enumerated in hs Prodromus as belonging to the oak family, at least two-thirds are provisional species, that is, are not known strictly to fulfil the definition above given of a true species. It should be added that De Candolle no longer believes that species ae immutable creations, but concludes that the derivative theory is the most natural one, 'and the most accordant with the known facts in paleontology, geographical botany and zoology, of anatomical structure and classification.'" pg 65

 

Darwin concludes:

"Certainly no clear line of demarcation has as yet been drawn between species and sub-species --that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at, the rank of species: or, again, between sub-species and well-marked varieties, or between lesser varieties and individual differences. These differences blend into each other by an insensible series; and a series impresses the mind with the idea of an actual passage." Charles Darwin, Origin of the Species, 6th edition, pg 66