Help - a question on new species

[julianm]

There is a question about evolution that has been bugging me for a while, and I will be very grateful if someone can help with an explanation.

 

I suppose we define new species as distinct ones that cannot mate with their ancestor species. But if it was random mutation that created a new species, surely it needed to occur at the same time in a breeding pair. If it occurred in only one animal, then the mutation wouldn’t be able to procreate and start that new species.

 

But mutations occurring in a couple (or more individuals) simultaneously seem very very unlikely.

 

What am I getting wrong?

[Greg H.]

You're neglecting the idea of dominant and recessive genes. If the mutation occurred on a dominant gene, then only one of the pair would need to have it in order for it to spread to off spring.

 

That aside, a single point mutation isn't going to create a new species, regardless. It may turn out the new mutation isn't fit, and dies out long before that happens, or that despite the mutation the species remain the same (pale complexions versus dark complexions). It is the accumulation of changes over time that results in new species.

[delboy]

For one species to become another a very large number of mutations would need to build up over many generations to create sufficient difference in the new population. Most mutations create very small differences (or no difference at all) and would never create a new species on their own.

Mutations occur at cell division, so they would exist in the sperm and egg cells - meaning the new cells do not contain exactly the same information as the original cell they divided from. The mutation in either a sperm or an egg will be passed to the offspring regardless of any mutations in the other cell since the genetic material is combined when they come together to create the embryo.

[CharonY]

That is a bit misleading. One has to look at speciation on the population level. In principle a species does not evolve into another one. Instead, there must be a split within the population of sorts that results in reproductive isolation. I.e. generally one species does not become another over time but it splits (though one or the other can get extinct).

In some cases, relatively few mutations are required as for example recently demonstrated for butterfly species (see Kronforst et al Cell reports 2013). Here the authors showed that initial divergence required only changes in a dozen or so loci though after the split genetic divergence accelerated (again, keeping in my mind that we are talking about populations here).

[chadn737]

The problem with understanding speciation is that people do not think in terms of "trees" and time. It is quite common during the course of speciation for two populations to interbreed or at least be capable of interbreeding. Even those populations that are obviously distinct "species" may be capable of hybridization and even producing reproductively viable offspring. This is especially common in plants.

 

Take for instance this representation.

 

 

Think of the circles as individuals or lineages....you can see that as the populations begin to diverge, there is an imperfect separation of different lineages. Early in the divergence of the populations, interbreeding will still be possible, but it will be reduced. With time there will be less and less such mixing, until you reach a point where the two simple stop breeding altogether, even if still capable of such interbreeding.

 

The key to speciation is reproductive isolation. This can occur relatively rapidly...say colonization of an island by a population that is then cutoff from the original population, or it can occur over long periods of time. The true definition of a species, like so many things in Biology, is imperfect, a matter of convenience. Increasingly, I try to dispense with the idea of species in my thinking and instead think in terms of populations.

[julianm]

Thank you all for your answers, very informative!

 

chadn737, as for the definition of species, is it not the ability to interbreed that is the key to differentiating between species? If not, then my question is misleading indeed. If yes, then maybe speciation arises over a bumpy period of decreasing compatibility, with a series of less fertile offspring along the way…

 

Charon, I get the idea of reproductive isolation. Is that a necessary requirement for all new species? EDIT: the Kronforst study you mention answers this question. With a "no".

 

I wonder if I can look at this differently - it is not that new distinct species arise, it is that all intermediary individuals (hybrids, those in the "gap") simply die out or are not numerous enough.

Edited February 14, 2014 by julianm

[Ophiolite]

The concept of species is an artificial one created by humans to better understand the complex interplay of genetics within populations. The transition from one to another is rarely abrupt and so ambiguities will necessarily arise.

[chadn737]

Thank you all for your answers, very informative!

 

chadn737, as for the definition of species, is it not the ability to interbreed that is the key to differentiating between species? If not, then my question is misleading indeed. If yes, then maybe speciation arises over a bumpy period of decreasing compatibility, with a series of less fertile offspring along the way…

 

Charon, I get the idea of reproductive isolation. Is that a necessary requirement for all new species? EDIT: the Kronforst study you mention answers this question. With a "no".

 

I wonder if I can look at this differently - it is not that new distinct species arise, it is that all intermediary individuals (hybrids, those in the "gap") simply die out or are not numerous enough.

 

Interbreeding is the standard definition, but it is a very imperfect one. The problem of what is a species is long debated and even has its own name..."the species problem". Here are some papers on the species problem.

 

The concept of species goes back before evolution or genetics. The first person to give a definition for species based on biology was John Ray in his 1686 volume of Historia plantarum generalis. Thats nearly 200 years before Darwin or Mendel. Aristotle's ideas of essentialism still dominated.

 

Like most, if not all classification systems, this is definition is incomplete and somewhat arbitrary. Do you remember when Pluto was declassified as a planet? The cutoff line of what is a planet is somewhat arbitrary. Pluto for one doesn't care whether it is a planet or not, but we humans impose such standards of classifications onto nature because it helps us organize it and so reduce the complexity to something more easily understood. However, we should always keep in mind that these divisions are somewhat arbitrary, because when we begin to think they are actual and represent hard lines of division, then it leads to problems. This sort of essentialist thinking is one of the problems many Creationists have with grasping Evolution. They think that there is some sort of hard line and so often mistakenly believe that there had to be a point where species A became species B.

 

Perhaps a better way to think about it is with a color wheel (see attachment).

Now if we compare Blue to yellow....there is obviously a difference. But at what point does green and yellow become distinct? If you look at far enough points on the wheel, they are obviously distinct, but in between there is an area where it really becomes impossible to tell if the color is more green or more yellow.

 

Now if we think about the phylogenetic trees...if we compare the beginning and end of a tree...there is a clear distinction, but inbetween....it becomes almost impossible to tell. Our ability to call something species A and species B depends in no small part on the time in which they have been separated from each other and allowed to accumulate differences. Now there are rare exceptions to this rule. For instance, when two plant species hybridize to form a allopolyploid, that hybrid is essentially a new species in a single generation.

[delboy]

Thank you all for your answers, very informative!

 

chadn737, as for the definition of species, is it not the ability to interbreed that is the key to differentiating between species? If not, then my question is misleading indeed. If yes, then maybe speciation arises over a bumpy period of decreasing compatibility, with a series of less fertile offspring along the way…

 

Charon, I get the idea of reproductive isolation. Is that a necessary requirement for all new species? EDIT: the Kronforst study you mention answers this question. With a "no".

 

I wonder if I can look at this differently - it is not that new distinct species arise, it is that all intermediary individuals (hybrids, those in the "gap") simply die out or are not numerous enough.

 

I would also answer 'no' to the reproductive isolation question.

 

Intermediary individuals don't really exist because evolution is just a gradual process. As mentioned, species is an artificial concept, so an 'intermediary' is as much a species as any other. They do not die out but give rise to the next slightly different form - whether that be called a different species at any particular point in time is probably impossible to define, and rather artificial anyway.

[chadn737]

It depends on how we define reproductive isolation. There are three different types of speciation that are determined by geography and the degree of migration between populations.

 

Allopatric speciation does result from reproductive isolation. A population becomes split and reproductively isolated from each other by geography. Say, if you colonize an island and are then cutoff from the original population. These isolated groups then evolve along separate paths.

 

Parapatric speciation occurs when two populations occupy distinct geographical spaces, but migration and interbreeding between the two still occurs.

 

Sympatric speciation occurs when the two diverging populations occupy the same geographical space.

 

In Allopatric speciation, there is a clear reproductive isolation by geography. The question then is how do you get speciation in a sympatric scenario? The problem is, that occupying the same geographic space, its very easy for individuals to interbreed. Therefore you get gene flow between the populations, which maintains allele frequencies and thus preventing speciation. However, geography is not the only means of reproductive isolation. For instance, when individuals reproduce could be of importance. Think of a plant. It normally flowers in the spring. A mutation arises that causes it to flower in the summer, separating its flowering time from the rest of the population by months. Because they flower at different times, this creates a barrier to interbreeding, thus leading to reproductive isolation.

Edited February 14, 2014 by chadn737

[CharonY]

That is precisely the point (and also why I mentioned the Kronforst et al paper. We do make distinctions that are useful in many cases but they do not necessarily reflect the amount of changes in allele frequencies. They are somewhat somewhat necessary for example to look at phylogeny as you need these (somewhat arbitrary) cut-offs to calculate distances, e.g. based on sequence similarities.

This is especially relevant to keep in mind where reproduction is asexual.

 

 

Edit: Crossposted with chadn737.

Edited February 14, 2014 by CharonY

[delboy]

But surely some reproductive isolation is only necessary to create two species from one.

 

If a reproductive population is left for enough time the new individuals would not theoretically be able to reproduce with the original ones. Surely this must happen. So one new species has evolved from one original. Is this speciation or is there another name for it?

[chadn737]

But surely some reproductive isolation is only necessary to create two species from one.

 

If a reproductive population is left for enough time the new individuals would not theoretically be able to reproduce with the original ones. Surely this must happen. So one new species has evolved from one original. Is this speciation or is there another name for it?

 

It all depends upon gene flow between populations. If gene flow is limited enough, as in parapatric speciation what happens is that you get a continuous gradient along a population range, where there is a clear distinction between populations at two ends of a geographic range, but gradients in-between. A great example of this is the Ensatina salamanders in California. Subspecies at either end of the range cannot interbreed, but subspecies inbetween can.

Edited February 14, 2014 by chadn737

[delboy]

 

It all depends upon gene flow between populations. If gene flow is limited enough, as in parapatric speciation what happens is that you get a continuous gradient along a population range, where there is a clear distinction between populations at two ends of a geographic range, but gradients in-between. A great example of this is the Ensatina salamanders in California. Subspecies at either end of the range cannot interbreed, but subspecies inbetween can.

 

I'm not thinking of ring species. I'm thinking of a population with complete gene flow within the population.

[chadn737]

 

I'm not thinking of ring species. I'm thinking of a population with complete gene flow within the population.

 

How is such a population going to diverge unless gene flow is prevented. New mutations may arise, but in general, with gene flow, this recombination will break up any association and accumulation of mutations within a subpopulation. The one exception to this would be particularly large scale mutations that present potential reproductive barriers, such as chromosomal inversions...even in these circumstances, however, there may still be gene flow that counteracts the possibility of speciation.

[delboy]

 

How is such a population going to diverge unless gene flow is prevented. New mutations may arise, but in general, with gene flow, this recombination will break up any association and accumulation of mutations within a subpopulation. The one exception to this would be particularly large scale mutations that present potential reproductive barriers, such as chromosomal inversions...even in these circumstances, however, there may still be gene flow that counteracts the possibility of speciation.

 

I'm not thinking of divergence - one species leading to two - I'm thinking of one species evolving into one new species. This is a way for a new species to arise without reproductive isolation surely, which was the original question.

[chadn737]

 

I'm not thinking of divergence - one species leading to two - I'm thinking of one species evolving into one new species. This is a way for a new species to arise without reproductive isolation surely, which was the original question.

 

In a sense there is reproductive isolation....generational reproductive isolation. Our present day population is isolated from our ancestral populations 200,000 years ago, so there is no gene flow between modern humans and ancient humans. Time is the factor driving reproductive isolation.

Edited February 14, 2014 by chadn737

[julianm]

Thank you for all replies - extremely interesting!

 

So in short, speciation can occur whenever a factor creates a population subgroup. That factor can be geography (as in ring species), mating preferences (Charon's butterflies) or time, all leading to a sort of reproductive isolation.

[Andrew Polyak]

I might be reiterating previous responses, but there are usually speciation events that take place, that prevents gene flow between two groups of organisms.

 

For example:

• A physical barrier could appear between groups of individuals
• Selective mating: such as a blue fly only wanting to mate with other blue flies. Even though a yellow fly can still mate with a blue fly, it chooses not to!
• etc.

 

This lack of gene flow could eventually lead to speciation.