General, overarching concepts?

[Mokele]

We all know in physics and chemistry that there are several simple, mathematical laws/rules/concepts that apply to everything, such as E=mc^2 or that oppositely-charged atoms/molecules attract each other. However, biology seems to have things tougher: evolution has given us such a cornucopia of diversity that general laws are hard to find.

 

So, here's the challenge of this post: find general, over-arching concepts that pervade either all of biology or at least a major subset of it (all animal life, all plant life, all molecular biology, all ecology), and explain them.

 

I'll go first, with a rule near and dear to my heart: Size matters.

 

Every living organism moves at some stage of it's life, be it seed, spore, larvae or adult, and, furthermore, every organism is subject to phsyical interaction with the environment around it and to physical forces such as gravity or drag. But which forces and interactions predominate are, in a large part, determined by the size of the organism.

 

Image we have a cube of matter, and we hit it with a sci-fi "enlarging ray", doubling it's linear dimensions. A cube that was 1 inch on each side is now two inches on each side. But consider the cross-sectional area and surface area: the former will have gone from 1 square inch to 4, and the latter from 6 square inches to 24. And what of volume, which, if density remains constant, equals mass? 1 cubic inch of volume becomes 8 cubic inches.

 

Now, apply that to an animal. If you double the size of, say, a dog, it's surface areas will be increased 4-fold, including lung surface area, skin area (important in dissipating heat), bone cross-sectional area (determines the force a bone can take), and muscle cross-section (determines the force a muscle can exert). However, it's volume will be increased 8-fold, thus it's mass. So you have 4x the bone support for 8x the mass, and 4x the muscle c/s area moving 8x the mass. You also have 8x the volume of metabolically active, heat-generating cells, but only 4x the surface are to dissipate heat from. This is why animals do not scale up geometrically (with identical proportions); bigger animals have thicker bones and bigger muscles.

 

But it extends to more than just how big your bones are. Gravity and inertial forces act on mass, but fluid florces from air or water act on surface area. As you shrink an animal, the effect of gravity and inertia is reduced cubically, while fluid forces are reduced to the square. When you're small enough, fluid forces predominate, which is why dust particles tend to remain in the air. Baby spiders make cunning use of this fact of scaling. Newly hatched spiderlings are big enough that gravity keeps them down, but only barely. In order to disperse to new places, they climb to the top of some vegetation, and extend a long line of silk. This silk line doesn't increase their mass (it's coming from inside them, after all), but it drastically increases their surface area, enough for the viscous forces to predominate, sweeping the spiderlings off to exciting new areas with plenty of bugs to eat in a dispersal method called 'balooning'.

 

A thermal consequence is apparent when comparing the sizes of mammals to those of lizards. While some mammals are small, the mean size is typically larger than lizards, frogs and salamanders. The reason for this is simple: mammals need to stay warm, generating heat with their metabolic processes, which are proportional to their volume (number and volume of cells). However, they lose heat from their skin. As mammals shrink, there's more surface area per gram of body weight, making heat retention difficult. While there are many secondary adaptations to deal with this, most truly small mammals live on a metabolic razor's edge, feeding nearly continuously to fuel their bodies. In contrast, small ectotherms have an advantage: the same high ratio of surface area to body that causes heat-retention problems for mammals allows these small animals to warm up with only minimal basking in the sunlight. Because mammals have trouble with small size, ectotherms have moved into the vacant ecological niche of truly tiny animals, feeding on tiny insects such as termites or ant larvae.

 

Of course, there's a myriad of other implications of size, from growth to reproductive capacity to population sizes, but I'll leave it here for now.

 

 

So, anyone else? General ideas that seem to pervade biology?

 

Mokele

[bascule]

Wow, an awesome topic! And, as it happens, the friend of mine I leant The Ancestor's Tale to returned it to me just this day!

 

One of the overarching themes of Richard Dawkins work is: progress accumulates. That should be coupled with another important point: progress is often obliterated by mass extinctions, but not completely. Dawkins describes this pattern as looking like a "sawtooth": in "arms races" between predators and prey, both evolve progressively to become more adept at the tasks of catching prey and evading predators respectively, until both are wiped out in a mass extinction event.

 

Dawkins also describes the accumulation of "watershed" events in which lower-level problems evolution has encountered but is unable to solve are, for whatever reason, spontaneously solved, allowing for the development of more "advanced" creatures. Some of these include things like sexual reproduction, which gave birth to the concept of a species and allows novel adaptations to be shared among a population (a concept developed by Dawkins in his book River out of Eden)

 

Dawkins still rejects the intuitive linear and teleological view of "the evolution of man" and contends that one cannot consider the development of any species in isolation. Rather, the entire gestalt of the biosphere is the result of a web of interconnected evolutionary events, each one consisting of a haphazardly occuring adaptation with no concept of the result of what all such adaptations would ultimately amount to. The interplay of the results of these isolated events is what drove the evolution of mankind, which represents the best of what evolution managed to produce in terms of brain designs.

 

I think the point Dawkins was trying to get at was that evolution will inevitably, at some point, stumble upon higher level solutions to problems it has encountered since the dawn of life, and unless the ancestors of the organisms who garner these adaptations all go extinct, they will persist, and accumulate. The result is an emergent, progressive element within the evolutionary system, in the form of increasingly better reality-modelers, ultimately resulting in conscious entities.

 

Dawkins ponders just such a proposition at the end of The Ancestor's Tale:

 

Convergent evolution also inspired the Cambridge geologist Simon Conway Morris, whose provocative book Life's Solution: Inevitable Humans in a Lonely Universe presents exactly the opposite case to Gould's 'contingency'. Conway Morris means his subtitle in a sense which is not far from literal. He really thinks that a rerun of evolution would result in a second coming of man: or something extremely close to man. And, for such an unpopular thesis, he mounts a defiantly courageous case. The two witnesses that he repeatedly calls are convergence and constraint.

 

Convergence we have met again and again throughout this book, including in this chapter. Similar problems call forth similar solutions, not just twice or three times bit, in many cases, dozens of times. I thought I was pretty extreme in my enthusiasm for convergent evolution, but I have met my match in Conway Morris, who presents a stunning array of examples, many of which I had not met before. But whereas I usually explain convergence by invoking similar selection pressures, Conway Morris adds the testimony of his second witness, constraint. The materials of life, and the processes of embryonic development, allow only a limited range of solutions to a particular problem. Given any particular evolutionary starting situation, there is only a limited number of ways out of the box. So if two reruns of a Kauffman experiment encounter anything like similar selection pressures, developmental constraints will enhance the tendency to arrive at the same solution.

 

So, what's my overarching theme to evolution? It's "looking" for a way out of the box. Not looking the way a conscious entity would, but performing a stochastic search, performing a random walk of all paths from the common ancestor to a potential route out of "the box", randomly and haphazardly transcending all limitations of the system imposed by the common ancestor, and eventually it may happen that one line of descendants affords such a route.

 

In terms of the history of life systems, humans are extremely close to escaping the box. I think that's the whole thrust of transhumanism. Ray Kurzweil describes The Singularity as "When Humans Transcend Biology". I think The Singularity represents the point at which we finally escape "the box" and break free of all limitations imposed by our evolutionary legacy.

[Martin]

didn't see this until too late this evening to reply.

getting sleepy. but it's interesting.

 

what examples of convergent evolution, bascule?

I think of the wing and the single-lens eye. something evolving a few or a handful of times. I'd be interested to know of something that evolved independently on the order of 10 times. Legs?

 

one very big concept, for me, is co-evolution.

simple example being fruit and flowers----which coevolved with animals that pollinated and dispersed seeds, I guess.

[bascule]

what examples of convergent evolution, bascule?

 

Heh, I'm sure Mokele can do better than me at this:

 

The fins of whales and the fins of fish

The wings of bats and the wings of birds, or the wings of insects

The bill of the Duck Bill Platypus and the paddle of the Paddlefish

Dozens of examples between South American marsupials and North American placental mammals (the former were wiped out following the formation of a landbridge between the continents)

 

http://en.wikipedia.org/wiki/Convergent_evolution

 

I think of the wing and the single-lens eye. something evolving a few or a handful of times.

 

According to Dawkins the eye appeared in some 40 different varieties throughout the course of evolution. He called this "achingly eager to evolve eyes" or something to that effect.

 

I'd be interested to know of something that evolved independently on the order of 10 times. Legs?

 

Eyes certainly did

 

one very big concept, for me, is co-evolution.

simple example being fruit and flowers----which coevolved with animals that pollinated and dispersed seeds, I guess.

 

One of the most interesting examples of this I learned about recently was the Portugese Man of War, which is in fact four different animals who live together in a colony, all providing different functions.

[Mokele]

Legs are one thing, but so is the loss of them. It's occured 6 times in lizards (one being snakes, the others being lesser-known limbless or near-limbless forms), 3 times in extant amphibians, and at least once more in prehistoric amphibians.

 

I both agree and disagree with Dawkins on "rewinding the tape". Solutions would occur again, yes, but not necessarily the *same* solutions. Some mechanism of force generation like muscles may evolve again, but maybe it would push rather than pull? Intelligent species, maybe, but it likely wouldn't look like us. I think the two are talking past each other: Gould was saying that the morphology and biology of life might be very different a second time around (solving problems in new ways), while Dawkins is noting that the same problems will still have to be tackled.

 

Mokele

[daneeka]

....mammals need to stay warm, generating heat with their metabolic processes, which are proportional to their volume (number and volume of cells). However, they lose heat from their skin. As mammals shrink, there's more surface area per gram of body weight, making heat retention difficult. While there are many secondary adaptations to deal with this, most truly small mammals live on a metabolic razor's edge, feeding nearly continuously to fuel their bodies.

Mokele

 

Metabolic rate could also influence speciation rates because it can have have a major effect on rates of molecular evolution - metabolism generates by-products that can oxidise DNA (like free radicals) and consequently cause mutations. So a higher metabolic rate results in increased DNA oxidation and therefore greater rates if molecular evolution; and, because molecular evolution is needed for speciation in both sympatric and allopatric populations, increased rates would therefore result in a greater potential for speciation. Further, generation time is also influenced by metablic rate (isn't it?) - a small rodent, with an extremly high metabolic rate, maintains a much faster generatioin time than large bodied mammals - and this too could have an effect on speciation because shorter generation time means a greater number of germ cell devision, and consequently greater chance of replication errors per unit time.

 

The consequences of this, to drag this back on topic, is that in highly productive areas (like low latitudes) you get greater metobolic rates which increases speciation rates which in tern increases species richness. A general concept in biology is that species richness increases along a latitudinal gradient, from high to low lattitudes.

[Mokele]

On top of all of that, small species are able to make more use of microhabitats and may be able to divide niches that larger species cannot (think of all the hundreds of species of Anolis lizards specialized for living on various parts of the same tree), and are more suceptible to geographic barriers to reproduction (a 5 foot wide stream is nothing to a horse, but could isolate shrew populations on each side).

 

Mokele

[FreeThinker]

I will follow the trend of this post, and quote Dawkins “If we wiped out most of the living organisms on this planet besides bacteria, we would still preserve around 95% of life’s diversity”. (Actually, I don’t think Dawkins invented the phase, but he used it in his book The Ancestors Tale). To be successful, and to have a long linage on the evolutionary tree, you have to be prokaryotic. If we go back to when the first single cellular organisms appeared, than suddenly jump to the present, we could be forgiven for assuming that they are the only organisms on the planet (with a few exceptions to the rule).

 

I don’t know what this rule would be ‘summed up” as. Maybe, simplicity is vital for long term survival.

[bascule]

I don’t know what this rule would be ‘summed up” as. Maybe, simplicity is vital for long term survival.

 

I think there's a more important question we can ask about this:

 

Let's say we wipe out everything except for prokaryotes

 

1) Would we see a new eukaryote-like organism evolve?

2) Would multicellular organisms evolve?

3) Would sexual reproduction evolve?

4) Would animals with mammal-like intelligence evolve?

5) Would consciousness evolve?

 

I see these as inevitabilities, provided life doesn't go completely extinct.

 

I also admit the conceit of hindsight and the teleological phrasing of the question. Perhaps a better way to phrase these than "Would X evolve?" is "Would circumstances inevitably arise so that X is favored?"

[Martin]

 

I see these as inevitabilities, provided life doesn't go completely extinct.

I assume you mean LIFE ON EARTH.

 

You say "if life on earth doesnt go extinct, then such and such is inevitable"

 

 

logically not too meaningful bascule because it does not have some kind of timescale

 

we know that life on earth WILL go extinct (e.g. red giant, but long before that with say a 20 percent hotter sun)

 

your statement is like saying the following trivially true statement

 

"if 2+2 = 5 then it is inevitable that Mozart's Magic Flute will be performed here at my house"

 

a false premise can be used to imply anything one wishes.

 

===========================

 

Life on earth has a limited time to get its act together.

In terms of evolutionary time, it is not all that much.

============================

 

Are you familiar with the proposals to move the earth farther and farther from the sun in order to keep it in the habitable zone?

 

I don't remember the time-table, but it is known that the sun will get some 10 percent hotter on a timescale which is less than a billion years. I don't remember what it is. It could be on a timescale of 100s of millions of years. And it will keep on getting hotter.

 

this is not the "end of life" stuff, the red giant stuff, that I am talking about. there is an arc in stellar output as part of the normal way stars act.

=================

 

a planet does not have an infinite period of habitability.

the life on a planet does not get unlimited chances.

 

I think you probably know all this, maybe you were talking about something else with different assumptions

[bascule]

logically not too meaningful bascule because it does not have some kind of timescale

 

That's the problem with epiphenomena... they don't operate within set timescales. Since evolution depends on a number of different events which don't occur with any sort of regularity, so it becomes impossible to specify a timescale. Although, if you do want a rough estimate, I'd say a billion years or so.

 

your statement is like saying the following trivially true statement

 

"if 2+2 = 5 then it is inevitable that Mozart's Magic Flute will be performed here at my house"

 

a false premise can be used to imply anything one wishes

 

I admit the unbounded timescale poses a problem, as do issues like a changing sun. I do hope you see the point I'm getting at, despite the flawed way I'm expressing it. I'm getting back to the whole "rewinding the tape" idea and how the constraints of biological systems will ultimately lead them towards the same innovations, although the specific nature of those innovations would of course be dramatically different from what exists today. A "nookaryote" would have a radically different internal architecture from a eukaryote, but the basic "goals" it would accomplish with its internal architecture would be highly similar.

 

Life on earth has a limited time to get its act together.

In terms of evolutionary time, it is not all that much.

 

On the evolutionary timescale, it's a blink of an eye.

 

Are you familiar with the proposals to move the earth farther and farther from the sun in order to keep it in the habitable zone?

 

No. That sounds silly. I don't think any humans will be around by the time it becomes an issue.

 

I don't remember the time-table, but it is known that the sun will get some 10 percent hotter on a timescale which is less than a billion years. I don't remember what it is. It could be on a timescale of 100s of millions of years. And it will keep on getting hotter.

 

Within the constraints of biological systems, life has an amazing ability to adapt.

 

this is not the "end of life" stuff, the red giant stuff, that I am talking about. there is an arc in stellar output as part of the normal way stars act.

 

a planet does not have an infinite period of habitability.

the life on a planet does not get unlimited chances.

 

I think you probably know all this, maybe you were talking about something else with different assumptions

 

I'd be lying if I said it were possible to talk about life systems in these sorts of teleological terms with any degree of certainty. What I'm discussing are trends, not certainties. However I think the constraints of biological systems cause evolution to naturally tend towards particular configurations in similar situations, given a similar starting point. It's constraint that drives convergence, and I think with enough blind stumbling life systems (on earth, yes, on earth. Earth conditions are one of the constraint) will iterate over the same list of "solutions" I outlined above over and over again. Whether or not the planet will remain habitable enough is a different matter entirely, but I'd expect similar timetables overall to what happened in the first place.

 

I was quite astounded by St. Edward's University chemist Eamonn Healy, who describes in the film Waking Life the acceleration of evolution by breaking it down into "two billion years for life, six million years for the hominid, a hundred-thousand years for mankind as we know it" then describes the acceleration of human cultural evolution as being ten thousand years for agriculture, four hundred years for the scientific revolution, and one hundred fifty years for the industrial revolution.

 

Evolution has always seemed like a system asymptotically approaching a final endpoint to me...

[DaveC426913]

Evolution has always seemed like a system asymptotically approaching a final endpoint to me...

That because you make the huge and mostly incorrect assumption that the drivers for evolution are fixed.

 

If their environment and other factors did not change, you would be correct, organisms would reach a peak evolved state and evolve little more. In fact, this does happen on small scales and over certain time periods (Well, OK bacteria have done pretty good at large scale long-term stability**.)

 

The lion's share of ecosystems have countless factors that change all the time (which is why it is literally impossible with current computing power to simulate it except in the most sketchy way.)

 

**OK. And sharks.

And Algae.

Ferns. Coelocanths. Cockroaches.

 

(All right, but other than that - what have the Romans done for us?)

[JHAQ]

An overarching theory to correlate in a coherent manner the natural selection (NS) of memes equivalent to to that which happens with genes . I feel there are many parallels in that both create & are in response to CHANGE .

[bascule]

That because you make the huge and mostly incorrect assumption that the drivers for evolution are fixed.

 

No, I don't.

 

If their environment and other factors did not change, you would be correct, organisms would reach a peak evolved state and evolve little more.

 

I don't think you understood what I said at all. Never did I say there was a "peak evolved state" at which evolution would stop...

 

And nothing I said assumes static evolutionary conditions

[SkepticLance]

For those who are interested, it has been calculated that life on Earth will go extinct in less than 500 million years. This is due to the sun heating up, to the point where the entire planet gets a lot hotter than 100 C. The sun will swell and actually engulf the Earth in about 5 billion.

 

Most life on Earth will be gone in 400 million, with just a few extremophiles left. This is a small time period against the 3 to 4 billion years life has existed.

 

If humanity survives long enough, though, Earth life should by then be well established on other worlds.