Parasite Fitness

_{July 22, 2011

Intelligent Design}

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Over at PhysOrg.com, a recent article on ecosystems has shown up. It seems that, studying parasites, of all things, a new “rule” of ecosystems has been established.

A team from UCSB studied various ecosystems on the California coast and discovered that the ‘number’ of individuals in any species population can be predicted not only by its ‘size (an already known ‘rule’), but also by where it resides (top end, or bottom) on the feeding heirarchy.

How, then, does this fit in with Darwin’s Malthusian thoughts? Darwin wants to tell us that fitness is related to ecological niches, and that differential reproduction (NS) is what brings this fitness about. [Malthus, of course, pointed out that many more members of a species is produced each generation then survives to reproductive age.]

But if a species size and location on the feeding order determines how many “numbers” of the species will be found in the ecosystem, then doesn’t this mean that population of any particular species will remain relatively constant from one generation to another?

If so, then what, exactly, does ‘fitness’ mean in such a scenario? What does “differential reproductive success” mean in a, more or less, static population?

The “rule” of the ecosystem isn’t that as you become more fit, you increase in numbers; rather, it is simply that given your size and eating habits, there will be only so many of you in any particular ecosystem. Under these circumstances, then, what is the motivation for changing your phenotype?

This seems, overall, to be an argument for stasis. And, of course, this is what overwhelmingly is seen in the fossil record.

Let the “just-so” stories begin!

Comments

Just so stories can fix anything.Mung_{July 23, 2011
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TaslemGuy @2:
Darwinian evolution does not suggest that a “better species” will have greater populations. It suggests that organisms within a population which are more capable of passing on hereditary information will have a larger effect on the gene-pool.
But what, then, is driving selection? What is selection working on? IOW, the 'size' of the population is independent of its alleles, other than these alleles determine its size and feeding habits. But once that's established, what's the point of changing the alleles? Can you think of one? The Hardy-Weinberg Law, in fact, teaches that the total number of alleles stays constant in an inbreeding, constant size population. But that is what we have here. So, what, exactly, is the motivation for innovation? I see none. "If it ain't broken, don't fix it." But, of course, "just-so" stories area always fascinating!PaV_{July 23, 2011
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Well, no, PaV :) But it's an over-arching theory, and what you use it to predict depends on which bit of it you want to test. And although in Darwin's original formulation he does indeed have Malthusian scenarios in mind, they are not the only scenarios that work with his theory. To take your OP (and thanks for the link, it's an interesting article!:
Over at PhysOrg.com, a recent article on ecosystems has shown up. It seems that, studying parasites, of all things, a new “rule” of ecosystems has been established. A team from UCSB studied various ecosystems on the California coast and discovered that the ‘number’ of individuals in any species population can be predicted not only by its ‘size (an already known ‘rule’), but also by where it resides (top end, or bottom) on the feeding heirarchy. How, then, does this fit in with Darwin’s Malthusian thoughts? Darwin wants to tell us that fitness is related to ecological niches, and that differential reproduction (NS) is what brings this fitness about. [Malthus, of course, pointed out that many more members of a species is produced each generation then survives to reproductive age.]
Well, I think there's a bit unwarranted idea-splitting here. It's not so much that "fitness" is "brought about" by NS. What NS is is heritable differential fitness where fitness is defined very tightly as the ability to produce viable offspring.
But if a species size and location on the feeding order determines how many “numbers” of the species will be found in the ecosystem, then doesn’t this mean that population of any particular species will remain relatively constant from one generation to another?
It could do. Malthusian boom-bust is a bistable form of equilibrium but non-oscillating equilibria are just as possible. As is non-equilibrium, leading to extinction.
If so, then what, exactly, does ‘fitness’ mean in such a scenario? What does “differential reproductive success” mean in a, more or less, static population?
Exactly the same as it means in a non-static population - that that traits born by those who produce the most viable offspring in any generation will be best represented in the next. There is no reason why this will not reach an optimum, at which the death rate equals the birthrate, especially if there are viability penalties for larger numbers of offspring.
The “rule” of the ecosystem isn’t that as you become more fit, you increase in numbers; rather, it is simply that given your size and eating habits, there will be only so many of you in any particular ecosystem. Under these circumstances, then, what is the motivation for changing your phenotype?
Well, in a Darwinian framework, of course, "Motivation" has to be heavily scarequoted! To keep things simple, let's consider a two-species population - grass and browsers - in which the grass is restricted by some kind of sub-Artesian water-supply. So we have a field of grass in a dessert, fed on by some browsing animal. And let's start with a single pair. As the pair breed, and the pair's offspring breed, eventually the population grows to a numerical size at which there is only just enough grass, then overshoots. And let's suppose that there is are various alleles governing size in the population. One of two things could happen next: smaller offspring survive better on the scarcer food supply because they have less body mass to support; larger offspring survive better because they barge the smaller ones out of the way. Now, let's consider two scenarios: in one, the patch of grass is fed by many small water holes. In the other, the patch of grass is surrounded not by a desert but by a lake (it's an island, in other words). In the first, the offspring that do best, once the patch becomes overcrowded, are the big ones, because they have the muscle power to shunt their smaller siblings out of the way in order to reach the water. In the second, size is no advantage - all the animals can reach the lake shore, and the key factor is not having much body mass to support. And so, over time, the first population evolves to be larger (alleles for largeness come to dominate) until the very largest start to dehydrate because their water-requirement is simply too great. At which point, the populating stabilises at a mean body size and population N that can be supported by that patch of grass. Meanwhile, the second population, on the island evolve to have smaller body size, down to the size where the very smallest are at a disadvantage of some other factor that I can't think of right now - maybe they just get trampled on more easily. And the body size and population N again stabilise at values that can be supported by that patch of grass. And I think what the paper is saying is that the animal biomass supported by the grass will be the same in both cases, but in the first it will be a smaller population of larger animals, and in the second, a larger population of smaller animals. And that this general rule applies to parasites as well.
This seems, overall, to be an argument for stasis. And, of course, this is what overwhelmingly is seen in the fossil record.
Yes indeed. Once a population is optimally adapted for its environmental niche, there is nowhere to go but down, and NS becomes a conservative "force" i.e. any novelty is less successful than the status quo and tends not to propagate through the population. Indeed that's why "most mutations are deleterious" - not because there is something inherently deleterious about mutations (most are near-neutral) but because in an adapted population, most novelty is less good than what is already there. However, if the environment changes, either because of climactic or geological change, or because the population moves (either in its entirety or a sub-population migrates), then novelty may well be often better than the status quo and the eek will be punked. At least that's what the theory says - where, in your view, is the flaw? Or have I been completely persuasive? :)Elizabeth Liddle_{July 23, 2011
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Ilion: Darwinism predicts almost nothing. That's why it's so hard to 'falsify'. It's an ex post facto pseudoscience.PaV_{July 23, 2011
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Atom: I've fixed the link. Thanks for pointing that out. As to your question, I suspect that beetles, being of smallish body size, and living off of what, detritus, should be rather abundant in any given ecosystem, given the 'rules'. In terms of the "Designer", it seems that,over and over again, nature presents us with cyclic phenomena. There's perpetual recycling. I would imagine that beetles have a significant role to play in ecosystem recycling, hence explaining their abundance. It would be interesting to see if I'm close to being right on this.PaV_{July 23, 2011
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Does this say anything about the abundance of different species at any level of the food chain? For example, could this help explain why the "designer" had an "inordinate fondness for beetles"? Also, the link to the article seems to have an extra http on the front. (Broken link).Atom_{July 23, 2011
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Your description of Darwinian evolution is incorrect. Darwinian evolution does not suggest that a "better species" will have greater populations. It suggests that organisms within a population which are more capable of passing on hereditary information will have a larger effect on the gene-pool. Darwinian evolution doesn't suggest or predict that changes in population will constantly occur. This finding supports, not detracts, from the theory of evolution.TaslemGuy_{July 22, 2011
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"A team from UCSB studied various ecosystems on the California coast and discovered that the ‘number’ of individuals in any species population can be predicted not only by its ‘size (an already known ‘rule’), but also by where it resides (top end, or bottom) on the feeding heirarchy." So, Darwinism "predicts" that there will be more gazelles than lions, and more grass plants than gazelles?Ilion_{July 22, 2011
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