Evolution theory mechanisms
I recently had a discussion with someone more expert on evolutionary theory than I. When I cited natural selection as the mechanism by which evolution works, he told me that natural selection is only one such mechanism, and that there others. By the time he finished talking, I'd forgotten to ask what I'm asking here.
What ARE the other mechanisms, if any?
(And a related question, while I'm here. I've been told that "survival of the fittest" is not an apt description of how natural selection works, and that a better description is "survival of the most adaptable". Is that correct?)
Re: Evolution theory mechanisms
Quote:
Originally posted by Liberal
I recently had a discussion with someone more expert on evolutionary theory than I. When I cited natural selection as the mechanism by which evolution works, he told me that natural selection is only one such mechanism, and that there others. By the time he finished talking, I'd forgotten to ask what I'm asking here.
What ARE the other mechanisms, if any?
(And a related question, while I'm here. I've been told that "survival of the fittest" is not an apt description of how natural selection works, and that a better description is "survival of the most adaptable". Is that correct?)
Sexual selection is, I think, typically treated as something separate from natural selection. Then there's also genetic drift -- that is, the tendency, in the absence of selective pressure, for the commonness of different alleles to change, just purely by chance.
Anyone got any others? Have Darwin's Finch or Stranger on a Train joined the Dome?
Re: Evolution theory mechanisms
Sexual selection is indeed another evolutionary pressure. To expand on that a little, consider the peacock. The male grows huge, gaudy, colorful feathers, which help him attract a mate. However, those feathers aren't exactly the best idea for avoiding predators. If your feathers are too big and shiny, you're a bigger target for predators, but if your feathers are too small, you won't attract a mate. So you have natural selection and sexual selection competing against each other.
Re: Evolution theory mechanisms
I'm not sure I'd really refer to natural selection as being a mechanism so much as an underlying principle of evolutionary synthesis, i.e. that evolution tends to favor organisms who are most successful at surviving to reproduction. (This may sound like a simplistic tautology, but in fact this very concept was not widely accepted until Darwin's famous publication on the topic and subsequent extensive studies.) Sexual selection--that is, the preferential selection of mates based upon some expressed phenotype or interspecific competition, and the wide conduction of said phenotype to successive generations--is but one form of natural selection. Environmental selection--due to the ability of an organism to survive to sexual maturity and successful conduct gamete transfer--is another subset of natural selection. Under these categories is a wide variety of specifically defined mechanisms (some more widely accepted than others) that provide the actual impetus for evolutionary change and speciation.
There is also artificial selection, which indicates an intentional and exterior selection of traits that are bred for or culled from the population in question. Natural and artificial selection can complement each other; for instance, it is speculated that the forebearers of domestic canids evolved to live near and scavenge from people, and the co-beneficial relationship eventually led to domestication of dogs by humans. From an extreme gene centric viewpoint the same could be said for most domestic species as they have benefitted (in terms of gene proliferation) from their subservient cooperation with humans, and if one wanted to go out on a limb and talk about "extended phenotypes", domestic flora and fauna are actually a part of our natural selection as a species.
Other non-selective contributors to evolution include mutation (from environmental sources or replication error), genetic drift, population gene flow, and direct lateral gene transfer. However, it is widely agreed that natural selection and the specific mechanisms thereof is the dominant contributor to evolutionary change in complex organisms, and that the other categories listed above simply provide for greater variation and beneficial but incremental uniqueness to evolutionary development.
Stranger
Re: Evolution theory mechanisms
Thanks for all that information, Stranger! One follow up. At the beginning of your first paragraph, you discourage usage of the term "mechanism", but at the end of it you use the term yourself. Is it that the overarching entity, natural selection, is not a mechanism, but its child entities are? And while I'm at it, what does "mechanism" mean in biology that makes it different from common usage.
Finally, could you address the parenthetical question in the OP? Thanks very much!
Re: Evolution theory mechanisms
What is meant by "mechanism" in the context of evolutionary biology is actually an interesting and somewhat contentious epistemological question. In the simplest terms, a mechanism can be defined in this context as a discrete process that has a deterministic cause and resultant effect; not just "natural selection", but a specific causal chain of events. For instance, we can speak conceptually about a herd of herbivores and pack hunters that feed on them; the herbivores that are the slowest or have the least stamina or are (for whatever reason) forced to the outskirts of the herd will be the first culled. Similarly, the predators with the greatest speed or agility, or are best able to defend or share a kill, will be the most successful. In the end, the tendency will be toward faster and stronger prey and predator, up to the point that such fitness becomes a detriment in another way. We can model this mechanism mathematically with differential equations and game theory to determine what is the optimax solution for both prey and predator, and absent of other effects we will expect that the real world problem will come to the same solution (or at least oscillate close around the locus of the ideal solution). This is an example of predator-prey coevolution.
The reality, of course, is that these mechanisms don't function in isolation, and in attempting to simplify biosystems to make them sufficiently simple to model mathematically may remove influences which seem minor but are actually critical. See keystone species as an example of this; species that comprise relatively little in terms of population numbers or biomass relative to the entire system, and which are usually at the top of the food cycle and last on the energy flow, so you would think that fluctuations of their numbers would have the least effect upon the system as a whole, but because the provide some critical feedback reduction or elimination can have dramatic effects.
There should also be some caution inserted here with regard to the term, which in non-technical parlance is generally considered to refer to an artificial device or simple machine. This should not be taken to mean in the context of biology that mechanisms imply teleological intent; merely that they obey a consistent set of rules for causal events that allow prediction of results.
As for "survival of the fittest", it isn't so much wrong as ambiguous. What, evolutionarily speaking, is fitness? The predator that can run the fastest might bring down more prey, but if another competing organism manages to successfully feed off of the corpse who is better for it? Success in evolution can be measured in how widely and prevalently ones genes have been spread about the gene pool; eating better, being smarter, or jumping higher may be a measure of fitness of the organism, but having the most children that themselves are capable of successfully procreating, ad nauseam, is the only measure that really matters in the sense of evolutionary success. And groups or species which are best able to adapt to changing environmental conditions (be they predation, changing food sources, competition pressure, et cetera) will ultimately proliferate more than those which do not adapt as well.
Stranger
