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Selective Pressures reducing population diversity

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Selective Pressures reducing population diversity

Postby DeaconRazorblades » Thu May 20, 2010 12:59 pm

I'm having a debate with a creationist who also claims to be a physician. His argument is that all selective pressures reduce the diversity of populations. Here is a snippet:

Selection pressures do reduce the diversity of populations, which is a mathematical and empirical fact of life. Selection pressures drive to extinction those variants that can not reproduce in the presence of these selection pressures. The failure of certain members of a population to reproduce in the presence of selection pressures reduces the diversity of the population. Over generations, the most fit members tend to take over the population. All you need to do to prove me wrong is give a single example where a selection pressure increases the diversity of a population.

Now, me being more inclined to physics the actual mechanisms of evolution still elude me to a certain degree. Is this individual right? He is asking for a single example where a selection pressure increases the diversity of a population. However, I think he is wrong. Even with the influence of selective pressures you will still have point mutations and genetic drift, which I believe in itself would cause population diversity even in dense populations. Any examples would be appreciated or any knowledge at all. Here is the url for the full debate (I should warn you that it is a creationist website and it quickly derailed from cosmic evolution which was the OP to evolutionary biology and selective pressure): http://www.evolutionfairytale.com/forum/index.php?showtopic=3357&st=0

Thanks in advance!
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Postby plasmodesmata11 » Fri May 21, 2010 12:58 am

I don't know if this is what you're looking for exactly, but disruptive selection can give way to more than one regular phenotype.
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Postby JackBean » Fri May 21, 2010 1:50 pm

imagine some birds eating some seeds. These will extinct and the birds will be pushed to find other source of food. You could imagine that part of population will specialize into nuts and the other into nectar... ;)

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Postby Eous » Fri May 21, 2010 7:21 pm

While selective pressure tends to hinder a populations diversity, as you'll notice by the bird example above (as classic darwin finches), selective pressures such as limited food supplies, can push a population towards speciation; one group continues with the first food source while the other finds and speializes for another. Even as a less extreme example, a part of the population of birds, being birds, could fly to another island (perhaps a little bit farther than they casually fly) to find the same food source, but in greater supply.

In both of those, selective pressure pushed a single population to become multiple populations. Due to subsequent adaptation to the new area or food source (which might include differing selective pressures), the populations will diversify more. Is this an acceptable increase in diversity due to selective pressure on a population, even though the result involves the seperation of the population?

Also, on a smaller scale, selective pressure will naturally promote the survivaly of lineages that include more diversity, because chances are that one of those diverse offspring will be more fit for the conditions. If the capability to produce more diverse offspring is also an inhereted characteristic, then the selective pressure is effectively driving the population towards greater diversity so that there is a greater chance of individuals surviving and passing on those genes that promote greater diversity. This is obviously more applicable to populations of microbes(with short generation times), or species that have multitudes of offspring at once.
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Postby Darby » Fri May 21, 2010 9:28 pm

The creationist is kind of right but is missing an important detail: selection works on multiple fitness traits, preserving diversity in new combinations for long periods, but environments CHANGE over long periods, shifting the selection.

You do see reduced diversity in well-adapted species in very stable niches. There just aren't a lot of those.
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Re: Selective Pressures reducing population diversity

Postby Beckett » Tue May 25, 2010 4:40 pm

The creationist is NOT right. It is not true that all selective pressures reduce the diversity of a population. He is only considering Directional Selection and/or Stabilizing Selection, but not considering Disruptive Selection, which favours the extreme cases of the traits and in fact increase the diversity of the population as a whole.
Of course that if you get, after a process of evolution, 2 population from 1 each of them might be lees diverse than the original one but you don´t have to be fooled of hoe to measure diversity (that is generally the problematic point). If it was measured over the whole initial population at the beginning, do it the same in the end, which means considering both resulting populations.
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Re: Selective Pressures reducing population diversity

Postby smilodon1 » Mon Jul 12, 2010 7:59 pm

Interestingly, your creationist friend just stated the "theory of evolution", i.e., natural selection. Given an unchanging environment, natural selection will decrease diversity. It's only when there is a change in the environment that natural selection will increase diversity as an "attempt" to adapt the organism to the change. He denies evolution while accepting the mechanism of evolution.
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Re: Selective Pressures reducing population diversity

Postby Riddles » Wed Jul 14, 2010 7:41 am

You could consider

For example, there are three male morphs in the side-blotched lizard See eg:

Also, more generally, there are mutations and in sexually reproducing systems there is meiosis and recombination which produces unique gametes to make unique individuals time after time. There is a new distribution of genes/alleles across unique individuals in each generation.
As the environment is constantly changing then in each generation selection will be a little different and over time can either move one way and then back again or another way or more long-term in one direction. And if populations are reproductively divided then their outcomes can be quite different.
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