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If a population isn't evolving because it's in Hardy-Weinberg (HW) equilibrium, then I know that both genotype and allele frequencies must stay constant.

My question is, can evolution still not occur even if Hardy-Weinberg conditions aren't met? In all of my books, the conclusion seems to be that if HW conditions are met, then evolution doesn't occur. However, they don't clarify if HW conditions being met is both a necessary and sufficient condition for evolution to not occur (they only imply that it's sufficient).

Any thoughts?

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  • $\begingroup$ Could you clarify what specially you mean by 'evolution'? Are you referring to the evolution of a morphological trait, molecular evolution, evolution as a general response to selection? I'm also a little bit confused as to how this relates to the title of the post. $\endgroup$ – user438383 Jul 27 '20 at 17:26
  • $\begingroup$ That’s important because by definition, if HWE assumptions are all met then there can’t be any selection occurring. $\endgroup$ – user438383 Jul 27 '20 at 17:40
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Where do you get that populations aren't evolving?

A population might be in HWE for a particular trait if there is no selection on that trait, but that doesn't mean that there is no selection or drift happening on any trait!

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  • $\begingroup$ True but that wasn't the main purpose of my question. The more important part is regarding whether HW is necessary not just sufficient $\endgroup$ – Ally Jul 27 '20 at 17:14
  • $\begingroup$ @Ally Maybe an example of what you are thinking of would be helpful. As-is, to me this question just raises my eyebrows. I think you might have confusion somewhere but I'm having trouble pinpointing where that is. $\endgroup$ – Bryan Krause Jul 27 '20 at 17:24
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    $\begingroup$ If a trait is under selection, by definition it's not in HWE. $\endgroup$ – swbarnes2 Jul 27 '20 at 17:33
  • $\begingroup$ Yeah so reiterating the question in an if..then format might make it more clear. Right now, I'm getting that "if HW, then no evolution", which also implies "if evolution (aka if a trait is under selection), then no HW". However, from the original premise it does not necessarily follow that "if no evolution, then HW". So my question was basically asking if there was no evolution occurring, would that necessitate the population being in HW or if there was some other mechanism / set of conditions that would allow no evolution to occur $\endgroup$ – Ally Jul 28 '20 at 18:07
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    $\begingroup$ The HW principle is not a mechanism. They are equations that build a rather simplistic model that sometimes is applicable and useful in real life. $\endgroup$ – swbarnes2 Jul 28 '20 at 20:57
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Let me focus on the necessary and sufficient part: such a language is suitable in mathematics, but does not really have much meaning in evolutionary/statistical context. Instead we talk about rejecting the null hypothesis, which does not mean accepting the alternative hypothesis. Equally, a failure to reject the null hypothesis does not mean that this hypothesis is true.

Hardy-Weinberg is a null model of evolution with about a dozen of assumptions. It is not a model of "no evolution", but rather a model of how evolution happens all the time (but it seems like "no evolution" in the sense that not much interesting is happening). Rejecting this null model/hypothesis means that some of its assumptions are violated. However, if the genotype frequencies do satisfy HWE, it doesn't automatically mean that the assumptions of this model are satisfied.

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