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My understanding is that the possible mechanisms of evolution are:

  • Environmental changes
  • Cultural/mating preferences
  • Population Immigration
  • Genetic Mutation

Am I missing anything? I've heard that population shifts within a existing populaces will effect evolution, but imagining the most simplicity scenario, it's hard to see why the would make a difference.

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It's not really clear what 'factors of evolution' you are referring to... there are many factors that play a role in evolution - availability of nutrients, availability of reproductive partners, space,... Could you clarify a bit what kind of factors you mean? – Armatus May 22 '12 at 20:13
@Armatus: "availability of nutrients" would be an environmental change. I'm not sure about "availability of reproductive partners", though I believe that if the availability partners was stable, it would not be a factor, and I already stated in the question that it is unclear to me if "population shifts" beyond population immigration play any significant role. – blunders May 22 '12 at 20:21
@Armatus: My list is based on this video, the "Five Fingers of Evolution" - though I've edited it's meaning to make more sense to me; for example, "Environmental changes" I believe are called adaption, which to me is less clear the saying the changes are a result of the environment changing. If it's still not clear what I mean, please attempt to be more description on were exactly things are clear, and unclear. Thanks! – blunders May 22 '12 at 20:35
I've changed the question title slightly from active to passive phrasing. The reason being is that not all of these directly affect changes in the gene pool (ie, environmental factors). Furthermore, evolution can be semantically argued to have differing definitions. – user560 May 23 '12 at 0:06
The first version of the question was vastly superior. In the current form, it’s much too long, needlessly detailed, contains errors, and no longer is a question. I’d suggest reverting to something much closer to the original form. – Konrad Rudolph May 23 '12 at 9:22
up vote 9 down vote accepted

Evolution is defined as a change in the allele frequency of population through time. The Hardy-Weinberg model predicts that the allele frequency of a population will not change (i.e., evolution will not occur) if the following conditions are met:

  • no natural or sexual selection

  • no gene flow (immigration or emigration from the population)

  • no mutatation

  • no genetic drift (changes in allele frequency due to random events)

* random mating (all gametes are equally likely to combine)

EDIT based on comments from Remi.b: Non-random mating alone is not sufficient to cause evolution if the other conditions exist.

So we can conclude that if any of the above conditions are not met then there is a change in allele frequency and thus evolution, and thus that factor is the cause of evolution.

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+1 @DQdlM: Awesome! Knowing now that Hardy-Weinberg appears to be the source of the logic was a huge help, and you're answers was within my range of plain-English. Taking a step back from "bio-lingo", would it be correct to say that the "inputs" for natural selection are only mutation(s) and/or statistically significant changes in the way the organism interacts with the environment? Also, assuming mating was completely random, why would emigration affect evolution? Meaning it would appear that other conditional changes would be the source of the evolutionary change, not emigration. Thanks! – blunders May 23 '12 at 1:39
@blunders natural selection requires heritable variation and a survival or reproductive advantage based on that variation. The ultimate source of all variation is mutation but note that the thing that evolves is a population, so you can change the allele frequency by having individuals enter or leave a population. This would be an evolutionary event unto itself but would also potentially add more heritable variation that natural selection could act upon. – KennyPeanuts May 23 '12 at 10:57
The answer is actually partially wrong. I agree that under all of the above conditions, no change in allele frequency can occur. However, this is true even if you drop the last condition (random mating). In absence of random mating (and in presence of the other conditions), genotype frequencies may change through time (and differ from Hardy-Weinberg's predictions) but allele frequencies would still remain constant. – Remi.b Jun 12 at 23:25
@Remi.b thanks for your comments. I agree with your feedback so I have edited the answer in response. Thanks. – KennyPeanuts Jun 13 at 14:40

What is evolution?

The first step is to remind ourself of the definition of the term "evolution". Evolution is most often defined as "any change in allele frequency in a population".

Forces that drive evolution

Categorizing the processes that affect allele frequencies might be subject to issues of semantics. Without going into the details, we generally recognize 4 forces that drives evolution

  1. Natural selection
    • Natural selection refers to the deterministic change in allele frequency due to a differential in fitness among different genotypes.
  2. Genetic Drift
    • Genetic Drift refers to the stochastic sampling process of individuals
  3. Mutations
    • A mutation refers to any spontaneous change (substitution, indel, chromosome duplication, etc...) in an individual's genotype.
  4. Gene flow (aka. migration)
    • Gene flow refers to the transfer (migration) of DNA sequences among populations.

KennyPeanuts's answer, random mating and hardy-weinberg equilibrium

In his answer, @KennyPeanuts also talk about random mating. Random mating refers to the condition where the probability of two individuals to mate depends only on their respective fitness. Many people phrase random mating as absence of mate choice but it actually refers to the absence of variation for mate choice in the population.

Hardy-Weinberg states that under the above four conditions and random mating, then the frequency of the genotype that has the allele $i$ derived from the mother and the allele $j$ derived from the father, where $x_i$ and $x_j$ are the frequency of these alleles is $\cdot x_i \cdot x_j$. This means that for a bi-allelic locus, the allele frequency of the genotypes AA, AB, BA and BB are $x^2$, $x(1-x)$, $x(1-x)$ and $(1-x)^2$, respectively where $x$ is the frequency of the allele A. For the heterozygotes (AB and BA), we often care little which of the two allele is inherited by the mother and which is inherited by the father (assuming there are genders) and we therefore call AB both AB and BA genotypes (which can eventually be confusing). As such, the frequency of the AB genotype is $2 x(1-x)$.

The condition of random mating ensure that there is no deviation of genotype frequencies from the Hardy-Weinberg's expectations and it ensure that there is no change in genotype frequencies from the first to the second generation considered (after one generation, the equilibrium genotype frequency is immediately reached). Random mating is therefore not a condition for evolution to not occur.

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