We've learned that population genetics is the study of genetic variation within a population. Now, if you recall, genetic variation arises from differences in the genetic code and can be measured as observable differences in the characteristics of an organism.
For instance, a mutation can alter the genotype of a gene and create a new allele. That allele, in turn, can result in the creation of a new characteristic, or phenotype. That means that, at its core, population genetics is a study of the allelic, genotypic, and phenotypic variation within a population. This information can then be used to determine whether or not a population is evolving and make a prediction regarding cause of the evolution.
For our research project, let's see how the traits we've been studying, like coat color and fire-breath, affect a wild flying hamster population.
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Instead of analyzing the progeny of a single cross like we did earlier with Punnett squares, this time we're going to analyze the genotype and phenotype of all of the progeny of an entire population of flying hamsters. Adrian suggests we use the Hardy-Weinberg equation for our studies.
The Hardy-Weinberg equilibrium equation describes genotypic frequency in a population. When a population is in Hardy-Weinberg equilibrium, allelic and genotypic frequency can be predicted by the equation. However, for a population to be in Hardy-Weinberg equilibrium, several criteria must be met.
First, mating must be random. The genotype of individuals at the gene being studied can't affect mate choice.
Second, the population size must be very large, virtually infinite. If you're considering a population of four individuals, random death of a single individual would significantly alter the genetic makeup of that population. In contrast, a random death in a population of, say, 10,000 individuals, would not.
The third criterion for Hardy-Weinberg equilibrium is that there can be no migration between populations. If individuals enter or leave the population being studied, the genetic makeup of the population is altered.
Fourth, there can be no mutations. Mutations could introduce new alleles or alter the effect of alleles currently being studied.
Finally, natural selection cannot affect the alleles under consideration. If individuals with a specific genotype are more likely to survive and reproduce, the genetic makeup of the population can be significantly altered.
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If it seems like an awful lot of restrictions for this equation to work, you're right. Would all of these conditions ever be met in a population that we'd find in the wild? The answer is almost surely, 'no.' If that's the case, then what's the point in learning this fancy equation?
The answer is that using this equation can tell us whether or not an evolutionary agent is affecting a population we're studying. That is, allele frequency within a population should not change from generation to generation unless an evolutionary agent changes it. Therefore, if we can establish that a population in nature has deviated from Hardy-Weinberg equilibrium, we can conclude that the genetic makeup of the population is changing, and maybe we can conduct further studies to infer the major evolutionary agents at work.
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