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Population Genetics

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Jeans vs. Genes. What's the difference?

Plenty. One you have for life, the other just lasts a few years. One is the basis for the passing of traits from one generation to the next. Some jeans you change frequently. But what happens when you change a gene's frequency? Essentially, evolution is a change in gene frequencies within a population.

Genes in Populations

Darwin knew that heritable variations are needed for evolution to occur. However, he knew nothing about Mendel’s laws of genetics. Mendel’s laws were rediscovered in the early 1900s. Only then could scientists fully understand the process of evolution. We now know that variations of traits are heritable. These variations are determined by different alleles . We also know that evolution is due to a change in alleles over time. How long a time? That depends on the scale of evolution.

  • Microevolution occurs over a relatively short period of time within a population or species. The Grants observed this level of evolution in Darwin’s finches (see the "Biogeography" concept).
  • Macroevolution occurs over geologic time above the level of the species. The fossil record reflects this level of evolution. It results from microevolution taking place over many generations.

Remember that individuals do not evolve. Their genes do not change over time. The unit of evolution is the population. A population consists of organisms of the same species that live in the same area. In terms of evolution, the population is assumed to be a relatively closed group. This means that most mating takes place within the population. The science that focuses on evolution within populations is population genetics . It is a combination of evolutionary theory and Mendelian genetics.

Gene Pool

The genetic makeup of an individual is the individual’s genotype . A population consists of many genotypes. Altogether, they make up the population’s gene pool. The gene pool consists of all the genes of all the members of the population. For each gene, the gene pool includes all the different alleles for the gene that exist in the population. For a given gene, the population is characterized by the frequency of the different alleles in the gene pool.

Allele Frequencies

Allele frequency is how often an allele occurs in a gene pool relative to the other alleles for that gene. Look at the example in the Table below . The population in the table has 100 members. In a sexually reproducing species, each member of the population has two copies of each gene. Therefore, the total number of copies of each gene in the gene pool is 200. The gene in the example exists in the gene pool in two forms, alleles A and a . Knowing the genotypes of each population member, we can count the number of alleles of each type in the gene pool. The table shows how this is done.

Genotype Number of Individuals in the Population with that Genotype Number of Allele A Contributed to the Gene Pool by that Genotype Number of Allele a Contributed to the Gene Pool by that Genotype
AA 50 50 × 2 = 100 50 × 0 = 0
Aa 40 40 × 1 = 40 40 × 1 = 40
aa 10 10 × 0 = 0 10 × 2 = 20
Totals 100 140 60

Let the letter p stand for the frequency of allele A . Let the letter q stand for the frequency of allele a . We can calculate p and q as follows:

  • p = number of A alleles/total number of alleles = 140/200 = 0.7
  • q = number of a alleles/total number of alleles = 60/200 = 0.3
  • Notice that p + q = 1.

Evolution occurs in a population when allele frequencies change over time. What causes allele frequencies to change? That question was answered by Godfrey Hardy and Wilhelm Weinberg in 1908 (see the Hardy-Weinberg Theorem concept).

Summary

  • Microevolution occurs over a short period of time in a population or species. Macroevolution occurs over geologic time above the level of the species.
  • The population is the unit of evolution.
  • A population’s gene pool consists of all the genes of all the members of the population.
  • For a given gene, the population is characterized by the frequency of different alleles in the gene pool.

Practice

Use this resource to answer the questions that follow.

  1. What is the genetic mechanism of evolution?
  2. How does genetic variation among a species occur?
  3. Why do populations evolve, but not individuals?
  4. How is the genetic makeup of a population determined?
  5. How is allele frequency calculated?

Review

  1. Compare microevolution to macroevolution.
  2. Why are populations, rather than individuals, the units of evolution?
  3. What is a gene pool?
  4. Assume that a population of 50 individuals has the following numbers of genotypes for a gene with two alleles, B and b : BB = 30, Bb = 10, and bb = 10. Calculate the frequencies of the two alleles in the population’s gene pool.

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