6 Evolution and Population Genetics

From inheritance to evolution

Population genetics examines how genetic variation is distributed within populations and how it changes across generations. Evolution is a change in allele frequencies in a population. Individuals develop and reproduce, but populations evolve genetically over generations.

An allele is an alternative form of a gene. In diploid organisms, individuals typically inherit two alleles for each gene, one from each parent. Their genotype is their genetic makeup; their phenotype is an observable trait shaped by genotype and often by environmental conditions.

How alleles are inherited

During meiosis, the two alleles of a gene separate into different gametes. This pattern is the law of segregation. Alleles of different genes can assort independently when the genes are unlinked or far apart on a chromosome. Nearby genes may be inherited together through linkage, although crossing over can recombine them.

For a simple inheritance example, let $A$ be a dominant allele and $a$ a recessive allele. A cross between two heterozygotes, $Aa \times Aa$, has these expected genotype frequencies:

With complete dominance, the expected phenotype ratio is three individuals with the dominant phenotype to one with the recessive phenotype. These are probabilities for many offspring, not guarantees for a small family. Dominance describes how alleles affect a phenotype; it does not mean that an allele is more common or advantageous. Some traits show incomplete dominance or codominance, and many are affected by multiple genes and environmental conditions.

Where genetic variation comes from

A population’s Gene pool contains all its alleles. Mutation creates new alleles, while sexual reproduction reshuffles existing alleles through crossing over, independent assortment, and fertilization. Migration can also bring alleles into a population from elsewhere.

Genetic variation can contribute to trait differences, but not all variation is inherited: environmental conditions can also affect phenotype. Heritable differences can be passed to offspring and may influence reproductive success. Mutation is the ultimate source of new alleles, although a particular mutation may be neutral, harmful, or beneficial depending on context.

Four processes that change populations

Four major processes can change allele frequencies, and they can operate at the same time:

• Mutation produces new genetic variants.

• Natural selection occurs when heritable differences are associated with differences in reproductive success. Alleles that contribute to greater reproductive success in a particular environment tend to become more common. Selection acts on organisms’ traits; populations change genetically across generations.

• Genetic drift changes allele frequencies by chance. A bottleneck follows a sharp reduction in population size. A founder effect occurs when a new population is established by a small number of individuals. Both can make chance especially influential.

• Gene flow moves alleles between populations through migration and reproduction. It can add variation to a population and make neighboring populations more genetically similar.

Selection is nonrandom with respect to reproductive success, whereas drift changes allele frequencies by chance. Neither process needs to act alone.

A baseline for allele frequencies

The Hardy–Weinberg principle provides a baseline for a population in which allele and genotype frequencies remain constant from generation to generation. For a gene with two alleles, $A$ and $a$, let their frequencies be $p$ and $q$. Because these are the only alleles:

Under Hardy–Weinberg conditions, expected genotype frequencies are:

For example, when $p = 0.7$ and $q = 0.3$, the expected frequencies are $0.49$ for $AA$, $0.42$ for $Aa$, and $0.09$ for $aa$.

The model assumes a very large population, random mating, and no mutation, migration, or natural selection. Real populations may not meet these assumptions. A departure from expected frequencies can prompt investigation, but does not by itself identify which process is responsible.

Putting the ideas together

Inheritance passes alleles between generations, while mutation creates new alleles and recombination reshuffles existing variation. Evolution is population-level change in allele frequencies. Mutation, natural selection, genetic drift, and gene flow can all contribute to that change. Hardy–Weinberg expectations provide a useful comparison point, not a complete explanation of why a population has changed.