19.1 Population Evolution

Population Genetics and Evolution

Changes in allele frequencies over time

Evolution, at its core, is about changes in allele frequencies within a population over time. Four main forces drive these changes:

• Natural selection favors individuals with advantageous traits, increasing the frequency of associated alleles across generations. The classic example is the peppered moth: during England's Industrial Revolution, dark-colored moths became more common as soot darkened tree bark, giving them a survival advantage over light-colored moths.
• Genetic drift causes random changes in allele frequencies, and its effects are strongest in small populations. An allele can reach fixation (100% frequency) or disappear entirely just by chance. Two common scenarios produce this:
  • The founder effect occurs when a small group splits off to start a new population, carrying only a fraction of the original gene pool.
  • The bottleneck effect occurs when a population is drastically reduced (by a disaster, for example), and the survivors' alleles may not represent the original population's diversity.
• Mutation introduces entirely new alleles into a population. Mutation rates are low, but over many generations, new alleles accumulate and become raw material for evolution. The sickle cell allele, for instance, arose from a single point mutation in the hemoglobin gene.
• Gene flow transfers alleles between populations through the movement of individuals or gametes (such as pollen carried by wind). This can increase genetic variation in a receiving population by introducing new alleles, or it can make two populations more genetically similar over time.

Application of Hardy-Weinberg principle

The Hardy-Weinberg principle describes what a non-evolving population looks like genetically. It acts as a null model: if a population meets all of its assumptions, allele frequencies stay constant generation after generation. Any deviation from this equilibrium tells you that evolution is occurring.

The five assumptions are:

1. No mutation
2. No gene flow (migration)
3. No natural selection
4. Random mating
5. Infinitely large population size (no genetic drift)

No real population meets all five, which is exactly the point. The model gives you a baseline to measure evolutionary change against.

The two Hardy-Weinberg equations:

• $p + q = 1$ where $p$ is the frequency of one allele and $q$ is the frequency of the other allele (for a two-allele system)
• $p^2 + 2pq + q^2 = 1$ where $p^2$ is the frequency of homozygous dominant individuals, $2pq$ is the frequency of heterozygotes, and $q^2$ is the frequency of homozygous recessive individuals

How to use these equations (step-by-step):

1. Start with what you know. Often you're given the frequency of the homozygous recessive phenotype, since those individuals are identifiable. That value equals $q^2$.
2. Take the square root of $q^2$ to find $q$.
3. Subtract $q$ from 1 to find $p$ (since $p + q = 1$).
4. Plug $p$ and $q$ into the genotype equation to calculate expected genotype frequencies.
5. Compare your expected frequencies to the observed data. If they don't match, one or more of the five assumptions is being violated, and the population is evolving.

For example, sickle cell anemia shows heterozygote advantage in malaria-prone regions: carriers ($2pq$) have higher fitness than either homozygous group, which keeps both alleles in the population and produces genotype frequencies that deviate from what you'd expect under simple Hardy-Weinberg conditions.

Factors affecting genetic variation

Three major factors shape how much genetic variation exists in a population: mutation, migration, and selection.

Mutation is the ultimate source of all new genetic variation. Most mutations are neutral (no effect on fitness) or deleterious (harmful). Rarely, a mutation is beneficial. Even neutral mutations matter because they add to the genetic diversity that selection or drift can act on later.

Migration (gene flow) can either increase or decrease variation depending on context:

• New alleles arriving from a genetically different population increase variation (think of an invasive species entering a new ecosystem and interbreeding with a local population).
• If immigrants are genetically similar to the resident population, gene flow won't add much new variation and can even homogenize populations over time.

Selection pressure causes differential survival and reproduction based on traits. There are three main patterns of selection, and each affects the distribution of traits differently:

1. Directional selection favors one extreme of a trait distribution, shifting the population mean in that direction. This reduces variation over time by eliminating alleles on the unfavored end. Antibiotic resistance in bacteria is a clear example: the selective pressure of the antibiotic consistently favors resistant individuals.

2. Stabilizing selection favors intermediate trait values and selects against both extremes. This narrows the distribution around the mean. Human birth weight is the textbook case: very small and very large babies have lower survival rates, so intermediate weights are favored.

3. Disruptive selection favors both extremes over intermediate values, which can widen the trait distribution and potentially split a population into two distinct groups. Beak size in Galápagos finches illustrates this: birds with very large or very small beaks may each be better suited to different food sources than birds with medium beaks, pushing the population toward two peaks.

Population Genetics and Fitness

Fitness, in evolutionary biology, doesn't mean physical strength. It's a measure of an individual's reproductive success relative to others in the population. An organism with high fitness passes more alleles to the next generation.

Effective population size is the number of individuals actually contributing offspring to the next generation. This is often smaller than the total census population because not every individual breeds. A population of 10,000 might have an effective population size of only 2,000 if most reproduction is done by a small subset. Effective population size matters because smaller effective sizes make genetic drift more powerful.

Adaptive radiation occurs when a single ancestral species rapidly diversifies into many species, each adapted to a different ecological niche. Darwin's finches are a famous example: one ancestral finch species colonized the Galápagos Islands and gave rise to over a dozen species with different beak shapes, diets, and habitats. This process tends to happen when organisms encounter many open niches, such as after a mass extinction or upon colonizing new territory.