19.2 Population Genetics

Sources and Impacts of Genetic Variation in Populations

Population genetics studies how allele frequencies change within populations over time. It connects the mechanics of inheritance you've already learned (Mendelian genetics, meiosis) to the larger picture of how evolution actually works at the population level.

Sources of Genetic Variation

Genetic variation is the raw material evolution works with. Without it, natural selection has nothing to act on. There are several ways variation enters and gets reshuffled within a population.

Mutation introduces changes in DNA sequence. These are the ultimate source of all new alleles.

• Point mutations affect single nucleotides:
  • Silent mutations don't change the amino acid sequence (the protein stays the same)
  • Missense mutations swap in a different amino acid, which may or may not affect protein function
  • Nonsense mutations create a premature stop codon, usually producing a nonfunctional, truncated protein
• Frameshift mutations insert or delete nucleotides, shifting the entire reading frame downstream. These tend to be severely disruptive.
• Chromosomal mutations involve large-scale changes:
  • Deletions remove segments of a chromosome
  • Duplications copy segments (sometimes providing raw material for new gene functions)
  • Inversions reverse the orientation of a segment
  • Translocations move segments between non-homologous chromosomes

Recombination during meiosis shuffles existing genetic material into new combinations.

• Independent assortment randomly distributes maternal and paternal chromosomes into gametes. For humans with 23 chromosome pairs, this alone produces $2^{23}$ (over 8 million) possible gamete combinations.
• Crossing over exchanges segments between homologous chromosomes, creating recombinant chromosomes that didn't exist in either parent.

Gene flow (migration) introduces new alleles when individuals move between populations and breed. This can bring in alleles that weren't previously present.

Sexual reproduction and random fertilization combine gametes in unpredictable ways, generating unique allele combinations in every offspring.

Natural Selection's Impact on Traits

Fitness measures an individual's reproductive success in a given environment. It's not about strength or speed in the abstract; it's about how many viable offspring you leave.

Natural selection acts on phenotypic variation and can shift allele frequencies in different patterns:

• Directional selection favors one extreme phenotype, shifting the population mean in that direction. A classic example: bacteria exposed to antibiotics, where resistant individuals survive and reproduce while susceptible ones die off.
• Stabilizing selection favors intermediate phenotypes and reduces variation. Human birth weight is a good example: babies that are too small or too large have lower survival rates, so the population clusters around a moderate weight.
• Disruptive selection favors both extremes over the intermediate phenotype, which can increase variation. In African seedcracker finches, birds with very large or very small beaks feed efficiently on different seed types, while medium-beaked birds struggle with both.
• Frequency-dependent selection occurs when an allele's fitness depends on how common it is. For instance, a rare color morph in prey species may have an advantage because predators don't form a search image for it. As that morph becomes more common, the advantage disappears.

Genetic Drift, Bottlenecks, and Diversity

Effects of Drift and Bottlenecks

Genetic drift is the random fluctuation of allele frequencies from one generation to the next due to chance alone. Think of it this way: not every individual reproduces, and even those that do pass on a random sample of their alleles. In large populations, these random fluctuations average out. In small populations, they don't.

• Drift can lead to fixation (an allele reaches 100% frequency) or loss (an allele drops to 0%) purely by chance, regardless of whether the allele is beneficial or harmful.
• Drift reduces genetic diversity over time because alleles are randomly lost and never come back (unless reintroduced by mutation or gene flow).

Two special cases of drift are particularly important:

• Founder effect: A small group splits off and establishes a new population. That small group carries only a fraction of the original population's genetic diversity. For example, the Amish population in Pennsylvania descended from a small number of German settlers, and certain rare alleles (like those causing Ellis-van Creveld syndrome) are unusually common in that community.
• Bottleneck effect: A catastrophic event (disease, natural disaster, hunting) drastically reduces population size. The survivors carry only a subset of the original genetic diversity. Northern elephant seals were hunted down to about 20 individuals in the 1890s; even though the population has recovered to over 100,000, genetic diversity remains extremely low. Cheetahs show a similar pattern.

Both bottlenecks and founder effects can increase inbreeding, which raises homozygosity and exposes harmful recessive alleles.

Hardy-Weinberg Equilibrium

Hardy-Weinberg equilibrium describes a theoretical, non-evolving population. It serves as a null model: if you want to know whether evolution is occurring, you compare your real population to Hardy-Weinberg predictions.

The model requires five conditions:

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

If all five hold, allele frequencies stay constant across generations. The equation is:

$p^2 + 2pq + q^2 = 1$

where $p$ is the frequency of one allele and $q$ is the frequency of the other (for a two-allele system), and $p + q = 1$. The terms $p^2$, $2pq$, and $q^2$ represent the expected frequencies of the homozygous dominant, heterozygous, and homozygous recessive genotypes, respectively.

No real population meets all five conditions, which is exactly the point. When observed genotype frequencies deviate from Hardy-Weinberg predictions, one or more evolutionary forces must be at work.

Forces Shaping Genetic Diversity

Several forces push allele frequencies around in populations. Here's how each one affects genetic diversity:

Natural selection increases the frequency of beneficial alleles and decreases the frequency of harmful ones, adapting populations to their environments. It's the only evolutionary force that consistently produces adaptive change.

Genetic drift causes random allele frequency changes and is strongest in small populations. Unlike selection, drift is directionless: it can eliminate beneficial alleles or fix harmful ones purely by chance.

Gene flow transfers alleles between populations. It increases diversity within a population (new alleles arrive) but decreases differences between populations (they become more genetically similar over time).

Mutation is the only process that creates entirely new alleles. Most mutations are neutral or harmful, but occasionally one is beneficial. Mutation rates are generally low, so mutation alone changes allele frequencies very slowly.

Non-random mating doesn't change allele frequencies by itself, but it does change genotype frequencies:

• Inbreeding (mating between close relatives) increases homozygosity across the genome. This exposes recessive deleterious alleles, which is why inbred populations often show reduced fitness (inbreeding depression).
• Assortative mating (mating between phenotypically similar individuals) increases homozygosity for the specific traits involved and can contribute to divergence within a population.

The interplay of all these forces determines a population's genetic structure. Large populations with gene flow tend to maintain high diversity. Small, isolated populations are vulnerable to drift and inbreeding, which erode diversity. Genetic variation matters because it's what allows populations to adapt when environments change. A population with little variation has fewer options when new selective pressures arise.