4.5 Mechanisms of Evolution
Key Takeaways
- Natural selection acts on heritable variation: directional selection shifts the mean, stabilizing selection narrows variation, and disruptive selection favors the extremes.
- Genetic drift causes random allele-frequency changes that matter most in small populations, with the bottleneck effect (sharp reduction in numbers) and founder effect (small colonizing group) as classic examples.
- Hardy-Weinberg equilibrium predicts genotype frequencies (p^2 + 2pq + q^2 = 1) only when five conditions hold: no mutation, no migration, no selection, random mating, and a very large population.
- Speciation can be allopatric (geographic isolation) or sympatric (without geographic separation, via mechanisms such as polyploidy in plants), and reproductive isolation can be prezygotic (no mating or fertilization) or postzygotic (hybrid problems).
- Sexual selection - mate choice and male-male competition - can favor traits that reduce survival, as in the peacock's tail, illustrating that fitness is reproductive success, not longevity.
Defining Evolution Precisely
For the Praxis, evolution is a change in allele frequencies in a population over time. Individuals do not evolve; populations do. Five mechanisms - selection, drift, gene flow, mutation, and non-random mating - drive those frequency changes.
Natural Selection
Natural selection acts on heritable variation: individuals whose traits improve survival and reproduction in a given environment leave more offspring, so their alleles become more common.
| Mode | What it Does | Example |
|---|---|---|
| Directional | Shifts the mean toward one extreme | Increase in dark-peppered moth frequency during industrial pollution |
| Stabilizing | Reduces variance; favors the average | Human birth weight clusters near 7 lb because both extremes raise infant mortality |
| Disruptive (diversifying) | Favors both extremes; reduces intermediates | African seedcracker finches with either large or small beaks specialize on different seed sizes |
Fitness
Fitness is reproductive success, not how strong or long-lived an organism is. A trait that helps survival but reduces mating success will be selected against.
Genetic Drift
Genetic drift is the random change in allele frequencies due to sampling error in finite populations. Drift is strongest in small populations and can fix or eliminate alleles regardless of their fitness value.
- Bottleneck effect: a sharp population reduction (disease, disaster, overhunting) leaves a small surviving group whose allele frequencies differ randomly from the original population. Cheetahs show extremely low genetic diversity attributed to an ancient bottleneck.
- Founder effect: a small group colonizes a new area, carrying only a fraction of the original allele pool. The Amish population shows elevated frequencies of certain recessive disorders because of a founder effect.
Gene Flow (Migration)
Gene flow transfers alleles between populations through migration of individuals or gametes. It tends to homogenize populations and counteracts the genetic differences produced by selection or drift. High gene flow can prevent speciation; reduced gene flow is often a prerequisite for it.
Mutation
Mutation is the ultimate source of new genetic variation. Per-locus mutation rates are low, but with thousands of genes and large populations, mutations supply the raw material that the other mechanisms then sort.
Non-Random Mating and Sexual Selection
Non-random mating does not change allele frequencies on its own, but it changes genotype frequencies (for example, increasing homozygosity through inbreeding) and sets up conditions for selection.
Sexual selection is a special case in which traits affecting mate choice (intersexual selection, e.g., peacock plumage) or same-sex competition (intrasexual selection, e.g., antlers in stags) are favored, even when those traits reduce survival. This is why males in many species are more conspicuous or weaponized than females.
Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle states that allele and genotype frequencies remain constant from generation to generation only if five conditions hold:
- No mutation
- No migration (gene flow)
- No natural selection
- Random mating
- A very large (effectively infinite) population
When those conditions hold, with two alleles at frequencies p and q (where p + q = 1):
- p^2 = frequency of homozygous dominant
- 2pq = frequency of heterozygous (carriers)
- q^2 = frequency of homozygous recessive
- p^2 + 2pq + q^2 = 1
Worked Example
In a population, 1 in 10,000 individuals is affected by an autosomal recessive disease. What fraction of the population is carriers?
- q^2 = 1/10,000 = 0.0001 -> q = 0.01
- p = 1 - q = 0.99
- 2pq = 2 x 0.99 x 0.01 ~= 0.0198, or about 1.98%
So roughly 1 in 50 individuals is a carrier - more than 198 times higher than the disease frequency. This is why Hardy-Weinberg shows up on the Praxis: carriers vastly outnumber affected individuals for rare recessive diseases.
Speciation
A species under the biological species concept is a group whose members can interbreed and produce fertile offspring in nature.
Allopatric vs Sympatric
- Allopatric speciation ('other country'): a physical barrier (river, mountain, glaciation, lava flow) separates a population. Selection and drift act independently on each side until they can no longer interbreed. Most documented speciation events are allopatric.
- Sympatric speciation ('same country'): new species arise without geographic isolation. Common in plants through polyploidy (doubling of chromosome number, e.g., wheat). Also possible through ecological niche divergence and sexual selection.
Reproductive Isolation
Once two populations cannot exchange genes, they are reproductively isolated. Mechanisms divide into two camps.
| Prezygotic (before fertilization) | Postzygotic (after fertilization) |
|---|---|
| Habitat isolation (different microhabitats) | Hybrid inviability (zygote fails to develop) |
| Temporal isolation (mate at different times) | Hybrid sterility (offspring viable but sterile - mules) |
| Behavioral isolation (different courtship rituals) | Hybrid breakdown (F2 generation weak or sterile) |
| Mechanical isolation (incompatible reproductive structures) | |
| Gametic isolation (sperm cannot fertilize egg) |
Sickle cell disease is caused by a recessive allele. In a West African population, the disease (homozygous-recessive) frequency is approximately 1 in 100. Assuming Hardy-Weinberg equilibrium, what fraction of the population are heterozygous carriers?
A volcanic eruption isolates a small group of beetles on a newly formed island. Over many generations, the island population becomes a distinct species from the mainland beetles. Which mechanism BEST describes this process?