Evolutionary Mechanisms

Core Explanation

Evolution is defined as a change in allele frequencies in a population over generations. Hardy-Weinberg equilibrium describes the conditions under which NO evolution occurs. The five major mechanisms that cause deviation from equilibrium — that is, the mechanisms of evolution — are:

1. Mutation — creates new alleles
2. Genetic drift — random changes, especially in small populations
3. Gene flow — movement of alleles between populations
4. Natural selection — differential reproductive success
5. Nonrandom mating — shifts genotype frequencies

Mutation

Mutation is the ultimate source of all new genetic variation. Without mutation, there would be no new alleles for other evolutionary mechanisms to act upon.

• Mutations are random with respect to fitness — they do not arise because an organism "needs" them.
• Most mutations are neutral or deleterious; very few are beneficial.
• Mutation rates are typically low (on the order of 10⁻⁵ to 10⁻⁹ per gene per generation), so mutation alone changes allele frequencies very slowly.
• However, mutation combined with selection or drift can produce substantial evolutionary change over time.
• Mutations in germ-line cells are heritable; mutations in somatic cells are not (though they contribute to cancer).

Genetic Drift

Genetic drift is the random change in allele frequencies due to chance events — which individuals survive and reproduce, and which alleles happen to be passed to the next generation. It is NOT adaptive and does NOT lead to adaptation.

Drift is stronger in small populations. In a large population, chance fluctuations average out. In a small population, a single random event (e.g., an individual with a rare allele failing to reproduce) can substantially alter allele frequencies or even eliminate alleles.

Consequences of genetic drift:

1. Allele frequencies fluctuate randomly over time.
2. Genetic variation within populations tends to decrease.
3. Genetic differences between populations tend to increase.
4. Alleles can become fixed (frequency = 1.0) or lost (frequency = 0.0) purely by chance.

Bottleneck Effect

A population bottleneck occurs when a population's size is drastically reduced by a catastrophic event (fire, flood, disease, habitat destruction, overhunting). The survivors represent a small, random sample of the original population's genetic diversity.

• Example: The northern elephant seal was hunted to near extinction (~20 individuals in the 1890s). Though the population has rebounded to over 100,000, genetic diversity remains drastically reduced compared to the closely related southern elephant seal — a lasting genetic signature of the bottleneck.

Founder Effect

The founder effect occurs when a small group of individuals colonizes a new habitat. The founding population carries only a fraction of the original population's genetic diversity.

• Example: The Amish population in Lancaster County, Pennsylvania, was founded by a small number of individuals. Certain otherwise-rare recessive disorders (e.g., Ellis-van Creveld syndrome) occur at elevated frequencies because the founders happened to carry those alleles — and subsequent genetic isolation preserved them.
• Example: Darwin's finches on the Galápagos — each island was colonized by a small number of founders, and the allele frequencies in each population reflected the random sample of alleles the founders carried.

Gene Flow

Gene flow (migration) is the transfer of alleles between populations through the movement of individuals or their gametes (e.g., pollen dispersal).

• Gene flow tends to reduce genetic differences between populations — it homogenizes allele frequencies.
• It can introduce new alleles into a population that mutation has not produced.
• Gene flow can also oppose natural selection if immigrants carry alleles maladapted to the local environment.
• In humans, increasing global mobility has dramatically increased gene flow between previously isolated populations.

Natural Selection

Natural selection is the only evolutionary mechanism that consistently leads to adaptation — traits that improve survival and reproduction in a specific environment.

Modes of Selection

Heterozygote advantage is a special case of balancing selection. The classic example: in regions where malaria is endemic, individuals heterozygous for the sickle-cell allele (HbA/HbS) have a survival advantage — they are protected from severe malaria (unlike HbA/HbA homozygotes) but do not suffer from sickle-cell disease (unlike HbS/HbS homozygotes). Both alleles are maintained in the population.

Sexual Selection

Sexual selection is a form of natural selection in which individuals with certain traits are more likely to obtain mates. It often produces traits that appear maladaptive for survival:

• Intrasexual selection: Competition within one sex (usually males) for access to mates. Examples: antlers in deer, large body size in elephant seal males, ritualized combat.
• Intersexual selection: Mate choice — typically females choosing among males based on traits that signal genetic quality or resource-providing ability. Examples: peacock tail feathers, bowerbird nest decorations, frog calling.

Sexual selection explains the evolution of sexual dimorphism — phenotypic differences between males and females of the same species.

Nonrandom Mating

Nonrandom mating does NOT directly change allele frequencies, but it DOES shift genotype frequencies:

• Inbreeding: Mating between relatives increases homozygosity and decreases heterozygosity. This exposes deleterious recessive alleles and reduces fitness (inbreeding depression).
• Assortative mating: Individuals mate preferentially with others that are phenotypically similar (positive assortative mating) or dissimilar (negative assortative mating).

Common Misconceptions and Exam Traps

• Misconception: "Genetic drift is just weak natural selection." Drift is RANDOM with respect to fitness — it does not produce adaptation. A deleterious allele can drift to fixation in a small population.
• Exam trap: Confusing the bottleneck effect with the founder effect. Bottleneck = population crashes (everyone affected); Founder = small group leaves to colonize (voluntary or dispersal-driven). Both reduce diversity, but the mechanism differs.
• Misconception: "Gene flow always helps populations." Gene flow can introduce maladaptive alleles, disrupting local adaptation. It can also homogenize distinct populations, reducing biodiversity.
• Exam trap: Forgetting that nonrandom mating shifts genotype frequencies but does NOT directly change allele frequencies. This is a classic distinction — only the other four mechanisms change allele frequencies.
• Misconception: "Sexual selection and natural selection are completely separate." Sexual selection IS a form of natural selection — it's selection for traits that increase mating success, which is a component of fitness.