Evolutionary Processes: Mechanisms of Evolution and Their Effects on Populations

Ch. 23 – Evolutionary Processes

Introduction to Evolutionary Processes

Evolution is defined as the change in allele frequencies in a population over time. While natural selection is a key mechanism, three other processes—genetic drift, gene flow, and mutation—also contribute to evolutionary change. Understanding these processes is fundamental to modern evolutionary biology and population genetics.

• Natural Selection: Differential survival and reproduction based on heritable traits.

• Genetic Drift: Random changes in allele frequencies, especially significant in small populations.

• Gene Flow: Movement of alleles between populations through migration and breeding.

• Mutation: Introduction of new alleles through changes in DNA sequence.

Modern Synthesis integrated Darwinian evolution with Mendelian genetics, leading to the field of population genetics, which quantifies how these processes affect allele and genotype frequencies.

The Hardy–Weinberg Principle

Hardy–Weinberg Equilibrium as a Null Model

The Hardy–Weinberg Principle provides a mathematical model to predict genotype frequencies in a non-evolving population. It serves as a null hypothesis for detecting evolutionary change.

• Equation: $p^2 + 2pq + q^2 = 1$

• p: Frequency of the dominant allele

• q: Frequency of the recessive allele

• Assumptions: No selection, no mutation, no migration, large population size, and random mating

Deviations from Hardy–Weinberg equilibrium indicate that one or more evolutionary processes are at work.

Nonrandom Mating

Inbreeding and Its Effects

Nonrandom mating occurs when individuals do not mate randomly within a population. Inbreeding, or mating between close relatives, is a common form of nonrandom mating and has significant genetic consequences.

• Increases homozygosity: More individuals become homozygous for alleles, reducing heterozygosity.

• Does not change allele frequencies directly, but alters genotype frequencies.

• Inbreeding depression: Reduced fitness due to increased expression of deleterious recessive alleles.

Example: Inbreeding depression is observed in small populations such as Florida panthers and purebred dogs, where increased homozygosity leads to health problems and reduced survival.

Sexual Selection

Sexual selection is a form of nonrandom mating where individuals with certain traits are more likely to obtain mates. It can lead to changes in allele frequencies and often results in sexual dimorphism.

• Intersexual selection: Mate choice, often by females selecting for 'honest signals' of genetic quality.

• Intrasexual selection: Competition among individuals of the same sex for mates.

Example: Peacock tails and elephant seal size are classic examples of traits shaped by sexual selection.

Natural Selection

Modes of Selection

Natural selection can act in several patterns, each with distinct effects on genetic variation and population adaptation.

• Directional Selection: Favors one extreme phenotype, shifting the population mean and reducing genetic variation.

• Stabilizing Selection: Favors intermediate phenotypes, reducing variation but maintaining the mean.

• Disruptive Selection: Favors extreme phenotypes at both ends, increasing variation and potentially leading to speciation.

• Balancing Selection: Maintains genetic diversity through mechanisms such as heterozygote advantage and frequency-dependent selection.

Mode of Selection	Effect on Phenotype	Example	Effect on Genetic Variation
Directional	Shifts mean phenotype	Finch beak size during drought	Decreases
Stabilizing	Favors intermediates	Human birth weight	Decreases
Disruptive	Favors extremes	Gill raker number in fish	Increases
Balancing	Maintains diversity	Color patterns in guppies	Maintained

Genetic Drift

Random Changes in Allele Frequencies

Genetic drift refers to random fluctuations in allele frequencies due to chance events. It is especially significant in small populations and can lead to the fixation or loss of alleles.

• Founder Effect: When a new population is established by a small group, allele frequencies may differ from the source population.

• Bottleneck Effect: Sudden reduction in population size due to catastrophe, reducing genetic diversity.

Example: Cheetahs and Florida panthers have experienced bottlenecks, resulting in low genetic diversity.

Gene Flow

Movement of Alleles Between Populations

Gene flow occurs when individuals migrate between populations and breed, introducing new alleles and homogenizing genetic differences between populations.

• Increases genetic diversity within populations but reduces differences between populations.

• Can counteract effects of selection, drift, and inbreeding.

Example: Introduction of captive-bred fish into wild populations can reduce the fitness of wild populations due to maladapted alleles.

Mutation

Source of New Genetic Variation

Mutation is the ultimate source of genetic variation, creating new alleles. While most mutations are neutral or deleterious, some can be beneficial and increase in frequency through selection.

• Rare and slow in most eukaryotes, but significant in organisms with short generation times (e.g., bacteria).

• Types: Point mutations, insertions, deletions, gene duplications, and lateral gene transfer.

Example: Aphids acquired genes for carotenoid synthesis from fungi via lateral gene transfer, a rare but impactful evolutionary event.

Summary Table: Effects of Evolutionary Processes

Process	Effect on Allele Frequencies	Effect on Genetic Variation	Effect on Fitness
Natural Selection	Non-random change	Can increase or decrease	Usually increases
Genetic Drift	Random change	Decreases	Random
Gene Flow	Homogenizes populations	Can increase or decrease	Random
Mutation	Introduces new alleles	Increases	Usually neutral or deleterious