Evolutionary Processes: Mechanisms and Patterns of Evolution

Evolutionary Processes

Overview of Evolutionary Mechanisms

Evolution in populations is driven by several mechanisms, each with distinct consequences for genetic variation and adaptation. The four primary processes are natural and sexual selection, genetic drift, gene flow, and mutation.

• Natural & Sexual Selection: Favor certain alleles, leading to adaptation.

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

• Gene Flow: Movement of alleles between populations, increasing genetic variation.

• Mutation: Introduction of new alleles, restoring genetic diversity.

Sources of Genetic Variation

Mechanisms Creating Genetic Diversity

Genetic variation is essential for evolution, providing the raw material for natural selection. Variation arises from several sources:

• Mutation: Formation of new alleles by changes in DNA sequence.

• Chromosomal Mutations: Alteration in gene number or location.

• Rapid Reproduction: Short generation times increase the impact of mutation rates.

• Sexual Reproduction: Recombination of chromosomes during meiosis creates new allele combinations.

Example: Variation in eye color and shell patterns among individuals demonstrates genetic diversity.

Mutations

Role and Impact of Mutations

Mutations are changes in the DNA sequence that create new alleles, restoring genetic diversity. They are random with respect to fitness and generally occur slowly compared to other evolutionary processes.

• Most mutations are deleterious: Lower fitness and are eliminated by natural selection.

• Rarely, beneficial mutations occur: These can increase in frequency due to natural selection.

Equation: Mutation rate per generation: $\mu$

Example: In a population, mutation introduces new alleles, but most do not significantly change allele frequencies in a single generation.

Natural Selection

Mechanism and Consequences

Natural selection is the process by which individuals with higher fitness (better adapted to their environment) reproduce more successfully, passing on advantageous alleles to the next generation.

• Differential reproductive success: Certain alleles become more common.

• Not a chance event: The environment selects for traits that confer reproductive success.

Example: DDT resistance in fruit flies increased after widespread pesticide use, demonstrating adaptation.

Genetic Drift

Random Fluctuations in Allele Frequencies

Genetic drift refers to random changes in allele frequencies from one generation to the next, especially pronounced in small populations.

• Usually affects neutral alleles: Little or no effect on fitness.

• Does not result in adaptation: Changes are due to chance.

• Occurs in every population: Causes allele frequencies to drift randomly over time.

Equation: Probability of allele fixation in a population: $p = \frac{1}{2N}$ (for a new mutation in diploids, where $N$ is population size)

Genetic Drift in Small Populations

Genetic drift has a disproportionate effect in small populations, leading to rapid fixation or loss of alleles.

• Oscillations in allele frequency: More rapid and pronounced in small populations.

• Conservation concern: Small populations in nature reserves or zoos are especially vulnerable.

Founder Effect

The founder effect occurs when a few individuals start a new population in a new area, often resulting in a change in allele frequency and reduced genetic diversity compared to the source population.

• Disproportionate effect: Small founder populations are subject to strong genetic drift.

• Example: Ellis-van Creveld Syndrome is unusually prevalent among the Old Order Amish due to a small founding population and limited gene flow.

Population Bottleneck

A population bottleneck is a sudden decrease in population size due to events like disease outbreaks or natural catastrophes, allowing for a disproportionate effect of genetic drift.

• Reduces genetic variation: Only a small subset of alleles survive the bottleneck.

Gene Flow

Movement of Alleles Between Populations

Gene flow occurs when individuals or gametes move from one population to another, increasing genetic variation and reducing genetic differences among populations over time.

• Can increase or decrease fitness: Introduction of new alleles may be beneficial or detrimental.

• Example: In Parus major (great tit), immigration from mainland brings alleles that reduce fitness in central populations, while populations without high migration rates have higher fitness.

Summary of Evolutionary Mechanisms

Comparison Table

Process	Definition and Notes	Effect on Genetic Variation	Effect on Average Fitness
Selection	Certain alleles are favored	Can lead to maintenance, increase, or reduction	Can produce adaptation
Genetic Drift	Random changes in allele frequencies; most important in small populations	Tends to reduce, via loss or fixation of alleles	Usually reduces
Gene Flow	Movement of alleles between populations	May increase by introducing new alleles, may decrease by removing alleles	May increase by introducing high-fitness alleles; may decrease by introducing low-fitness alleles
Mutation	Production of new alleles	Increase by introducing new alleles	Random with respect to fitness; most mutations lower fitness

Types of Selection

Patterns of Natural Selection

Natural selection can occur in several patterns, each affecting genetic diversity and trait distribution differently:

• Directional Selection: Favors one extreme phenotype, increasing frequency of a particular allele and reducing genetic diversity over time.

• Stabilizing Selection: Favors intermediate traits, reducing genetic diversity but maintaining the average value of the trait.

• Disruptive Selection: Favors extreme phenotypes over intermediates, maintaining genetic diversity and potentially leading to speciation.

Directional Selection

• Increases frequency of favored allele: Can lead to fixation (frequency = 1.0 or 100%).

• Reduces genetic diversity: Disadvantageous alleles decline in frequency.

• Example: Selection for larger body size in a population.

Stabilizing Selection

• Intermediate traits favored: Individuals with average trait values reproduce more.

• Reduces genetic diversity: Extremes are selected against.

• No change in average trait value: Trait mean remains constant.

• Example: Human birth weight—infants with intermediate weights have higher survival rates.

Disruptive Selection

• Extreme phenotypes favored: Intermediates selected against.

• Maintains genetic diversity: Both extremes persist in the population.

• Can cause speciation: If individuals with one extreme trait mate preferentially with similar individuals.

Sexual Selection

Mechanisms and Effects

Sexual selection is a special form of natural selection where individuals differ in their ability to attract mates. Favored individuals produce more offspring, and favored traits increase in frequency.

• Asymmetry of sex: Females invest more resources in offspring; eggs are expensive, sperm are cheap.

• Types of sexual selection:

  • Female choice: Females select mates based on physical characteristics or resources provided.

  • Male-male competition: Males compete for access to females, often through combat or territorial disputes.

• Sexual dimorphism: Traits differ between males and females, often due to sexual selection.

Example: Peacock tail feathers are favored by female choice; lion manes and antlers are results of male-male competition.

Review/Important Concepts

• Understand the four processes of evolution and their functions.

• Know how types of selection (directional, stabilizing, disruptive) affect populations.

• Recognize the mechanisms and consequences of sexual selection, including female mate choice and male-male competition.