Evolution in Populations

Introduction to Evolutionary Mechanisms

Evolution within populations is driven by changes in allele frequencies over time. These changes can occur through several mechanisms, including natural selection, genetic drift, and gene flow. Understanding these processes is essential for explaining how populations adapt and evolve.

• Natural selection favors individuals with advantageous traits, increasing their frequency in the population.

• Genetic drift involves random changes in allele frequencies, especially significant in small populations.

• Gene flow is the movement of alleles between populations, introducing new genetic variation.

• Genetic variation is the foundation for evolutionary change, providing the raw material for these mechanisms to act upon.

• Example: During a drought on Daphne Major, only finches with certain beak sizes survived, demonstrating natural selection in action.

Genetic Variation and Its Importance

Genetic and Phenotypic Variation

Genetic variation refers to differences in DNA sequences among individuals, while phenotypic variation refers to observable traits. Both are crucial for evolution, but only genetic variation is heritable and subject to evolutionary forces.

• Discrete traits (e.g., flower color) are usually controlled by a single gene.

• Continuous traits (e.g., height) are influenced by multiple genes and environmental factors.

• Gene variability is measured as the percentage of loci that are heterozygous in a population.

• Nucleotide variability measures differences at the DNA sequence level, most of which do not affect phenotype.

• Phenotypic plasticity is the ability of a genotype to produce different phenotypes in different environments; only genetically determined variation is heritable.

Sources of Genetic Variation

• Mutation: Random changes in DNA, including point mutations, insertions, and deletions. Most are neutral or harmful, but some can be beneficial.

• Gene duplication: Duplications of DNA segments can lead to new gene functions.

• Sexual reproduction: Shuffles alleles through crossing over, independent assortment, and fertilization, greatly increasing genetic diversity.

• Example: High mutation rates in viruses like HIV lead to rapid evolution and drug resistance.

Additional info: In humans, independent assortment alone produces (8.4 million) possible chromosome combinations per parent.

The Hardy-Weinberg Principle

Modeling No Evolution

The Hardy-Weinberg equation provides a mathematical model for a population where no evolution is occurring at a specific gene locus. It predicts genotype frequencies from allele frequencies under certain conditions.

• Population: A group of interbreeding individuals of the same species in a given area.

• Gene pool: The total collection of alleles at all loci in a population.

• Allele frequency: The proportion of a specific allele among all alleles at a locus in the population.

The Hardy-Weinberg Equation

• For two alleles, (frequency of dominant allele) and (frequency of recessive allele):

• Genotype frequencies: (homozygous dominant), (heterozygous), (homozygous recessive)

• Hardy-Weinberg equation:

Assumptions of Hardy-Weinberg Equilibrium

• No mutations

• Random mating

• No natural selection

• Extremely large population size (no genetic drift)

• No gene flow (no migration)

If any of these conditions are violated, evolution may occur at that locus.

Application Example: Phenylketonuria (PKU)

• PKU is a recessive disorder occurring in 1 out of 10,000 births.

• Frequency of affected individuals ():

• Frequency of recessive allele ():

• Frequency of dominant allele ():

• Frequency of heterozygous carriers (): (about 2%)

Mechanisms That Alter Allele Frequencies

Natural Selection

Natural selection is the only mechanism that consistently leads to adaptive evolution by increasing the frequency of alleles that enhance survival and reproduction.

• Acts on phenotypes, indirectly affecting genotypes.

• Example: DDT resistance in Drosophila melanogaster increased after DDT use.

Genetic Drift

Genetic drift is the random fluctuation of allele frequencies, especially significant in small populations.

• Can lead to loss of alleles and reduced genetic variation.

• Founder effect: A new population started by a small group may have different allele frequencies than the source population.

• Bottleneck effect: A drastic reduction in population size can change allele frequencies and reduce genetic diversity.

• Harmful alleles can become fixed by chance.

Gene Flow

Gene flow is the movement of alleles between populations due to migration of individuals or gametes.

• Reduces genetic differences between populations.

• Can introduce new alleles, increasing genetic variation within populations.

• Example: Gene flow maintains banded coloration in island water snakes despite selection for unbanded forms.

Modes of Natural Selection

Types of Selection

• Directional selection: Favors individuals at one extreme of a trait (e.g., deeper beaks in finches during drought).

• Disruptive selection: Favors individuals at both extremes over intermediates (e.g., beak size in seedcracker finches).

• Stabilizing selection: Favors intermediate phenotypes, reducing variation (e.g., human birth weight).

Sexual Selection

• Sexual selection: Favors traits that increase mating success.

• Sexual dimorphism: Differences in appearance between males and females due to sexual selection.

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

• Intersexual selection (mate choice): One sex chooses mates based on certain traits (e.g., female gray tree frogs prefer long-calling males).

Balancing Selection

• Balancing selection: Maintains multiple alleles in a population.

• Frequency-dependent selection: Fitness of a phenotype depends on its frequency (e.g., left- vs. right-mouthed scale-eating fish).

• Heterozygote advantage: Heterozygotes have higher fitness than either homozygote (e.g., sickle-cell trait confers malaria resistance).

Case Study: The Sickle-Cell Allele

• Sickle-cell disease is caused by a point mutation in the hemoglobin gene, resulting in abnormal hemoglobin and sickle-shaped red blood cells.

• Homozygotes develop sickle-cell disease; heterozygotes are resistant to malaria and usually healthy.

• High frequency of the sickle-cell allele in malaria-prone regions is due to heterozygote advantage.

Limits of Natural Selection

Why Perfect Organisms Do Not Exist

• Natural selection can only act on existing variation; new advantageous mutations may not arise when needed.

• Evolution is constrained by historical factors; new traits evolve by modifying existing structures.

• Adaptations are often compromises between different functions.

• Chance events and environmental changes can limit adaptive evolution.

Summary Table: Mechanisms of Evolution

Key Equations

• Allele frequencies:

• Genotype frequencies:

• Calculating allele frequency from genotype counts:

Concept Checks and Applications

• Genetic variation is essential for evolution; without it, populations cannot adapt to changing environments.

• Tracking allele frequencies over generations reveals whether evolution is occurring.

• Natural selection is the only mechanism that consistently increases adaptation; genetic drift and gene flow can change allele frequencies but do not always improve fitness.