Population Genetics and Natural Selection

Introduction

This section explores the mechanisms of evolution, focusing on population genetics and natural selection. It covers the evidence supporting evolutionary theory, the processes by which evolution occurs, and summarizes key concepts for understanding how populations change over time.

Natural Selection

Definition and Mechanism

Natural selection is the process by which certain heritable traits become more common in a population due to differential reproductive success. This occurs as organisms interact with their environment, leading to some individuals producing more offspring than others.

• Differential success in reproduction: Individuals with advantageous traits are more likely to survive and reproduce.

• Overproduction of offspring and limited resources: More offspring are produced than can survive, leading to competition.

• Variation in population: Members of a population differ from one another in their traits.

• Inheritance: Many differences among individuals are heritable and based on genetic variation.

• Differential reproductive success: Better adapted individuals survive and produce more offspring, increasing the frequency of favorable traits.

Example: In a population of giraffes, those with longer necks may be better able to reach food and thus survive and reproduce more successfully than those with shorter necks.

Summary of Natural Selection

Key Points

• Natural selection acts on individuals, but evolution is observed as changes in populations over generations.

• Individuals do not evolve; populations do.

• Acquired characteristics during an individual's lifetime are not inherited.

• Environmental factors can change, making previously favorable traits less advantageous.

Evidence of Evolution

Types of Evidence

Multiple lines of evidence support the theory of evolution:

• Natural selection in action: Observable changes in populations, such as the evolution of drug-resistant HIV.

• Fossil record: Fossils embedded in sedimentary rock strata provide a timeline of evolutionary change. Each stratum represents a different era, with transitional fossils showing intermediate forms.

• Biogeographic evidence: The Earth is divided into distinct biogeographical regions, each with unique species. Barriers prevent species from migrating, and the distribution of fossils and living species helps reconstruct evolutionary history.

• Anatomical evidence: Homologous structures (similar anatomy and function) indicate common ancestry among species.

• Biochemical evidence: All organisms share basic biochemical molecules (DNA, ATP, enzymes). Similarities in DNA and protein sequences reflect evolutionary relationships.

Example: The evolution of drug-resistant HIV demonstrates natural selection in real time, as resistant viruses proliferate when exposed to anti-HIV drugs.

The Process of Evolution

Population Genetics

Evolution occurs at the population level through changes in gene frequencies over time. This is known as microevolution.

• Population: All members of a species occupying a particular area at the same time.

• Gene pool: The sum total of all alleles of all genes in a population.

Example: The frequency of light- and dark-colored moths changes in response to environmental pollution, demonstrating evolution in action.

Agents of Evolutionary Change

Several mechanisms drive changes in allele frequencies within populations:

• Mutations: Random, permanent genetic changes that introduce genetic diversity. For example, the Arabidopsis plant may have 200-300 mutations per generation; humans have approximately 60.

• Genetic drift: Changes in allele frequencies due to chance events, especially in small populations.

• Gene flow: Movement of alleles between populations through migration.

• Nonrandom mating: Individuals select mates based on phenotype or genotype, affecting allele frequencies. For example, the Amish population in Pennsylvania has a higher frequency of a recessive allele for dwarfism than the general population.

• Natural selection: Populations adapt to their environment over time as favorable traits become more common.

Requirements for Natural Selection

• Overproduction of offspring and limited resources: Leads to competition for survival.

• Variation in population: Genetic differences arise from mutations and crossing-over during meiosis.

• Inheritance: Differences must be heritable.

• Differential reproductive success: "Survival of the fittest"—better adapted individuals leave more offspring.

Summary of Main Points

• Phenotypic variation among individuals in a population results from the combined effects of genes and environment.

• Hardy-Weinberg equilibrium describes the genetic structure of a non-evolving population.

• Natural selection and other evolutionary processes drive changes in populations.

• Multiple lines of evidence support the theory of evolution.

Table: Agents of Evolutionary Change

Key Equations

Hardy-Weinberg Equation:

Where p and q are the frequencies of two alleles in a population.

Additional info: The Hardy-Weinberg equilibrium provides a baseline for detecting evolutionary change in populations. Deviations from equilibrium indicate that one or more agents of evolution are at work.