Population Genetics and Natural Selection

Introduction

This section covers the fundamental concepts of population genetics and natural selection, focusing on the mechanisms of evolution, evidence supporting evolutionary theory, and the processes that drive genetic change in populations. These topics are central to understanding how species adapt and evolve over time.

Natural Selection

Definition and Mechanism

Natural selection is the process by which certain traits become more common in a population due to differential reproductive success. It is a key mechanism of evolution, first described by Charles Darwin.

• Differential success in reproduction: Individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation.

• Interaction with environment: The environment determines which traits are favorable.

• Example: In a population of giraffes, those with longer necks may survive better when food is scarce, leading to an increase in long-necked individuals over generations.

How Natural Selection Works

Natural selection operates through several key steps:

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

• Variation in population: Members of a population differ from one another due to genetic variation.

• Inheritance: Many differences are heritable, resulting from genetic differences passed from parents to offspring.

• Differential reproductive success: Individuals better adapted to their environment survive and produce more offspring.

Additional info: Genetic variation arises from mutations, genetic recombination during meiosis, and gene flow.

Summary of Natural Selection

Key Points

• Individuals do not evolve: Evolution occurs at the population level, not within individual organisms.

• Evolution is measured as changes in proportions of heritable traits: Over generations, the frequency of certain traits changes within a population.

• Acquired characteristics are not inherited: Only genetic traits are passed to offspring, not traits acquired during an individual's lifetime.

• Environmental factors: A trait that is favorable in one environment may be detrimental in another.

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 older layers beneath younger ones.

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

• Anatomical evidence: Similar structures (homologous structures) in different species indicate common ancestry. For example, the forelimbs of humans, whales, and bats have similar bone structures but different functions.

• Biochemical evidence: All organisms share basic biochemical molecules such as DNA and ATP. Similarities in DNA and protein sequences reflect evolutionary relationships.

Example: Evolution of Drug Resistance

• Drug-resistant HIV strains multiply rapidly when exposed to anti-HIV drugs, demonstrating natural selection in action.

The Process of Evolution

Population Genetics

Evolution occurs at the population level through changes in gene frequencies over time, a process 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.

Agents of Evolutionary Change

Several mechanisms drive changes in allele frequencies within populations:

• Mutations: Random, permanent genetic changes that introduce new genetic variation. For example, the Arabidopsis thaliana 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. 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 through differential survival and reproduction.

Example: Industrial Melanism in Moths

• In non-polluted areas, light-colored moths are more common because they are less visible to predators on light tree trunks.

• In polluted areas, tree trunks darken, and dark-colored moths become more common due to increased survival.

Summary of Main Points

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

• Hardy-Weinberg equilibrium describes a non-evolving population (not covered in detail here).

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

• Multiple lines of evidence support the theory of evolution, including fossil, anatomical, biochemical, and biogeographical data.