Population Genetics and Natural Selection: Study Notes

Population Genetics and Natural Selection

Introduction

Population genetics and natural selection are central concepts in evolutionary biology. They explain how genetic variation within populations leads to evolutionary change and adaptation through mechanisms such as mutation, genetic drift, gene flow, nonrandom mating, and natural selection.

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.

• Differential success in reproduction: Individuals with advantageous traits produce more offspring.

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

How Natural Selection Works

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

• Variation in population: Individuals differ in traits due to genetic variation.

• Inheritance: Many differences are heritable and passed to offspring.

• Differential reproductive success: Better-adapted individuals survive and reproduce more.

Example

• Giraffes with longer necks survived competition for food, leading to predominance of the long-necked trait.

Summary of Natural Selection

• Natural selection occurs through interactions between individuals and their environment.

• Over time, it increases adaptation to environments.

• New environments can lead to new selective pressures and potentially new species.

• Important Clarifications:

  • Individuals do not evolve; populations do.

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

  • Environmental factors may change which traits are favorable.

Evidence of Evolution

Types of Evidence

• Natural selection in action: Observable changes in populations, e.g., drug resistance in HIV.

• Fossil record: Fossils in sedimentary rock strata show changes over geological time; transitional fossils link groups.

• Biogeographic evidence: Distinctive species distributions across Earth's six biogeographical regions due to barriers and migration.

• Anatomical evidence: Homologous structures (e.g., limbs of vertebrates) indicate common ancestry.

• Biochemical evidence: All organisms share basic molecules (DNA, ATP); similarity in DNA and protein sequences reflects relatedness.

Example: Natural Selection in Action

• Drug-resistant HIV: Resistant viruses multiply quickly when treated with anti-HIV drugs.

Example: Fossil Evidence

• Strata in sedimentary rocks represent eras; transitional fossils show evolutionary links.

Example: Biogeographical Evidence

• Barriers such as oceans and mountains prevent species migration, leading to unique regional species.

Example: Anatomical Evidence

• Homologous structures in birds, bats, whales, cats, horses, and humans show evolutionary relationships.

• Embryological development similarities among vertebrates further support common ancestry.

Example: Biochemical Evidence

• Degree of similarity in cytochrome c protein among species reflects evolutionary distance.

The Process of Evolution

Population Genetics

Population genetics studies genetic variation within populations and how gene frequencies change over time.

• Population: All members of a species in a specific area.

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

• Microevolution: Change in gene frequencies within a population over time.

Example: Peppered Moth

• Light-colored moths are more visible on unpolluted trees and are eaten by birds; dark-colored moths increase in polluted areas due to camouflage.

Agents of Evolutionary Change

There are five main agents that drive evolutionary change in populations:

• Mutations: Random, permanent genetic changes; source of genetic diversity.

  • Example: Thale cress plant has 200-300 mutations per generation; humans have about 60.

• Genetic drift: Change in allele frequencies due to chance events (e.g., natural disasters).

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

• Nonrandom mating: Individuals select mates based on phenotype or genotype, affecting allele frequencies.

  • Example: Amish population in Pennsylvania has higher frequency of certain alleles due to nonrandom mating.

• Natural selection: Populations adapt to their environment over time; better-adapted individuals have higher reproductive success.

Conditions for Natural Selection

Natural selection requires:

1. Overproduction of offspring and limited resources: Competition for survival.

2. Variation in population: Genetic differences due to mutations and crossing-over in meiosis.

3. Inheritance: Heritable genetic differences.

4. Differential reproductive success: 'Survival of the fittest'—better-adapted individuals produce more offspring.

Summary of Key Points

• Phenotypic variation in populations results from genes and environment.

• Hardy-Weinberg equilibrium describes conditions under which allele frequencies remain constant.

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

• Multiple lines of evidence support the theory of evolution.

Additional info:

• Hardy-Weinberg equilibrium: In a non-evolving population, allele and genotype frequencies remain constant from generation to generation. The equation is: $p^2 + 2pq + q^2 = 1$ where $p$ and $q$ are the frequencies of two alleles.