Evolution and Natural Selection: Study Guide

Evolution and Natural Selection

Introduction

Evolution is the process by which populations of organisms change over generations through variations in traits, often driven by natural selection. This unit explores the mechanisms, evidence, and outcomes of evolutionary change, as well as the classification of biodiversity.

Natural Selection

Darwin's Theory of Natural Selection

• Charles Darwin developed the theory of natural selection after his voyage to the Galápagos Islands, where he observed variations among species.

• Key Components:

  • Variation: Individuals in a population vary in their traits.

  • Overproduction: More offspring are produced than can survive.

  • Competition: Organisms compete for limited resources.

  • Differential Survival and Reproduction: Individuals best suited to their environment survive and reproduce.

  • Descent with Modification: Favorable traits become more common in subsequent generations.

• Adaptation: A heritable trait that increases an organism's fitness in a particular environment.

Lamarck's Theory

• Jean Baptiste Lamarck proposed that organisms could acquire traits during their lifetime and pass them to offspring (inheritance of acquired characteristics).

• This theory is incorrect because acquired traits do not alter genetic material passed to offspring.

Types of Natural Selection

• Directional Selection: Favors one extreme phenotype, shifting the population mean.

• Stabilizing Selection: Favors intermediate phenotypes, reducing variation.

• Disruptive Selection: Favors both extreme phenotypes, potentially leading to speciation.

Example: In a population of birds, directional selection may favor larger beaks if large seeds become the main food source.

Evidence for Evolution

Types of Evidence

• Fossil Record: Shows changes in organisms over time and transitional forms.

• Biogeography: Study of the geographic distribution of species; similar species are often found in nearby regions.

• Comparative Anatomy:

  • Homologous Structures: Similar structures due to common ancestry (e.g., vertebrate limbs).

  • Vestigial Structures: Reduced or nonfunctional features inherited from ancestors (e.g., human appendix).

  • Analogous Structures: Similar function but different evolutionary origins (e.g., wings of birds and insects).

• Embryology: Similar embryonic development among related species.

• Molecular Biology: Comparison of DNA and protein sequences reveals evolutionary relationships.

Forces of Evolution

Sources of Genetic Variation

• Mutation: Random changes in DNA that introduce new alleles.

• Genetic Recombination: Shuffling of genes during meiosis increases diversity.

Mechanisms Driving Evolution

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

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

  • Bottleneck Effect: Drastic reduction in population size alters allele frequencies.

  • Founder Effect: A new population is established by a small group, leading to different allele frequencies.

• Artificial Selection: Humans select for desirable traits in organisms (e.g., dog breeding).

• Non-random Mating: Mates are chosen based on specific traits, affecting allele frequencies.

Hardy-Weinberg Equilibrium

• Describes a non-evolving population where allele and genotype frequencies remain constant.

• Five Assumptions:

  • No natural selection

  • No mutation

  • No migration (gene flow)

  • Large population size

  • Random mating

• Equation:

$p^2 + 2pq + q^2 = 1$ $p + q = 1$

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

Speciation

Species and Speciation

• Species: A group of organisms that can interbreed and produce fertile offspring.

• Speciation: The formation of new species through reproductive isolation.

Isolating Mechanisms

• Reproductive Isolation: Barriers that prevent interbreeding between populations.

• Types of Isolation:

  • Behavioral Isolation: Differences in mating behaviors prevent reproduction.

  • Geographical Isolation: Physical barriers separate populations.

  • Temporal Isolation: Populations breed at different times.

Biodiversity and Classification

Biodiversity

• Refers to the variety of life forms in an ecosystem or on Earth as a whole.

Taxonomy and Classification

• Taxonomy: The science of naming, describing, and classifying organisms.

• Phylogeny: The evolutionary history and relationships among species.

• Clade: A group of organisms that includes an ancestor and all its descendants.

• Cladogram: A diagram that shows evolutionary relationships among groups.

Structure Type	Description	Example
Homologous	Similar structure, different function; common ancestry	Human arm and whale flipper
Analogous	Different structure, similar function; no common ancestry	Bird wing and insect wing
Vestigial	Reduced or nonfunctional structure inherited from ancestors	Human appendix

Summary

• Darwin's observations led to the theory of natural selection, explaining how evolution occurs.

• Multiple lines of evidence support evolutionary theory, including fossils, anatomy, and molecular data.

• Genetic variation and evolutionary forces drive changes in populations.

• Speciation results from reproductive isolation mechanisms.

• Biodiversity is organized through taxonomy and illustrated by phylogenetic trees and cladograms.