SHU_1211_Ch19_Evolution

Chapter 19: Processes of Evolution

Evolutionary Foundations

• “Nothing in biology makes sense except in the light of evolution.”-- Theodosius Dobzhansky, Geneticist

Key Concepts

• 19.1 Evolution is both factual and serves as the basis of broader theory.

• 19.2 Mechanisms of Evolution: Mutation, Selection, Gene Flow, Genetic Drift, Nonrandom Mating.

• 19.3 Evolution is measured by changes in allele frequencies.

• 19.4 Types of Selection: Stabilizing, Directional, Disruptive.

• 19.5 Selection can maintain variation within and among populations.

• 19.6 Evolution is constrained by historical and trade-off factors.

Evolution: A Factual Basis

Definition of Evolution

• Evolution refers to the change in the genetic composition of populations over time, supported by fossil records, lab experiments, and natural observations.

• Observations across geological, morphological, and molecular data provide substantial support for evolutionary theory.

Historical Context

• Pre-Darwinian ideas proposed by biologists suggested species change, but no mechanism was identified.

• Charles Darwin's voyage (1831-1836) on HMS Beagle uncovered evidence leading to evolutionary theory.

Nature and Classification of Species

• Aristotle: Proposed organisms were perfectly formed and unchanging.

• Old Testament: Believed species were individually designed by a creator.

• Carolus Linnaeus: Laid groundwork for taxonomy and binomial nomenclature, viewing adaptations as evidence of design.

Fossils and Catastrophism

• Fossils (remains or traces) provide evidence for evolutionary changes over time.

• French scientist Georges Cuvier developed palaeontology and the concept of catastrophism—catastrophes lead to species extinctions.

• Geologists James Hutton and Charles Lyell introduced gradualism and uniformitarianism, influencing Darwin's thought.

Observations by Darwin in the Galápagos Islands

• Noted species variations between islands, indicating adaptations to local environments.

• Darwin’s finches exhibited different beak shapes based on food sources, leading to the concept of adaptive radiation.

Mechanisms of Evolution

Mutation

• Mutations are changes in DNA sequences, serving as the origin of genetic variation.

• Can be beneficial, neutral, or harmful, and may restore genetic variation over time.

Gene Flow

• Migration introduces new alleles, modifying allele frequencies within populations.

Genetic Drift

• Random changes in allele frequencies, leading to significant effects in small populations.

• Bottleneck Effect: Reduces genetic variation due to environmental pressures.

• Founder Effect: Occurs in populations that colonize new regions and lack original genetic diversity.

Nonrandom Mating

• Prevalence of mate selection based on phenotype.

  • Sexual selection influences traits leading to reproductive success; advantageous traits may compromise survival.

Types of Selection

Stabilizing Selection

• Favors average phenotypes; reduces variation without shifting the mean.

• Example: Human birth weights.

Directional Selection

• Favors phenotypes at one extreme of the spectrum, increasing certain traits in the population.

• Example: Horn evolution in longhorn cattle due to predation pressures.

Disruptive Selection

• Favors phenotypes at both extremes; can lead to increased variation within a population.

• Example: Bill size in seedcrackers.

Maintaining Variation

• Frequency-dependent selection can maintain polymorphism based on trait fitness.

• Heterozygote advantage: In fluctuating environments, heterozygous individuals outperform homozygotes, leading to sustained genetic diversity.

• Geographic variation affects trait distribution across populations based on environmental conditions.

Evolutionary Constraints

• Evolution limited by historical events and trade-offs; adaptations may not always be beneficial in every context.

• Example: Rough-skinned newts produce a neurotoxin, but predatory adaptations can lead to varying success.

Summary of Evolutionary Theory

• Evolution is a descent with modification, driven by natural selection, mutation, gene flow, genetic drift, and nonrandom mating.

• Microevolution reflects changes in allele frequencies over time, while macroevolution accounts for larger evolutionary patterns influenced by rare events.