Evolution and Natural Selection: Study Guide for General Biology

Evolution and Natural Selection

Introduction to Evolution

Evolution is a central concept in biology, describing how species change over time through various mechanisms. Understanding evolution helps explain the diversity of life and the adaptation of organisms to their environments.

• Definition: Evolution is the change in the heritable characteristics of biological populations over successive generations.

• Key Points:

  • Evolution is not always improvement; it is change over time.

  • Natural selection is a primary mechanism driving evolution.

  • Other mechanisms include genetic drift, mutation, and gene flow.

• Example: The development of antibiotic resistance in bacteria is an example of evolution by natural selection.

Charles Darwin and the Voyage of the Beagle

Charles Darwin's observations during his voyage on the HMS Beagle were foundational to the development of evolutionary theory.

• Darwin's Birth: 1809

• Voyage Duration: The Beagle's expedition lasted approximately 5 years.

• Purpose of the Voyage:

  • To collect evidence for the theory of evolution by natural selection.

  • To study new species of animals and plants, especially in the Galapagos Islands.

  • To survey the coast of South America.

• Key Observations:

  • Variation among species on different islands.

  • Fossils resembling modern species.

  • Adaptations to specific environments.

Darwin's Main Ideas

Darwin proposed several key concepts that form the basis of modern evolutionary biology.

• Species change over time.

• Living species have arisen from earlier life forms.

• Descent with modification: Modern species arise through gradual changes from ancestral species.

• Natural selection: The process by which organisms better adapted to their environment tend to survive and produce more offspring.

Mechanisms of Evolution

Several mechanisms contribute to evolutionary change in populations.

• Natural Selection: Traits that enhance survival and reproduction become more common in a population over time.

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

• Gene Flow: Movement of genes between populations.

• Mutation: Changes in DNA that introduce new genetic variation.

Types of Natural Selection

Natural selection can act in different ways on the distribution of traits within a population.

• Directional Selection: Favors one extreme phenotype.

• Stabilizing Selection: Favors intermediate phenotypes, reducing variation.

• Disruptive Selection: Favors both extreme phenotypes over intermediates.

• Sexual Selection: Favors traits that enhance mating success.

Evidence for Evolution

Multiple lines of evidence support the theory of evolution.

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

• Comparative Anatomy: Homologous structures indicate common ancestry.

• Embryology: Similarities in early development suggest evolutionary relationships.

• Molecular Biology: Similarities in DNA and protein sequences among species.

Homologous and Analogous Structures

Comparing anatomical features helps distinguish evolutionary relationships.

• Homologous Structures: Features inherited from a common ancestor (e.g., the wing of a bat and the flipper of a whale).

• Analogous Structures: Features that serve similar functions but evolved independently (e.g., wings of birds and insects).

Population Genetics and Variation

Genetic variation within populations is essential for evolution to occur.

• Sources of Variation:

  • Mutation

  • Sexual reproduction (crossing over, independent assortment, random fertilization)

• Genetic Drift: Bottleneck and founder effects can reduce genetic diversity.

• Selection: Heterozygote advantage and balancing selection can maintain variation.

Key Terms and Definitions

• Uniformitarianism: The idea that geological processes occur at the same rate now as in the past.

• Gradualism: Evolutionary change occurs slowly and steadily over time.

• Artificial Selection: Human-driven selection of traits in domesticated species.

• Bottleneck Effect: A sharp reduction in population size due to environmental events.

• Founder Effect: Reduced genetic diversity when a population is descended from a small number of colonizing ancestors.

Important Equations

• Hardy-Weinberg Equation: Describes allele and genotype frequencies in a non-evolving population. $p^2 + 2pq + q^2 = 1$ $p + q = 1$

Table: Types of Selection and Their Effects

Type of Selection	Description	Effect on Population
Directional	Favors one extreme phenotype	Shifts population mean
Stabilizing	Favors intermediate phenotypes	Reduces variation
Disruptive	Favors both extremes	Increases variation, may lead to speciation
Balancing	Maintains multiple alleles	Preserves genetic diversity

Limitations of Natural Selection

Natural selection cannot produce perfectly adapted organisms due to several constraints.

• Adaptations are often compromises.

• Chance events and historical constraints limit outcomes.

• Selection can only act on existing variation.

Applications and Examples

• Antibiotic resistance in bacteria demonstrates rapid evolution due to strong selection pressure.

• Darwin's finches in the Galapagos Islands show adaptive radiation and speciation.

• Population bottlenecks in endangered species can reduce genetic diversity and adaptability.