Chapter 13: How Populations Evolve

Defining Evolution and Its Scales

• General Definition of Evolution: In its broadest sense, evolution refers to change or growth that occurs in a particular order.

• Biological Definition of Evolution: In biology, evolution refers to the processes that have transformed life on Earth from its earliest forms to the vast diversity observed today. It is essentially characterized by heritable changes.

• Microevolution: Refers to generation-to-generation changes in the frequencies of alleles or genotypes within a population.

• Macroevolution: Refers to large-scale evolution occurring over very long periods of time. This occurs at or above the species level and involves the formation of new taxonomic groups or the extinction of existing taxonomic groups.

Historical Perspectives on Species

• Aristotle (Ancient Greece): Proposed that species are fixed and do not evolve. He believed all forms of life could be arranged on a scale of increasing complexity, known as the Continuum of organization or the Natural Scale of Life. In this hierarchy, insects and small animals were positioned at the bottom, while humans sat at the top.

Charles Darwin and the Voyage of the Beagle

• The Expedition ($1831-1836$): Darwin served on the Voyage of the Beagle, a five-year expedition primarily to South America.

• Significant Locations Visited:

  • Atlantic Ocean: Great Britain, Europe, Africa, Cape of Good Hope.

  • South America: Andes mountains, Tierra del Fuego, Cape Horn.

  • Pacific Ocean: Galapagos Islands, Australia, Tasmania, New Zealand.

  • Galapagos Island Specifics: The map identifies several islands including Pinta, Genovesa, Marchena, Santiago, Daphne, Fernandina, Pinzón, Isabela, Santa Cruz, Santa Fe, San Cristobal, Florenza, and Española. The scale of the islands is noted as approximately $40\,km$ (or $40\,miles$).

• Observations and Collections:

  • Collected exotic and diverse fossils, fauna, and flora from South America.

  • Found many unusual fossils.

  • Noted that South American plants and animals had very unique characteristics, distinct from European species.

  • Galapagos Specifics: Observed that species on the islands were found nowhere else in the world, were unique to different islands within the chain, and were similar but not identical to species on the nearby mainland (while being quite different from those on far mainlands).

Evidence Supporting Evolution

Evolution is supported by several lines of evidence, including fossils, homology, vestigial structures, comparative embryology, and molecular biology.

• Fossils: Preserved remains of organisms from the past that document differences between past and present life and provide evidence of extinction.

  • Mineralized Fossils: Most fossils consist of mineralized bones, skulls, or teeth.

  • Casts or Mold Fossils: Formed when an organism is buried and decays, leaving an empty hole that is eventually filled with sediment or minerals.

  • Trace Fossils: These are fossils of an organism's activity, such as footprints, burrows, and tracks.

  • Organic Material Preservation: Occasionally, organic material is retained. Examples include fossilized leaves containing remnant chlorophyll and insects trapped in amber (fossilized resin).

• Homology (Morphological Divergence): Anatomical similarities between species represent variations on an anatomical structure from a common ancestor.

  • Homologous Structures: Examples include forelimbs that have different uses but similar underlying structures due to remodeling for specific functions (descent with modification).

• Morphological Convergence (Analogy): Analogous structures have similar functions but different structures and do not infer evolution from a common ancestor. These structures evolved independently.

  • Examples: Bat wings, butterfly wings, and bird wings.

• Vestigial Structures: Body parts that have no apparent current function, such as human tail bones. These suggest that species have changed over time.

• Comparative Embryology: Organisms within a group often share similar patterns of development and the same master genes directing that development.

• Molecular Biology: Similarities in DNA and protein sequences reflect evolutionary relationships. Comparisons of specific amino acid sequences (e.g., in proteins like cytochrome c) show varying degrees of similarity between:

  • Honeycreepers ($10$ species)

  • Song sparrow

  • Gough Island finch

  • Deer mouse

  • Asiatic black bear

  • Bogue (a fish)

  • Human

  • Thale cress (a plant)

  • Baboon louse

  • Baker's yeast

Theoretical Influences and the Development of Natural Selection

• Charles Lyell: Author of The Principles of Geology. He proposed the Uniformity Theory and Gradualism, suggesting the Earth changed slowly over millions of years through gradual processes.

• Thomas Malthus: Correlated human population decreases with disease, famine, and war. He proposed that humans can run out of resources because reproductive capacity can exceed the environment's ability to sustain them.

• Alfred Russel Wallace: Independently developed a theory of evolution by natural selection while working in Indonesia in $1858$.

• Charles Darwin's Theory: By the early $1840$s, Darwin had drafted his theory but did not publish until completing On the Origin of Species by Means of Natural Selection in $1859$.

  • Core Concept: Descent with modification—living species descended from earlier forms and changed over time.

  • Mechanism: Natural Selection.

Principles of Natural Selection in Modern Terms

• Observations about Populations:

  • Natural populations have an inherent capacity to increase in size.

  • Resources (food, living space) eventually become limited.

  • Individuals must compete for these limited resources.

• Observations about Genetics:

  • Individuals share certain traits, but vary in the details of those traits.

  • Shared traits have a heritable basis in genes (alleles).

• Inferences:

  • Certain forms of a shared trait make a bearer better able to survive.

  • Individuals better able to survive tend to leave more offspring.

  • Adaptive traits (alleles) become more common in the population over time.

The Evolution of Populations

• Population: A localized group of individuals belonging to the same species with the potential to interbreed.

• Gene Pool: All alleles at all gene loci in all individuals of a population.

• Allele Frequency: The abundance of a particular allele among members of a population.

• Fitness: The degree of adaptation to an environment, measured by genetic contribution to future generations.

• Sources of Genetic Variation:

  • Mutation: The source of new alleles.

  • Crossing over (Meiosis I): Introduces new combinations of alleles.

  • Independent Assortment (Meiosis I): Mixes maternal and paternal chromosomes.

  • Fertilization: Combines alleles from two parents.

  • Changes in Chromosome Number/Structure: Often causes dramatic functional changes.

The Hardy-Weinberg Genetic Equilibrium

This principle states that allele and genotype frequencies remain constant generations despite meiosis and sexual reproduction under five specific conditions:

1. The population must be very large.

2. No gene flow between populations.

3. No mutations occur.

4. Mating is random.

5. No natural selection (all individuals survive and reproduce equally).

Mathematics of Hardy-Weinberg:

• Frequency of dominant allele: $p$

• Frequency of recessive allele: $q$

• Genotype Frequencies:

  • Homozygous dominant: $p^2$

  • Homozygous recessive: $q^2$

  • Heterozygote: $2pq$

• Equations:

  • $p + q = 1$

  • $p^2 + 2pq + q^2 = 1$

Drivers of Microevolution

Microevolution is driven by four primary mechanisms: Mutation, Natural Selection, Genetic Drift, and Gene Flow.

1. Natural Selection and Phenotypic Distribution

Natural selection affects the frequency of heritable traits in three distinct ways:

• Directional Selection: Shifts the range of variation in one direction, favoring variants at one extreme.

  • Example: Mouse fur color becoming darker as a landscape is shaded by trees.

• Stabilizing Selection: Favors intermediate forms of a trait and removes extreme variants, reducing phenotypic variation.

  • Example: Eliminating unusually light or dark mice to favor an intermediate color.

• Disruptive Selection: Favors forms at both extremes of a range, favoring variants at opposite ends over intermediate individuals.

  • Example: Mice colonizing a habitat with light soil and dark rocks.

2. Sexual Selection

Refers to an advantage in securing a mate.

• Sexual Dimorphism: Visible differences between males and females of a species.

• Examples:

  • Male elephant seals fighting for access to female clusters.

  • Male birds of paradise using flashy courtship displays for choosy females.

  • Female stalk-eyed flies preferring males with the longest eyestalks (a trait with no survival advantage other than attractiveness).

3. Genetic Drift

A change in the gene pool due to chance, which reduces genetic variation. It is more pronounced in small populations.

• Bottleneck Event: A drastic reduction in population size (e.g., due to earthquakes or fires), resulting in a surviving population with reduced genetic variation.

• Founder Effect: Occurs when a small group colonizes a new habitat (e.g., an island), leading to less genetic variation and sometimes a high frequency of inherited disorders.

4. Gene Flow

Refers to the genetic exchange between populations, contributing to microevolutionary change.