Evolution, Evolutionary Processes, Speciation, and Sexual Selection: Study Notes

Chapter 22 – Evolution by Natural Selection

Darwin's Major Ideas

Charles Darwin revolutionized biology with two foundational concepts: Descent with Modification and Natural Selection.

• Descent with Modification: All species are related by common ancestry and have changed over time.

• Natural Selection: The mechanism by which individuals with advantageous traits reproduce more successfully, leading to adaptation.

Historical Context and Influences

• Scale of Nature: Aristotle's idea of a fixed hierarchy of life forms.

• Species as Biological Units: Recognition that species are distinct entities.

• Linnaean Classification: System for naming and grouping organisms based on similarities.

• Species Change: Early recognition that species can change over time.

• Malthusian Principle: Populations grow faster than resources, leading to competition.

• Uniformitarianism (Lyell): Geological processes are slow and continuous, implying Earth's great age.

• Lamarck’s Theory: Proposed inheritance of acquired traits (use and disuse), now known to be incorrect.

Population-Level Evolution

• Evolution occurs at the population level, not in individuals.

Descent with Modification and Genetic Relationships

• All life shares a single origin; genetic relationships among species are revealed by DNA similarities.

• Sources of variation include mutation, meiosis, bi-parental inheritance, and random fertilization.

Homologies as Evidence for Evolution

• Genetic Homology: Similar DNA sequences among species.

• Developmental Homology: Similar embryonic development patterns.

• Structural Homology: Similar anatomical structures (e.g., limb bones in vertebrates).

• These homologies support common ancestry.

Mechanism of Natural Selection

• Individuals in a population vary in traits.

• Some traits confer higher reproductive success in a given environment.

• Natural selection is not random; it favors traits that increase fitness.

• Fitness: The ability to survive and reproduce.

• It is reproduction (not mere survival) that matters for evolution.

Evidence Supporting Evolution

• Extinction: Fossil record shows species disappear over time.

• Geographic Proximity: Related species are often found near each other.

• Homoplasy: Similar traits arising independently (convergent evolution).

• Transitional Fossils: Fossils showing intermediate forms between groups (e.g., Archaeopteryx between reptiles and birds).

• Vestigial Traits: Structures with reduced or no function (e.g., human appendix).

• Artificial Selection: Human-driven selection (e.g., dog breeds) demonstrates evolutionary principles.

Common Misconceptions about Evolution

• "Evolution occurs in individuals": False; only populations evolve as allele frequencies change.

• "Evolution is goal-directed": False; evolution is not purposeful or striving for perfection.

• "Evolution creates perfect organisms": False; evolution involves trade-offs and constraints.

• "Evolution is only in the past": False; evolution is ongoing (e.g., antibiotic resistance in bacteria).

Chapter 23 – Evolutionary Processes

Population Genetics

Population genetics merges Mendelian inheritance (discrete traits) with Darwinian evolution (continuous variation).

• Population: The smallest unit of evolution.

• Evolution is defined as a change in allele frequencies in a population over time.

Genes, Alleles, and Gene Pools

• Gene: A segment of DNA coding for a trait.

• Allele: Different forms of a gene; individuals have two alleles per gene (one from each parent).

• Fixed Allele: Only one allele exists at a locus in a population; no variation.

• Small populations are more likely to become fixed for alleles, which can be maladaptive.

• Gene Pool: All alleles present in a population.

Mechanisms of Evolution

• Five processes cause evolution (change in allele frequency):

  1. Genetic Drift: Random changes, especially in small populations.

  2. Non-random Mating: Mating not at random (e.g., inbreeding).

  3. Mutation: New alleles arise by mutation.

  4. Migration (Gene Flow): Movement of alleles between populations.

  5. Natural Selection: Differential reproductive success based on traits.

Genetic Drift

• Random, unpredictable changes in allele frequencies.

• Two main mechanisms:

  • Bottleneck Effect: Sudden reduction in population size (e.g., disaster).

  • Founder Effect: Small group colonizes a new area, carrying only a subset of alleles.

Hardy-Weinberg Equilibrium

• Provides a mathematical model for allele and genotype frequencies in a non-evolving population.

• If a population is in Hardy-Weinberg equilibrium, allele frequencies do not change (no evolution).

• Serves as a null hypothesis for detecting evolution.

Hardy-Weinberg Equation:

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

$p + q = 1$

• Where p and q are the frequencies of two alleles.

Natural Selection and Phenotypes

• Natural selection is adaptive and acts on phenotypes (observable traits).

• Three modes of selection:

  • Directional Selection: Favors one extreme phenotype.

  • Stabilizing Selection: Favors intermediate phenotypes.

  • Diversifying (Disruptive) Selection: Favors both extremes over intermediates.

Preservation of Genetic Diversity

• Diploidy: Hides recessive alleles in heterozygotes.

• Heterozygote Advantage: Heterozygotes have higher fitness (e.g., sickle cell trait).

• Negative Frequency-Dependent Selection: Rare alleles are favored.

• Neutral Variation: Genetic variation with no effect on fitness.

Chapter 24 – Speciation

Speciation and Species Concepts

• Speciation: The process by which new species arise (macroevolution).

• Can occur abruptly or gradually.

• Defining "species" is challenging; three main concepts:

  1. Biological Species Concept: Species are groups of interbreeding populations reproductively isolated from others.

  2. Morphospecies Concept: Species are defined by morphological differences.

  3. Phylogenetic Species Concept: Species are the smallest monophyletic groups on a phylogenetic tree.

Biological Species Concept

• Focuses on reproductive isolation and production of viable offspring.

• Not applicable to fossils or asexual organisms.

Reproductive Barriers

• Prezygotic Barriers (before fertilization):

  • Before mating: Temporal, habitat, behavioral isolation.

  • After mating attempt: Mechanical isolation, gamete incompatibility.

• Postzygotic Barriers (after fertilization):

  • Hybrid viability (hybrids do not develop or survive well).

  • Hybrid sterility (hybrids are sterile, e.g., mule).

  • Hybrid breakdown (hybrid offspring are weak or sterile in subsequent generations).

• Postzygotic barriers are more "expensive" because resources are invested before failure is realized.

Limitations of Species Concepts

• Biological Species Concept: Limited by hybrids, asexual species, and historical/geographic isolation.

• Morphospecies Concept: Easy to apply but problematic with polymorphic or cryptic species; subject to taxonomic interpretation.

• Phylogenetic Species Concept: Logical and widely applicable but limited by data availability and may over-split species.

Steps to Speciation

• Gene flow is interrupted.

• Genetic differentiation occurs.

• Reproductive isolation is established.

Allopatric vs. Sympatric Speciation

• Allopatric Speciation: Occurs when populations are geographically separated.

• Sympatric Speciation: Occurs without geographic separation, often via disruptive selection or polyploidy.

Outcomes of Secondary Contact

• Fusion (populations merge)

• Extinction (one population disappears)

• Reinforcement of divergence

• Hybrid zone formation

• New species formation

Sexual Selection

Sexual vs. Natural Selection

• Sexual Selection: Selection for traits that increase mating success.

• Natural Selection: Selection for traits that increase survival and reproduction.

• Resource distribution and parental investment influence mating systems and selection pressures.

Mating Systems

System	Description
Polygyny	One male, multiple females
Polyandry	One female, multiple males
Monogamy	One male, one female
Promiscuity	Multiple males and females

Resource Distribution and Mating Strategies

• Polygyny: Large/strong males compete for access to females; showy males attract mates.

• Monogamy: Increased parental care leads to pair bonding; little sexual dimorphism.

• Polyandry: Abundant resources allow females to mate with multiple males; can lead to complex genital morphology and larger females.

• Polygamy: Complex social dynamics influence mating patterns.

Cheating in Mating Systems

• Cheating (e.g., extra-pair copulations) is rare because natural selection favors mechanisms to detect and prevent it.

Haploid-Diploid System and Relatedness

• In haplodiploid systems (e.g., bees), sisters share on average 75% of their genes.