Evolution and Sexual Selection

Anisogamy and Isogamy

Anisogamy refers to a form of sexual reproduction involving the fusion of two dissimilar gametes (e.g., large eggs and small sperm), while isogamy involves gametes of similar size and form.

• Anisogamy: Characteristic of most animals and plants; leads to differentiation of male and female sexes.

• Isogamy: Seen in some algae and fungi; no distinct male or female gametes.

• Evolution of Anisogamy: Thought to arise due to disruptive selection favoring both very small and very large gametes, but not intermediate sizes.

Example: Human reproduction is anisogamous, with large, nutrient-rich eggs and small, motile sperm.

Types of Natural Selection

• Directional Selection: Favors one extreme phenotype, causing a shift in the population's trait distribution.

• Disruptive Selection: Favors both extremes over intermediate phenotypes, potentially leading to speciation.

• Stabilizing Selection: Favors intermediate phenotypes, reducing variation.

Sexual Selection Theories

• Trivers’ Parental Investment Theory: The sex investing more in offspring (usually females) becomes a limiting resource for which the other sex competes.

• Bateman’s Principle: Variance in reproductive success is greater in males than females, leading to stronger sexual selection on males.

Sexual Selection Mechanisms

• Intrasexual Selection: Competition within one sex (often males) for access to mates. Can occur pre-copulation (fighting), during copulation (sperm competition), or post-copulation (cryptic female choice).

• Intersexual Selection: Mate choice, often by females, based on traits indicating genetic quality or resources.

• Leks: Aggregations where males display for females, who choose mates based on displays.

• Good Genes vs. Fisherian Arbitrary Choice: Good genes hypothesis suggests females choose mates with traits indicating genetic fitness; Fisherian hypothesis suggests traits are chosen for attractiveness alone ("sexy sons").

• Direct & Indirect Benefits: Direct benefits include resources or protection; indirect benefits include genetic advantages for offspring.

• Sexual Dimorphism: Differences in appearance between sexes due to sexual selection.

Examples and Case Studies

• Gray Tree Frogs: Call duration in males is an indicator of genetic quality.

• Barn Swallow Experiment: Tail length affects mating success, supporting sexual selection.

• Human Mating Strategies: Vary by sex and context, influenced by evolutionary pressures.

Population Genetics

Hardy-Weinberg Equilibrium (HWE)

The Hardy-Weinberg principle describes a non-evolving population where allele and genotype frequencies remain constant from generation to generation.

• Assumptions: No mutation, random mating, no gene flow, infinite population size, and no selection.

• If Assumptions Met: Allele and genotype frequencies remain constant.

• If Assumptions Not Met: Evolution occurs; frequencies change over time.

• Equation:

• p: Frequency of dominant allele

• q: Frequency of recessive allele

Forces of Evolution

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

• Selection: Differential survival and reproduction changes allele frequencies.

• Mutation: Introduces new alleles into a population.

• Balancing Selection: Maintains genetic diversity; includes frequency-dependent selection and heterozygote advantage.

Example: Prairie Chicken populations experienced reduced genetic diversity and fitness due to drift and habitat loss.

Speciation

Species Concepts

• Biological Species Concept: Groups of interbreeding natural populations reproductively isolated from others.

• Morphological Species Concept: Based on structural features.

• Ecological Species Concept: Based on ecological niche.

• Phylogenetic Species Concept: Smallest group sharing a common ancestor.

Steps in Speciation

1. Isolation of populations

2. Divergence (genetic, ecological, or behavioral)

3. Reproductive isolation

Modes of Speciation

• Allopatric Speciation: Geographic separation leads to divergence.

• Sympatric Speciation: Occurs without geographic isolation, often via polyploidy or sexual selection.

• Dispersal & Vicariance: Dispersal involves movement to new areas; vicariance is splitting by physical barriers.

• Autopolyploidy & Allopolyploidy: Polyploidy from same species (auto-) or hybridization (allo-), common in plants.

• Example: Apple maggot flies diverged on different host plants (sympatric speciation).

• Sexual Selection: Can drive sympatric speciation, e.g., cichlid fish with different mate preferences.

Reproductive Barriers

• Prezygotic Barriers (prevent mating): Habitat, temporal, and behavioral isolation.

• Prezygotic Barriers (prevent fertilization): Mechanical and gametic isolation.

• Postzygotic Barriers: Reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.

Examples: Anole lizards show reproductive isolation; hybrid zones and ring species illustrate ongoing speciation.

Ecology

Definition and Levels of Study

Ecology is the study of interactions between organisms and their environment. Levels include:

• Organismal

• Population

• Community

• Ecosystem

• Biosphere

Climate

• Seasons: Caused by Earth's axial tilt; seasonality increases with latitude.

• Global Precipitation Patterns: Driven by solar energy and atmospheric circulation; wet tropics, dry subtropics.

• Rain Shadows: Mountains block moist air, creating dry areas on leeward sides.

• Uneven Warming: Equator receives more direct sunlight than poles.

Climate Change

• Temperature Change: Global temperatures have risen over the past 200 years.

• Greenhouse Effect: Greenhouse gases (CO2, CH4, etc.) trap heat by absorbing infrared radiation.

• Is the Effect Man-Made? The greenhouse effect is natural, but human activities have increased greenhouse gas concentrations.

• Atmospheric Composition: Greenhouse gases are a small but impactful component.

• CO2 Trends: Atmospheric CO2 has increased significantly since the Industrial Revolution.

• Abiotic Changes: Melting glaciers, sea level rise, altered precipitation patterns.

• Biotic Changes: Shifts in species ranges, altered phenology (timing of biological events).

Biomes

• Biomes: Major ecological communities defined by climate and dominant vegetation.

• Precipitation & Temperature: Biome distribution is determined by these factors ("nose graph").

• Winter & Drought Deciduous: Plants lose leaves in response to cold or dry seasons.

• Needle Leaf & Broad Leaf Evergreen: Needle leaf evergreens (conifers) retain leaves year-round; broad leaf evergreens common in tropics.

Population Biology/Ecology

• Metapopulation: A group of spatially separated populations connected by dispersal.

• Local/Subpopulation: Individual populations within a metapopulation.

• Geographical Range: The area a species occupies; can include native, exotic/invasive, and migrating species.

• Population Structure: Age and sex distribution, often visualized with population pyramids.

• Density Estimation Methods:

  • Sample & Estimate: Counting individuals in sample plots.

  • Mark-Recapture: Marking and recapturing individuals to estimate total population.

  • Index of Population Size: Indirect measures (e.g., tracks, nests).

• Types of Dispersion: Clumped, uniform, random.

Demography and Population Growth

• Life Tables: Summarize survival and reproductive rates by age.

• Survivorship Curves:

  • Type I: High survival early, drops in old age (e.g., humans).

  • Type II: Constant mortality (e.g., birds).

  • Type III: High mortality early, survivors live long (e.g., oysters).

• Exponential Growth: Population grows at a constant rate under ideal conditions.

• Logistic Growth: Growth slows as population approaches carrying capacity (K).

• Conditions: Exponential growth occurs with unlimited resources; logistic growth occurs with resource limitation.