Evolution by Natural Selection, Evolutionary Processes, and Speciation

Chapter 22 – Evolution by Natural Selection

Darwin's Major Ideas

• 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: The concept that species are distinct and real entities.

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

• Species Change: Recognition that species can change over time.

• Malthusian Principle: Populations grow faster than resources, leading to competition ('struggle for existence').

• Uniformitarianism: Geological processes are constant over time (Lyell).

• Lamarck’s Theory: Use and disuse of traits leads to inheritance of acquired characteristics (now known to be incorrect).

Population-Level Evolution

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

Descent with Modification

• All life shares a single origin; genetic relationships among species are due to shared ancestry.

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

Homologies as Evidence for Evolution

• Genetic Homology: Similar DNA sequences among different species.

• Developmental Homology: Similar embryonic development patterns.

• Structural Homology: Similar anatomical structures (e.g., vertebrate limbs).

• These homologies support the theory of evolution by indicating common ancestry.

Natural Selection Explained

• Individuals in a population vary in traits.

• Some traits confer a reproductive advantage in a given environment.

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

• "Survival of the fittest" refers to reproduction, not just survival.

Evidence Supporting Evolution

• Extinction: Fossil record shows species disappear over time.

• Geographic Proximity: Related species are often found in the same region.

• Homoplasy: Similar traits evolve 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 change.

Common Misconceptions about Evolution

• "Evolution occurs in individuals": False; evolution is a change in population allele frequencies.

• "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

• Combines Mendelian inheritance (traits as particles/genes) with Darwinian quantitative traits (continuous variation).

• The population is the smallest unit of evolution.

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

Genes and Alleles

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

• Allele: Different versions of a gene.

• Individuals have two alleles for each gene (diploid organisms).

• 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.

Processes That Cause Evolution

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

• Non-random Mating: Mating not at random (e.g., inbreeding, assortative mating).

• Mutation: Source of new genetic variation.

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

• Natural Selection: Differential reproductive success based on traits.

Genetic Drift Mechanisms

• Bottleneck Effect: Sudden reduction in population size leads to loss of genetic diversity.

• Founder Effect: Small group establishes a new population with different allele frequencies.

Hardy-Weinberg Equilibrium

• Mathematical model to test if evolution is occurring.

• If allele and genotype frequencies remain constant, the population is in equilibrium (no evolution).

• Equation:

  • Allele frequencies: $p + q = 1$

  • Genotype frequencies: $p^2 + 2pq + q^2 = 1$

• Assumes no mutation, migration, selection, genetic drift, or non-random mating.

• Serves as a null hypothesis; rarely met in nature.

Natural Selection and Phenotypes

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

• Three modes:

  • Directional Selection: Favors one extreme phenotype.

  • Stabilizing Selection: Favors intermediate phenotypes.

  • Diversifying (Disruptive) Selection: Favors both extreme phenotypes.

Preservation of Genetic Diversity

• Diploidy: Maintains hidden recessive alleles.

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

• Negative Frequency-Dependent Selection: Rare phenotypes favored.

• Neutral Variation: Genetic variation with no fitness effect.

Chapter 24 – Speciation

Speciation Overview

• Speciation: Formation of new species (macroevolution).

• Source of biological diversity; can be abrupt or gradual.

Species Concepts

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

• Morphospecies Concept: Species defined by morphological differences.

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

Concept	Advantages	Disadvantages
Biological	Focuses on reproductive isolation	Not applicable to fossils/asexuals; hybrids exist
Morphospecies	Widely applicable, easy to use	Polymorphic/cryptic species; subjective
Phylogenetic	Logical, widely applicable	Requires detailed data; may over-split species

Reproductive Isolation Mechanisms

• Prezygotic Barriers (before fertilization):

  • Before mating: Temporal, habitat, behavioral isolation

  • After mating: Mechanical, gametic incompatibility

• Postzygotic Barriers (after fertilization):

  • Hybrid inviability

  • Hybrid sterility

  • Hybrid breakdown

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

Steps to Speciation

• Interruption of gene flow

• Genetic differentiation

• Reproductive isolation

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 (increased divergence)

• Hybrid zone (stable hybrids)

• 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 sexual selection intensity.

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, little sexual dimorphism.

• Polyandry: Abundant resources, females larger or with complex genitalia.

• Polygamy: Complex social dynamics.

Cheating in Mating Systems

• Cheating (e.g., extra-pair copulations) is rare because natural selection favors detection and prevention mechanisms.

Haploid-Diploid System and Relatedness

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

• This high relatedness influences social behavior and evolution of eusociality.