Chapter 22: Evolution by Natural Selection

Typological Thinking, Lamarckian Evolution, and Darwinian Evolution

• Typological Thinking (Plato): The belief that every organism is a perfect, unchanging type created by a divine being. Variation is considered unimportant.

• Aristotle's Scale of Nature: Organisms are arranged in a fixed sequence based on increasing size and complexity, with humans at the top. This is also known as the "Great Chain of Being."

• Lamarckian Evolution: Proposed that species change over time through the inheritance of acquired characteristics. For example, giraffes stretch their necks to reach leaves, and this trait is passed to offspring.

• Darwin and Wallace’s Theory of Natural Selection: Species change over time due to natural selection, where individuals with advantageous traits survive and reproduce more successfully. Variation within populations is essential.

Evidence Supporting the Theory of Evolution

• Change Through Time: Fossil records, transitional forms, and observable changes in living species (e.g., antibiotic resistance in bacteria) demonstrate that species are not static.

• Descent from a Common Ancestor: Homologous structures (similar anatomy in different species), molecular similarities (DNA, proteins), and vestigial traits support common ancestry.

• Internal Consistency: Multiple independent lines of evidence (fossils, genetics, anatomy) all support the same evolutionary patterns.

How Natural Selection Causes Evolution

• Artificial Selection: Humans selectively breed organisms for desired traits (e.g., dog breeds, crop varieties), demonstrating how selection can change populations.

• Darwin's Postulates (Criteria for Natural Selection):

  1. Variation exists among individuals in a population.

  2. Some of these variations are heritable.

  3. More offspring are produced than can survive; there is a struggle for existence.

  4. Individuals with advantageous traits are more likely to survive and reproduce.

• Evolutionary Terms vs. Everyday English: In biology, "fitness" refers to reproductive success, not physical strength; "adaptation" is a heritable trait that increases fitness.

Applying Darwin’s Postulates: Real-World Scenarios

• Drug Resistance: Bacteria with mutations that confer resistance survive antibiotic treatment and reproduce, leading to a population of resistant bacteria.

• Finch Beaks in the Galápagos: During droughts, finches with deeper beaks survive better because they can eat harder seeds, leading to an increase in deep-beaked individuals.

Common Misconceptions about Natural Selection and Evolution

• Natural Selection Changes Populations, Not Individuals: Individuals do not evolve; populations evolve over generations.

• Natural Selection Is Not Goal-Directed: Evolution does not work toward a predetermined goal; it is a response to environmental pressures.

• Natural Selection Does Not Lead to Perfection: Traits that increase fitness in one environment may be disadvantageous in another; evolution is constrained by historical and genetic factors.

Chapter 23: Evolutionary Processes

The Hardy–Weinberg Principle and Population Evolution

• Gene Pool Concept: The total collection of alleles in a population at any one time.

• Hardy–Weinberg Equilibrium: A population is in equilibrium if allele and genotype frequencies remain constant from generation to generation in the absence of evolutionary forces.

• Assumptions of Hardy–Weinberg Principle:

  • No mutation

  • No gene flow

  • No genetic drift (infinite population size)

  • No selection

  • Random mating

• Hardy–Weinberg Equations:

  • Allele frequencies:

  • Genotype frequencies:

Effect of Nonrandom Mating

• Nonrandom Mating: Mating that is not random (e.g., inbreeding, assortative mating) changes genotype frequencies but not necessarily allele frequencies.

• Inbreeding: Increases homozygosity and can expose deleterious recessive alleles, often reducing fitness (inbreeding depression).

• Interaction with Natural Selection: Inbreeding can increase the effectiveness of selection against deleterious alleles.

Natural Selection and Hardy–Weinberg Assumptions

• Natural Selection: Violates Hardy–Weinberg by favoring certain alleles, changing allele and genotype frequencies over time.

• Types of Selection:

  • Directional Selection: Favors one extreme phenotype, shifting the average trait value.

  • Stabilizing Selection: Favors intermediate phenotypes, reducing variation.

  • Disruptive Selection: Favors both extreme phenotypes, increasing variation.

  • Balancing Selection: Maintains multiple alleles in the population (e.g., heterozygote advantage).

• Effects on Phenotype and Genetic Variation:

  • Directional: Decreases variation

  • Stabilizing: Decreases variation

  • Disruptive: Increases variation

  • Balancing: Maintains variation

Genetic Drift Compared to Other Evolutionary Processes

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

• Effect on Genetic Variation: Reduces genetic variation within populations.

• Effect on Fitness: Can increase or decrease fitness randomly.

• Sampling Error: Random events can cause allele frequencies to fluctuate unpredictably.

• Founder Effect: A new population is started by a small number of individuals, leading to different allele frequencies than the original population.

• Bottleneck Effect: A sudden reduction in population size (e.g., due to disaster) drastically reduces genetic diversity.

Gene Flow Compared to Other Evolutionary Processes

• Gene Flow: Movement of alleles between populations through migration.

• Effect on Genetic Variation: Increases genetic variation within populations, decreases differences between populations.

• Effect on Fitness: Can increase or decrease fitness depending on the alleles introduced.

• Gene Flow vs. Genetic Drift: Gene flow homogenizes populations, while drift causes divergence.

Mutation Compared to Other Evolutionary Processes

• Mutation: Random changes in DNA sequence; the ultimate source of genetic variation.

• Effect on Genetic Variation: Introduces new alleles into a population.

• Effect on Fitness: Most mutations are neutral or deleterious; a few are beneficial.

• Importance to Evolution: Without mutation, evolution would eventually stop because there would be no new variation for selection to act upon.

Chapter 24: Speciation

Mechanisms of Reproductive Isolation

• Prezygotic Isolation: Prevents individuals from different species from mating.

  • Temporal (different breeding times)

  • Habitat (different environments)

  • Behavioral (different mating behaviors)

  • Mechanical (incompatible reproductive structures)

  • Gametic (incompatible gametes)

• Postzygotic Isolation: Mating occurs, but offspring are inviable or sterile (e.g., mule hybrids).

Species Concepts: Advantages and Disadvantages

• Biological Species Concept: Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups.

  • Advantage: Focuses on reproductive isolation.

  • Disadvantage: Not applicable to asexual organisms or fossils.

• Morphospecies Concept: Species are identified by differences in morphological features.

  • Advantage: Useful for fossils and asexual species.

  • Disadvantage: Subjective; may miss cryptic species.

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

  • Advantage: Applicable to all organisms; based on evolutionary history.

  • Disadvantage: Requires detailed genetic information.

Allopatric Speciation

• Definition: Speciation that occurs when populations are geographically separated.

• Processes Affecting Speciation: Genetic drift, natural selection, and mutation can cause divergence.

• Allopatric Speciation by Dispersal: A few individuals colonize a new habitat, leading to divergence.

• Allopatric Speciation by Vicariance: A physical barrier (e.g., river, mountain) splits a population, leading to divergence.

• Example: Formation of new species of squirrels on opposite sides of the Grand Canyon.

Sympatric Speciation

• Definition: Speciation that occurs without geographic separation.

• Disruptive Selection: Favors extreme phenotypes, leading to reproductive isolation within the same area.

• Polyploidization: Chromosome duplication (especially in plants) can instantly create reproductive isolation.

• Example: Polyploid speciation in plants such as wheat.

Outcomes of Secondary Contact

• Fusion: Populations interbreed and differences are erased.

• Extinction: One population outcompetes the other, leading to extinction.

• Reinforcement: Natural selection increases reproductive isolation.

• Hybrid Zones: Regions where interbreeding occurs and hybrids are common.

• Hybrid Speciation: Hybrids may form a new species if they are fit and reproductively isolated from parent species.

Additional info: These notes expand on the learning objectives by providing definitions, examples, and context for each evolutionary process and speciation mechanism. Equations are provided for Hardy–Weinberg calculations, and a comparative table summarizes selection types.