Evolution and the Rise of Evolutionary Thought

Introduction to Evolution

Evolution is both a pattern and a process. The pattern is revealed by scientific data, while the process consists of mechanisms that produce the observed changes in living organisms over time.

• Typological Thinking: Plato and Aristotle believed species were unchanging types, with Aristotle adding a 'scale of nature' from simple to complex.

• Lamarck: Proposed that species change through time and can move up the scale of nature.

• Darwin and Wallace: Introduced the concepts of change through time and common ancestry, laying the foundation for modern evolutionary biology.

Evidence for Evolution

• Species Change Through Time: Fossil evidence shows life has changed over billions of years, with extinction events altering species composition.

• Transitional Features: Fossils with traits intermediate between ancestral and derived species support evolutionary change.

• Vestigial Traits: Reduced or nonfunctional structures similar to functional organs in ancestors (e.g., human tailbone).

• Homology: Similarity due to common ancestry, including:

  • Genetic Homology: Similar DNA, RNA, or protein sequences.

  • Developmental Homology: Similar embryonic structures (e.g., pharyngeal pouches in vertebrate embryos).

  • Structural Homology: Similar adult morphology (e.g., limb bones in vertebrates).

Mechanisms of Evolution

Natural Selection

Natural selection is the process by which individuals with advantageous traits survive and reproduce more successfully, increasing the frequency of those traits in the population.

• Variation: Individuals vary in their traits.

• Heritability: Some trait differences are heritable.

• Differential Survival and Reproduction: More individuals are born than can survive; those with beneficial traits are more likely to survive and reproduce.

• Fitness: The ability to produce surviving, fertile offspring relative to others.

• Adaptation: A heritable trait that increases fitness in a particular environment.

• Artificial Selection: Humans selectively breed organisms for desired traits (e.g., pigeons).

The Hardy-Weinberg Model

The Hardy-Weinberg equilibrium describes a non-evolving population where allele and genotype frequencies remain constant.

• Alleles: Different versions of a gene.

• Genotypes: Combinations of alleles in individuals.

• Phenotypes: Observable traits.

• Gene Pool: All alleles in a population.

Hardy-Weinberg Equations:

• Allele frequencies:

• Genotype frequencies:

Assumptions:

• No selection

• No mutation

• No migration

• Large population size

• Random mating

Modes of Selection

• Directional Selection: Favors one extreme phenotype, shifting the population mean.

• Stabilizing Selection: Favors intermediate phenotypes, reducing variation.

• Disruptive Selection: Favors both extremes over intermediates.

• Balancing Selection: Maintains multiple phenotypes in a population.

• Sexual Selection: Traits that increase mating success (e.g., bright plumage in birds).

Other Evolutionary Processes

• Genetic Drift: Random changes in allele frequencies, especially in small populations (e.g., founder effect, bottleneck effect).

• Gene Flow: Movement of alleles between populations, reducing genetic differences.

• Mutation: The ultimate source of genetic variation; most mutations are neutral or deleterious, few are beneficial.

Speciation and Species Concepts

Species Concepts

• Biological Species Concept: Groups of actually or potentially interbreeding populations that are reproductively isolated from others.

• Morphospecies Concept: Species are identified by morphological differences.

• Phylogenetic Species Concept: Species are the smallest monophyletic groups on a phylogenetic tree, identified by synapomorphies.

Mechanisms of Reproductive Isolation

• Prezygotic Isolation: Barriers before fertilization (temporal, habitat, behavioral, mechanical, gametic).

• Postzygotic Isolation: Barriers after fertilization (hybrid inviability, hybrid sterility).

Allopatric and Sympatric Speciation

• Allopatric Speciation: Geographic isolation leads to divergence (by dispersal or vicariance).

• Sympatric Speciation: Speciation without geographic isolation, often via polyploidy (autopolyploidy, allopolyploidy).

Outcomes of Secondary Contact

• Fusion of populations

• Extinction of one population

• Reinforcement of divergence

• Hybrid zone formation

• Formation of new species via hybridization

History of Life and Phylogenies

Fossil Types and Formation

• Intact Fossils: Organic remains preserved without decomposition.

• Compression Fossils: Organic material compressed into a thin film.

• Cast Fossils: Cavity filled with minerals forms a cast of the organism.

• Permineralized Fossils: Minerals infiltrate cells, hardening into stone.

• Trace Fossils: Indirect evidence (e.g., footprints, burrows).

Biases in the Fossil Record: Habitat, taxonomic/tissue, temporal, and abundance biases affect fossilization likelihood.

Geologic Timeline

• Precambrian: ~4.6 bya to 541 mya; origin of life, first green algae, and simple animals.

• Phanerozoic Eon: 541 mya to present; includes Paleozoic, Mesozoic, and Cenozoic eras.

• Paleozoic: Diversification of animals, plants, fungi; ends with mass extinction.

• Mesozoic: Age of reptiles and gymnosperms; ends with dinosaur extinction.

• Cenozoic: Age of mammals and flowering plants.

Adaptive Radiation

• Rapid production of many descendant species from a single lineage, often following ecological opportunity or key innovations (e.g., Cambrian Explosion).

Prokaryotes: Bacteria and Archaea

Three-Domain System

• Woese's three-domain tree: Bacteria, Archaea, Eukarya.

Prokaryotic Diversity

• Found in extreme environments (extremophiles).

• Most are harmless; some are pathogens.

Koch's Postulates

1. Agent present in diseased, absent in healthy organisms.

2. Agent isolated and grown in pure culture.

3. Agent causes disease in healthy host.

4. Agent reisolated from experimental host.

Gene Transfer in Prokaryotes

• Transformation: Uptake of DNA from environment.

• Transduction: Transfer via bacteriophages.

• Conjugation: Direct transfer via plasmids or chromosomal segments.

Prokaryotic Morphology and Metabolism

• Varied size, shape, motility, and cell wall composition (Gram-positive vs Gram-negative).

• Metabolic diversity:

  • Phototrophs: Use light energy.

  • Chemoorganotrophs: Use organic molecules.

  • Chemolithotrophs: Use inorganic molecules.

  • Autotrophs: Use CO2 as carbon source.

  • Heterotrophs: Use organic carbon.

Protists

Defining Features

• All eukaryotes except plants, animals, and fungi.

• Possess mitochondria (or related genes), nucleus, endomembrane system, and cytoskeleton.

Endosymbiosis Theory

• Mitochondria originated from engulfed alpha-proteobacteria.

• Chloroplasts originated from engulfed cyanobacteria (primary endosymbiosis); some lineages acquired chloroplasts via secondary endosymbiosis.

Protist Diversity and Importance

• Key innovations: multicellularity, sexual reproduction, alternation of generations.

• Ecological roles: primary producers, consumers, decomposers.

• Medical and economic importance: malaria, harmful algal blooms, crop diseases.

Plants

Green Plants and Land Colonization

• Viridiplantae: Green algae and land plants.

• First land plants appeared ~475 mya; green algae are older.

• Nonvascular plants (e.g., mosses) are gametophyte-dominant; seedless vascular plants (e.g., ferns) are sporophyte-dominant.

Adaptations to Land

• Cuticle, UV-absorbing compounds, and sporopollenin for protection.

• Evolution of lignin and vascular tissue enabled upright growth.

Reproduction and Life Cycles

• Alternation of generations: multicellular haploid (gametophyte) and diploid (sporophyte) stages.

• Homospory (one spore type) vs heterospory (two spore types: microspores and megaspores).

• Seeds and pollen are key innovations in gymnosperms and angiosperms.

• Flowers and fruits enhance pollination and seed dispersal.

Animals

Defining Features

• Multicellular, heterotrophic eukaryotes lacking cell walls.

• Ingest food before digestion.

• Most have neurons and muscle cells.

Major Evolutionary Innovations

• Origin of tissues (diploblasts vs triploblasts).

• Bilateral symmetry, cephalization, and nervous system development.

• Body cavity (coelom) evolution.

• Protostome (mouth first) vs deuterostome (anus first) development.

Animal Diversity

• Protostomes: Lophotrochozoans (e.g., flatworms, annelids, molluscs) and Ecdysozoans (e.g., nematodes, arthropods).

• Deuterostomes: Echinoderms (e.g., sea stars, sea urchins) and chordates (e.g., vertebrates).

Key Animal Groups

• Flatworms: No coelom, high surface-area-to-volume ratio, many are parasitic.

• Annelids: Segmented worms with coelom and bristles (chaetae).

• Molluscs: Foot, visceral mass, mantle; diverse forms (snails, clams, squids).

• Arthropods: Segmented bodies, jointed limbs, exoskeleton; includes insects, crustaceans, myriapods, chelicerates.

• Echinoderms: Radial symmetry in adults, water vascular system, tube feet.

• Chordates: Pharyngeal slits, dorsal hollow nerve cord, notochord, post-anal tail.

Vertebrate Evolution

• Jawless Vertebrates: Hagfishes and lampreys.

• Gnathostomes: Jawed vertebrates (cartilaginous fishes, ray-finned fishes, coelacanths, lungfishes, tetrapods).

• Tetrapods: Amphibians (first land vertebrates), amniotes (reptiles, birds, mammals).

• Key Adaptations: Jaws, paired appendages, amniotic egg, endothermy, flight (in birds).

Viruses

Definition and Importance

• Viruses are infectious agents with a DNA or RNA genome, protein capsid, and sometimes a lipid envelope; lack ribosomes and depend on host cells for replication.

• Major causes of disease and drivers of evolution via natural selection and lateral gene transfer.

Viral Replication Cycle

1. Viral genome enters host cell.

2. Viral genome is transcribed and translated.

3. Viral genome is replicated.

4. New viral particles assemble.

5. Virions exit the host cell (budding or lysis).

6. Virions infect new hosts.

Genome Replication Strategies

• DNA Viruses: Use host or viral DNA polymerase.

• RNA Viruses: Use viral RNA-dependent RNA polymerase or reverse transcriptase (for retroviruses).

Origins of Viruses

• Regression Hypothesis: Evolved from free-living ancestors that lost cellular components.

• Escape Hypothesis: Originated from mobile genetic elements.

• Virus-First Hypothesis: Originated before or alongside earliest cells.

Key Terms

• Epidemic: Localized outbreak.

• Pandemic: Global outbreak.

• Virulence: Degree of pathogenicity.

• Uncoating: Release of viral genome inside host cell.