Unit 7: Natural Selection – Foundations and Mechanisms of Evolution

Unit 7: Natural Selection

7.1: History of Evolutionary Thought

The development of evolutionary theory has involved contributions from many scientists and philosophers, each adding to our understanding of how species change over time.

• Plato: Proposed that all species are perfect and unchanging.

• Aristotle: Suggested that species could be arranged in a hierarchy of increasing complexity, known as the "Scala Naturae" or "Great Chain of Being."

• Taxonomy: The science of classifying organisms, foundational for understanding evolutionary relationships.

• Carolus Linnaeus: Founder of modern taxonomy; developed the binomial nomenclature system. He believed species were fixed and did not change.

• Georges-Louis Leclerc (Comte de Buffon): Proposed that species could change due to environmental factors and speculated that Earth was older than 6,000 years.

• Erasmus Darwin: Noted vestigial structures and suggested that competition and selection could improve species.

• Georges Cuvier: Founded paleontology, opposed evolution, and supported catastrophism (major events shape Earth's features and cause extinctions).

• Catastrophism: The idea that sudden, short-lived, violent events, such as volcanoes, earthquakes, and floods, have shaped Earth's surface.

• James Hutton: Proposed the concept of "deep time" and gradual geological processes.

• Charles Lyell: Outlined uniformitarianism, the idea that the same geological processes observed today have occurred throughout Earth's history.

• Uniformitarianism: Slow, uniform, and continuous processes are responsible for Earth's features. "The present is the key to the past."

• Jean-Baptiste de Lamarck: First to propose a testable hypothesis for evolution. Correctly suggested that adaptations drive evolution and that characteristics are inherited, but incorrectly believed in inheritance of acquired traits ("use and disuse").

• Thomas Malthus: Economist who argued that populations grow faster than resources, leading to competition and survival of the fittest.

7.2: Darwin

Charles Darwin's observations and studies laid the foundation for the theory of evolution by natural selection.

• Born in England in 1809, Darwin was interested in nature from a young age. He studied medicine and theology before becoming a naturalist.

• He became an assistant to botanist John Henslow and geologist Adam Sedgwick, gaining expertise in classification and geology.

• Darwin joined the HMS Beagle as a companion to Captain Fitzroy, with the main goal of mapping the South American coastline.

• Darwin observed geological formations and fossils in South America, including raised beaches and marine fossils in the Andes.

• His voyage to the Galapagos Islands was pivotal; he noted that species on each island were uniquely adapted to their environments.

• Darwin's finch observations led to the idea that species evolve to fit their environment, a process he called natural selection.

• Genetic Variation: Arises from mutations and meiosis, providing the raw material for evolution.

• Adaptation: Inherited traits that enhance survival and reproduction in a specific environment.

• Fitness: The ability of an organism to survive and reproduce in its environment.

7.3: Selection

Selection is the process by which certain traits become more common in a population due to differential reproductive success.

• Artificial Selection: Humans select for desirable traits in plants and animals.

• Natural Selection: Outlined by Darwin, it is the process where organisms with advantageous traits survive and reproduce more successfully.

• Key Points of Natural Selection:

  • Variation exists within populations.

  • Organisms compete for limited resources.

  • Individuals with advantageous traits have higher reproductive success.

  • Populations become better adapted over time.

7.4: Evidence of Evolution

Multiple lines of evidence support the theory of evolution, including fossils, anatomical structures, molecular data, and embryology.

• Fossils: Remains or traces of ancient organisms. The law of superposition states that older fossils are found in deeper layers.

• Transitional Fossils: Show intermediate forms between ancestral and derived species.

• Homologous Structures: Anatomically similar structures inherited from a common ancestor.

• Analogous Structures: Different structures with similar functions, evolved independently.

• Vestigial Structures: Structures that have lost their original function in a species.

• Molecular Evidence: DNA and protein similarities reflect evolutionary relationships. For example, humans share 98% of their DNA with chimpanzees.

• Embryological Evidence: Similar patterns of development suggest common ancestry.

7.5: Hardy-Weinberg Principle

The Hardy-Weinberg principle provides a mathematical model to study genetic variation in populations.

• Evolution: Defined as a change in allele frequencies over time.

• Gene Pool: The total collection of alleles in a population.

• Allele Frequencies: Denoted as p (dominant) and q (recessive), with the equation $p + q = 1$.

• Genotype Frequencies: $p^2 + 2pq + q^2 = 1$, where $p^2$ = homozygous dominant, $2pq$ = heterozygous, $q^2$ = homozygous recessive.

• Hardy-Weinberg Equilibrium: Occurs when allele frequencies remain constant if no evolutionary forces act on the population.

• Conditions for Equilibrium: No mutations, no migration, large population, random mating, and no selection.

7.6: Genetic Drift and Gene Flow

Genetic drift and gene flow are mechanisms that alter allele frequencies in populations.

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

• Bottleneck Effect: A sharp reduction in population size reduces genetic diversity.

• Founder Effect: A new population started by a small group may have different allele frequencies than the original population.

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

7.7: Types of Natural Selection

Natural selection can take different forms depending on which phenotypes are favored.

• Stabilizing Selection: Favors intermediate phenotypes; extremes are selected against. Example: Human birth weight.

• Directional Selection: Favors one extreme phenotype. Example: Evolution of horse size.

• Disruptive Selection: Favors both extremes over the intermediate phenotype. Example: Mice coloration in patchy environments.

7.8: Other Selection Mechanisms

Other forms of selection include sexual selection, exaptation, and heterozygote advantage.

• Sexual Selection: Traits that increase mating success become more common.

• Sexual Dimorphism: Distinct differences between sexes in appearance or size.

• Exaptation: Traits evolved for one function are co-opted for another (e.g., feathers for flight).

• Heterozygote Advantage: Heterozygotes have a selective advantage (e.g., sickle cell trait and malaria resistance).

7.9: Evolution and Species Concepts

Evolution occurs at both small (microevolution) and large (macroevolution) scales. The biological species concept defines species based on the ability to interbreed and produce fertile offspring.

• Microevolution: Changes in allele frequencies within a population.

• Macroevolution: Large-scale changes that can result in new species.

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

7.10: Speciation

Speciation is the process by which new species arise.

• Allopatric Speciation: Occurs when populations are geographically separated.

• Sympatric Speciation: Occurs without geographic separation, often through genetic changes.

• Adaptive Radiation: Rapid evolution of many species from a common ancestor, often when new habitats become available.

• Punctuated Equilibrium: Species remain unchanged for long periods, punctuated by rapid evolutionary changes.

• Gradualism: Species evolve through a slow and steady accumulation of small changes.

7.11: Isolating Mechanisms

Reproductive isolation maintains species boundaries.

• Prezygotic Mechanisms: Prevent mating or fertilization (habitat, temporal, behavioral, mechanical, gamete isolation).

• Postzygotic Mechanisms: Prevent hybrid offspring from developing into fertile adults (hybrid inviability, infertility, breakdown).

7.12: Life History and the Origin of Life

The origin of life involved several key stages, from the formation of simple molecules to self-replicating systems.

• Big Bang Theory: Universe began 13.8 billion years ago; Earth formed 4.5 billion years ago.

• Miller-Urey Experiment: Simulated early Earth conditions and produced amino acids, supporting the idea that organic molecules could form naturally.

• Stages of Life: Monomers → Polymers → Protocells → Self-replicating systems (RNA world hypothesis).

• LUCA: Last Universal Common Ancestor, shared by all life.

7.13: Extinctions

Extinction is the loss of all members of a species. Mass extinctions are rapid decreases in biodiversity, with five major events in Earth's history.

7.14: Taxonomy

Taxonomy is the science of naming and classifying organisms.

• Binomial Nomenclature: Two-part scientific naming system (Genus species), always italicized or underlined.

• Linnaean System: Hierarchical classification: Species, Genus, Family, Order, Class, Phylum, Kingdom, Domain.

7.15: Three Domain System

All life is classified into three domains based on genetic and cellular differences.

• Bacteria: Single-celled, prokaryotic, cell walls with peptidoglycan.

• Archaea: Single-celled, prokaryotic, cell walls without peptidoglycan, often in extreme environments.

• Eukarya: Cells with a nucleus, includes protists, fungi, plants, and animals.

7.16: Mapping Out Evolution

Evolutionary relationships are depicted using diagrams such as cladograms and phylogenetic trees.

• Cladogram: Shows relationships based on shared characteristics; branch lengths are arbitrary.

• Phylogenetic Tree: Shows evolutionary history with branch lengths indicating time since divergence.

• Clade: A group consisting of a common ancestor and all its descendants.

• Node: Represents a speciation event.

• Outgroup: A species or group outside the clade of interest, used for comparison.

• Ancestral Traits: Traits present in the common ancestor.

• Derived Traits: Traits that evolved after the common ancestor.

7.17: Species Relations

Comparative methods help clarify evolutionary relationships.

• Convergent Evolution: Independent evolution of similar traits in unrelated lineages due to similar environmental pressures.

• Molecular Comparisons: DNA and protein sequence similarities are used to infer evolutionary relationships and estimate divergence times (molecular clocks).

• Protein Comparisons: Differences in protein sequences can help determine phylogeny.