Darwin and Evolution

15.1 The History of Evolutionary Thought

  • Prior to Darwin

    • The view of nature was predominantly based on long-held beliefs rather than experimental evidence.

    • Gradual acceptance of evolutionary ideas about species changing over time.

    • Evolution serves as the unifying principle of biology, explaining:

    • Unity and diversity of life.

    • Similarities between organisms indicating recent common ancestry.

    • Dissimilarities indicating ancient common ancestry.

  • Mid-Eighteenth-Century Influences

    • Taxonomy development:

    • Count Buffon:

      • French naturalist who cataloged all known plants and animals in a 44-volume work.

      • Suggested mechanisms for evolution including environmental influences and the struggle for existence.

    • Linnaeus:

      • Advocated the fixity of species.

      • Proposed that each species has:

      • An ideal structure and function.

      • A place in the scala naturae (a hierarchy of life).

      • Developed the binomial system of nomenclature for classifying living organisms.

  • Late-Eighteenth-Century Influence:

    • Cuvier:

    • Pioneered comparative anatomy for classifying animals and founded paleontology.

    • Proposed catastrophism:

      • Earth’s strata reflect changes due to local catastrophes.

      • Repopulation by species from surrounding regions after each catastrophe indicates change over time.

  • Late-Eighteenth/Early-Nineteenth-Century Influences:

    • Lamarck:

    • First to propose evolution, linking diversity to environmental adaptation.

    • Asserted that complex organisms arise from simpler ones and that acquired characteristics are inherited.

    • Charles Lyell:

    • Proposed uniformitarianism, arguing that geological change is gradual and constant.

    • Malthus:

    • Studied population dynamics and connected famine, war, and epidemics to resource limitations.

    • Provided a framework for Darwin's natural selection concept.

15.2 Darwin’s Theory of Natural Selection

  • Early Life and Influences:

    • Darwin’s childhood fascination with nature and insect collecting shaped his interests.

    • College experiences enhanced his skills in plant identification through his mentor, John Henslow.

    • Conducted fieldwork with geologist Adam Sedgewick in 1831.

  • Voyage on the HMS Beagle:

    • Darwin served as a naturalist on a five-year voyage.

    • Collected evidence for descent with modification from common ancestors.

    • Understood diversity results from adaptations to various environments and pondered the origin of new species.

  • Observations of Change Over Time:

    • Witnessed geological changes on the voyage:

    • Found raised beaches and fossilized shells in sedimentary layers.

    • Collected fossils of Glyptodon and Mylodon (giant sloth).

    • Discovered marine shells at high altitudes in Andes, suggesting life forms descend from extinct relatives.

  • Biogeographical Observations:

    • Biogeography: Study of species distribution geographically reflects historical events.

    • Observed similar species in analogous environments (e.g., Patagonian cavy vs. European rabbit, greater rhea vs. lesser rhea).

    • Concluded that environmental differences modify related species.

  • Galápagos Islands Observations:

    • The islands showed varied species adapted to local environments.

    • Variations in tortoise neck length linked to vegetation types on different islands.

    • Observed significant beak differences among finch species adapted to feeding on various sources (seeds vs. insects).

  • Natural Selection and Adaptation:

    • Variation is crucial for natural selection.

    • Genetic variation can be harmful, helpful, or neutral, with harmful variations filtered out by natural selection.

    • Beneficial mutations lead to greater reproductive success, though natural selection lacks a goal of improvement.

  • Competition for Resources:

    • Darwin applied Malthus’s population concepts to animals, realizing limited resources mean not all offspring survive.

    • Example: Calculated elephant offspring survival leading to unsustainable populations (up to 19 million descendants in 750 years).

  • Differential Reproductive Success:

    • Individuals with favorable traits can outcompete others, leading to differential reproductive success.

    • Fitness: Defined as reproductive success relative to population members; affected by environmental factors.

  • Adaptation Explanation:

    • Adaptations increase organism fitness.

    • Can cause convergent evolution, where unrelated organisms display similar traits (e.g., flippers in manatees, penguins, turtles).

  • Artificial Selection:

    • Humans can modify traits via selective breeding.

    • Example: Domestic dog diversity resulted from breeding wolves with specific traits; Darwin equated this to natural selection processes in nature.

  • Ongoing Observations of Natural Selection:

    • Research by Peter and Rosemary Grant on finches shows beak size adapts to changing weather conditions.

15.3 Evidence for Evolution

  • Fossil Evidence:

    • Fossils, including remains and traces (e.g., footprints), document life's history, showing succession from simple to complex forms.

    • Transitional fossils trace descent among different groups.

  • Anatomical Transitions:

    • Evolutionary transitions observed in whales from fossils (e.g., Ambulocetus, Basilosaurus).

  • Biogeographical Evidence:

    • Reflects evolutionary patterns consistent with geographic isolation and evolution in specific locales (e.g., marsupials in Australia and South America).

  • Anatomical Evidence:

    • Homologous structures: Similar bone structures in vertebrates inherited from a common ancestor but adapted to different functionalities (e.g., forelimbs of various animals).

    • Embryological development: Shared embryonic features in vertebrates (e.g., postanal tails and pharyngeal pouches).

  • Biochemical Evidence:

    • All organisms use the same biochemical building blocks, including DNA triplet codes and amino acids.

    • Genetic similarities indicate common descent; differences suggest phylogenetic separation.

  • Developmental Biology Evidence:

    • Hox genes: Regulate body plan development in animals; variations lead to diverse body plans (example: vertebrae number in chickens vs. snakes).

  • Criticisms of Evolution:

    • Evolution is a well-supported scientific theory, not merely a hypothesis.

    • It is misunderstood as a theory of life's origin rather than diversity post-origins.

    • Transitional fossils exist in number, countering the idea that they are absent.

  • The Tree of Life Project:

    • Collaborative project depicting the relationships among all life, supporting Darwin's evolution theory and its unifying role in biology.