BIO112 Lec6 Evolution by Natural Selection 1 2025

Lecture 6: Evolution by Natural Selection

  • Overview of the concept of evolution.

  • Sources: OpenStax chapter 18; Freeman chapter 25.

Historical Context

Plato and Typological Thinking (428-348 BC)

  • Plato's belief: every organism exemplifies a perfect essence created by a God.

  • Modern interpretation: species as unchanging types; variations deemed unimportant.

Aristotle and the Great Chain of Being (384-322 BC)

  • Aristotle categorized organisms in a linear hierarchy.

  • Fixed types of species; higher species viewed as more complex.

  • Humans positioned at the top of this chain.

  • Aristotle's ideas maintained popularity into the 1700s across various circles.

Evolutionary Theories

Lamarck's Contributions (1809)

  • Proposed inheritance of acquired traits influenced by environmental changes.

  • Suggested simple organisms arise from spontaneous generation and evolve upward over time towards complexity.

  • Viewed evolution as a progressive movement leading to "better" species.

Darwin and Wallace (1858)

  • Both presented ideas of natural selection before the Linnean Society.

  • Key insights:

    • Evolution by natural selection is not linear.

    • Variability among individuals in populations is critical.

  • Darwin conceived his theory around 1838; Wallace's essay catalyzed the joint presentation.

Common Ancestry and Tree of Life

Framework of Evolution

  • All species emerge from a common ancestor.

  • No species is inherently "higher" than another; branches denote relatedness.

  • Critique of Eurocentric narratives in evolutionary history; recognizing other cultural contributions.

The Revolutionary Nature of Natural Selection

  1. Challenged static existence of species.

  2. Shifted from typological to population thinking.

  3. Provided a testable scientific mechanism for change over time.

Components of Evolution

Essential Concepts

  • Descent with Modification (Darwin)

  • Change in allele frequencies across generations (modern definition).

  • Status of pattern (species change) and process (natural selection).

    • Predicts:

      1. Species evolve through time.

      2. Common ancestry linking species (speciation).

      3. Occurrence of extinction.

Evidence of Change

  • Fossil records and geological time scales illustrate evolution through:

    • Older rock layers with primitive species like trilobites.

    • Younger layers showing more complex life forms.

Extinction Events

  • Over 99% of species that have existed are now extinct.

  • Marked by significant mass extinctions and constant background extinction.

Evolution Patterns and Predictions

  • Evidence supports the theory:

    1. Change occurs in species over time.

    2. Relatedness through common ancestry (speciation).

    3. Extinction events reinforce species changes.

  • Observable relatedness in homologies, transitional features, and vestigial traits.

Transitional Features in Evolutionary History

  • Examples include the evolution of tetrapod limbs from lungfishes, illustrating gradual complexity.

  • Key transitional fossils:

    • Tulerpeton, Acanthostega, Tiktaalik, Eusthenopteron.

Vestigial Traits

  • Examples include:

    • Spider monkey tails and human coccyx indicate common ancestry.

    • Demonstrate retained features from evolutionary history.

Speciation through Radiation

  • Recent genetic data affirm common ancestry among Galápagos mockingbird species differing across islands.

  • Diversification of populations over time leads to distinct species.

Homology in Evolution

  • Shared traits signify common ancestry, studied at:

    1. Genetic Level.

    2. Developmental Level.

    3. Structural Level.

  • Example: Eyeless gene homology between fruit flies and humans.

Contemporary Evolutionary Examples

  • Illustrated by phenomena such as:

    • Evolution of bacterial resistance to drugs.

    • Changes in fish populations due to harvesting practices.

    • Darwin's finches on the Galápagos provide live examples of adaptation.

Human-Induced Evolution

  • Discussion on how human harvesting alters phenotypic traits in wildlife.

  • Can unintentionally promote "unnatural" selection against desirable traits.

  • Examples of traits affected:

    • Age and size at sexual maturation, body size, sexual dimorphism.

Research Findings on Ground Finches

  • Hypothesis testing on beak characteristics in response to environmental changes, specifically drought.

  • Summary data supports the influence of natural selection on measurable characteristics over time in populations.

Common Misconceptions about Evolution

  • Evolutionary change is a population phenomenon, not individual.

  • Natural selection does not aim for perfection or progress but reacts to existing genetic variation.