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.