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Evolutionary Biology: Darwin's Evidence

Introduction to Evolutionary Biology: BIO 200 - Lecture 4: Darwin's Evidence

Lecture Overview

This lecture, presented by Dr. Heather Williams, focuses on Charles Darwin's hypothesis of natural selection and the evidence supporting it, both from his lifetime and modern discoveries. Key areas covered include:

  • Darwin's hypothesis of natural selection.

  • Evidence available to Darwin during his time.

  • Modern evidence supporting the hypothesis.

  • Implications of Darwin's hypothesis.

  • Evidence supporting these implications.

Darwin's Hypothesis of Natural Selection

Charles Darwin, in Origin of the Species, articulated the core idea of natural selection:

  • Original Quote: "As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form."

Three-Part Model of Darwin's Hypothesis

  1. There must be variation in the population.

  2. Variation must lead to differences among individuals in lifetime reproductive success.

  3. Variation must be transmitted to the next generation (i.e., heritable).

Five-Part Model of Darwin's Hypothesis (More Detailed Explanation)

  1. Individuals vary.

  2. Populations tend to overbreed relative to available resources, leading to a struggle for survival.

  3. Better variations have better survival (often termed "survival of the fittest").

  4. Survivors will reproduce, and non-survivors won't.

  5. Traits leading to better survival and reproduction must be heritable.

Equivalence of the Models

  • The three-part and five-part models are fundamentally the same, with the five-part model offering a more granular explanation of the underlying mechanisms. For example, point #5 in the five-part model (heritability) is necessary for point #2 (differences in reproductive success) to have evolutionary significance.

Evidence for Darwin's Hypothesis: Both Contemporary and Modern

1. Do We See Individual Variation in Natural Populations?

  • Answer: YES!

  • Examples:

    • Powelliphanta land snails display variation.

    • Vincetoxicum rossicum (Dog strangling vine) shows individual differences.

2. Do We See Overbreeding, Leading to a Struggle for Survival in Natural Populations?

  • Answer: YES!

  • Example: Oak (Quercus sp.) Seedlings:

    • A parent oak tree produces numerous acorns.

    • Estimates suggest only 0.01 - 0.1% of seedlings are expected to survive to adulthood.

    • This implies that if 10,000 acorns are produced, only 1 to 10 oaks might survive to maturity, highlighting the intense struggle for existence due to limited resources relative to overproduction.

3. Do Better Variations for a Particular Environment Have Higher Survival Rates?

  • Darwin's Knowledge: Darwin did not have direct, observational proof of this during his lifetime.

  • Modern Evidence: Grant and Grant's Work on Daphne Major (Medium Ground Finch - Geospiza fortis):

    • Rosemary and Peter Grant conducted long-term studies on the Galápagos island of Daphne Major.

    • They observed significant variation in head and beak size among the medium ground finches.

    • 1977 - Severe La Niña Event:

      • Characterized by extreme drought conditions.

      • Resulted in extreme population decline among finches.

      • Vegetation withered, and small, soft seeds were quickly consumed.

      • Only large, hard seeds remained, requiring larger, stronger beaks to crack.

      • Survival Data (from P. T. Boag and P. R. Grant in Science 214:82, 1981):

        • Dead birds: Average beak length 10.68 mm, average beak depth 9.42 mm.

        • Survivors: Average beak length 11.07 mm, average beak depth 9.96 mm.

        • Difference: Survivors had an average beak length increase of +0.39 mm and beak depth increase of +0.54 mm compared to dead birds.

        • This clearly demonstrates that individuals with larger beaks, better suited to the available food source, had higher survival rates.

    • 1984/1985 - El Niño Event:

      • Characterized by unusually wet conditions.

      • Led to abundant vegetation and a high concentration of small, soft seeds.

      • Large, hard seeds became less common.

      • Selection was observed again, likely favoring finches with smaller beaks more efficient at consuming the abundant small seeds, though specific data for this event isn't detailed here, the general observation confirms selection acted in the opposite direction.

    • Conclusion: The Grants' work provides direct, irrefutable evidence that better variations (e.g., beak size) lead to higher survival rates under specific environmental conditions, demonstrating