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Natural Selection Overview

• Author: P.R. Grant

Learning Objectives

• Define fitness:

  • Contrast between Darwin’s original usage and modern understanding.

• Calculate changes in allele frequencies:

  • Utilize selection coefficients of the alleles.

• Describe mechanism of natural selection:

  • Explain selective changes of finches on Galápagos island of Daphne Major.

Key Quotes on Natural Selection

• Charles Darwin:

  • "Can we doubt that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? This preservation of favorable and rejection of injurious variations I call natural selection." - The Origin of Species, 1859

• Alfred Russel Wallace:

  • "I have been struck by the utter inability of many intelligent persons to understand…the effects of natural selection, therefore, I wish to suggest to you…‘Survival of the Fittest’.” - Letter to Darwin, 1866

Concept of 'Fittest'

• Darwin’s version of fitness:

  • Not technical; based on an organism's adaptation to its environment (not physical fitness).

• Modern fitness interpretation:

  • Defined as the number of offspring left behind by individuals with specific traits or alleles.

• Relative fitness (ω):

  • Describes the comparative offspring output of groups possessing different traits.

Population Genetics

• Eclipse of Darwin's theory:

  • Initial rejection of natural selection based on belief it could not change populations quickly.

• Mathematical contributions:

  • R.A. Fisher (1918) and J.B.S. Haldane (1924) demonstrated that selection acts faster than previously thought.

Calculating Selective Changes

• Formula:

  • Δp = sp(1-p)

  • Where:

    • Δp = change in frequency of allele p over one generation

    • s = selection coefficient

    • p = current frequency of allele p

The Selection Coefficient

• Definition:

  • Measure of the relative strength of selection favoring a beneficial allele.

• Examples:

  • s = 0 (equal fitness of alleles)

  • s = 0.01 (101 offspring vs 100)

  • s = -0.01 (99 offspring vs 100)

Genetic Variation Component

• Definition:

  • Product of the frequency of one allele and the other. Higher variation allows faster changes.

• Maximum variation:

  • Achieved when p = 0.5.

Case Study: CCR5-delta32 Mutation Example

• Setup:

  • Initial frequency = 0.2, selection coefficient = 0.1

  • Calculate Δp = sp(1-p)

• Calculations:

  • Δp = 0.1 * 0.2 * (1 - 0.2)

  • Δp = 0.016

  • New frequency: p’ = Δp + p = 0.216 (after one generation)

• Generational Spread:

  • Time for Δ32 allele to spread to frequency of 0.9: 4/s = 4/0.1 = 40 generations (≈ 800 years).

Case Study: CitT Mutation Example

• Initial frequency: 0.1, selection coefficient: 0.1

• Calculations:

  • Δp = 0.1 * 0.1 * (1 - 0.1) = 0.009

• Generational Spread:

  • Again, 4/s = 4/0.1 = 40 generations.

Strong Selection Examples

• Peppered Moths:

  • Selection coefficient estimated at 0.3 (J.B.S. Haldane, 1924).

• Finches and Mice:

  • Hoekstra et al. (2004): Selection coefficient for mice matching background = 0.09-0.39.

Observations on Natural Populations

• Darwin's Finches Study:

  • Conducted by Rosemary and Peter Grant from 1973 until 2012, yielding insights into natural selection.

• Daphne Major:

  • Small volcanic island, home to various finch species, dramatically affected by climate and food sources.

Evolutionary Changes Observed in G. fortis (1976-1978)

• Population Decline:

  • From 1,400 individuals in 1976 to 200 by 1978 due to lack of food sources (seed abundance drop).

• Beak Depth Adaptation:

  • Increased mean beak depth from 9.42 mm to 9.96 mm in one generation.

• Size Increase:

  • Average beak depth and body size grew ~4% from 1976 to 1978.

• Macroevolutionary Perspective:

  • Hypothesized rapid evolution, suggesting that G. fortis could evolve into G. magnirostris size in ~200 years under consistent environmental pressures.

Effects of Environmental Changes on Beak Depth

• Drought Conditions:

  • Increased competition for food, leading to the struggle of smaller birds.

• Wet Conditions:

  • Enhanced seed production and success for smaller birds, resulting in better reproductive success for them.

Genetic Insights on Beak Size and Shape

• Key Genes Identified:

  • ALX1 and HMGA2 linked to beak shape variability.

• Epigenetic Variation:

  • Being studied for its role in finch variation.

Conclusion on Natural Selection

• Natural selection leads to higher reproductive success for certain heritable traits.

• Today's measurements of fitness are based on offspring success.

• Calculations of future population states are made possible through selection coefficients derived from relative fitness.

• Direct observations show strong and oscillating selection among Galápagos finches.

Upcoming Topics

• Rare recessive alleles in Amish populations: 1 in 14 carries a recessive allele for dwarfism, as opposed to 1 in 1000 in general populations. Explore the implications of why such traits persist despite not appearing adaptive.