2 Adaptive Landscape

THE ADAPTIVE LANDSCAPE

  • Concept Origin: The metaphor for evolutionary change was introduced by Sewall Wright in 1932.

    • Describes a population occupying a position on an ADAPTIVE LANDSCAPE defined by allele frequencies.

  • Characteristics of the Adaptive Landscape:

    • Peaks: Represent genetic compositions for which w (average fitness) is high.

    • Troughs: Represent genetic compositions for which ŵ (the alternative average fitness) is low.

  • Movement on the Adaptive Landscape:

    • As gene frequencies change and w increases, the population moves uphill to rest on an adaptive peak.

    • Multiple peaks exist; some are higher than others, but natural selection can only improve fitness by moving uphill.

    • Changes in the environment can alter the fitness of genotypes, thereby changing the adaptive landscape.

  • Impacts of Environmental Change:

    • Genetic compositions previously in troughs may become peaks.

    • Natural selection may then drive the population to this new peak.

    • Generally, populations move to local peaks rather than the global highest peak.

INTERACTIONS OF EVOLUTIONARY FACTORS

  • Factors Affecting Allele Frequency:

    • Deterministic Factors: Include mutation, gene flow, and selection. They consistently affect allele frequency across populations if conditions are equal.

    • Stochastic Factors: Genetic drift is random and affects allele frequencies unpredictably.

  • Equilibrium Composition:

    • A group of populations with identical starting allele frequencies and equal rates of mutation, gene flow, and selection achieve the same equilibrium composition.

    • Wright (1931) defined the equations for these probability distributions, which are complex.

  • Probability and Selection Impact:

    • The probability of an allele frequency deviating from a deterministic equilibrium increases when:

    • Selection is weaker.

    • Effective population size is smaller.

  • General Rules:

    • If 4N_es >> 1: Selection influences allele frequency more strongly.

    • If 4N_es << 1: Genetic drift has a greater influence.

  • Consequences of Selection and Drift:

    1. A slightly advantageous mutation is less likely to fix in a small population than in a large one.

    2. Allele frequency may fluctuate around equilibrium set by factors like heterozygous advantage, sometimes drifting to fixation.

    3. In small populations, deleterious alleles may increase in frequency due to drift.

  • Adaptive Landscape Dynamics:

    • If a population operates on one adaptive peak and genetic drift shifts allele frequencies downhill, selection may then act to bring the population back uphill to a different peak.

    • This dynamic can lead to achieving a higher peak than originally reached, showcasing the interaction between genetic drift and selection.

SEWALL WRIGHT'S SHIFTING BALANCE THEORY

  • The theory explains variations in chromosomal rearrangements among populations and species, accounting for why those rearrangements may reduce fertility in heterozygous individuals.

  • While selection alone cannot elevate the frequency of chromosomal rearrangements, genetic drift can enable them to increase to a critical frequency (around 0.5) where selection can then lead to fixation.

  • Estimate of Chromosome Variant Fixation Rates:

    • Lande (1979) estimated fixation rates of chromosome variants using proportions of related species.

    • Factors utilized include strength of selection against heterozygotes and mutation rates to chromosome variants.

    • Observed that effective population sizes in various mammals and insects are typically in the range of tens to a few hundreds of individuals. This aligns with ecological estimates of effective population sizes.

VISUAL REPRESENTATIONS

  • Figure 13: Illustrates average fitness () as it varies with allele frequencies.

    • The mean fitness of the population is the average of three genotype fitnesses, weighted by their frequencies at different loci.

  • Figure 14: Shows the impact of selection, mutation, and genetic drift on probability distributions for allele frequencies of a deleterious allele in populations of varied sizes: small (A), intermediate (B), and large (C).

    • The probability curves reflect different selection strengths (solid, dashed, and dotted lines) against the deleterious allele.

    • Key Takeaway: Gene frequencies are more stable and likely near the deterministic equilibrium in larger populations with strong selection, whereas smaller populations show greater variability and less predictability in terms of allele frequency shifts, particularly when selection is weak.