Principles of Ecology: Adaptation and Evolution - Part I Study Notes

Principles of Ecology: Adaptation and Evolution - Part I

Introduction to Evolution

• Dobzhansky's Quote: "Nothing in Biology makes sense except in the light of evolution."

Darwin's Theory of Evolution

• Influences on Darwin:

  • Charles Lyell's Principles of Geology.

  • Principle of Uniformitarianism: The same processes occurring today also occurred in the past at the same rate. This principle forms the foundation for understanding how natural selection operates over long periods.

• Darwin's Basic Formula for Evolution (Basis of Natural Selection):

  1. Heritable variation exists in a population: Individuals within a population exhibit differences that can be passed down to offspring, with a genetic underpinning.

  2. Resources are limited; competition exists: Due to finite resources, individuals must compete for survival and reproduction.

  3. Differential Reproduction: As a result of (1) and (2), individuals with variations that enable them to better acquire resources will produce more offspring. This leads to a differential representation of genotypes in future generations.

Defining Evolution, Fitness, and Adaptation

• Natural Selection: Differential representation of genotypes in future generations resulting from heritable differences among individuals in survival and reproduction.

• Fitness: The ability of an individual to survive and reproduce relative to other individuals in the population. It is a measure of reproductive success.

• Adaptation: A heritable trait that increases an individual's fitness in a specific environment.

• Evolution (Defined): Genetic change in a population over time. It's crucial to note that individuals do not evolve; populations evolve.

• Population: A group of conspecifics (individuals of the same species) residing in a specific place at a specific time.

• Variation: Introduced by mutation, these are random changes in the DNA sequence of a gene.

Modelling Changes in the Gene Pool: Hardy-Weinberg Equilibrium

• Hardy-Weinberg Equilibrium (H-W Eq.) Defined: A mathematical representation of genotype frequencies of a population that is not changing (i.e., not evolving). It is based on probability theory and sums all alleles in the population.

• Utility of Hardy-Weinberg Equations:

  • Allows us to determine if a population is evolving by comparing predicted genotype frequencies to observed genotype frequencies.

  • If a population is NOT evolving: Predicted genotype frequencies will match observed genotype frequencies.

  • If a population IS evolving: Predicted genotype frequencies will not match observed genotype frequencies. ($ext{predicted Genotype freq.} <br>eq ext{Observed Genotype freq.}$).

• Assumptions of Hardy-Weinberg Equilibrium (Conditions for No Evolution):

  1. Large Population: The population size is large enough that random chance events (genetic drift) do not significantly alter allele frequencies.

  2. No Migration (No Gene Flow): There is no movement of individuals or alleles into or out of the population.

  3. No New Mutations: No new alleles are introduced into the gene pool.

  4. No Differential Mortality or Reproduction (No Natural Selection): All individuals have equal chances of survival and reproduction.

  5. Random Mating: Individuals mate randomly, without any preference for certain genotypes.

• Key Idea: A population in H-W Eq. will stay in H-W Eq. as long as these five assumptions remain true, meaning no evolution is occurring.

Mechanisms of Evolution

Mechanisms of evolution represent violations of the Hardy-Weinberg assumptions.

1. Genetic Drift (Violation of Assumption #1: Large Population)

• Defined: Random or stochastic shifts in allele frequencies, most important in small populations.

  • Chance events are more likely to alter allele frequencies significantly in smaller populations.

  • Small populations often do not mate randomly, further violating assumptions.

• Effective Population Size ($N_e$):

  • Defined: The number of individuals in a population that effectively takes part in random mating. This value is crucial because the effect of genetic drift depends on $N_e$.

• Consequences of Genetic Drift:

  • Decreases variation within a population.

  • If $N<em>e$ is large, genetic drift happens less, and its effects might be ignored. Conversely, if $N</em>e$ is small, genetic drift is more pronounced.

  • The overall effect of drift is a reduction in genetic diversity.

2. Gene Flow (Violation of Assumption #2: No Migration)

• Defined: The gain or loss of alleles due to migration. This can involve random movement, leading to disproportionate migration (i.e., the number of individuals entering does not equal the number leaving).

• Effects of Gene Flow:

  • Selection is often most effective in isolated populations, as gene flow can homogenize allele frequencies between populations, reducing local adaptations.

  • Gene flow can either oppose or reinforce the effects of selection and genetic drift depending on the direction and magnitude of allele transfer.

3. Mutation Pressure (Violation of Assumption #3: No New Mutations)

• Defined: Change in allele frequency due to the origin of a new allele.

• Role of Mutations:

  • Mutations are generally rare events and have a minor, direct effect on allele frequencies. They are often ignored in the immediate term for population genetic models.

  • Crucial Role: Mutations generate the new genetic variation upon which other mechanisms like genetic drift, gene flow, and natural selection can act. Thus, more mutations lead to more genetic variation.

4. Natural Selection (Violation of Assumption #4: No Differential Mortality or Reproduction)

• Mechanism: Occurs whenever there is differential survival and/or reproduction, meaning one genotype performs better than others in a given environment.

• Quantified by the Selection Coefficient ($s$):

  • Defined: The proportional reduction of a genotype represented in the next generation due to death or reproductive failure.

  • Values of $s$ range from $0$ to $1$. $s=0$ indicates no selection (all individuals survive and reproduce as expected); $s=1$ indicates $100\%$ failure or sterility for that genotype.

  • In many contexts, $s$ refers specifically to recessive genotypes.

• Effect on Allele Frequencies (for dominant/recessive traits):

  • A dominant allele frequency ($p$) tends to be pushed towards $1$, meaning the population increasingly exhibits the dominant trait.

  • Heterozygote Refuge: The recessive allele can persist in the population within heterozygotes, as its expression is masked by the dominant allele. This means natural selection cannot completely remove a recessive allele from a population if it's maintained in heterozygotes. Other mechanisms of evolution might be needed to fully remove it.

• Forms of Selection (most traits are quantitative):

  • Directional Selection: Favors one extreme phenotype over others, shifting the population mean towards that extreme (e.g., larger body size preferred).

  • Stabilizing Selection: Favors intermediate phenotypes, reducing variation and maintaining the status quo (e.g., average birth weight).

  • Disruptive Selection: Favors both extreme phenotypes over intermediate forms, potentially leading to two distinct sub-populations (e.g., favoring very small or very large beak sizes, but not intermediate).

Natural Selection and Fitness

• Modeling Selection: The success of a genotype is measured by its fitness.

• Relative Fitness: Measures the success of a genotype in reference to a single trait, and it is directly affected by the selection coefficient ($s$).

  • If s > 0 (meaning there is selection against the recessive trait), then the fitness of the dominant trait is greater than the fitness of the recessive trait.

Properties of Fitness

• Know These 3 Properties:

  1. Property of the Genotype: For a given environment, the same genotype will consistently have the same fitness.

  2. Property of the Environment: Different environments can lead to different fitness values for the same genotype. A genotype highly fit in one environment may be poorly fit in another.

  3. Measured Over More Than One Generation: Fitness is not a single-generation measurement; it requires observing changes in genotype frequencies over multiple generations to accurately assess reproductive success and survival.

Summary of Key Points

• Darwin's basic formula provides the foundation for understanding Natural Selection.

• Evolution acts on populations, not individuals.

• The Hardy-Weinberg equations are fundamental tools for modeling evolution and assessing if a population is changing.

• Violations of the Hardy-Weinberg assumptions result in the various mechanisms of evolution:

  • Genetic Drift: Random shifts in allele frequencies.

  • Gene Flow: Changes in allele frequencies due to movement of individuals.

  • Mutation Pressure: The original source of all new genetic variation.

  • Natural Selection: Differential reproductive success based on fitness.

• These mechanisms of evolution do not act in isolation; they can interact with each other.

• Fitness has three core properties: it's a property of the genotype, it's environment-dependent, and it's measured over multiple generations. These properties are essential for understanding evolutionary dynamics.