5. selection

Study Guide - Smart Notes

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Mechanisms of Evolution

Overview of Evolutionary Mechanisms

Evolutionary change in populations occurs through several mechanisms that alter allele frequencies over time. These mechanisms include mutation, genetic drift, gene flow, and selection. Of these, selection is the only process that consistently leads to adaptation, where traits that confer higher fitness become more common.

Mutation: Random changes in DNA that introduce new alleles.
Genetic Drift: Random fluctuations in allele frequencies, especially in small populations.
Gene Flow: Movement of alleles between populations through migration.
Selection: Change in allele frequencies due to differences in fitness among individuals.

Fitness is defined as the lifetime reproductive success, or the number of viable offspring produced. An adaptation is a trait that increases fitness relative to other traits in the population.

Natural Selection: Postulates and Types

Three Postulates of Natural Selection

Natural selection operates when three main conditions are met:

Variation: Individuals in a population vary in their traits.
Heritability: The variation in traits is heritable.
Fitness Correlation: Fitness correlates with heritable variation; individuals with certain traits have higher reproductive success.

Traits can be quantitative (continuous, e.g., height) or qualitative (discrete, e.g., coat color). Most traits are quantitative and polygenic, meaning they are influenced by multiple genes.

Graph of human height as a quantitative trait Jaguar with spotted coat color, a discrete trait Black jaguar with solid coat color, a discrete trait

The Modern Synthesis

The Modern Synthesis integrates Darwin's theory of natural selection with Mendel's principles of inheritance, forming the foundation of modern evolutionary biology. It emphasizes that most traits are polygenic and that most mutations have small effects, leading to gradual evolutionary change.

Portrait of a scientist associated with the Modern Synthesis

Fitness: Direct, Indirect, and Inclusive

Fitness can be measured in several ways:

Direct Fitness: Number of offspring produced by an individual.
Indirect Fitness: Number of offspring produced by close relatives, weighted by relatedness.
Inclusive Fitness: Sum of direct and indirect fitness.

Relative fitness is often scaled from 0 to 1, where 1 represents the highest fitness in the population.

Modes of Selection

Three Main Types of Selection

Selection can act in different ways, affecting the distribution and mean of trait values in a population:

Stabilizing Selection: Favors intermediate phenotypes, reducing variance without changing the mean.
Directional Selection: Favors one extreme phenotype, shifting the mean and reducing variance.
Disruptive Selection: Favors both extremes, increasing variance and potentially leading to speciation.

Graph showing modes of selection and trait value changes

Stabilizing Selection Example

Stabilizing selection is illustrated by human birth weight, where very small and very large babies have higher mortality, resulting in a narrower distribution of birth weights.

Graph showing stabilizing selection on birth weight Graph showing mortality and birth weight distribution

Trait Distribution Changes

Selection affects both the mean and variance of trait distributions:

Stabilizing selection: Mean does not change, variance decreases.
Directional selection: Mean changes, variance decreases.
Disruptive selection: Mean does not change, variance increases.

Kin Selection and Altruism

Altruistic Traits and Kin Selection

Altruism is behavior that decreases the fitness of the actor but increases the fitness of the recipient. Kin selection explains how altruistic traits can evolve when they benefit close relatives, increasing the actor's inclusive fitness.

Ants as an example of altruistic behavior

Belding’s Ground Squirrel: Alarm Calling

Females are more likely to give alarm calls when close kin are nearby, supporting the kin selection hypothesis.

Ground squirrels, alarm calling behavior Graph showing frequency of alarm calling by sex and age Graph showing alarm calls directed toward relatives

White-fronted Bee-eater: Helping at the Nest

Young bee-eaters often help at the nest, delaying their own reproduction to assist close kin. Helpers are usually offspring from previous seasons, and group size increases the number of offspring fledged.

White-fronted bee-eaters, cooperative breeding White-fronted bee-eater at nest site Graph showing probability of helping by natal and in-law status

Sexual Selection

Sexual Selection: Mechanisms and Effects

Sexual selection is a form of natural selection that arises from differences in mating success. It often leads to the evolution of exaggerated, sexually dimorphic traits, especially in males, due to greater intensity of selection for traits that increase mating success.

Sexual Asymmetry: Females typically invest more in reproduction (larger gametes, more parental care), so males benefit more from increased mating success.
Exaggerated Traits: Traits such as size, color, and weapons are often more pronounced in males.

Examples include long tails in widowbirds, bright coloration in peacocks, and antlers in deer.

Testing Sexual Selection Hypotheses

Experimental tests, such as artificially lengthening or shortening tails in widowbirds, can demonstrate the effect of trait variation on mating success.

*Additional info: The notes expand on the original outline by providing definitions, examples, and context for each mechanism and mode of selection, as well as the role of kin selection and sexual selection in evolutionary biology.*