Evolutionary Theory: Natural and Sexual Selection

Posted on Jan 22, 2025 in Biology

Evolutionary Theory and Evolutionary Psychology

Darwin

Darwin proposed the first evolutionary theory to explain how change might take place over time, especially how seemingly purposeful or functional structures might have arisen.

Adaptation

Improbably usefulness (too precisely functional to have arisen by chance only).

Williams proposed the following criteria for invoking adaptation:

  1. Reliability: Does the mechanism regularly develop in most or all members of the species across normal environments and perform dependably in the contexts in which it is designed to function?
  2. Efficiency: Does the mechanism solve a particular adaptive problem well?
  3. Economy: Does the mechanism solve the problem without huge costs to the organism?

Darwin’s Natural Selection Theory

Adaptations that arose as a consequence of successful survival:

  1. Variance: The “raw materials for evolution.”
  2. Inheritance: Only some variations are inherited (passed down reliably from generation to generation). In order for inheritance to occur, an organism must reproduce.
  3. Selection: Organisms with some heritable variants leave more offspring because these attributes aid survival or reproduction.

Differential reproductive success: Reproductive success relative to others.

Darwin’s Sexual Selection Theory

This theory answers seeming contradictions to the theory of natural selection (e.g., “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!”).

Adaptations that arose as a consequence of successful mating.

Primary mechanisms:

  1. Intrasexual competition: (e.g., stags locking horns) Competition between members of one sex for mating access to the other sex.
  2. Intersexual selection: Preferential mate choice. If members of one sex have some consensus about the qualities desired in members of the opposite sex, individuals of the opposite sex who possess those qualities will be preferentially chosen as mates (Darwin: “female choice”).

What Natural Selection Is and Is Not

  1. Natural selection is the primary cause of evolutionary change and the only known cause of adaptations, but it is not the only cause of evolutionary change. Genetic drift (mutations, founder effects – a small portion of the population establishes a colony elsewhere, genetic bottlenecks – e.g., population shrinks because of a disaster) can also lead to change.
  2. Natural selection is not forward-looking or intentional to meet distant needs.
  3. Natural selection is gradual relative to the human lifespan. Rates of change differ. Punctuated equilibrium: long periods of no change, then relatively sudden change.

Clarifying Natural Selection and Sexual Selection

  1. Human behavior is not just genetically determined. Human behavior cannot occur without 1) evolved adaptations and 2) environmental input that triggers the development and activation of these adaptations (e.g., callus).

Genes and Particulate Inheritance

Mendel showed that inheritance is particulate, not blended. Genes, rather than genotypes, are passed down. A gene is the fundamental unit of inheritance.

The Ethology Movement

The study of the proximate mechanisms and adaptive value of animal behavior (e.g., duck imprinting) oriented biologists to focus on the importance of adaptation and the role of biology in the study of human behavior.

The Inclusive Fitness Revolution

Hamilton proposed that classical fitness (an individual’s direct reproductive success) is too narrow. Natural selection favors characteristics that cause genes to be passed on, not just through an individual’s offspring.

Inclusive fitness = classical fitness + the effects of an individual’s action on the reproductive success of genetic relatives.

Group Selection Theory

Adaptations evolved for the benefit of the group via the differential survival or reproduction of groups. Only species with traits beneficial to their group survived.

Williams challenged this theory in his book Adaptation and Natural Selection. Consider a bird species with two types: selfish and self-sacrificing. Who survives? Selection operating on individual differences within a group is greater than selection operating at the group level.