Lesson 22: Macroevolution and Speciation

Sympatric Species

Species that live in the same geographic area and often utilize different parts of the habitat.

Allopatric Species

Species that live in different geographic locations, separated by physical barriers.

Reproductive Isolation

A condition occurring when individuals cannot mate or produce fertile offspring, critical for speciation.

Pre-zygotic Mechanisms

Mechanisms that prevent the formation of a zygote, including ecological, behavioral, temporal, mechanical isolation.

Post-zygotic Mechanisms

Mechanisms affecting hybrid zygotes after fertilization, including hybrid inviability and hybrid infertility.

Biological Species Concept (BSC)

A species is defined as groups of actually or potentially inter-breeding natural populations that are reproductively isolated from other such groups.

Phylogenetic Species Concept (PSC)

A concept that defines a species as groups of populations that have been evolving independently, based on phylogenies.

Gradualism

The hypothesis that speciation occurs through slow, continuous changes over long periods.

Punctuated Equilibrium

The hypothesis that species experience long periods of stasis interrupted by rapid bursts of evolutionary change.

Ecological Isolation

A pre-zygotic mechanism where species occupy different habitats or niches within the same geographic area, preventing contact.

Behavioral Isolation

A pre-zygotic mechanism where differences in courtship rituals or other behaviors prevent successful mating between species.

Temporal Isolation

A pre-zygotic mechanism where species breed during different times of day, different seasons, or different years, preventing interbreeding.

Mechanical Isolation

A pre-zygotic mechanism where structural differences in reproductive organs prevent successful mating between species.

Hybrid Inviability

A post-zygotic mechanism where hybrid offspring do not complete development or are frail and cannot survive.

Hybrid Infertility

A post-zygotic mechanism where hybrid offspring survive but are sterile and cannot reproduce.

Reinforcement

continuous improvement of pre-zygotic isolation mechanisms, occurs when partially sterile hybrids are selected against, favoring alleles that prevent hybridization

Outcome of Gene Flow Between Partially Isolated Populations

Hybrids of partially isolated populations can promote gene flow, which slows down or prevents speciation. For speciation to occur and for populations to remain separate species, gene flow must cease.

Geographic Isolation

greatly reduces gene flow between populations, making it more likely for genetic drift and natural selection to result in reproductive isolation and consequently, speciation.

Adaptive Radiation

The rapid evolution of a group of species from a common ancestor, adapting to different environmental niches and leading to increased diversity over a relatively short period (e.g., Galapagos finches, Cichlids in Lake Victoria).

Allopatric speciation

occurs when populations become geographically isolated, ceasing gene flow and leading to the evolution of distinct species.