Chapter 7 - Speciation

Microevolution

Change in allele frequencies in a population over generations; microevolutionary processes alone do not explain the origin of new species.

Macroevolution

Evolutionary change above the species level, including the origin of new taxonomic groups, key adaptations, and mass extinction events.

Speciation

The evolutionary process by which one species splits into two or more distinct species, bridging the gap between microevolution and macroevolution.

Biological Species Concept (BSC)

Defines a species as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring.

Reproductive Isolation

The existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile offspring.

Hybrids

Offspring that result from the interspecific mating of individuals from two different species.

Prezygotic Barriers

Reproductive barriers that prevent mating or block fertilization between species if mating is attempted.

Postzygotic Barriers

Reproductive barriers that operate after a hybrid zygote has formed, preventing it from developing into a viable, fertile adult.

Habitat Isolation

Prezygotic barrier where two species occupy different habitats within the same area and rarely or never encounter each other.

Temporal Isolation

Prezygotic barrier where species breed during different times of the day, different seasons, or different years.

Behavioral Isolation

Prezygotic barrier where unique courtship rituals or other behaviors are effective mate recognition signals unique to a species.

Mechanical Isolation

Prezygotic barrier where morphological differences prevent successful mating or pollen transfer.

Gametic Isolation

Prezygotic barrier where sperm of one species is unable to fertilize the eggs of another species due to biochemical incompatibility.

Reduced Hybrid Viability

Postzygotic barrier where the genes of different parent species interact in ways that impair the hybrid's development or survival.

Reduced Hybrid Fertility

Postzygotic barrier where hybrid offspring are healthy and viable but completely or largely sterile (e.g., a mule).

Hybrid Breakdown

Postzygotic barrier where first-generation hybrids are viable and fertile, but when they mate, offspring of the next generation are feeble or sterile.

Morphological Species Concept

Distinguishes a species by structural and anatomical features; applies to sexual and asexual organisms without needing gene flow data.

Ecological Species Concept

Defines a species in terms of its ecological niche, focusing on how members interact with living and nonliving parts of their environment.

Phylogenetic Species Concept

Defines a species as the smallest group of individuals that share a common ancestor, forming one branch on the tree of life.

Allopatric Speciation

Speciation that occurs when gene flow is interrupted when a population is divided into geographically isolated subpopulations.

Sympatric Speciation

Speciation that occurs in populations that live in the same geographic area without physical isolation.

Dispersal

Mechanism of geographical isolation where a few individuals colonize a new isolated area away from the main population.

Vicariance

Mechanism of geographical isolation where a physical barrier splits a widespread population into isolated subgroups.

Polyploidy

A condition in which an organism has extra sets of chromosomes due to an accident during cell division; a major cause of sympatric speciation in plants.

Autopolyploid

An individual that has more than two chromosome sets that are all derived from a single species.

Allopolyploid

A fertile polyploid individual produced when two different species interbreed and combine their chromosomes.

Sexual Selection

A form of natural selection where individuals with certain inherited traits are more likely to obtain mates; can drive sympatric speciation.

Habitat Differentiation

Sympatric speciation mechanism where a subpopulation exploits a habitat, food source, or resource not used by the parent population.

Hybrid Zone

A geographic region in which members of different species meet and mate, producing at least some offspring of mixed ancestry.

Reinforcement

An outcome in a hybrid zone where natural selection strengthens prezygotic barriers, reducing the formation of unfit hybrids over time.

Fusion

An outcome in a hybrid zone where substantial gene flow weakens reproductive barriers, causing two species to fuse back into a single species.

Stability

An outcome in a hybrid zone where hybrids continue to be produced routinely, maintaining a stable narrow zone without full fusion or reinforcement.

Punctuated Equilibria

Model of evolutionary tempo describing long periods of apparent stasis interrupted by brief periods of rapid evolutionary change during speciation.

Gradualism

Model of evolution suggesting that species diverge and evolve gradually from a common ancestor over long periods of time.

Grolar Bear (Polar Bear x Grizzly Bear)

An example of climate-change-induced habitat overlap leading to interspecific hybridization between two distinct bear species.

Apple Maggot Fly (Rhagoletis pomonella)

A classic example of sympatric speciation driven by habitat differentiation after switching host plants from hawthorn to domestic apples.

Cichlids of Lake Victoria

A classic example of sympatric speciation driven by female mate choice and sexual selection based on male coloration.

Mosquitofish (Gambusia hubbsi)

Example of allopatric speciation in different ponds where body shape adapted to presence or absence of predators, creating mechanical isolation.

Ensatina Salamanders

A classic example of a ring species exhibiting progressive divergence and reproductive isolation around California's Central Valley.

Mule (Horse x Donkey)

A classic example of reduced hybrid fertility; the mule is a viable hybrid organism that is sterile due to differing chromosome counts.

Bread Wheat (Triticum aestivum)

A classic agricultural example of an allopolyploid crop species derived from hybridization events among wild grasses.

Ecological Niche

The specific set of biotic and abiotic resources that a species uses in its environment.