Speciation, Hybridization, and the Evolution of Reproductive Isolation

Manual: General · Subject: Evolutionary Biology

This lesson explores how new species arise, how barriers evolve, and why gene flow can accompany divergence.

What is a species?

Species concepts

No single species concept is universally sufficient. The biological species concept emphasizes reproductive isolation, the phylogenetic species concept emphasizes diagnosable lineages, and the ecological species concept emphasizes niche differentiation. Empirical work often requires integrating multiple concepts.

Mechanisms of divergence

Speciation can proceed with or without geographic isolation. Allopatric divergence is common, but sympatric and parapatric speciation are also studied, especially in systems with strong ecological specialization, assortative mating, or chromosomal change. Hybrid zones reveal both the permeability and the strength of reproductive barriers.

Barriers to gene flow

Prezygotic barriers

Mating time, habitat, behavior, mechanical or gametic isolation

Postzygotic barriers

Hybrid inviability, sterility, or reduced fitness

Reinforcement

Selection strengthens prezygotic isolation when hybrids are costly

Introgression

Persistent exchange of alleles between lineages

Speciation modes

Allopatric

Divergence after geographic separation
Often simplest to model

Sympatric

Divergence without geographic separation
Requires strong ecological or sexual selection

Which barrier acts before fertilization?

Hybrid sterility Hybrid inviability Gametic isolation Reduced hybrid viability

What is reinforcement in speciation?

Evidence used in speciation studies

Evidence	What it can show
Assortative mating	Prezygotic isolation
Hybrid fitness assays	Postzygotic barriers
Genomic clines	Locus-specific introgression
Ecological experiments	Niche divergence

Why can hybridization be evolutionarily important?

It always prevents speciation It can transfer adaptive alleles across lineages It eliminates all phylogenetic signal It only occurs in domestic species

Investigating a hybrid zone

1

Sample across the geographic transect and phenotype gradient.
2

Genotype markers that distinguish parental lineages.
3

Estimate clines and test for locus-specific deviations.
4

Measure hybrid fitness or mating preference.
5

Infer whether selection, dispersal, or barrier architecture explains the pattern.

Give one reason why a biological species concept can be difficult to apply in practice.

Neuroanatomy Explorer

Memory Deck

Match the Pairs

Concept Constellation