MODULE 12: Speciation, Hybridization, Ecological Speciation, & Adaptive Radiation

What is the Biological Species Concept?

Takes species as groups of interbreeding populations that are reproductively isolated from other groups; emphasizes reproductive isolation.

What is the Morphospecies Concept?

Defines species based on morphological differences; can be subjective and fails with cryptic species or limited traits.

What is the Phylogenetic Species Concept?

Defines species based on monophyly and genetic divergence; different genes may give different evolutionary histories.

What are cryptic species?

Species that are morphologically indistinguishable but genetically distinct.

What are Evolutionarily Significant Units (ESUs)?

Genetically distinct populations within a species that are important for conservation.

Three major steps of speciation

(1) Initial isolation, (2) Divergence by selection/drift/mutation, (3) Secondary contact & formation of reproductive isolation.

What is allopatric speciation?

Speciation that occurs due to geographic isolation separating populations.

Dispersal (as a mechanism of isolation)

A few individuals colonize a new area, creating founder effects and rapid divergence; seen in Hawaiian Drosophila.

Vicariance (as a mechanism of isolation)

A physical barrier splits a population; example: snapping shrimp isolated by the formation of the Isthmus of Panama.

Isolation by time (temporal isolation)

Populations breed at different times and cannot interbreed; example: Japanese winter moths in cold vs warm regions.

Isolation by mutation

Genetic changes such as polyploidy cause instant reproductive isolation, especially in plants.

Role of genetic drift in speciation

Drift in small populations can cause divergence and create Dobzhansky-Muller incompatibilities.

Dobzhansky-Muller incompatibility

Genetic incompatibility that evolves between isolated populations due to independent mutations.

Role of natural selection in speciation

Different environments produce different adaptations that drive divergence; example: freshwater vs marine sticklebacks.

Prezygotic isolation

Reproductive barriers that prevent mating or fertilization (habitat, temporal, sexual isolation, immigrant inviability).

Postzygotic isolation

Reproductive barriers after fertilization (hybrid inviability, hybrid sterility, low hybrid fitness).

What is reinforcement?

Natural selection strengthens prezygotic isolation when hybrids have low fitness, preventing wasteful hybridization.

Why is allopatric speciation considered the default?

Most empirical evidence supports geographic isolation as the most common mechanism.

What is micro-allopatric speciation?

Speciation without obvious large-scale barriers; tiny habitat differences create isolation (e.g., rock-dwelling African cichlids).

What is sympatric speciation?

Speciation occurring without geographic barriers, often driven by strong ecological or sexual selection.

Example of sympatric speciation in fish

Nicaraguan crater lake cichlids diverging within a single lake.

Benthic vs. limnetic sticklebacks

Two forms in the same lake diverge ecologically, evolving reproductive isolation as a byproduct.

How does strong selection enable sympatric speciation?

If intermediates have low fitness, populations split into distinct adaptive peaks.

What is hybridization?

Interbreeding between genetically distinct groups; common in recently diverged lineages.

Possible outcomes of hybridization

Reinforcement, fusion (collapse), hybrid speciation, or stable hybrid zones.

Hybrid speciation

Hybrids form a new species if they have higher fitness or exploit a new ecological niche.

Hybrid zones

Regions where hybrids persist because they have higher fitness in intermediate environments.

Ecological speciation

Speciation caused directly by adaptation to different environments, which produces reproductive isolation.

Ecological speciation case study: Sulfide spring fish

Fish adapt to toxic H₂S; adaptation leads to reproductive isolation through habitat choice, immigrant inviability, and hybrid inviability.

What is adaptive radiation?

Rapid diversification of one lineage into many species occupying different ecological niches.

Three typical steps of adaptive radiation

(1) Habitat differences, (2) Feeding specialization, (3) Communication/behavioral divergence.

Lake whitefish radiation

Species diverge in size, body shape, trophic ecology, and physiology; 2-5 species per lake.

African cichlid radiation

Up to thousands of species differing in morphology, ecology, and sexual ornaments.

Hawaiian honeycreeper radiation

Diverse bill morphologies evolved to exploit different food sources across islands.

What is a key innovation?

A trait that allows access to new resources and promotes ecological diversification (e.g., pharyngeal jaws in cichlids).

Ecological opportunity

Access to unexploited resources that enables rapid diversification.

Transgressive segregation

Hybrids express extreme traits outside parental ranges; expands available phenotypes for selection.

Hybrid swarms

Groups where most genetic variation is shared but species differ in key adaptive traits, producing many new combinations.

How hybridization contributes to adaptive radiation

It creates novel phenotypes, increases variation, and accelerates divergence.