Reproductive Isolation

Reproductive isolation comprises the biological barriers that prevent members of different species from successfully interbreeding, and its evolution is the central event of speciation.

Definition

Reproductive isolation is the set of heritable barriers that reduce or prevent gene flow between populations. Prezygotic barriers act before fertilization, while postzygotic barriers reduce the viability or fertility of hybrids after fertilization.

Scope

This topic covers the classification of reproductive barriers into prezygotic and postzygotic categories, the genetic basis of hybrid inviability and sterility through Dobzhansky-Muller incompatibilities, the role of reinforcement in strengthening prezygotic isolation, and how barriers accumulate as populations diverge.

Core questions

• How are reproductive barriers classified as prezygotic versus postzygotic?
• What is the genetic basis of hybrid inviability and sterility?
• How can selection strengthen prezygotic isolation through reinforcement?
• In what order do reproductive barriers typically accumulate during divergence?

Key theories

Dobzhansky-Muller incompatibilities

Hybrid inviability and sterility arise from negative interactions between genes that diverged separately in each population and had never been tested together, explaining postzygotic isolation without any individual being unfit in its own lineage.

Reinforcement

When hybrids are unfit, natural selection can favor stronger prezygotic isolation by selecting against costly hybridization, accelerating the completion of speciation.

Mechanisms

Prezygotic barriers include ecological or habitat isolation, temporal isolation in breeding timing, behavioral isolation through mate choice, mechanical incompatibility, and gametic isolation. Postzygotic barriers include intrinsic hybrid inviability and sterility, which arise from Dobzhansky-Muller incompatibilities between independently evolved alleles, and extrinsic barriers where hybrids are poorly adapted to available niches. Where hybridization is costly, reinforcement strengthens prezygotic barriers. Total isolation is the cumulative effect of all barriers acting in sequence, so even partial barriers can substantially restrict gene flow.

Clinical relevance

Understanding reproductive barriers aids the management of hybridization between domestic and wild species, the control of pest and vector species complexes, and conservation decisions where hybridization threatens the genetic integrity of endangered taxa.

History

Dobzhansky in 1937 and Muller in the 1940s independently explained hybrid incompatibility through interactions between diverged genes. Coyne and Orr's comparative analyses later quantified how isolation accumulates with genetic distance and clarified the conditions under which reinforcement operates.

Debates

How important is reinforcement?

Whether selection against hybrids commonly drives the final strengthening of prezygotic isolation, versus isolation arising mainly as a byproduct of divergence, remains an active question.

Key figures

• Theodosius Dobzhansky
• Hermann J. Muller
• Jerry Coyne
• H. Allen Orr

Seminal works

• dobzhansky1937
• coyneOrr2004
• futuyma2017

Frequently asked questions

What is the difference between prezygotic and postzygotic isolation?

Prezygotic barriers prevent mating or fertilization from occurring, while postzygotic barriers reduce the viability or fertility of hybrids after a zygote has formed.

Why are many hybrids sterile or inviable?

Often because of Dobzhansky-Muller incompatibilities: genes that evolved independently in each parent species interact poorly when combined in a hybrid, even though each works fine in its own genetic background.