Speciation & Reproductive Isolation

You’ll be able to

Define a biological species in terms of reproductive isolation
Contrast allopatric and sympatric speciation by the role of geography
Classify prezygotic and postzygotic barriers to reproduction

What makes a species?

Under the biological species concept, a species is a group of populations whose members can interbreed and produce fertile offspring, and who are reproductively isolated from other such groups. Speciation — the origin of a new species — happens when gene flow between populations is cut off long enough that they diverge into groups that can no longer successfully interbreed. Reproductive isolation, not mere physical difference, is the dividing line: two populations that look different but still interbreed freely remain one species, while two look-alike populations that cannot interbreed are two species.

Allopatric vs. sympatric speciation

Allopatric speciation ("other homeland") begins with a geographic barrier — a river changes course, a mountain range rises, a founder population colonizes an island. The barrier stops gene flow; the separated populations then accumulate different mutations and experience different selection and drift until they can no longer interbreed even if reunited. Sympatric speciation ("same homeland") occurs without geographic separation — populations diverge while living in the same area. In plants especially, errors in cell division can double the chromosome number (polyploidy), instantly producing offspring that can breed with each other but not with the parent population. Other routes include divergence by habitat or by mate preference within one location.

Barriers before and after the zygote

Reproductive isolation is maintained by barriers grouped by when they act. Prezygotic barriers prevent mating or fertilization, so no zygote ever forms: temporal isolation (breeding at different times), habitat isolation (living in different microhabitats), behavioral isolation (different courtship signals), mechanical isolation (incompatible reproductive structures), and gametic isolation (sperm and egg cannot fuse). Postzygotic barriers act after a hybrid zygote forms, preventing it from becoming a fertile adult: reduced hybrid viability (the hybrid dies young), reduced hybrid fertility (the hybrid survives but is sterile, like a mule), and hybrid breakdown (first-generation hybrids are fine but their offspring are weak or sterile).

The engine of speciation

reduced gene flow + divergence (mutation, selection, drift) → reproductive isolation

Cut off gene flow long enough and independently accumulating genetic differences eventually make interbreeding impossible — that is a new species.

Worked example

A donkey and a horse can mate and produce a mule, but the mule is sterile and cannot reproduce. Does this reproductive barrier act before or after fertilization, what specific type is it, and does it keep horses and donkeys as separate species?

1.A viable hybrid (the mule) is actually produced, so fertilization occurred and a zygote formed — the barrier must act after the zygote, making it postzygotic.
2.The mule survives to adulthood but cannot produce offspring of its own, which is the definition of reduced hybrid fertility (hybrid sterility).
3.Because genes cannot flow between horses and donkeys through the sterile mule, the two remain reproductively isolated and therefore separate species.

Answer: It is a postzygotic barrier — reduced hybrid fertility (hybrid sterility). The sterile mule blocks gene flow, so horses and donkeys stay reproductively isolated as distinct species.

Checkpoint

Two populations of squirrels live on opposite rims of the Grand Canyon. The canyon prevents them from meeting, and over time they diverge into distinct forms. This is an example of:

Watch out

Sort barriers by when they act, not by how dramatic they seem. If no zygote ever forms (mating or fertilization is blocked) it is prezygotic; if a hybrid zygote forms but fails to become a fertile adult it is postzygotic. A dead or sterile hybrid is always postzygotic, because a hybrid had to form first.

Checkpoint

Two closely related frog species live in the same pond, but one breeds in early spring and the other in late summer, so they never mate. Which barrier is this?

On the exam

For any isolation scenario on the exam, ask two questions in order: (1) Did a hybrid zygote form? No → prezygotic; Yes → postzygotic. (2) Then name the specific mechanism. Being able to justify the pre-/post- split earns more than just labeling it.

Answer the 2 checkpoints as you read.