Explain how reproductive isolation leads to speciation, the formation of new species from an existing population (Ohio's Learning Standards for Science, Biology, B.E.2).

What this topic is asking

Ohio standard B.E.2 (Speciation) is about how new species form and how variation within a species, driven by genetics and changing gene frequencies, can eventually split one species into two. The Ohio Biology EOC turns this into items where you explain how isolation leads to speciation, usually from a scenario (a barrier appears, or two groups stop interbreeding). The crosscutting idea is cause and effect plus stability and change: a barrier to gene flow is the cause; divergence into new species is the effect. Build on natural selection, which is the engine that drives the populations apart.

What a species is

The Ohio standards use the idea of a species you can test with breeding. Under the biological species concept, a species is a group of organisms that can interbreed in nature and produce fertile offspring. The key word is fertile: a horse and a donkey can mate and produce a mule, but the mule is sterile, so horses and donkeys are still separate species. Two groups become separate species at the point where they can no longer interbreed to produce fertile offspring.

Reproductive isolation

For one species to become two, the two groups must stop exchanging genes. Anything that prevents interbreeding is a reproductive isolating mechanism, and it cuts off gene flow (the movement of alleles between populations). Once gene flow stops, the two groups evolve independently.

Isolating mechanisms come in two broad kinds:

• Geographic isolation. A physical barrier separates the populations: a canyon forms, a river changes course, sea level rises and cuts off an island, or a glacier advances. The groups simply cannot reach each other.
• Reproductive (biological) isolation without geography. The groups live in the same area but still do not interbreed because they breed at different times (different season or time of day), use different mating behaviors or signals, or are otherwise incompatible. These prevent mating even with no barrier on a map.

How isolated populations diverge

Once two populations are isolated, three processes pull them apart genetically over many generations.

• Different mutations. New mutations arise independently in each population, adding different new alleles to each.
• Different natural selection. The two environments may differ (climate, predators, food), so selection favors different traits in each group.
• Genetic drift. In each population, allele frequencies also change by chance, especially if a population is small.

Because gene flow no longer mixes the two gene pools, these differences build up instead of being shared. Given enough time, the populations differ so much that they can no longer interbreed, and speciation is complete.

Allopatric speciation, step by step

The most commonly tested route is allopatric speciation (allo = different, patric = place): a population is split by a geographic barrier.

Try this

Q1. State the biological definition of a species. [1]

• Cue. A group of organisms that can interbreed in nature to produce fertile offspring.

Q2. Explain why a difference in breeding season can lead to speciation between two populations living in the same area. [2]

• Cue. Breeding at different times stops the two groups from interbreeding, so gene flow stops; the isolated populations then accumulate different genetic changes and can diverge into separate species.