Speciation and reproductive isolating mechanisms

What a species is

• Under the biological species concept, a species is a group of populations whose members can interbreed in nature and produce fertile offspring, and cannot do so with other groups.
• Speciation is the process that turns one species into two: the gene pools stop exchanging alleles and become different enough that they could not mix again.
• A reproductive isolating mechanism (RIM) is anything that stops two groups producing fertile offspring together; speciation is complete when the isolation holds even where the groups meet.
• The concept struggles with bacteria and other asexual organisms, with fossils that cannot be tested, and with groups that still hybridise now and then, so biologists also use shared DNA and shared features to define species.

Allopatric speciation: split by a barrier

• A geographic barrier, such as a rising mountain range, a sea strait or advancing glaciers, splits one population into two and stops gene flow.
• Each side gets its own mutations, its own drift and its own selection pressures, so the two gene pools change in different directions.
• Over many generations the differences build up in mating behaviour, timing, body shape and chromosomes, and these become isolating mechanisms.
• If the barrier later disappears and the groups meet, they no longer interbreed, or their hybrids fail: they are separate species.
• Allopatric speciation is thought to be the most common mode in animals, and the uplift of the Southern Alps and the Pleistocene glaciations split many New Zealand populations this way.

Sympatric speciation: no barrier at all

• In sympatric speciation a new species forms inside the range of its parent species, with no geographic barrier to stop gene flow.
• In plants the main route is polyploidy, which isolates offspring from their parents in a single generation.
• In animals it usually starts with disruptive selection on a new food or habitat in the same place, plus mating within the group, so gene flow between the two forms falls even though they live side by side.
• Examples include cichlid fish in small crater lakes, where open-water and bottom-feeding forms have split, and insects that switch to a new host plant and then mate on it.
• Sympatric speciation is harder than allopatric because any interbreeding mixes the gene pools again, so isolation has to evolve while the groups are still in contact.

Try it: allopatric or sympatric?

Prezygotic and postzygotic isolating mechanisms

• Prezygotic mechanisms stop mating or fertilisation, so no zygote forms; they waste no energy on hybrids and so are favoured by selection.
• Postzygotic mechanisms act after fertilisation: the hybrid dies, is sterile, or its offspring are weak or sterile.
• When hybrids are less fit, selection favours individuals that avoid mating with the other group, so prezygotic barriers grow stronger; this is called reinforcement.

Try it: prezygotic or postzygotic?

Polyploidy: a new species in one generation

• A polyploid has more than two full sets of chromosomes, such as 3n, 4n or 6n; it usually arises when chromosomes fail to separate in meiosis (non-disjunction), giving a gamete with a full diploid set.
• Autopolyploidy doubles the chromosomes of one species; allopolyploidy follows a hybrid between two species, whose sets cannot pair, so the hybrid is sterile until its chromosome number doubles.
• After doubling, every chromosome has an identical partner, so meiosis works and the plant is fertile, but it cannot breed back with either parent: a cross gives an odd-numbered, sterile offspring.
• That is instant reproductive isolation in the same place, so polyploidy is sympatric speciation, and it is a sudden jump, not a slow change.
• It is common in plants because many self-pollinate or reproduce asexually, so a single new polyploid can found a population; cord grass, Spartina anglica, a weed of New Zealand estuaries, arose this way in England in the 1800s.

New Zealand examples: kiwi

• There are five kiwi species, and DNA studies show they split within the last few million years as their populations were repeatedly cut apart.
• During cold periods of the Pleistocene, glaciers and treeless land divided South Island forests into isolated refuges, stopping gene flow; each refuge population drifted and was selected separately.
• Rowi at Ōkārito and the tokoeka further south live on the same island but stayed genetically distinct, which supports allopatric splits followed by little gene flow.
• Kiwi of different species can hybridise when people put them together, which is why conservation managers keep species and even regional forms apart.

New Zealand examples: wētā and hebes

• Tree wētā (Hemideina) have different species in Auckland, Wellington and Canterbury, and Auckland tree wētā have several chromosome races that meet in narrow zones, a possible early stage of speciation by chromosome change.
• Mountain stone wētā live on separate South Island ranges, and populations on different ranges have diverged genetically, because the valleys between act as barriers, like islands in the sky.
• Hebes (Veronica) radiated into about a hundred species in New Zealand as the mountains rose, many of them separated by habitat: coast, forest edge, bog or alpine rock.
• Hebes hybridise readily where species meet, so their isolation is mostly prezygotic and ecological, and several hebes are polyploids with double or triple the usual chromosome number.

What Achieved, Merit and Excellence look like

• Achieved defines the terms and names the isolating mechanism; Merit explains how it stops gene flow in this context; Excellence links the mechanism to the mode of speciation and compares it with the alternative.