speciation, including allopatric and sympatric speciation, the role of reproductive isolating mechanisms (prezygotic and postzygotic), and the biological species concept

What this dot point is asking

VCAA wants the biological species concept, the two main modes of speciation (allopatric and sympatric), and the reproductive isolating mechanisms (prezygotic and postzygotic) that maintain species boundaries.

The answer

Speciation is the evolutionary process by which one species splits into two or more new species. It is the central question of biodiversity: how do new species arise?

The biological species concept

A biological species is a group of organisms whose members can interbreed in nature to produce viable, fertile offspring, and which are reproductively isolated from other such groups.

The biological species concept works well for sexually reproducing animals in the present day. It struggles with:

• Asexual organisms (bacteria, many plants) that do not interbreed at all.
• Fossil species that cannot be tested for interbreeding.
• Hybrid zones where related species do produce some fertile offspring.

For these cases, biologists use alternative concepts (morphological, phylogenetic, ecological), but the biological species concept is the focus at VCE level.

Allopatric speciation

Allopatric speciation (allo = other, patric = place) occurs when a population is split by a geographical barrier.

The sequence is:

1. Geographical isolation. A barrier (river, mountain range, glacier, lava flow, sea level change, climate change, human road) separates one population into two. Gene flow stops.
2. Independent evolution. Each sub-population accumulates different mutations, experiences different selection pressures and undergoes genetic drift (especially if one population is small).
3. Reproductive isolation. Over many generations, enough genetic difference accumulates that, even if the barrier is removed, the two populations no longer interbreed successfully.
4. Two species. The original species has split.

Examples. Kaibab and Abert's squirrels on opposite rims of the Grand Canyon; Galapagos finches diverging from a single colonising population; Australian marsupials diverging from the rest of Pangaean placentals after continental drift.

Allopatric speciation is the most common form.

Sympatric speciation

Sympatric speciation (sym = same) occurs within the same geographical area, without a physical barrier. It is less common but well documented in some groups.

Mechanisms include:

• Polyploidy. A reproductive error doubles the chromosome number (autopolyploidy) or combines chromosome sets from two parent species (allopolyploidy). The polyploid offspring cannot breed with the parent species because their chromosomes cannot pair properly during meiosis, but it can breed with other polyploids. Common in plants (wheat, cotton, strawberries).
• Ecological isolation. A sub-population begins to exploit a different niche (for example, apple maggot flies switching from native hawthorn to introduced apple trees in North America). Different feeding times and habitats reduce interbreeding.
• Sexual selection. Female preference for certain male traits diverges in a sub-population, creating a behavioural barrier (cichlid fish species in African lakes).

Reproductive isolating mechanisms

Once two populations stop interbreeding, reproductive isolating mechanisms maintain the separation. They are grouped by when they act, relative to fertilisation.

Prezygotic mechanisms prevent fertilisation, so no zygote forms.

• Habitat isolation. Populations live in different habitats and rarely meet (water snakes that prefer different water types).
• Temporal isolation. Breeding seasons or times of day differ (fly species that emerge in different months).
• Behavioural isolation. Different courtship displays, songs or pheromones (firefly flashing patterns; bird songs).
• Mechanical isolation. Reproductive structures incompatible (snail shells coiling in opposite directions; differently shaped flowers attracting different pollinators).
• Gametic isolation. Sperm and egg fail to fuse (sea urchin sperm proteins not recognised by another species' egg surface).

Postzygotic mechanisms allow fertilisation, but the hybrid is disadvantaged.

• Hybrid inviability. The hybrid embryo fails to develop or dies young.
• Hybrid sterility. The hybrid lives but cannot reproduce. The mule (horse x donkey) is sterile because horse and donkey chromosomes cannot pair properly in meiosis.
• Hybrid breakdown. First-generation hybrids are fertile, but their offspring are weak or sterile.

Prezygotic mechanisms are evolutionarily more efficient because they avoid wasting energy on a doomed offspring. Where two related species coexist, selection often favours stronger prezygotic isolation (reinforcement).

How long does speciation take?

Speciation in animals typically takes hundreds of thousands to millions of years. Plant polyploidy can produce a new species in one generation. Bacterial speciation can be observed in laboratory experiments within decades because of rapid generation times.

Worked example

The mosquito species Culex pipiens has diverged into two forms in London. After tunnels for the Underground were dug in the late 19th century, a sub-population of mosquitoes colonised them. Over about a century, these mosquitoes:

• Geographically isolated. They almost never leave the tunnels and surface mosquitoes rarely enter.
• Diverged in behaviour. Underground mosquitoes feed on mammals (humans, rats); surface mosquitoes feed on birds. Underground mosquitoes breed year-round; surface mosquitoes need a winter diapause.
• Reproductively isolated. Studies show the two forms now rarely interbreed and produce few fertile offspring when forced.

This is allopatric speciation playing out fast enough to study within a human lifetime.

Common traps

Saying speciation happens "to an individual". Speciation is a population-level process. Individuals do not speciate.

Confusing reproductive isolation with geographical isolation. Geographical isolation is one cause; reproductive isolation is the outcome that defines a new species.

Saying postzygotic mechanisms "prevent fertilisation". They do not. They allow fertilisation but disadvantage the hybrid.

Calling polyploidy "rare". It is uncommon in animals but very common in plants and is responsible for many crop species.

Forgetting that the biological species concept fails for some groups. It works for sexually reproducing animals; it cannot be applied to asexual or fossil species.

In one sentence

Speciation is the process by which one species splits into two or more reproductively isolated species, occurring most commonly through allopatric mechanisms (a geographical barrier separates populations, which then diverge through selection, drift and mutation) and less commonly through sympatric mechanisms (polyploidy, ecological or sexual selection within one area), with prezygotic isolating mechanisms (habitat, temporal, behavioural, mechanical, gametic) and postzygotic mechanisms (hybrid inviability, sterility, breakdown) maintaining the species boundary.