Speciation and Isolation

Before we can understand how new species form, we need to know what a species is. The most widely used definition in biology is the biological species concept: a species is a group of organisms that can interbreed in nature to produce fertile, viable offspring.

This definition works well for many animals, but it has limitations:

• Asexual organisms (like bacteria) do not interbreed, so the concept does not apply.
• Fossils cannot be tested for interbreeding — palaeontologists use physical similarities instead.
• Hybrids sometimes occur between species (e.g., ligers from lions and tigers, or mules from horses and donkeys), but hybrids are usually sterile, so gene flow does not continue.
• Ring species exist where neighbouring populations interbreed, but populations at opposite ends of the ring do not — creating a grey area.

Despite these limitations, the biological species concept is a useful starting point for Stage 5. What matters most is reproductive isolation: when two populations can no longer exchange genes, they are on separate evolutionary paths and may become different species.

For a new species to form, gene flow between populations must stop or be severely reduced. This is called reproductive isolation, and it can happen in several ways.

Geographical isolation (allopatric speciation)

A physical barrier separates populations. This could be a mountain range rising, a river changing course, a glacier expanding, or an ocean forming. Each isolated population experiences different selection pressures and accumulates different mutations. Over time, they become genetically distinct. If they later come back into contact, they may no longer be able to interbreed.

Behavioural isolation (sympatric speciation)

Populations live in the same geographical area but no longer interbreed because of differences in behaviour. Examples include:

• Different mating calls — two frog species may occupy the same forest but breed at different times or respond to different calls.
• Different courtship rituals — birds may have dances or displays that only attract mates of the same population.
• Host preference — some insects only lay eggs on one plant species; if two populations prefer different plants, they rarely meet to mate.

Other forms of isolation include temporal isolation (breeding at different times of year) and mechanical isolation (physical incompatibility of reproductive organs). At Stage 5, geographical and behavioural isolation are the main types to understand.

When Australia separated from Gondwana around 50 million years ago, it carried a cargo of marsupial mammals. Isolated from the placental mammals that dominated other continents, Australian marsupials underwent one of the most spectacular adaptive radiations in evolutionary history.

Adaptive radiation occurs when a single ancestral species diversifies into many new forms, each adapted to a different ecological niche. In Australia, marsupials filled niches that placental mammals filled elsewhere:

• Kangaroos and wallabies — large herbivores equivalent to deer and antelope
• Tasmanian devils — scavenging carnivores equivalent to hyenas
• Thylacines (now extinct) — pursuit predators equivalent to wolves
• Wombats — burrowing herbivores equivalent to badgers
• Koalas — arboreal leaf-eaters equivalent to sloths
• Quokkas and possums — small generalists equivalent to squirrels
• Numbats — insect-eaters equivalent to anteaters

This radiation was possible because Australia lacked competing placental mammals. Natural selection favoured different traits in different environments: hopping for open plains, climbing for forests, burrowing for grasslands. Over millions of years, one ancestral marsupial gave rise to dozens of distinct species.

Wallabies and kangaroos are an excellent example of recent speciation. They share a recent common ancestor and can sometimes interbreed, but they are classified as different species or genera based on size, chromosome number and ecological preferences. The swamp wallaby (Wallabia bicolor) and red-necked wallaby (Notamacropus rufogriseus) look similar but have different chromosome numbers and do not produce fertile offspring together — a clear sign of reproductive isolation.