Defining Species & Reproductive Isolation

A species is a group of organisms with similar characteristics that share a common gene pool. For sexually reproducing organisms, members of the same species can (actually or potentially) breed to produce fertile offspring, but members of different species cannot. Fertile means the offspring can produce their own offspring once sexually mature. A sexual species can also be described as the largest gene pool that can occur under natural conditions.

This definition fails for organisms that reproduce asexually, so other criteria are used — each with its own strengths and drawbacks:

Today, DNA sequencing is the more likely method of identification, because its data is highly accurate and reliable.

REPRODUCTIVE ISOLATION

Reproductive isolation is the prevention of the exchange of genetic material between organisms of different species. It preserves each species as a separate gene pool. Isolation matters because mating between different species cannot produce fertile offspring — so if a new species is to form and remain a species, some form of reproductive isolation must exist and be maintained.

Two main types of mechanism produce reproductive isolation:

1. Pre-zygotic mechanisms, which prevent a zygote from forming.
2. Post-zygotic mechanisms, which allow a zygote to form but prevent a fertile hybrid from developing.

Pre-zygotic barriers stop a zygote forming; post-zygotic barriers stop fertile hybrids developing.

PRE-ZYGOTIC MECHANISMS

• Temporal isolation - species breed at different times. Australia's 55 species of kangaroos and wallabies often share habitats but cannot interbreed because their mating seasons differ; some flowers accept pollen only in a brief window (morning but not afternoon); many frogs and insects mate at different times of year.
• Behavioural isolation - courtship signals are answered only by members of the same species, such as species-specific frog mating calls, or crickets calling by vibrating their wings.
• Mechanical isolation - reproductive structures are incompatible. This is common in insects and reptiles (bats show great diversity in penis structure); in plants, flower shape matches one pollinator, so a bee-pollinated flower will not be pollinated by a bird.
• Habitat isolation - species occupy different microhabitats (for example, rainforest canopy and forest floor) and never meet.
• Gamete isolation - gametes meet but cannot fuse: the egg's chemical signals are not recognised, so sperm or pollen never travels to it; the male gamete cannot penetrate the egg; penetration occurs but the nuclei fail to fuse; or, in external fertilisers, gametes released into water never find each other.

POST-ZYGOTIC MECHANISMS

• Hybrid inviability - the zygote forms and starts developing, but the hybrid is often weak, a poor competitor for food and resources, more susceptible to disease and predators, and may have birth defects. It is unlikely to reach reproductive age or contribute to the gene pool.
• Hybrid sterility - the hybrid survives but cannot reproduce. Examples include the liger (male lion crossed with female tiger) and the mule (female horse crossed with male donkey).

The mule is the standard worked example. A horse has 64 chromosomes and a donkey 62, so the mule receives 32 + 31 = 63 chromosomes. Because 63 is an odd number, homologous pairs cannot form during meiosis, so no gametes are produced — the mule is infertile. Mules are still deliberately bred, because they are strong and can carry large cargo for their body size.

The mule receives 32 chromosomes from the horse and 31 from the donkey — 63 in total. An odd number cannot pair in meiosis, so no gametes form.

SUMMARY OF ISOLATING MECHANISMS