96 Prezygotic Reproductive Isolating Mechanisms

Key Idea: Reproductive isolating mechanisms prevent interbreeding between different species. Prezygotic isolating mechanisms occur before fertilisation can take place. Reproductive isolation prevents successful interbreeding between species and is crucial to maintaining species' integrity. Prezygotic isolating mechanisms operate before fertilisation can occur and prevent "gamete wastage". They are the most common type of isolating mechanism and may be associated with behaviour, morphology, or reproductive timing. Single barriers to gene flow (such as geographical barriers) are usually insufficient to isolate a gene pool, so most species commonly have more than one type of barrier. Geographical barriers are not strictly a reproductive isolating mechanism, because they are not part of the species’ biology, although they are usually a necessary precursor to reproductive isolation in sexually reproducing populations.

Geographical isolation

Geographical isolation describes the isolation of a species population (gene pool) by some kind of physical barrier, for example, mountain range, water body, isthmus, desert, or ice sheet. Geographical isolation is a frequent first step in the subsequent reproductive isolation of a species. Example: Geological changes to the lake basins has been instrumental in the proliferation of cichlid fish species in the rift lakes of East Africa (far right). Similarly, many Galápagos Island species (e.g. iguanas, finches) are now quite distinct from the Central and South American mainland species from which they separated.

Ecological (habitat) isolation

Ecological isolation describes the existence of a prezygotic reproductive barrier between two species (or sub-species) as a result of them occupying or breeding in different habitats within the same general geographical area. Ecological isolation includes small scale differences (e.g. ground or tree dwelling) and broad differences (e.g. desert vs grasslands). Ecological isolation often follows geographical isolation, but in many cases the geographical barriers may remain in part. Example: Hochstetter’s and Archey’s frogs (right) are sympatric (occur in the same geographical area) in the Coromandel region, but occupy different habitats within that range. Archey’s frog has no webbing between the toes and is found in forested areas away from water where it climbs into damp vegetation. Hochstetter's frog has partial toe webbing and prefers stream margins.

Lake Victoria

Lake Tanganyika

Lake Malawi

Hochstetter’s frog

NASA Earth Observatory

Archey’s frog

Photo: DOC-CV

David M. Green cc2.5

Temporal isolation

Temporal isolation means isolated in time, and it prevents species interbreeding because they mate or they are active at different times. For example individuals from different species do not mate because they are active during different times of the day (e.g. one species is active during the day and the other at night) or in different seasons.

Example: Closely related animal species may have different breeding seasons or periods of emergence to prevent interbreeding. The periodical cicadas (Magicicada genus) are an excellent example of this. Periodical cicadas are found in North America. There are several species and some have an overlapping distribution. Most of their life is spent underground as juveniles, emerging to complete their development and to mate. To prevent interbreeding, the various species spend either 13 or 17 years underground developing. Emergence of a single species is synchronised so the entire population emerges at the same time to breed.

Periodical cicada

Periodical cicada emerging

Bruce Marlin

Lorax

The gametes (eggs and sperm) from different species are often incompatible, so even if the gametes meet, fertilisation is unsuccessful. Gamete isolation is very important in aquatic environments where the gametes are released into the water and fertlisation occurs externally (e.g. reproduction in frogs, fish, and coral). Where fertilisation is internal, the sperm may not survive in the reproductive tract of another species. If the sperm does survive and reach the egg, chemical differences in the gametes prevent fertilisation. Chemical recognition is also used by flowering plants to recognise pollen from the same species. Pollen from a different species is recognised as foreign and it does not germinate.

Example: Two species of sea urchin, the red sea urchin (Strongylocentrotus franciscanus) and the purple sea urchin (Strongylocentrotus purpuratus), share the same geographic range. Sea urchins release their gametes into the sea water, but the two species do not interbreed because their gametes are not compatible.

Behavioural Isolation

In many species, courtship behaviours are a necessary prelude to successful mating. These behaviours may include dances, calls, displays, or the presentation of gifts. The displays are very specific and are unique to each species. This means that mates of the same species recognise and are attracted to the individual performing the behaviour, but members of other species do not recognise or pay attention to the behaviours.

Birds exhibit a wide range of courtship displays. The use of song is widespread but ritualised movements, including nest building, are also common. Example: Galápagos frigatebirds have an elaborate display in which they inflate a bright red throat pouch to attract a mate.

Mechanical (morphological) isolation

Structural differences (incompatibility) in the anatomy of reproductive organs prevents sperm transfer between individuals of different species. This is an important isolating mechanism preventing breeding between closely related species of arthropods.

Example: The sexual organs of empid flies have a lock-and-key mechanism. Without the right shaped genitalia, individuals cannot mate.

Many flowering plants have coevolved with their animal pollinators and have flower structures to allow only that insect access. Structural differences in the flowers and pollen of different plant species prevents cross breeding because pollen transfer is restricted to specific pollinators and the pollen itself must be species compatible.

Species A Species B

Red sea urchin

Male frigatebird display

Frog calling

Empid flies mating

Orchid

(b) What role do isolating mechanisms have in maintaining the integrity of a species?

2. What is a prezygotic isolating mechanism?

3. (a) Why is geographical isolation not regarded as a reproductive isolating mechanism?

(b) Explain why, despite this, it often precedes, and is associated with, reproductive isolation:

4. Distinguish between geographical and ecological isolation:

5. Identify the type(s) of reproductive isolation described in the following examples:

(a) Two species of butterfly (right) coexist in the same habitat but have different breeding seasons:

(b) Male bowerbirds construct elaborate bowers (shelters) to attract a mate. One species, the MacGregor's bowerbird builds a tall structure and decorates it with charcoal. A second species, the satin bowerbird, decorates its bower with bright blue objects:

Breeding season for species A Breeding season for species B

J F M A M J J A S O N D

(c) Two species of New Zealand skinks, Oligosoma smithi and O. suteri are sympatric (same area) in north-eastern New

Zealand. O. smithi is diurnal and gives birth to live young. O. suteri is nocturnal and lays eggs.

(d) The blackbird (Turdus merula) and the ring ouzel (Turdus torquatus) are two closely related species found in Europe. The blackbird is a woodland species and the ring ouzel tends to inhabit highlands:

(e) Two species of sage plants coexist in a region of

Southern California. Black sage (Salvia mellifera) has small flowers and is pollinated by small bees while white sage (S. apiana) has larger flowers providing a larger landing platform for its larger pollinator, carpenter bees.

The two species of sage remain reproductively isolated.

White sage Black sage