Reproductive Isolation (5) Flashcards
What is reproductive isolation
Reproductive isolation is a mechanism that prevents two or more species from exchanging genes (interbreeding) and producing viable, fertile hybrids even though they are not geographically separated. Such species that live together are called sympatric species.
What do geographic barriers start? (4)
- As has been explained, geographic barriers often start the development of new species, especially in the case of animals. If the isolated groups never come into contact again, they will almost certainly accumulate enough genetic differences over time to become separate species. This can take 3 000 to 250 000 years for complete reproductive isolation.
- Once these separate species come into contact again most will not interbreed. This is probably because some of the differences accumulated during speciation will prevent breeding it.
- These differences were probably incidental adaptations to local conditions during geographic isolation (allopatry). They did not arise for the purpose of preventing breeding between different species.
- These ‘by-products’ of allopatric speciation, which cause reproductive isolation and are probably the final stage of speciation.
There are several mechanisms (methods) of reproductive isolation, which can operate either pre-zygotically (pre-mating) or post-zygotically (post-mating). Most prevent mating or fertilisation from taking place. What are they? (4)
- Breeding at different times of the year
- Species-specific courtship behaviour
- Adaptation of plants to different pollinators
- Infertile offspring
What does breeding different times of the year prevent?
Different animal species often have different mating seasons and plants flower at different times of the year, which prevents mating opportunities.
What are some examples of breeding at different times of the year? (3)
- Two parrot species, the critically endangered Cape Parrot and the Grey-headed Parrot occur in areas quite close to each other. Both species share similar breeding habitats but there has been no record of hybridisation as they breed in different seasons – the Cape Parrot breeding from August to February and Grey-headed from April to August.
- Two species of eagle that live in the same territory, the Tawny Eagle and the Bateleur,breed in different seasons: the former in winter, the latter in summer.
- Two fish species of the kob family both spawn in the water off the Kwa-Zulu Natal coast; silver kob spawns between August and December and the squaretail kob between June and September.
What is species-specific courtship behaviour?
Courtship behaviour is an animal activity to signal sexual readiness for pair formation, mating and reproduction. It is important as an isolating mechanism as it prevents different species from interbreeding even though their territories overlap.
What are some examples of courtship behaviour? (4)
- breeding display, e.g. soaring and calling with occasional claw grappling of the well-known African fish eagle.
- stridulating songs by male cicadas (Christmas beetles), during pair formation and courtship.
- secretion of pheromones (chemical compounds that bring creatures together to mate), which may be secreted by special glands or incorporated into other substances, e.g. urine. Pheromones are widespread among insects and vertebrates but not birds.
- breeding plumage, metallic green of the male malachite sunbird.
What are adaptation by plants to different pollinators ?
Plants and their flowers have groups of traits (pollination syndromes) that are an adaptation for particular pollinators. These traits are floral features that attract particular animals to visit the flowers to receive and deposit pollen. The features include flower shape, size, colour, scent, reward type (nectar and/or pollen), timing of flowering, etc.
What are some examples of adaptation by plants to different pollinators? (5)
- The flower is designed so that only one specific pollinator can get to the pollen. or example, some orchids attract their pollinators by sexual deception and mimicry, e.g. oils that mimic sexual pheromones and flower shapes that imitate the female of an insect species so that they may attract the males, which, in the process, pick up and deposit pollen.
- Flowers that are visited by flies can be: −foul smelling, e.g. Stapelia gariepensis, a carrion flower that attract flies by mimicking smell of rotten meat. −tubular flowers with the narrow opening just wide and long enough for a pollinator’s tongue to reach the nectar, e.g. Erica irbyana. Horse flies and bee flies with a long proboscis are examples of such pollinators.
- Dull-coloured flowers with strong, fruity or musty scents open at night e.g. baobabs, bananas or mangoes attract pollinators such as bats or mice.
- The African Lily (Massonia depressa) of the Succulent Karoo region is pollinated by nocturnal rodent species. Its ground level flowers are adapted to support non-flying mammal pollinators by being very sturdy.
- Birds such as sunbirds with long beaks are attracted to unscented flowers
What are infertile offspring?
Even if two different species mate and produce hybrid offspring that are vigorous, the species are still reproductively isolated if the hybrid offspring are sterile, i.e. they cannot produce offspring. This is an example of post-zygotic reproductive isolation as it occurs after fertilisation.
What is an example of infertile offspring?
The classic example is the mule, which is the result of a cross between a donkey stallion and a horse mare. Mules are vigorous, healthy animals, but they are sterile, i.e. they are unable to reproduce successfully because they are unable to produce normal gametes as the chromosomes do not pair and cross over correctly during meiosis. Female mules sometimes produce viable eggs but males are infertile.
