The biological species concept defines a species as a population whose members can produce fertile offspring with one another, but not with members of other groups. The failure of different species to breed successfully is not a single event, but a series of biological checkpoints that prevent the flow of genes between groups. These barriers are collectively known as reproductive isolation, and they ensure that distinct species maintain their unique genetic identities. Reproductive isolation mechanisms act at various stages, from preventing the initial meeting of individuals to rendering the resulting offspring infertile.
Preventing Initial Contact: Isolation Before Mating
The most efficient mechanisms for reproductive isolation occur before mating even takes place, preventing the expenditure of energy on an attempt that is destined to fail.
Habitat Isolation
Habitat isolation occurs when species live in different parts of the same geographic area and thus rarely encounter one another. For instance, two species of garter snakes might share a territory, but one lives predominantly in the water while the other remains on land. This difference in habitat use limits their opportunities to interact.
Temporal Isolation
Temporal isolation occurs when species breed during different times of day or in different seasons. The American toad and the Fowler’s toad, for example, have overlapping ranges but do not hybridize. This is because the American toad mates in early summer while the Fowler’s toad mates later in the season, acting as a complete reproductive block.
Behavioral Isolation
Behavioral isolation relies on distinct courtship rituals or signals that prevent recognition between species. Female fireflies will only respond to the unique flash patterns emitted by males of their own species. The elaborate “sky-pointing” dance performed by the male blue-footed booby is another example, as females select a partner who executes this species-specific ritual correctly.
Failure to Fertilize: Mechanical and Gametic Blocks
If individuals from different species overcome the initial isolating barriers and attempt to mate, incompatibility mechanisms can still prevent the formation of a fertilized egg, or zygote. These mechanisms occur after contact but before fertilization is complete.
Mechanical Isolation
Mechanical isolation describes the physical incompatibility between the reproductive structures of the two species. In many insect species, the size or shape of the genitalia differs so significantly that copulation is anatomically impossible. A similar block occurs in plants, such as two species of sage, where the flower structure is specialized to deposit pollen onto a specific body part of a particular type of pollinator. If a different insect visits the flower, the pollen will not contact the stigma of the other species, preventing fertilization.
Gametic Isolation
Gametic isolation acts at the cellular level, where the sperm of one species is unable to fertilize the egg of another. This is common in aquatic organisms that release their gametes into the water, such as sea urchins. The egg’s surface possesses specific receptor proteins that must chemically recognize complementary proteins on the sperm head to allow penetration. If the proteins do not match, the sperm is blocked.
When Hybrids Fail: Issues After Fertilization
Should all pre-zygotic barriers fail and fertilization successfully occur, the resulting hybrid offspring face a final set of post-zygotic hurdles that reduce their viability or fertility.
Hybrid Inviability
Hybrid inviability occurs when the fertilized egg fails to develop properly or the hybrid offspring is frail and does not survive to reproductive maturity. This failure is often due to conflicting parental genes that interfere with the complex, coordinated process of embryonic development. For example, a cross between a sheep and a goat will result in an embryo that dies in the early developmental stages.
Hybrid Sterility
Hybrid sterility is a post-zygotic barrier where the hybrid survives but cannot produce functional gametes. The classic example is the mule, the offspring of a female horse (64 chromosomes) and a male donkey (62 chromosomes), resulting in a hybrid with 63 chromosomes. During meiosis, chromosomes must pair up precisely to be divided into gametes. The mule’s odd number of chromosomes, coupled with structural dissimilarities, prevents this proper alignment, rendering the mule sterile.
Hybrid Breakdown
In some cases, the first-generation hybrid (F1) is both viable and fertile, but subsequent generations show a rapid decline in fitness, known as hybrid breakdown. This is common in certain plants, where the F1 hybrid may be vigorous, but the F2 generation produces seeds that fail to germinate or grow into weak, sterile adults. This breakdown is caused by the shuffling of incompatible genes from the two parent species, which interact poorly in new combinations.