Speciation, the process by which new species arise, is a fundamental concept in evolutionary biology. This process requires a complete and permanent halt to gene flow between populations. This condition is known as reproductive isolation, which describes biological barriers that prevent two groups from successfully interbreeding and producing fertile offspring. Reproductive isolation is the mechanism that enforces their identity as separate species, beginning with divergence and the evolution of barriers.
The Context: Geographic Isolation and Divergence
The evolutionary journey toward reproductive isolation often begins with separation, allowing populations to diverge genetically. The most common pathway is allopatric speciation, where a physical barrier divides a single population into two geographically isolated groups. Barriers like a new mountain range, a wide river, or movement to a distant island obstruct gene flow. Once separated, natural selection, genetic drift, and new mutations accumulate independently, causing the populations to adapt differently to their respective environments.
An alternative path is sympatric speciation, which occurs without physical separation, meaning the diverging groups remain in the same geographic area. This is less common in animals but frequent in plants, often driven by a sudden genetic change like polyploidy, where an organism possesses more than two sets of chromosomes. Sympatric speciation can also arise through niche partitioning, where a subset of the population specializes in a different resource or habitat within the same range. For example, the North American apple maggot fly evolved different populations based on whether they fed on hawthorn fruit or introduced apples.
Prezygotic Barriers: Preventing Fertilization
Reproductive isolation mechanisms that act before fertilization are termed prezygotic barriers. They are highly effective because they prevent the wasted energy of an unsuccessful mating attempt.
Habitat Isolation
Habitat isolation occurs when two species live in the same general area but occupy different ecological niches, such as two insect species living on different host plants.
Temporal Isolation
Temporal isolation involves species having different breeding schedules, mating during different seasons or times of the day. For instance, two frog species in the same pond may never interbreed if one reproduces in early spring and the other in late summer.
Behavioral Isolation
Behavioral isolation is widespread, relying on species-specific courtship rituals or signals that allow only members of the same species to recognize each other as potential mates. The elaborate dances of certain birds or the unique flashing patterns of fireflies are examples where a breakdown in communication prevents mating.
Mechanical Isolation
If mating is attempted, mechanical isolation may intervene, resulting from a physical incompatibility between the reproductive structures of the two species. The differing morphology of insect genitalia, sometimes referred to as a “lock-and-key” system, can physically block the transfer of gametes.
Gametic Isolation
The final prezygotic barrier is gametic isolation, which acts at the molecular level, ensuring that the sperm and egg from different species cannot successfully fuse. This is relevant in aquatic species like sea urchins, which release their gametes into the water, where chemical signals only allow the correct species’ sperm to penetrate. These prezygotic barriers stop the reproductive process before a hybrid zygote can form, maintaining the separation of gene pools.
Postzygotic Barriers: Failure After Fertilization
When prezygotic barriers fail, and a hybrid zygote successfully forms between two different species, postzygotic barriers act after fertilization to reduce the viability or fertility of the hybrid offspring.
Reduced Hybrid Viability
The first barrier is reduced hybrid viability, where the hybrid either does not complete development or is frail and has a low chance of survival into adulthood. Genetic incompatibilities between the parent species can disrupt the hybrid’s developmental pathways, leading to early mortality, such as in the failed development of hybrid sheep and goat embryos.
Reduced Hybrid Fertility
If the hybrid survives, the next barrier is reduced hybrid fertility, meaning the organism is sterile and cannot produce its own offspring. The mule, the offspring of a male donkey and a female horse, is the most famous example; it is robust but sterile because the differing number of chromosomes prevents the proper pairing during meiosis. This inability to produce functional gametes ends the hybrid’s lineage, preventing gene flow between the parent species.
Hybrid Breakdown
The final postzygotic mechanism is hybrid breakdown, which occurs when the first-generation hybrids are viable and fertile, but subsequent generations lose fitness. These second-generation or later hybrids are often sterile or suffer from reduced viability, meaning that even if the initial cross is successful, the hybrid lineage quickly dies out. These postzygotic failures demonstrate that the two parent populations have diverged so significantly at the genetic level that their genes are fundamentally incompatible when combined.
The Outcome: Achieving Full Speciation
The culmination of these barriers is the achievement of full speciation, where the two populations are irreversibly separated. When two diverging groups come back into contact and produce unfit hybrids, a process called reinforcement may occur. Reinforcement is a form of natural selection that favors individuals who avoid interbreeding with the other group, thereby strengthening the prezygotic barriers. Selection pressures favor any trait—like a more discriminating mate choice—that prevents the wasteful outcome of producing low-fitness offspring.
This evolutionary pressure acts to “polish off” the speciation process, ensuring that the differences between the two incipient species become more pronounced in areas where they overlap. The process is complete when reproductive isolation is total and permanent, which aligns with the biological species concept. According to this concept, two populations are considered separate species when they can no longer successfully interbreed to produce viable, fertile offspring.