Biological isolation is a process in which groups of organisms are prevented from interbreeding. This separation of populations is a component of evolution because it stops the exchange of genes, also known as gene flow. When populations can no longer mix their genetic material, they begin to follow separate evolutionary paths. Over time, this divergence can lead to the formation of entirely new species.
Geographic Isolation as a Starting Point
The most direct way populations become isolated is through geographic separation, initiating a process called allopatric speciation. This occurs when a physical barrier, such as a mountain range or an ocean, divides a once-continuous population. Once separated, the groups cannot interbreed simply because they cannot reach each other. This lack of gene flow is the first step toward developing distinct species.
A classic example of this can be seen with the squirrels of the Grand Canyon. The Abert’s squirrel lives on the south rim, while the Kaibab squirrel is found only on the north rim. These two populations are believed to have descended from a single ancestral species that was split when the canyon formed. Over thousands of years of isolation, the Kaibab squirrel has developed distinct features, such as a white tail and black belly, that distinguish it from its relative on the south rim. The formation of the Grand Canyon created an impassable barrier for these small mammals, and while still genetically similar, the separation has allowed for independent evolutionary paths.
Mechanisms of Reproductive Isolation
Reproductive isolation involves biological barriers that prevent members of different groups from producing viable, fertile offspring. These barriers are categorized as either prezygotic or postzygotic, depending on whether they act before or after fertilization. Prezygotic barriers are mechanisms that block reproduction from taking place, preventing the formation of a fertilized egg, or zygote.
Prezygotic barriers can manifest in several ways:
- Habitat isolation: Two species live in the same general area but occupy different habitats.
- Temporal isolation: Species have different breeding schedules, such as mating during different seasons.
- Behavioral isolation: Animals have specific courtship rituals; for example, the unique songs of different bird species may only attract mates of the same species.
- Mechanical isolation: Due to incompatible reproductive organs.
- Gametic isolation: Involves sperm and egg cells that cannot fuse.
Postzygotic barriers come into play after a zygote has formed from the mating of two different species. One barrier is hybrid inviability, where the hybrid embryo cannot develop properly and does not survive. Another is hybrid sterility, where the hybrid offspring, such as a mule, survives to adulthood but is unable to reproduce. A third type, hybrid breakdown, occurs when first-generation hybrids are fertile, but their offspring are weak or sterile.
The Process of Speciation Through Isolation
Once populations are isolated, whether by a physical barrier or reproductive mechanisms, they begin to diverge genetically. This divergence is driven by natural selection and genetic drift. Natural selection causes populations to adapt to their specific environments. If two isolated populations face different environmental pressures, such as different food sources or predators, they will evolve different traits that are advantageous in their respective habitats.
Genetic drift involves random fluctuations in the frequencies of gene variants, or alleles, within a population. This process has a more significant impact in small, isolated populations, where chance events can lead to the loss of certain alleles and the fixation of others. Over many generations, these random changes can cause two populations to become genetically distinct from one another.
The combination of natural selection and genetic drift leads to the accumulation of genetic differences between the isolated groups. If these differences become substantial enough, they can give rise to reproductive barriers. At this point, even if the geographic barrier is removed and the populations come back into contact, they will no longer be able to interbreed successfully. This marks the completion of speciation, where one ancestral species has given rise to two or more distinct species.
Observing Isolation in the Natural World
The Galápagos Islands provide an example of speciation resulting from isolation. The various species of finches found there are believed to have descended from a single ancestral species that arrived from the mainland. The islands’ geographic isolation from one another allowed different finch populations to evolve independently. This process, known as adaptive radiation, resulted in about 18 different species, each with a unique beak shape and size adapted to a specific food source available on its island. Their distinct songs and appearances also act as reproductive barriers, keeping the species separate.
Another example is the speciation of Drosophila fruit flies in the Hawaiian Islands. From what is thought to be a single ancestral species that colonized the archipelago, hundreds of unique species have evolved. This diversification is a result of the founder effect, where new islands were colonized by a small number of individuals, and subsequent geographic isolation. The flies adapted to a wide array of ecological niches, from forests to lava flows, and different breeding substrates like specific fruits, flowers, and leaves, leading to reproductive isolation and the formation of many distinct species.