Speciation is the evolutionary process by which reproductively isolated populations evolve to become distinct species. There are four known methods of speciation, all of which have occurred naturally at some point.
- Allopatric: A population splits into two geographically isolated populations due to geographical changes, such as mountain formation. The isolated populations then evolve different genotypes and or phenotypes as they both experience different selective pressures, independently undergo genetic drift, or have different mutations occur.
- Peripatric: A subform of allopatric speciation in which new species are formed in isolated, smaller peripheral populations that are prevented from mating with the main population. Genetic drift is believed to play a larger role in this form.
- Parapatric: Individuals of each population may come in contact with each other occasionally due to only partial geographic isolation. The reproductive isolation occurs because of reduced fitness of the heterozygote, which leads to selection for behaviors or mechanisms that prevent interbreeding. This is modeled less on geographic isolation and more on the single habitat causing natural selection.
- Sympatric: New species arise from two or more decedents of a single ancestral species all occupying the same geographic location. This is often seen when different groups of a single species of insect all become dependent on different plants, eventually resulting in different species.
Once new species form through reproductive isolation, any interbreeding would create a new, hybrid species. The continuity of these distinct species is maintained by reproductive barriers, both prezygotic and postzygotic, that prevent this hybridization.
The main line of defense against disrupting the evolutionary process is prezygotic isolation. This represents barriers that prevent interaction of gametes, or sex cells, from two incompatible species from interacting to form a zygote. These prezygotic barriers can be genetic differences in reproductive organs, difference in mating times and rituals, or even geographical barriers.
Should the prezygotic barriers fail to prevent breeding, there is another evolutionary fail safe against hybridization. Postzygotic isolation occurs after the gametes form a hybrid zygote operates by one of three methods.
- Affecting Hybrid Viability: Gametes may be incompatible due to different numbers of chromosomes or even the size and shape of those chromosomes. This causes the gametes to form zygotes that generally can not survive long enough to finish developing. If it somehow beats the odds and does devolop, the hybrid typically has a number of developmental defects that aren't compatible with survival and reproduction, which causes premature death and prevents it from maturing and passing along its defects.
- Affecting Hybrid Fertility: Even when the rare hybrid does go on to become a fully developed adult, it typically has low or no fertility. No matter how healthy a mature hybrid individual is, without reproductive function, a new species can not begin. This is often seen when horses mate with donkeys. The resulting hybrid is completely infertile if male, meaning that no two horse – donkey hybrids can ever mate each other.
- Hybrid Breakdown: The evolutionary process that causes speciation in the first place happens for a reason. Each species evolved specifically to survive and thrive in its own specific environment. While hybrids often have benefits from both parent species, they also lack some or all of the specialized traits that ensured the parent species' success. The hybrids are often weaker than their parents, and represent a step back rather than a step forward in evolution. They often can not compete with other species and eventually die off.