- Habitat Isolation: Two organisms do not encounter each other in nature. This isolation can be caused by an obvious geographic barrier such as a canyon or mountain. However, the separation can also be subtler. For example, some species of birds prefer to spend time in the upper branches of a tree, while others prefer to perch in the brush at the base of a tree. Two species of birds may live in the same tree, yet are not likely to breed because they do not encounter one another.
- Temporal Isolation: Two organisms do not breed at the same time. This isolation can happen on any time scale. For example, individuals of two species may be active at different times of the day, or two species may become reproductively receptive during different times of the month. Alternatively, some flowering plants breed during different times of the year. Incompatible timing can prevent species from interbreeding.
- Behavioral Isolation: Two organisms are not sexually attracted to one another. This isolation may be as simple as not recognizing that the other individual is a potential mate. It can be more complicated in species with elaborate courtship displays or dances. For example, some birds participate in very specific rituals to attract mates. These may include complex dance moves, sounds, and decorated plumage for display. If an individual from another group is not attracted by these mate-attracting efforts, mating does not occur.
- Mechanical Isolation: Two organisms are not physically able to mate with one another. This isolation is more common with flower pollination, where the pollinator and flower must physically complement one another to maximize successful fertilization. For example, if a bee does not fit into a flower in a way that allows it to capture pollen, the bee cannot deliver the pollen to another flower for proper pollination. In addition, some insects have very complex genitalia that would preclude mating with another individual whose sexual structures are not compatible. Some organisms, such as an elephant and a mouse, have such a large size difference in reproductive structures that mating is impossible.
- Gametic Isolation: The sperm and egg of two organisms will not fuse to form a zygote. This isolation is especially relevant for organisms that undergo external fertilization, which happens when sperm and egg fuse outside of a body. Bottom-dwelling marine invertebrates, as well as some amphibians and fishes, participate in external fertilization, releasing their eggs and sperm into water to potentially intermix with the gametes of other species. Biochemical factors can prevent a sperm cell from entering the egg cell of another species. For example, sea urchin eggs are coated with a jelly that contains chemicals that preferentially activate sperm of the same species. Species with internal fertilization, where the sperm and egg fuse inside an organism's body, can also experience gametic isolation. The environment within the reproductive tract of the female, such as acidic conditions, may disable the sperm cell of the male of another species. Other biochemical factors may cause the egg and sperm to fail to fuse. Mammalian sperm and eggs contain proteins that preferentially bind to opposite-sex gametes of the same species.
- Reduced hybrid viability: The hybrid fails to develop normally and results in a weak, unhealthy adult. Sickly hybrids can sometimes be seen along a hybrid zone, where two species overlap and the formation of hybrids occurs. Some Rana frogs will mate with individuals of other Rana species, but the hybrids may be too weak to survive. Natural selection suggests these individuals will be preyed upon and not develop into adults.
- Reduced hybrid fertility: The hybrid develops normally into a healthy adult and may be a very strong and robust individual. However, the adult is infertile, or the hybrid makes gametes but those gametes will not fuse with the gametes of the previous generation. In this case, gene flow (change in the total set of genes of a population caused by individuals entering or leaving a population) is functionally blocked. The commonly known example is the mule hybrid formed by a horse and donkey. The horse and donkey have a different number of chromosomes. A chromosome is a structure that contains DNA, the genetic material that is passed from one generation to the next. The hybrid mule has an intermediate number of chromosomes, complicating the production of functioning sperm and eggs. Other hybrids have been formed by two species in captivity, such as tigers and lions, zebras and horses, and buffalo and cows. Some hybrids, such as those produced by polar bears and grizzly bears, have been seen in the wild. These pairings result in viable, healthy hybrids that are sterile (unable to reproduce).
- Hybrid breakdown: Over multiple generations, the hybrids fail to thrive or reproduce. In this case, the first-generation hybrid may seem normal. It is strong, healthy, and even fertile. However, with each new generation, hybrid health or fertility begins to diminish, and the hybrids eventually fail to thrive. Sunflowers and some varieties of cultivated rice undergo this process.
Sympatric versus Allopatric Speciation
There are two major mechanisms for speciation: allopatric speciation and sympatric speciation. Allopatric speciation is the formation of a new species due to a reduced gene flow in the presence of a geographic barrier. A physical barrier is the simplest way to minimize gene flow (change in the gene pool caused by individuals entering or leaving a population). If individuals of two groups do not physically encounter one another, interbreeding is impossible. Sometimes an obvious geographic barrier separates two groups of organisms. Rivers, lakes, oceans, mountains, canyons, and the movements of tectonic plates are examples of geographic features that prevent interbreeding among those living on either side. This is especially true for those organisms incapable of travel across such a barrier. When a new geographic barrier forms or an old one shifts position, the resulting isolation of a population can lead to the formation of a new species. However, a physical barrier is not necessary for reproductive isolation.Other mechanisms of reproductive isolation can prevent gene flow when groups reside in the same geographic region. Sympatric speciation is the formation of a new species in the absence of a geographic barrier that would prevent gene flow. The most common mechanism of sympatric speciation is polyploidy, a condition in which an organism has more than the normal number of copies of a set of chromosomes. For example, after thousands of years of hybridization, wheat has strains that are diploid (have two sets of chromosomes), tetraploid (have four sets of chromosomes), and hexaploid (have six sets of chromosomes). The process of sympatric speciation immediately reproductively isolates a new generation from the previous one.