How Speciation Happens

Reproductive barriers block gene flow between populations, allowing gene pools to diverge through the mechanisms of evolution.
The biological species concept revolves around the idea of successful reproduction within a species and reproductive isolation between species. In order for a new species to form, a group of organisms must become reproductively isolated from other groups. Reproductive isolation results from biological barriers to the formation of healthy, fertile offspring between individuals of two different species. By limiting the mixing of genes (the component of DNA that carries hereditary traits) by individuals moving between two populations (interbreeding groups), their evolutionary paths diverge. Each population will acquire mutations at random and in potentially different parts of the genome. A mutation is a change in DNA sequence. Genetic drift and natural selection may be different in each population, and the gene pool (the total set of genes of all individuals in a population) will become increasingly different. Additionally, the normal and expected evolution of a species will continue to occur independently within the separate groups. The composition of the gene pool may diverge dramatically, depending on their relative environments and circumstances. After some generations have passed, the populations may reunite in a hybrid zone, a geographic region where two species overlap and the formation of hybrids occurs. If this happens, individuals may attempt to interbreed, forming a hybrid, an offspring produced when members of two species interbreed. If viable hybrid offspring between the two groups are produced, it may lead to the populations reuniting as a single species. However, if too much divergence has occurred between the gene pools, hybrids may be infertile, unhealthy, or completely absent. In these cases, speciation is complete and the two populations (interbreeding groups of individuals in an area) are considered separate species. For example, the repeated glaciation in North America is believed to have created more than 30 distinct species of freshwater fish. Each time glaciation occurred, new areas formed that were isolated from others. Scientists believe two main areas served as the refuges for the fish. These area are the Mississippian refuge, located between Missouri and Illinois, and the Atlantic Coast refuge, found east of Appalachian Mountains, south of Long Island. Nine of the 30 species show DNA similarities with the Mississippian and Atlantic Coast populations. While most of the fish species look very similar to each other, scientists have been able to isolate the genetics of eight species to determine they are, indeed, distinct subspecies. These species have differences in body shape and behavior. With each glaciation event, the fish became more distinct from each other and less likely to interbreed due to geographic isolation. Over time, new species arose.
When a barrier separates a species into two different populations, speciation may occur. In North America, repeated glaciation (buildup and receding of glaciers) divided the area near the Great Lakes into several distinct regions. This caused the divergence of the species of freshwater fish now found in the area.

Reproductive Barriers

Prezygotic reproductive barriers prevent a zygote from forming, and postzygotic reproductive barriers prevent a hybrid zygote from developing into a healthy, fertile adult.
Reproductive isolation is the key to identifying separate sexually reproducing species. A reproductive barrier, or reproductive isolating mechanism, prevents the formation of healthy, fertile offspring between two groups of organisms. These barriers reduce gene flow, which is a change in the gene pool caused by individuals entering or leaving a population. This reinforces the genetic separation of a species. When a reproductive barrier appears between individuals of the same species, the process of speciation has begun. There are many steps involved in reproduction to form healthy, fertile offspring. If any of these steps is blocked by a reproductive barrier, the groups are reproductively isolated and considered separate species. Reproductive barriers are categorized by whether they block reproduction before or after a zygote, a fertilized egg, has formed following mating.

Prezygotic Barriers

A prezygotic barrier is a reproductive barrier at the time of mating or fertilization that impedes the formation of a hybrid zygote (a fertilized egg produced by individuals of two different species), which could develop into an offspring. Zygote formation can be blocked at many different stages, beginning with the probability of two organisms encountering one another. There are several types of prezygotic barrier.
  • 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.
Various prezygotic (before fertilization) barriers exist that might prevent individuals of two species from successfully producing a viable fertilized egg. For example, if two individuals are not attracted to each other, they will not mate.

Postzygotic Barriers

A postzygotic barrier is a reproductive barrier to the development of a hybrid zygote into a healthy, fertile adult. These barriers may not be immediately obvious because a hybrid fertilized egg formed between individuals of different species may develop normally and even reproduce. However, if a barrier is present, at some point the hybrid fails to either thrive or reproduce. In other cases, the first generation may seem healthy, but the second generation may not thrive or reproduce. These are the ways that postzygotic barriers can affect hybrids.
  • 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

Allopatric speciation occurs in the presence of a geographic barrier, such as a river or an area devastated by fire to prevent gene flow, and sympatric speciation occurs in the absence of a geographic barrier.
Speciation requires the reproductive isolation of groups of organisms. Once a group has become reproductively isolated by a prezygotic barrier, it will evolve independently from other groups. If the gene pools (total sets of genes) of two groups diverge enough, then they will become incapable of interbreeding if given the chance. At that point, they are considered separate species.

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.
Sympatric speciation occurs without an obvious geographic separation to minimize gene flow. Species inhabit the same region but are unable to reproduce. Allopatric speciation separates species geographically, preventing reproduction.

Speciation from Multiple Chromosomes

Polyploidy is instant sympatric speciation because the offspring have a different number of chromosomes than their parents.
Polyploidy is when a cell contains more than the normal number of copies of a set of chromosomes. It occurs when a mitotic or meiotic error changes the number of chromosomes (carriers of genetic information) in a gamete or zygote. Mitosis is the process of cell division that results in two cells that are genetic clones of the parent cell. This type of cell division is used for growth or body repair. Meiosis is a process in cell division during which the number of chromosomes decreases to half the original number by two divisions of the nucleus, resulting in the production of gametes (sperm or eggs). During meiosis, an error may fail to reduce the number of chromosomes. Another possible occurrence is that the zygote experiences an error in mitosis during development. Errors such as these produce an offspring whose number of chromosomes does not match that of the parent generation. The new generation is immediately reproductively isolated from the previous one. This is why polyploidy is sometimes referred to as a mechanism for "instant speciation." Sympatric speciation by polyploidy is more common in plant species that can self-fertilize. This is because individuals with abnormal gametes are unlikely to find another individual with gametes of the same number of chromosomes to breed with. Also, individuals grown from abnormal zygotes are unlikely to find an individual with matching gametes when they eventually mature and form gametes.
When a meiotic error occurs, gametes (sperm and eggs) with the wrong number of chromosome copies (polyploidy) can be produced. If these gametes fuse, the new zygote, or fertilized egg, will have a different number of chromosomes than the parent species.