Speciation refers to the genesis of a new species from an ancestral species. There are two basic ways this can happen. Anagenesis involves one species evolving into a different species. Cladogenesis occurs when one species splits into two or more species. Cladogenesis is of greater interest in terms of biodiversity and is the type of speciation discussed here. Speciation has two primary components: diversification and genetic isolation. The two principal types of cladogenesis are allopatric speciation and sympatric speciation.
Allopatric speciation is the better understood of the two types of cladogenesis. It occurs when one species is separated into two groups by some physical barrier, resulting from, for example, climate change, a geological event, or a human-induced change in the environment. For example, the uplift of a new mountain range might divide an ancestral species into two isolated groups. Once the species is separated into these groups, each group may accumulate genetic changes that serve to differentiate it from the other. This accumulation of changes may result from natural selection or from random events.
If the environment on either side of the barrier is different, natural selection may favor genes that produce different traits on either side of the barrier. Even if the environment on either side of the barrier is similar, it may be that when the two groups were separated, by chance one of the groups had a different subset of the total genetic diversity present in the species, and so that group has different "raw materials" for selection to act upon. It may also happen that a neutral mutation occurs in one of the groups, meaning it is neither favorable to the organism nor unfavorable to it. Selection will not act upon such a mutation, and it may persist in the population purely by chance. Also by chance, some versions of genes may become common in a small population while others disappear. This is called genetic drift.
So by a variety of processes, involving either selection or chance, two physically separated groups may accumulate differences between them. This is the first part of the process of speciation. The second part involves the lack of gene flow between the two groups. That is, individuals from one group do not cross the barrier to mate with individuals in the other group, resulting in the genetic isolation of each group from the other. This genetic isolation permits the development of differences in the two groups.
If these differences are to persist, there must be a persistent impediment to gene flow between them. If the two groups never come into contact again, they will almost certainly accumulate enough differences over time to become separate species. If they expand their ranges and come back into contact, some other mechanism must act to "preserve" the differences they evolved in isolation (allopatry). Traditionally, this mechanism is known as reproductive isolation , and it is a byproduct of the diversification that has already taken place.
Reproductive isolation can operate prezygotically (premating) or postzygotically (postmating). Prezygotic reproductive isolation prevents fertilization from taking place. It may be that members of the two groups breed at different times of the day or different times of the year or in different habitats. They may have developed mechanical differences that prevent copulation, or perhaps copulation takes place, but the two groups have become chemically incompatible so that fertilization does not occur. Postzygotic reproductive isolation acts after fertilization. The embryo may not develop normally, or the offspring may be unhealthy or infertile as adults. In all these ways, reproductive isolation may prevent the gene pools of the two groups from mixing, allowing them to continue on independent evolutionary trajectories.
There is some disagreement among scientists regarding the importance of reproductive isolation in the speciation process. If, as noted above, the two groups that have accumulated differences between them remain separated by a physical barrier preventing their members from ever meeting, it may not matter whether they develop reproductive isolating mechanisms. Proponents of the phylogenetic species concept, for instance, would say that the fact that the groups have accumulated diagnostic differences and are evolving independently is sufficient evidence to say that speciation has taken place.
The other principal type of cladogenesis is sympatric speciation. In this type of speciation, a species splits into two groups that diversify and become genetically isolated while remaining in the same place. "Same place" typically means that individuals from both groups meet in the same habitat during the breeding season. Most of the mechanisms by which sympatric speciation may occur are poorly understood. There must be some impediment to gene flow if differentiation into two groups is going to take place.
Sympatric speciation can happen if a mutation results in an immediate reproductive barrier in a segment of the species. The most common example of this is polyploidy in plants. In this case, errors in cell division may cause a doubling of the normal number of chromosomes, which instantaneously produces a reproductive barrier.
Another possible mechanism for sympatric speciation is disruptive selection, which takes place when a species has a trait that is manifested in two very different ways, such as two different coat colors. In this case, natural selection operating in a highly partitioned environment (dark versus light background, for instance) may favor one expression of the trait in one particular portion of the habitat and the other expression of the trait in a different portion of the habitat. Selection may thus compound the differences in the trait's expression and in this way result in differentiation.
Polyploidy in plants is also an example of how quickly speciation can take place, even in a single generation. Usually, however, speciation takes longer. Just how long is dependent on many variables, such as the generation time of the organisms involved, as well as factors of chance. There are two predominant schools of thought regarding the speed of speciation. "Gradualists," on the one hand, believe groups accumulate differences slowly over hundreds of thousands or millions of years. "Punctuationalists," on the other hand, believe that speciation takes place comparatively rapidly, over thousands of years, and little change occurs between these rapid bursts of differentiation.
Ann E. Kessen and Robert M. Zink
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Otte, Daniel, and John A. Endler, eds. Speciation and Its Consequences. Sunderland, MA: Sinauer Associates, 1989.
Quammen, David. The Song of the Dodo: Island Biogeography in an Age of Extinctions. New York: Simon & Schuster, 1996.