is the process of evolution by which new species arise. The key factor causing speciation is the appearance of genetic differences between two populations, which result from evolution by natural selection. Some species arise by the divergence of two or more new populations from a single common ancestral species. Others arise from hybridization events involving two parental species. can happen when a single population is split in two by some physical event, such as an earthquake. It can happen through random genetic mutations that arise within a population. In all cases, certain individuals will survive to reproduce while others do not. These reproduction events eventually result in new species. Criteria for Recognizing Species In order to discuss speciation, scientists must first define a species. Unfortunately, defining a species is not easy. There are actually several definitions in wide use. The best-known is the biological species concept, under which a species is a group of individuals that can successfully interbreed under natural conditions. 1
There is also an ecological species concept, in which a species is a group of individuals that can theoretically interbreed and that share the same ecosystem or habitat, which allows that interbreeding to be a reality. Under the phylogenetic species concept, a species is the smallest group on a phylogenetic tree that contains all the descendants of a common ancestor. Allopatric and Sympatric Allopatric speciation occurs when two populations of a species are geographically separated from one another so that they can no longer have physical contact. For example, if a mountain range rises in the middle of a population of one species, it will divide the population in two. The two populations will then breed only with the individuals close to them, and natural selection will result in the two groups adapting independently to their different environments. After a period of time, the two groups may no longer be able to interbreed. They would then be separate species under the biological species concept. 2
Under the phylogenetic species concept, speciation is complete when there are one or more genetically determined differences between the two populations, such that all individuals of one population are distinct from all individuals of the other. For example, all individuals on one island may have teeth on the margins of their leaves, while all of those on another island may lack such teeth. An intermediate stage in this process could be recognized when a particular characteristic is present in only one of the populations (possibly having arisen as a new mutation in that population), but this characteristic does not occur in all individuals of that population. Not all genetic changes that become common in a population are adaptations. Sometimes genetic changes are simply the effects of random mutations that have no adaptive value. This is particularly likely to occur when one of the populations is very small, as is likely to be the case when a new population is founded by a longdistance dispersal event. Although the newly formed population may grow quickly, all individuals are descended from the small number of individuals that founded the new population. A population of this sort 3
has experienced a founder event, in which it goes through a genetic bottleneck and characteristics that were rare in the ancestral population but happened to occur among the founding individuals of the new population may occur in all individuals of the new population under these various circumstances. Sympatric speciation occurs when a population splits in two but both groups remain in the same geographic location. This can occur through genetic mutation or behavioral quirks that then result in actual genetic changes through natural selection. For example, a plant may produce flowers that attract a particular pollinator. If one day a chance genetic mutation produces a differently-shaped flower that attracts a different pollinator, natural selection can then operate to reproduce that genetic variant. The new flower cannot use the pollinator of its ancestral plant, but it can thrive and reproduce plants that use the new pollinator. Plants often undergo polyploid speciation events, in which a plant produces seeds with twice the usual number of chromosomes. The polyploid offspring may not be able to reproduce with their parent species, but they can reproduce with one another. That is a form of genetic separation that is not caused by geographic separation. 4
Reproductive Isolation Reproductive Isolation is one of the keys to the evolution of new species. Isolation can occur in a number of ways. Geographic separation is literal physical separation. Phenetic separation involves timing; a plant that opens its flowers in April cannot reproduce with one that opens its flowers in June even if they grow right next to each other. Reproductive isolation can happen both before and after fertilization. Fertilization is a critical point in the reproductive cycle of plants, because this is the point at which an ovule either begins to develop into a seed or is lost to the population. One example of pre-fertilization reproductive isolation is the establishment of a different floral structure so that pollinating insects do not place the pollen from individuals of one plant species on the stigmas of individuals of another species, even if they visit plants of different species in succession. If a visit to the flower of one species results in the placement of pollen on the bee's back, but the stigma of another species is touched only by the underside of a visiting bee, crosspollination and cross-fertilization will not occur. There are many cases 5
of differing floral structure in the orchid family (Orchidaceae), in which natural pollinators do not cross-pollinate two closely related species, even when individuals of the two species grow side by side, but hybrids are easily generated when human investigators transfer the pollen from one species to the stigma of the other. Hybridization When a hybrid speciation event occurs, the newly derived species may have the same chromosome number as its parents (homoploid hybrid speciation), or it may have a higher number (polyploid hybrid speciation). In the latter case, the chromosome number of the newly derived species is usually the sum of those of its parents. In fact, polyploid hybrid speciation is one of the most frequent speciation mechanisms in plants. Reproductive isolation can also happen postfertilization. Hybrid offspring may die. Hybrid individuals may be viable yet they are steril and fail to produce gametes, and therefore fail to reproduce. In this case, natural hybrids may be present and even abundant in natural settings, but the two parents of these hybrids are recognized as belonging to separate species. 6