Speciation

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Speciation

Speciation can be defined, in a general way, as the various processes by which new species arise. Speciation mechanisms can be categorized in several ways. Some species arise by the divergence of two or more new species from a single common ancestral species (divergent speciation), while others arise from hybridization events involving two parental species (hybrid speciation). 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.

Criteria for Recognizing Species

Because two major criteria for the recognition of species are in wide use in biology, any discussion of speciation processes should refer to the criteria under which species are recognized. Criterion 1 is known as the phylo-genetic species concept. Criterion 2 is known as the biological species concept. Under either criterion (genetically distinct groups or reproductively isolated groups), species exist as one or more local populations . Most mating events involve individuals from just one population, and if local populations are small, all of the individuals within each one are closely related. However, there is often some degree of migration of individuals between populations by processes such as seed dispersal. Thus, the various local populations of a species may be loosely connected—by occasional interbreeding among them—into a larger population system, or reproductive community. Regardless of whether species are regarded as fully differentiated population systems (criterion 1), or population systems between which there is a genetically based barrier to reproduction (criterion 2), speciation processes involve the generation of two or more population systems from a common ancestor (divergent speciation) or the generation of a population system following one or more mating events involving individuals of different species (hybrid speciation). Because the two major species concepts recognize species on the basis of different criteria, a speciation event may be regarded as being complete when evaluated under criterion 1 while it is still in progress when evaluated under criterion 2.

Allopatric Speciation

Divergent speciation events can be categorized as those in which the populations that are separating into different species are geographically separated from each other (allopatric speciation) and those in which they are in close proximity (sympatric speciation). An allopatric speciation event begins with a single species that has allopatric (other land) populations, that is, populations that are geographically separated from each other and therefore in little or no contact with each other. This situation may arise when a long-distance dispersal event results in the founding of a new population that is distant from other populations of the species. An example of this would be an allopatric distribution of a species on two islands following the dispersal of one or more airborne seeds of a species from one island to the other. Allopatric distributions also arise when a widespread population system becomes fragmented into two or more allopatric systems by the formation of barriers to dispersal or by changes in climate. The rise of the Rocky Mountains and the corresponding formation of deserts and prairies in the North American interior have created unsuitable habitats for many plant species of temperate forest environments. These species, which at one time had continuous distribution ranges across the continent, became restricted to the forests of eastern and western North America. Once a species comes to have an allopatric distribution, the separate populations begin to evolve independently, and eventually they may become differentiated from each other. This differentiation may have an adaptive basis, as the populations evolve in response to different environmental conditions. Alternatively, the differentiation may be nonadaptive and simply reflect the random origins (by mutation) and genetic fixation of new characteristics that have little or 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 long-distance 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 (possibly just one individual). A population of this sort is described as having 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.

As differentiation of allopatric populations proceeds, the point is eventually reached at which two species are recognized. Under the phylogenetic species criterion (criterion 1), speciation can be regarded as having been completed as soon as 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.

Reproductive Isolating Barriers

In the example just presented, the two populations still may not be recognized as separate species under the biological species criterion (criterion 2) even after complete differentiation in one or more characteristics has occurred. Because criterion 2 involves the presence of a genetically determined reproductive isolating barrier (RIB), genetic differentiation in any number of characters, if maintained only by the geographic isolation of the populations, generally is regarded as an insufficient basis for the recognition of the two populations as separate species. Therefore, application of criterion 2 involves the assertion that genetically distinct and allopatric population systems may belong to the same species, and what is recognized as a speciation event under criterion 1 may be regarded as only the initial stages of a speciation event under criterion 2. However, if the two distinct populations later occur in sympatry—for example, following disperal of one of them into the range of the other—a test of sympatry occurs, and the presence or absence of RIBs can be evaluated. If these exist, criterion 2 is satisfied.

RIBs can be categorized in several ways. One useful distinction is between those that operate prior to fertilization (pre-fertilization RIBs) and those that operate afterward (post-fertilization RIBs). 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 a pre-fertilization RIB 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, cross-pollination and cross-fertilization will not occur. There are many cases 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. In a natural setting, the two species are genetically isolated by a pre-fertilization RIB, and thus are recognized as separate species under criterion 2 as well as under criterion 1. Another form of pre-fertilization RIB is temporal isolation (isolation by time). In this case, two closely related plant species may flower at different times of the year or day and cross-pollination therefore does not occur. However, under controlled environments, with appropriate day-lengths and temperatures, plants of two different species may be induced to flower at the same and cross-pollination may occur.

Hybridization

An example of a post-fertilization RIB is hybrid inviability. In this case, interspecific hybridization may occur under natural settings, but the off-spring of such crosses may die soon after seed germination. Another example of a post-fertilization RIB is hybrid sterility. In this case, the hybrid individuals may be viable yet they 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 under criteria 1 and 2. However, pollen and ovules are wasted by both species. This is a particular problem when two species occur in close proximity to each other and are isolated only by a post-fertilization RIB. In such cases, each of the two species is wasting some of its pollen and seeds and the reproductive potential of both species therefore is lessened. Furthermore, hybrid individuals, even if they are sterile, may compete with one or both of the parental species for habitat and pollinators. In such cases there will be selection against those individuals that cross with individuals of the other species, and any mutation that arises in one of the parental species and contributes to a pre-fertilization RIB is likely to become established in addition to the post-fertilization RIBs that already exist. In this manner, reproductive isolation can be reinforced between two species that are in geographic proximity yet are able to hybridize.

Hybrid speciation events must, of course, involve species that occur in sympatry. Although it may seem contradictory to speak of hybridization between species when (at least under criterion 2) species are reproductively isolated population systems, it is often the case that reproductive isolation is strong but not absolute, and in such cases viable, fertile hybrids may occasionally arise between sympatric species. In homoploid hybrid speciation events, hybridization occurs between two species and thereby generates plants with a combination of characteristics that does not occur in either of the parental species. The hybrids may be better adapted than either parental species to a particular habitat, and a new and successful lineage therefore may be initiated and may spread into a habitat that is unoccupied by the parental species. Eventually, if RIBs develop, a new species can be recognized under both criteria 1 and 2. At least three cases of homoploid hybrid speciation have been documented in native sunflowers of North America.

Polyploid hybrid speciation is extremely common in many plant groups, notably in ferns, the grass family (Poaceae), and the sunflower family (Asteraceae or Compositae). Like homoploid hybrid speciation, a polyploid speciation event is initiated by a hybridization event. However, reproductive isolation between the new species and both of its parents is usually established immediately, in the form of hybrid sterility, a post-fertilization RIB. Polyploid species usually have two complete sets of chromosomes from each parent, and any hybrids that are formed between the new species and either of the parental forms are likely to experience irregular meiosis and thus to be sterile.

see also Evolution of Plants; Hybrids and Hybridization; Polyploidy; Species.

Jerrold I. Davis