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.
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