Local Speciation in Plants: The Rule Not the Exception

Abstract

The concept that may arise through the gradual divergence of races has been and remains widely accepted by plant evolutionists. Evidence in its favor is circumstantial. It is unlikely that evolve through the gradual transformation of races, because there is no simple mechanism that can produce the collective transformation of a race and because some aspects of speciation involve stochastic processes. It is much more likely that evolve in local populations or metapopulations, mechanism of speciation notwithstanding. Dispersal and colonization from the locus of speciation bind populations of the neospecies through common descent. All populations share the unique features that distinguish the neospecies from its progenitor. There is no ecological or genetic relationship among or patterns within that cannot be explained by the theory of local speciation. Speciation occurs within a geographical context. A geographically defined group of individuals diverges from another or others and accumulates, through selective and stochastic processes, the numerous attributes (genetic and phenotypic) that distinguish it as a new species. This geographically defined group may be a single population, a collection of local populations or a widely distributed population system. The group may be geographically isolated from its relatives or sympatric, but ecologically divergent. The geographical race and the local population are thought to be the prime units of speciation in plants (Clausen 1951; Grant 1981; Stebbins 1950). The unit of speciation usually is inferred from the character of speciation. When speciation involves adaptive divergence, geographical races are considered to be the unit of speciation. The evolution of from geographical races is referred to as geographical speciation. When chromosomal evolution or other forms of change involving stochastic processes are involved in the early stages of divergence, local populations are thought to be the unit of speciation. The divergence of geographical races is thought to be the primary form of plant speciation. The paradigm of geographical speciation in plants was challenged by Raven (1980, 1986). He argued that gene flow was too restricted to permit geographical speciation; speciation must be a local process. He concluded that No particular pattern in nature can logically be regarded as the precursor of any other; races, subspecies and semispecies cannot be regarded as stages in the evolution of species (Raven 1980). The unit of speciation in animals also is a matter of debate. Many animal evolutionists (e.g., Carson 1987; Mayr 1982a, 1982b; Nevo 1989; Paterson 1985; Templeton 1981) and paleontologists (e.g., Eldredge and Gould 1972; Stanley 1979; Vrba 1980) contend that speciation occurs in isolated, local populations. They argue that some changes associated with speciation are most readily achieved by stochastic processes, and that the divergence of races is a form of phyletic gradualism, the efficacy of which is suspect with regard to speciation. Approaching the issue from another vantage point, Lynch (1989) investigated speciation models using phylogenetic evidence and distributional data. He inferred that most speciation events involved the divergence of major, geographically isolated population systems. The preponderance of the vicariant model of speciation is in agreement with findings of other workers (Cracraft 1982; Wiley and Mayden 1985). The manner in which and speciation are studied and phylogenies and biogeographies interpreted depend in part on the geographical unit of speciation. Thus it is important to evaluate the efficacy of geographical speciation and local speciation. In the following presentation, I will develop the case for local speciation in plants. First I will discuss factors, in addition to restricted gene flow, which reduce the likelihood of effective diffusion of evolutionary novelty across large population systems and thus mitigate against speciation through racial divergence. Then I will present evidence in favor of local speciation and discuss the character of the forthcoming species. Finally, I argue that local speciation is most likely to occur with-