Sympatric speciation: when is it possible?

Abstract

This paper is written to compare the results of theoretical investigations of sympatric speciation with the relevant experimental data. We understand sympatric speciation as a formation of species out of a population whose spatial structure is not important genetically. A necessary prerequisite for speriation is an action of disruptive selection on sufficiently polymorphic traits. The present analysis confirms the view that such a selection is ecologically realistic. The genetical part of speciation begins with a development of reproductive isolation between those individuals that are opposed in strme characters. It is shown that selection for reproductive isolation may be quite strong. Extinction of intermediate individuals, which completes speciation, proceeds under a wide range of ronditions, including those when the newly formed species differ in quantitative characters, though most of the genes arc likely to remain the same in both species. The whole process seems possible if differences i n srveral (up to 10) loci are sufficient to adapt the forming species to different niches and to establish reproductive isolation. It is shown that populations with bimodal distributions of some genetically determined quantitative characters can have a considerable life-time. Such distributions may he formed either as a transition stage of sympatric speciation or represent a stationary state under conditions close to those necessary to complete speciation. They are very important Tor experimental investigations. Sympatric speciation always follows the same principal course; it does not contradict the idea of a genome coadaptedness. The occurrence of sympatric speciation is different for different taxa depending rather on how frequently populations are subjected to the appropriate kind of selection than on their ability to obey it.

What, if anything, is sympatric speciation?

“…the evolution of a barrier to gene flow) among the members of an interbreeding population'. Kondrashov & Mina (1986): Sympatric speciation is the 'formation of species out of a population whose spatial structure is not important genetically'. 'We shall call speciation 'sympatric' if in its course the probability of mating between two individuals depends on their genotypes only'.…”

Behavioural correlates of genetic divergence due to host specialization in the pea aphid, Acyrthosiphon pisum

“…Knowledge of the genetic basis of the complex trait 'host use' would both permit us to understand how host use has evolved in the past and help us predict future evolutionary events related to host specificity. Debates over host shifts and sympatric speciation in herbivorous insects arise directly from differing assumptions concerning the genetic basis of host plant preference (Diehl & Bush, 1989;Kondrashov & Mina, 1985). Currently, these debates rage on in the absence of data on the genetic architecture of host specialization.…”

Sympatric speciation under incompatibility selection

“…9 appears to be an artifact (32). Indeed, if the resource is distributed continuously, substantial flattening of the population distribution can reduce competition for intermediate phenotypes and lead to stabilizing, instead of disruptive, selection (5). One-dimensional selection may also favor speciation under mutational pressure (35), but the importance of this effect remains unknown.…”

“…Allopatric speciation occurs because populations evolving in geographic isolation accumulate Dobzhansky-Muller incompatibilities, i.e., independently acquire new beneficial alleles that reduce fitness when combined in the same genotype (1,2). In contrast, the available theory of sympatric speciation by natural selection assumes that this process is driven by disruptive selection, which favors both of the opposite extreme values of one trait (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11).…”

“…Indeed, to cause sympatric speciation, natural selection must (i) protect genetic variability within the population and (ii) tear the population apart by eliminating intermediate phenotypes, such as those of hybrids between the incipient species (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11). These conditions can be met either by frequency-dependent disruptive selection acting on one trait or frequency-dependent incompatibility selection, acting on two traits and involving a fitness tradeoff between them (12)(13)(14)(15)(16)(17)(18)(19).…”