Lecture © Nik Tatarnic
BIOL 606 Session, University of Alberta, January 31, 2001
In 1979, Levin referred to species as "tools that are fashioned for characterizing organic diversity." However, if one is to consider the process of speciation, one must acknowledge that species are real and for the most part can be defined. Whether or not a speciation event is said to have occurred will depend on what species concept one uses. Most evolutionists, when discussing sexually reproducing organisms, adopt the Biological Species Concept (BSC), which defines species as "groups of potentially interbreeding natural populations which are reproductively isolated from other such groups" (Mayr 1963).
Under the BSC, speciation is considered complete once reproductive isolation halts gene flow. Thus, to study the mechanisms causing speciation one must concentrate on whatever forces are acting up until that moment. In practice, it is likely that several isolating mechanisms are active at the same time, and it is often difficult to determine which of these is most important (Coyne and Orr 1998). Coyne and Orr (1997, 1998) suggest that prezygotic isolation tends to evolve at a much quicker pace than postzygotic isolation when populations are sympatric, whereas in allopatry these evolutionary rates are more or less the same. This suggests the possibility of direct selection for sexual isolation in sympatry.
Recent studies have shown that one can observe and measure rapid evolutionary changes which may be instrumental in reproductive isolation. These include examples of rapid ecological character displacement in Darwin's finches (Grant and Grant 1989) and stickleback fish (Schluter and McPhail 1994), as well as field experiments demonstrating sexual selection against hybrids in the wild (Vamosi & Schluter 1999) and environment-dependent hybrid fitness (Hatfield & Schluter 1999).
The studies above show that rapid evolution of adaptive traits can occur in populations exposed to divergent habitats; however there are few empirical examples of reproductive isolation itself evolving equally rapidly. Based on data collected from introduced salmon, Hendry et al. (2000) provide evidence of reproductive isolation in only 13-14 generations, or 60-70 years. Hendry's team studied salmon introduced into Lake Washington, in Washington State, between 1937 and 1945. These initially formed a single large breeding colony in Cedar River, a large tributary to Lake Washington. In 1957, a second population, presumably derived from this initial population, was discovered spawning along Lake Washington Beach. Sockeye salmon are known to form distinct "ecotypes," in this case river breeders and beach breeders. Hendry's group found that almost 39 percent of all beach breeders were immigrants from the river, yet the populations remained genetically distinct. Indeed, gene flow from the river to the beach was found to be quite low, suggesting some mechanism preventing successful interbreeding, such as low mating success of immigrants or selection against hybrids.
It should be noted that the original hatchery population used for the introductions was made of beach and river ecotypes; therefore, current ecotype differences and reproductive isolation may in fact reflect a much longer history of divergence (Gustafson et al. 2000). Additionally, small sample sizes weaken arguments of morphological divergence, estimates of gene flow, and the natal origin of immigrants. Until these problems are addressed, conclusions regarding rapid reproductive isolation in this system cannot be drawn.
Coyne, J.A. and H.A. Orr. 1997 Patterns of speciation in Drosophila revisited. Evolution 51:295-303
Grant, R. and P. Grant, 1989. Evolutionary Dynamics of a Natural Population. University of Chicago Press, Chicago.
Gustafson, R.G., R. Waples, S.T. Kalinowski and G.A. Winans. 2001. Evolution of Sockeye Salmon Ecotypes. Science 291: 251-252b.
Hatfield, T. and D. Schluter. 1999. Ecological speciation in sticklebacks: environment-dependent hybrid fitness. Evolution 53: 886-873.
Hendry, A., J.K. Wenburg, P. Bentzen, E.C. Volk and T.P. Quinn. 2000. Rapid Evolution of Reproductive Isolation in the Wild: Evidence from Introduced Salmon. Science 290:516-518.
Levin, D.A. 1979. The nature of plant species. Science 204: 381-384.
Mayr, E. 1963. Animal Species and Evolution. Belknap Press, New York.
Schluter, D and J.D. McPhail. 1992. Ecological character displacement and speciation in sticklebacks. The American Naturalist 140(1):85-108.
Vamosi, S.M. and D. Schluter. 1999. Sexual selection against hybrids between sympatric stickleback species: Evidence from a field experiment. Evolution 53: 874-879.