Return to class home page

[Previous] [Next]

Speciation: controversy surrounding the origin and evolution of insects.

Session leader: John Kaltenbach.

As we all know the extent of insect diversity is astounding. What is it about insects that has lead to such an extraordinary number of species? Probably the most important factor is their relatively high degree of resource specialization, but also to be considered are behavioral differences in habitat preference and mate finding (Bush and Smith, 1997). The most contentious debate surrounding the origin and evolution of insects is the mechanism of speciation. Speciation is "more thoroughly awash in unfounded and often contradictory speculation than any other single topic in evolutionary theory" (Futuyma, 1983). Central to this debate is the controversy over the possibility that sympatric speciation, as opposed to the well documented allopatric speciation (Crozier and Pamillo, 1996), may have been quite common (Bernays and Chapman, 1994).

There have been many patterns of speciation proposed including allopatric, sympatric, peripatric, parapatric, and saltational or centrifugal speciation (Bush, 1975). However, Bush (1975) suggests that they fall into three broad categories, allopatric, parapatric and sympatric. This is not without controversy for it is always possible to invent a scenario involving allopatric isolation to explain speciation with a lack of geographic barriers (Godfray, 1994).

Allopatric Speciaton

Simply defined, allopatric speciation can occur in two different ways. First, a widely distributed species becomes subdivided into two or more isolated populations. The second, and probably more common way, occurs through the establishment of a new colony by a small number of founders (Carson, 1989). In both, . the resulting populations are reproductively isolated, and over time genetic changes accumulate, which lead to separate species. Most of the work has revolved around this theory which some maintain is central to specaiation (Carson, 1989).

Parapatric speciation

This occurs "whenever species evolve as contiguous populations in a continuous cline" (Bush, 1975). This is different from allopatric speciation in three ways (1) the is no spatial isolation required, (2) the mobility of the species is often quite low and (3) the reproductive isolating mechanisms arise by selection with the change in new habitat (pre and postmating reproductive isolation occurs concurrently with the movement) (Bush, 1975). This can be thought of as an edge-effect, where new populations arise on the edge of the range of the ancestral species.

Sympatric speciation

This is, most simply, the evolution of a new species within the dispersal area of its ancestor (Bush, 1975). It is distinguished from parapatric speciation in several ways, but the most important is that premating reproductive isolation occurs before the population moves to a new niche (Bush, 1975). Sympatric speciation occurs if gene flow is in some way prevented between populations of the same species within the same geographic locality .

Two views

Evolutionist are divided by these two views on the origin of species, "one emphasizing that populations can diverge in sympatry as a direct outcome of selection during the course of adaptation on different host plants or in different habitats, an the other maintaining that divergence can occur only during periods of geographic isolation" (Bush and Smith, 1997). Bush and Smith (1997) suggest that the reluctance to acceptance the possibility of sympatric speciation is a result of work focused primarily on either large, conspicuous vertebrates or showy, easily collected or reared invertebrates. In these species, preferences for habitat or host choice is not usually involved with the location of mates, as is the case with parasites, including specialist herbivores (Bush and Smith, 1997). Still, others hold rigidly to the allopatric speciation model as exemplified by Carson (1989) "The existence of multiple morphs in a natural population is more a reflection of elaboration of balanced polymorphism within a population rather than speciation, a process that requires separation and independent evolution of populations".

Evidence of sympatric speciation

Probably the most well documented example of sympatric speciation is in the Apple maggot fly, Rhagoletis pomonella, which is thought to have different host races which specialize on alternate hosts (Bush, 1975). In addition to the papers to be discussed, recent evidence has been presented of a sympatric speciation in the Imported Fire Ant, Solenopsis invicta (Shoemaker et. al, 1996).

We will discuss the evidence for sympatric speciation, as well as the mechanisms proposed for its occurrence, including host-shifting and the formation of different host-races, which will most assuredly add to the controversy and confusion. Population regulation, or selection pressure by natural enemies will also be an issue. However, "there is no consensus about the importance of interspecific competition for parasitoid population and community dynamics, let alone macroevolutionary dynamics." (Godfray, 1994).

Required papers

Full references list

Copyright 1998, John Kaltenbach.