It is a general rule ofthumb ofevolutionary biology that it is far easier to describe the maintenance of genetic variation than to describe its generation. This is particularly true for understanding the evolution of clinal variation. On the one hand, clines may represent the consequence of a recent collision of formally separated populations that evolved in allopatry. These situations, termed secondary introgression are thought to dominate systems with steep clines (Figure 1b) as when two species hybridize within a tension zone.
On the other hand, it is known that clines can be generated by spatially variable selection acting on a set of genetically identical subpopulations. One example of this comes from the Drosophila invader into North America (see Figure 1a), in which a cline was generated within two decades of the invasion. These situations reflect primary contact clines. The main problem for evolutionary ecologists is that it can be difficult to know whether a cline is the consequence of primary contact or secondary introgression. Recent authors indicate that cline theory can be utilized in either case, but these theoretical expectations provide little insight into the generation of the cline itself.
The mechanism of clinal generation is important to speciation, or the generation of species. Traditionally, natural historians believed that speciation across clines was commonplace, but today, there is far more skepticism largely because the evidence for such speciation is weak and elusive. The generation of 'good' species within a cline is called parapatric speciation and will proceed through the following steps. First, the environment generates strong local genetic differences among subpopulations in multiple traits and is reflected in a series of a steep clines. If such differences persisted, then hybrids of subpopulations from the endpoints of the cline will become less fit, a situation called a tension zone. The exact mechanism by which hybrids are less fertile or viable can be intrinsic (e.g., genetic incompatibilities) or extrinsic (e.g., less competitive in the environment). As a consequence of the fitness cost of hybridization, there will be strong selection for prezygotic traits that minimize cross-breeding and thereby lead to the generation of 'good' species, a process referred to as reinforcement.
However, to date, several of the predictions of para-patric speciation are poorly supported. There is growing evidence from molecular and other lines of evidence that most hybrid zones are not primary contact zones, but are instead the consequence of secondary introgression of formerly allopatric populations. Further, prezygotic isolation does not appear to be the primary force maintaining most hybrid zones.
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