ORIGIN OF SYMPATRIC SPECIES Species which live together in the same area at the same time, i.e., that are sympatric and synchronic, tend to be clearly differentiated in morphology, ecology and reproductive behavior. So common are congeneric species (closely related species which are included in the same genus) which are always sympatric that some biologists have argued that they must have evolved while part of the same species population (sympatric speciation). How they might have diverged genetically to the point of being reproductively isolated without being geographically isolated poses a severe problem for these theorists. An alternative view of the origin of sympatric species relies on the theory of allopatric speciation wherein a daughter species evolved from a parental species as an allopatric population and secondarily came into contact with the parental species through migration into its home range. Integrades between subspecies where their ranges overlap suggests that secondary contact after isolation is rather common. In this instance a peripheral isolate might have differentiated genetically (evolved) but not acquired a reproductive isolating mechanism. Consequently, it interbred with other populations after differentiating and reinvading the range of central populations through migration. Only if the peripheral population had developed a RIM while in isolation would the two species not produce integrades after range invasion by the new species. The acquisition of species status may prevent such sympatric species from interbreeding, but it will not prevent them from impacting on one another in an evolutionary relationship. Since the daughter species was once just a population of its parent species, the two species would be very similar. Any differences between them would have resulted from evolutionary processes acting on them while they were geographically isolated, but it is highly unlikely that these processes would have resulted in the usual level of differentiation observed among sympatric species. Further differentiation will result from the action of natural selection acting against individuals in both species which are very similar. Due to individual variation and the fact that these two species are closely related, we would expect considerable overlap in morphological, ecological and reproductive characteristics. Those individuals in both species outside of the area of overlap would not interact and so would be favored by natural selection; those in the area of overlap would interact and be eliminated from each population by the process of natural selection. This interaction could be either through competition for the same resources or in attempted mating. We will discuss the evolutionary significance of interspecific competition later in the course so will confine our discussion here to the effect of reproductive interaction between these two congeneric, sympatric species. Should a premating reproductive isolating mechanism evolve when daughter and parent species are allopatric (as was explain before) mutual range invasion would not result in any reproductive interaction between members of the two species. If, however, a postmating RIM evolved during allopatry, the ecological and ethological distinctions which serve to isolate well-differentiated sympatric species reproductively would not exist. These characters would be subjected to considerable overlap between the two species. Frogs, for example, use species-specific mating calls to facilitate successful reproduction. Male calls show some individual variation and female response to such calls also would be expected to vary. Each species would have a range of variation in both call and response to the call. Individuals in the zone of overlap would on occasion make mistakes and breed with members of the wrong species. Such mistakes would be fatal to the fitness of genes responsible for these phenotypic traits because successful reproduction would be prevented by the postmating RIM. These overlapping individuals would waste their gametes (no offspring would be produced), thus only those genes in both males and females which correctly recognize their own species would be passed on to the next generation. Natural selection would directly weed out these overlapping traits and cause the evolution of a premating RIM to reinforce the postmating one which evolved in allopatry. Evidence for this process can be seen in the phenomenon of character displacement. Allopatric species of frogs may overlap broadly in call characteristics without any selective penalty because the geographic separation prevents mistakes in mating. When these same two species are found in sympatry, call characteristics are sharply differentiated with no overlap. This differentiation is called character displacement because the mating call characters (visible on a sonograph) have been displaced where the two species are sympatric from their overlapping appearance where they are allopatric. Such comparisons between allopatric and sympatric species are only patterns observed at the same time but in different locations. How they were produced by the process of natural selection is an interpretation of the pattern. The term character displacement is simply a description of the observed difference in pattern despite the suggestion of process indicated by the word "displacement." The effect of natural selection in displacing characters in congeneric, sympatric species results in two completely different species distinguishable from one another morphologically, reproductively and ecologically. Once this process is over, the stage is now set for each species to speciate and produce a family of closely related species each adapated to a different facet of the environment they share. This pattern is called adaptive radiation and is the means wherby species diversity is built in a given area.