mating signals can diverge as the result of lowered hybrid viability or fertility. Such a process is called reinforcement (Butlin 1989) and can eventually lead to complete reproductive isolation, or speciation.
As in the case above, in areas where species overlap, mate-recognition
signals must be sufficiently different to prevent incorrect matings between
heterospecific individuals. As a result, selection may favor divergence
among the signals of different species where they occur in sympatry. The
term “reproductive character displacement” describes the outcome of such
a process (Butlin 1989), where traits used in mate recognition differ more
among sympatric than among allopatric populations because of the divergence of these characters in response to selection to reduce the probability of heterospecific matings. Reproductive character displacement has
proved to be difficult to demonstrate empirically.
The strongest evidence to date for reproductive character displacement
comes from examples of call divergence within sympatric populations
of related species of anurans. The tree frogs Litoria (Hyla) ewingi and L.
verrauxi can be found in largely disparate areas of southern Australia but
do overlap in some regions. In allopatry, the advertisement calls of the two
species are very similar. In sympatric populations, however, the calls of L.
ewingi and L. verrauxi are quite different. In these populations, the pulse
repetition rate and number of pulses per note in the calls of L. verrauxi
are shifted away from the values for L. ewingi (Littlejohn 1965). There is
evidence that these call characters are important in mate discrimination—
females of these sympatric populations show strong preferences for pulse
rates typical of conspecifics over pulse rates similar to those of heterospecifics (Loftus-Hills and Littlejohn 1971). Thus, the divergence of these
call characters could result in a reduced number of hybrid matings.
5.1.3. Neuroethological Mechanisms of Species Recognition
The importance of communication in speciation not only involved
behavior in evolutionary theory but also implicated neuroethology. The
behavioral preferences so crucial to species recognition emerge from an
interaction of stimulus variation and neural and cognitive processing.
Understanding how auditory systems decode and process species-specific
signals focuses the process of speciation on the nervous system. Studies on
crickets (e.g., Huber 1990), frogs (Capranica 1972), and birds (Margoliash
1983) have identified features of the auditory system that bias behavioral
responses toward the species’ own signal. Although the neuroethologists’
emphasis has been on the functional significance of these properties, they
are the underlying substrates to the behavior that must evolve if speciation
is to occur. Some neuroethological investigations, such as studies of neuralpattern generators that could potentially link properties of the signal with
signal recognition (Hoy et al. 1977), were motivated by evolutionary as well
as neurobiological issues (Doherty and Hoy 1985).
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M.J. Ryan and N.M. Kime
As in the case above, in areas where species overlap, mate-recognition
signals must be sufficiently different to prevent incorrect matings between
heterospecific individuals. As a result, selection may favor divergence
among the signals of different species where they occur in sympatry. The
term “reproductive character displacement” describes the outcome of such
a process (Butlin 1989), where traits used in mate recognition differ more
among sympatric than among allopatric populations because of the divergence of these characters in response to selection to reduce the probability of heterospecific matings. Reproductive character displacement has
proved to be difficult to demonstrate empirically.
The strongest evidence to date for reproductive character displacement
comes from examples of call divergence within sympatric populations
of related species of anurans. The tree frogs Litoria (Hyla) ewingi and L.
verrauxi can be found in largely disparate areas of southern Australia but
do overlap in some regions. In allopatry, the advertisement calls of the two
species are very similar. In sympatric populations, however, the calls of L.
ewingi and L. verrauxi are quite different. In these populations, the pulse
repetition rate and number of pulses per note in the calls of L. verrauxi
are shifted away from the values for L. ewingi (Littlejohn 1965). There is
evidence that these call characters are important in mate discrimination—
females of these sympatric populations show strong preferences for pulse
rates typical of conspecifics over pulse rates similar to those of heterospecifics (Loftus-Hills and Littlejohn 1971). Thus, the divergence of these
call characters could result in a reduced number of hybrid matings.
5.1.3. Neuroethological Mechanisms of Species Recognition
The importance of communication in speciation not only involved
behavior in evolutionary theory but also implicated neuroethology. The
behavioral preferences so crucial to species recognition emerge from an
interaction of stimulus variation and neural and cognitive processing.
Understanding how auditory systems decode and process species-specific
signals focuses the process of speciation on the nervous system. Studies on
crickets (e.g., Huber 1990), frogs (Capranica 1972), and birds (Margoliash
1983) have identified features of the auditory system that bias behavioral
responses toward the species’ own signal. Although the neuroethologists’
emphasis has been on the functional significance of these properties, they
are the underlying substrates to the behavior that must evolve if speciation
is to occur. Some neuroethological investigations, such as studies of neuralpattern generators that could potentially link properties of the signal with
signal recognition (Hoy et al. 1977), were motivated by evolutionary as well
as neurobiological issues (Doherty and Hoy 1985).
244
M.J. Ryan and N.M. Kime