Figure 5 Proportion of wild-caught tadpoles of each species expressing the carni

Question

Figure 5  Proportion of wild-caught tadpoles of each species expressing the carnivore morphology (height of bar) in nine different natural ponds: five high-elevation ponds where S. multiplicata was the only Spea species present, and four low-elevation ponds where both S. multiplicata and S. bombifrons were present; there were no S. multiplicata carnivores in ponds AP and SH. Number above each bar is the number of tadpoles of each species sampled.

Question:See Figure 5 above showing the frequency of the carnivore morph in the New Mexico spadefoot toad, S. multiplicata, and the Plains spadefoot toad, S. bombifrons, in both the high- and the low-elevation ponds in southeastern Arizona. What evidence supports character displacement operating on S. multiplicata?

9 25 24 S. multiplicata S. bombifrons 6 0.75 18 49 181 52 0.25 15 19 HS EM MB TR FE AP FT SD SH Pure-species ponds Mixed-species pondsH Pond

Explanation / Answer

Two species of spadefoot toads, Spea multiplicata and S. bombifrons, have undergone ecological character displacement and reproductive character displacement in south-eastern Arizona and south-western New Mexico, USA. In this region of north-south trending valleys and mountain ranges, both species are generally present in the valleys (such sites are designated as ‘sympatry’). However, only S. multiplicata breeds in ponds above 1350 m, and only S. bombifrons breeds in the lowest elevation playas (temporary ponds that form in the lowest areas of valleys; the latter two sites are designated as ‘allopatry’; Pfennig et al., 2006).

The two species have undergone ecological character displacement in tadpole trophic phenotype (Pfennig & Murphy, 2000, 2002, 2003; Pfennig et al., 2006, 2007; Rice et al., 2009). In allopatric ponds, both species produce similar, intermediate frequencies of two trophic phenotypes: an omnivore morph, which feeds mostly on organic detritus on the pond bottom; and a large-headed carnivore morph, which specializes on anostracan fairy shrimp in the water column (Pomeroy, 1981; Pfennig, 1990, 1992). However, in nearby sympatric ponds, S. multiplicata produces almost entirely omnivores, and S. bombifrons produces almost entirely carnivores (Pfennig & Murphy, 2000, 2002, 2003; Pfenniget al., 2006, 2007). Allopatric and sympatric S. multiplicata differ in tadpole morph production even when reared under common conditions (Pfennig & Murphy, 2002).

These canalized shifts in morph production likely evolved because of selection acting to lessen interspecific resource competition; i.e. they likely reflect ecological character displacement (Pfennig & Murphy, 2000, 2002, 2003; Pfennig et al., 2007; Rice et al., 2009). In particular, because S. bombifrons is the superior competitor for fairy shrimp (the resource on which the carnivore morph specializes), sympatric S. multiplicata is under strong selection to produce only omnivores (Pfenniget al., 2007). By contrast, in allopatry, disruptive selection favours production of both morphs as a means of lessening intraspecific competition for food (Pfennig et al., 2007; Martin & Pfennig, 2009).

Sympatric and allopatric S. multiplicata may experience ecologically dependent post-mating isolation. Offspring produced by matings between individuals from different selective environments (i.e. sympatry vs. allopatry; hereafter ‘between-selective-environment offspring’ or BSE offspring) grow less well than offspring produced by matings of individuals from the same selective environment (hereafter ‘within-selective-environment offspring’ or WSE offspring), in part because BSE offspring are competitively inferior to WSE offspring in both the allopatric and sympatric selective environments (Pfennig & Rice, 2007). Thus, sympatric and allopatric populations of S. multiplicata may be reproductively isolated from one another as an indirect by-product of ecological character displacement with S. bombifrons.

Spea multiplicata and S. bombifrons have also undergone reproductive character displacement. In particular, female S. multiplicata from allopatry prefer males with faster call rates in contrast to females in sympatry, which prefer males with slower calls (Pfennig, 2000, 2008; Pfennig & Pfennig, 2005). Presumably, female S. multiplicata from sympatry prefer slow-calling males as mates to avoid costly hybridization with fast-calling S. bombifrons males (Simovich et al., 1991; Pfennig & Simovich, 2002). Because S. multiplicata males from sympatry produce slower calls on average than S. multiplicata males in allopatry (Pierce, 1976; Pfennig, 2000), this difference in female mate preferences is likely to result in assortative mating within each selective environment.

As noted earlier, reduced gene flow between selective environments is best detected when overall levels of gene flow are intermediate, and S. multiplicata in the south-western USA appear to meet this requirement. On the one hand, gene flow between populations is likely not high. Adult Spea spend much of the year underground, emerging only during the summer rainy season to breed and feed (Bragg, 1944, 1945). Opportunities for dispersal are thus rare. Indeed, previous work has shown that both species exhibit significant population genetic structure (Rice, 2008; Rice & Pfennig, 2008). On the other hand, gene flow is likely not negligible. Because there are no physical barriers to gene flow between populations, and because running water from heavy rains can sweep S. multiplicata from one population to another (A. Rice, pers. obs.), some gene flow between populations is possible.

Thus, S. multiplicata has undergone both ecological and reproductive character displacement with S. bombifrons. Moreover, ecological character displacement appears to have resulted in the evolution of post-mating reproductive isolation between sympatric and allopatric populations of S. multiplicata(Pfennig & Rice, 2007). Finally, levels of gene flow between populations of S. multiplicata are likely intermediate. We therefore predicted that gene flow (measured with neutral markers) should be reduced between populations in these two distinct selective environments.

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