Following is a link to the paper itself, an extract from the paper discussing sympatric speciation, and a part on heterosis that we took out of the final text since one of the reviewers found it problematic. We did not have the courage or time to rewrite the part on heterosis, but there might be some value in sharing it, as indicated by Ben Haller’s comments below. Many thanks to Ben for allowing us to share his feedback.
Figure 2. A qualitative representation of Bergman’s rule on the unconstrained adaptive landscape (individual fitness peaks shift under the influence of environmental change).
Fitness of hybrids: Ηeterosis and outbreeding depression [this part was excerpted from the paper’s published version]
Heterosis and outbreeding depression describe crossbreeding events where hybrid fitness is, respectively, enhanced or depressed in relation to their parental forms. In populations that have had enough time to adapt to their environments, outbreeding depression is the most usual of both cases since any change would shift them away from their local optima (Nosil et al. 2005). Heterosis can be expected in the rare event that an intermediate fitness peak occupies the space between parental populations (Mallet 2007).
The fitness of hybrids is commonly measured in their parental environments by reciprocal transplant and phenotypic manipulations techniques (Edmans 1999; Rhode and Cruzan 2005). These methods, albeit indispensible on the practical level, have been criticized as having a critical blind spot. Hendry (2015) has noted that reciprocal transplants normally test hybrids and backcrosses in the parental environments, but those same hybrids and backcrosses would presumably have high fitness in intermediate environments. Thus, the experiments described above can’t reveal the presence of a fitness valley unless they also confirm the absence of intermediate environments. A similar line of argument is taken by Rundle and Whitlock (2001) who have highlighted the importance of determining the contribution of genetic and ecological mechanisms to hybrid fitness before any inferences concerning fitness valleys or speciation are to be made.
The following model presents a hybrid transplant experiment that could have leaded us to infer an adaptive valley for intermediate forms in what is actually an adaptive ridge. Consider the continuous population of Figure 3, where two parental forms (A and B) produce a hybrid form (H) which is assumed to be phenotypically intermediate. If we test the hybrid’s fitness in either of the parental environments, the outcome would be a significant fitness depression (fitness=0 in the graph) leading us to infer a fitness valley. However, in the intermediate environment the hybrid’s fitness would be equal or higher than the parental forms, indicating the presence of a continuous adaptive ridge (Rieseberg et al. 2003).
In vivo, the convergence of parents and the upbringing of their offspring in an intermediate environment provide a plausible physical mechanism for heterosis. Evidently, the shape of the eco-phenotypic landscape will play a defining role: A horseshoe of the likeness of Figure 5 is more likely to induce outbreeding depression than heterosis for intermediate forms. Overall, the eco-adaptive landscape may prove useful both for the conceptualization of naturally occurring hybridization processes and identification of the analytical scope of hybrid fitness experimental designs.
Benkman, C. W. 2003. Divergent selection drives the adaptive radiation of crossbills. Evolution 57: 1176–1181.