Monday, February 2, 2015
Reproductive Character Displacement or Alternative Explanations?
*Guest post by Santiago J. Sánchez-Pacheco
Closely related animal species are often so similar that it is hard to distinguish them. This immediately leads to the question of how the individuals of such species, when in sympatry, can recognize their conspecifics. Usually, the species differ in traits (i.e., species recognition signals; e.g., visual and sound signals) that are detectable by sensory mechanisms. Less is known, however, about how these phenotypic differences evolve. A common view is that hybrids suffer reduced fitness or cannot be produced whatsoever, and therefore selection should favor individuals with traits that avoid interspecific matings. By diverging in such traits, females and males of closely related species are less likely to waste energy in failed matings. This widely accepted assumption is usually referred to as “reproductive character displacement” (Losos, 2013).
When Brown and Wilson (1956) described character displacement, they proposed the following process: populations of two closely related species, after first coming into contact with each other, interact “in such a way as to diverge further from one another where they occur together”. Such divergence minimizes the chances of both competition and hybridization between the species, and therefore favors coexistence over exclusion.
While it is generally accepted that natural selection is the force increasing the frequency of the divergent traits, whether or not the resulting divergence is driven by the interaction between the two species (e.g., competition) remains uncertain. If a pattern of differences is consistently detected between populations of two closely related species when they are compared in allopatry versus sympatry, it seems reasonable to attribute this pattern to the interaction of both species. However, a number of processes other than a response to interspecific interaction may result in a “displacement-like” pattern—substantial differences of the environments between allopatry and sympatry, phenotypic plasticity or even random processes can all trigger differentiation (Kamath, 2014).
Based on six criteria (Box 1) established by Schluter & McPhail (1992) as general indicators to rule out alternative processes that might lead to a displacement-like pattern, recently Stuart & Losos (2013) pointed out that only a small portion (9 of 144 cases) of recent studies claiming evidence for ecological character displacement can conclude with a high degree of certainty that the interspecific interaction led to the observed divergence. According to Stuart & Losos, falsification of only one of these six criteria is enough evidence to determine that such divergence did not result from character displacement. Consequently, their findings suggest that most documented cases of ecological character displacement are equally consistent with other evolutionary and ecological phenomena. Although these two studies focus only on ecological character displacement, it is worth noting that the same eco-evolutionary principles underlie reproductive character displacement, so that alternative processes could also explain phenotypic differentiation presumably derived from interspecific interaction.
Despite the concept of character displacement having remained in the evolutionary literature for decades, this assumption has seldom been subjected to critical scrutiny. Indeed, it was not until recently that significant progress in designing thorough studies to rigorously test this adaptive hypothesis was achieved (e.g., Stuart et al. ).
Box 1: Modified from Stuart and Losos (2013). The six criteria for Ecological Character Displacement (ECD).
Brown Jr., W. L. and E. O. Wilson. 1956. Character displacement. Systematic Zoology 5(2): 49–64.
Kamath, A. 2014. http://www.anoleannals.org/2014/10/25/rapid-evolution-in-anolis-carolinensis-following-the-invasion-of-anolis-sagrei/
Losos, J. 2013. http://www.anoleannals.org/2013/08/12/reproductive-character-displacement-and-dewlap-color-in-haitian-anoles/
Schluter, D. and J. D. McPhail. 1992. Ecological character displacement and speciation in sticklebacks. The American Naturalist 140: 85–108.
Stuart Y. E. and J. B. Losos. 2013. Ecological character displacement: glass half full or half empty. Trends in Ecology and Evolution 28(7): 402–408.
Stuart Y. E., Campbell T. S., Hohenlohe, P. A., Reynolds, R. G., Revell, L. J. and J. B. Losos. 2014. Rapid evolution of a native species following invasion by a congener. Science 346: 463–466.
A blog post reviewing Stuart and Losos (2013) from a different perspective: