Predicting the Formation of Virtually Identical Species
A new study in Nature claims that scientists can predict the extent to which a clade will speciate by studying its environmental niche and the prevalence of sexual selection ("Ecological opportunity and sexual selection together predict adaptive radiation"). The news release at Science Daily, titled "Predicting the Formation of New Species," explains that the researchers
demonstrate for cichlids from 46 African lakes that the probability of diversification, or "adaptive radiation," depends on a combination of environmental factors and sexual selection. African cichlids are particularly suitable for this type of study because of the extremely high species richness that developed over time from what was originally a small number of species in large African lakes. For Lakes Victoria and Malawi alone, more than 800 endemic cichlid species have been recorded.Yes, it's true that there are many "species" of cichlids in these lakes -- if you define species according to the relevant biological concept, where a species is merely a reproductively isolated population. As we saw in my "Specious Speciation" series earlier this year:
Such definitions say nothing about the degree of morphological, behavioral, or genetic change that has evolved. Thus, such a definition of "species" does not necessarily imply that significant biological change has taken place between the two populations. In many cases, two populations may be termed different "species" under the biological species concept, yet the differences between the populations are small-scale and trivial.That seems to be precisely the case with many of these "species" of cichlids. Back in college, I recall writing a paper on cichlid speciation for an evolutionary biology course, and learned that the differences that constitute separate "species" among cichlids can be quite trivial.
For example, the cichlid genus Labeotropheus has two species, L. fuelleborni and L. trewavasae (pictured above, left and right respectively), which are almost morphologically identical. Yet they are considered to be different species largely because the more territorial L. fuelleborni is generally found in the upper 8 meters of lake water, while L. trewavasae wanders more during feeding and can be found as deep as 20 meters.1 Under natural conditions in the lake the two populations are not observed to interbreed, largely because they generally traverse different depths, and, technically speaking, meet the definition of the biological species concept. Thus they are considered separate "species" despite their extreme morphological similarity.
At least one textbook seems to agree. It discusses a similar study that found only "small degrees of ecological separation" among cichlid species, where the differences between species entailed "feeding preferences" which merely "put species in different places" in the lake.2
Incidentally, the Nature paper seems to explain why this is the case. The researchers found that speciation occurs in deep lakes (where there's lots of room for speciation) and also where there are lots of coloration differences which fosters sexual selection:
Cichlids are more likely to radiate in deep lakes, in regions with more incident solar radiation and in lakes where there has been more time for diversification. Weak or negative associations between diversification and lake surface area indicate that cichlid speciation is not constrained by area, in contrast to diversification in many terrestrial taxa. Among the suite of intrinsic traits that we investigate, sexual dichromatism, a surrogate for the intensity of sexual selection, is consistently positively associated with diversification. Thus, for cichlids, it is the coincidence between ecological opportunity and sexual selection that best predicts whether adaptive radiation will occur. These findings suggest that adaptive radiation is predictable, but only when species traits and environmental factors are jointly considered.3In other words, the factors that lead to speciation (read: establishment of reproductively isolated populations) include opportunities for lots of living space, and color differences between males and females. This means these cichlid "species" may often represent populations in different feeding zones in lakes, and or those with color differences that lead to sexually selected traits. These are small-scale differences that, in the end, do not show speciation mechanisms producing fundamentally new types of organisms.
[1.] A. J. Ribbink, A. C. Marsh, & B. J. Sharp, "The zoogeography, ecology and taxonomy of the genus Labeotropheus Ahl, 1927, or Lake Malawi (Pisces: Cichlidae)," Zoological Journal of the Linnaean Society, Vol. 79:223-243 (1983).
[2.] Sylva S. Mader, Essentials of Biology, p. 276 (Thomson Brooks/Cole, 2007) (emphases removed).
[3.] Catherine E. Wagner, Luke J. Harmon, and Ole Seehausen, "Ecological opportunity and sexual selection together predict adaptive radiation," Nature (2012).
Image credit: Wikicommons.