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Abstract:
In evolutionary biology, sexual selection is hypothesized to increase speciation
rates in animals, as theory predicts that sexual selection will contribute to
phenotypic diversification and affect rates of species accumulation at macroevolutionary
time scales. However, testing this hypothesis and gathering
convincing evidence have proven difficult. Although some studies have shown a
strong correlation between proxies of sexual selection and species diversity
(mostly in birds), this relationship relies on some assumptions on the link between
these proxies and the strength of sexual selection and is not detected in some
other taxa, making taxonomically widespread conclusions impossible.
In a recent study published in Current Biology [1], Ellis and Oakley provide
strong evidence that bioluminescent sexual displays have driven high species
richness in taxonomically diverse animal lineages, providing a crucial link between
sexual selection and speciation.
It was known that bioluminescence has evolved independently more than 40
times, with males often using it as a mating signal but with also some other possible adaptive functions including anti-predator defense and predation. Moreover, it has been
reported that small marine lanternfishes and sharks that use bioluminescence in mate identification
had a greater concentration of species than other deep-sea fishes that use bioluminescence for
defensive purposes [2-4]. But no one had ever determined whether this pattern is consistent across
diverse and distantly related animal groups living on sea and land.
Ellis and Oakley [1] explored the scientific literature for well-resolved evolutionary trees with
branches containing bioluminescent lineages and identified lineages that use light for courtship or
camouflage in a wide range of marine and terrestrial taxa including insects, crustaceans, cephalopods,
segmented worms, and fishes. The researchers counted the number of species in each
bioluminescent clade and found that all groups with light-courtship displays had more species and
faster rates of species accumulation than their non-luminous most closely related sister lineages or
ancestors. In contrast, those groups that used bioluminescence for predator avoidance had a lower
than expected rate of species richness on average.
Nicely encompassing a diversity of taxa and neatly controlling for the rate of species accumulation
of the encompassing clade, the results of Ellis and Oakley are clear-cut and provide the most
comprehensive evidence to date for the hypothesis that sexual displays can act as drivers of
speciation. One question this study incites is what is happening in terms of sexual selection in
species displaying defensive bioluminescence or no bioluminescence at all: do those lineages use no
mating signals at all or other mating signals that are less apparent, and will those experience lower
levels of sexual selection than bioluminescent mating signals, i.e. consistent with Ellis and Oakley
results? It would also be interesting to investigate the diversification rates in animal species using
other modalities, such as chemical, acoustic or any other type of signals used by males, females or
both sexes, to determine what types of sexual signals may be more generally drivers of speciation.
