Annotation:
Ecological theories of sexual reproduction assume that sexuality is advantageous in
certain conditions, for example, in biotically or abiotically more heterogeneous environments.
Such theories thus could be tested by comparative studies. However, the
published results of these studies are rather unconvincing. Here, we present the results
of a new comparative study based exclusively on the ancient asexual clades. The
association with biotically or abiotically homogeneous environments in these asexual
clades was compared with the same association in their sister, or closely related, sexual
clades. Using the conservative definition of ancient asexuals (i.e., age >1 million
years), we found eight pairs of taxa of sexual and asexual species, six differing in the
heterogeneity of their inhabited environment on the basis of available data. The difference
between the environmental type associated with the sexual and asexual species
was then compared in an exact binomial test. The results showed that the majority of
ancient asexual clades tend to be associated with biotically, abiotically, or both biotically
and abiotically more homogeneous environments than their sexual controls. In
the exploratory part of the study, we found that the ancient asexuals often have durable
resting stages, enabling life in subjectively homogeneous environments, live in the
absence of intense biotic interactions, and are very often sedentary, inhabiting benthos,
and soil. The consequences of these findings for the ecological theories of sexual
reproduction are discussed.
