In Asexual Organisms, Species Can Be Defined as Ecotypes

In asexual populations, Cohan (2002) has argued that ecotypes will tend to be genetically similar, so that they correspond to clusters of similar DNA sequence. This is so because when advantageous mutations arise, they will displace all of the individuals that occupy the same ecological niche. However, the new genotype will not be able to displace other ecotypes, because it will not carry the adaptations needed to exploit other niches (Fig. WN22.3). As we saw on pages 536–538, such selective sweeps eliminate genetic variation and produce a star genealogy (Fig. 19.15A). For example, Synechococcus bacteria, which live in thin mats around the Yellowstone hot springs, fall into distinct genotypes that correspond to different microhabitats (Ramsing et al. 2000). We saw a similar example of ecological divergence in laboratory populations of Pseudomonas (Fig. 18.21). The characteristic pattern of very closely related sequence clusters, kept homogeneous by occasional selective sweeps, allows ecotypes to be identified in samples of DNA taken from bacteria that cannot be grown in the laboratory. Typically, the groups named as “species” seem to contain several ecotypes, as defined by this pattern of sequence similarity.

The sporadic recombination typical of bacteria does not alter this correspondence between ecotypes and clustered DNA sequences, because only small segments of genome are transferred (Fig. 15.16; pp. 351–352). Occasional recombination may allow ecotypes to persist, because they can acquire globally advantageous mutations from other ecotypes (e.g., antibiotic resistance). In obligate sexually reproducing populations, however, it is hard for selection to maintain distinct ecotypes within any one area, because interbreeding continually breaks them down. Even if selection is strong enough to maintain them, traits not under direct selection will be reshuffled, and the genealogy across most of the genome will not reflect ecological clustering. (We see several examples of this throughout this chapter.) In sexual organisms, reproductive isolation is needed to hold specific combinations of genes and traits together in biological species.