The dynamics of species richness

The second characteristic that distinguishes the modes of co-evolution is whether speciation and extinction result from the interaction, leading to temporal dynamics of taxa. In some interactions neither speciation nor extinction is implied as a result of the co-evolutionary process. This is the case for mutual dependence. It is also the case for arms races. Arms races occur when quantitative traits in both species show reciprocal directional evolution. A well-known example is that of floral traits and those of their pollinators. For example, Rediviva bees in South Africa visit Diascia flowers to gather oils, which they collect from the base of flower spurs by inserting their forelegs (Figure 11.4). Some populations of bees have evolved extraordinary long forelegs, which match the long spurs of the flowers they visit (Steiner and Whitehead 1990). It is likely that both characters have co-evolved directionally: the bee to collect oils, and the flower to force more effective pollination service from the bee. In other cases, as we have already seen, co-evolution alters species richness. Escape-and-radiation co-evolution increases it. Another interaction that involves speciation is the aptly named 'diversifying co-evolution' (Figure 11.3). This occurs in some maternally inherited symbionts, such as Wolbachia and their hosts: the symbiont promotes reproductive isolation, through parthenogenesis or an incompatibility mechanism. As a result new taxa of hosts are produced, and, of course, new symbiont taxa too. As well as speciation, extinction can occur, as in co-evolutionary turnover (Figure 11.3).

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