Why Are There Species?

Biological species are defined by the ability of genes within them to recombine with each other. This definition tends to focus the study of speciation on the evolution of genetic differences that prevent interbreeding and gene exchange. However, it is not just gene flow and recombination that determine the pattern of divergence. Populations can diverge and remain distinct despite gene flow provided that selection is strong enough to overcome the mixing of populations (pp. 501–506); we have already seen many examples of extensive divergence despite interbreeding (recall the North American oaks of Fig. 22.6). Reproductive isolation is not necessary to allow divergence; indeed, if it were, it would be hard to understand how species evolve at all. Similarly, reproductive isolation is not the only reason why organisms cluster into distinct phenotypes—the phenomenon that gives us our intuitive notion of “species.” Predominantly asexual organisms also cluster into distinct forms, which allows us to make at least a rough classification of groups such as bacteria (e.g., Fig. 6.11). Some asexual groups are hard to classify and seem to form a more or less continuous range of phenotypes (e.g., Alchemilla; Sepp and Paal 1998). However, it is not clear that well-established asexual groups are in general any less sharply “clustered” than sexual groups (Holman 1987; Maynard Smith and Szathmary 1995, Chapter 9.5; Barraclough et al. 2003; Fontaneto et al. 2007). Reproductive isolation apart, what factors can explain clustering?

Such clustering may be an inevitable consequence of what Darwin called “descent with modification.” Suppose that individuals produce random numbers of offspring, and that these may differ slightly from their parents. We can imagine genetic lineages branching and going extinct over time and moving in a random walk across the space of possible phenotypes. This simple process leads to the buildup of ever-denser clusters, which look similar to each other simply because they are closely related (Fig. WN22.2). This process applies to asexual individuals or to nonrecombining genes. However, it cannot explain clustering of all features in sexual organisms, because different parts of the genome will have different ancestry, and so will show different patterns of clustering (recall pp. 427–432).

Usually, selection will favor particular combinations of characters, for example, adapting oaks to their various habitats (Fig. 22.6). This kind of selection for sets of characters that work well together leads directly to phenotypic clustering. Moreover, once some clustering is established, it may select for more clustering in other species. For example, the existence of distinct host species selects for specialized parasites and then for specialized parasites of those parasites. An especially important kind of selection is for the use of different limiting resources, because this allows diverse types to coexist. Divergence in the ability to use different resources is essential if a new species is to spread over a wide area alongside its progenitor. Otherwise, even if it is reproductively isolated, it will remain confined to a limited area by competition for the same resources.

So, selection can lead to clustering directly, by favoring combinations of traits that work well together. Moreover, if different trait combinations allow exploitation of different resources, then they may be able to coexist in the same place. However, if only those traits that are directly selected cluster together, we do not have a well-defined “species.” If we look at other traits, or at genealogical relationships, we will get a different classification. Here, we show how different features of organisms can come together in the same pattern of clustering.