Big evolution

As long as you're going to be thinking anyway, think big.

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Up until now the book has been concerned with the properties of individual lineages and how they change through time: so-called 'microevolution'. Our focus in microevolution has been mostly on how the average properties of a species may change and why they were of a certain observed value as opposed to some other hypothetical values. This is a natural stance to take, because intraspecific variation tends to be small in comparison with interspecific variation, hence, we can justify to some extent treating species as single points in the moving cloud of phenotype space (though see Chapter 11). Evolutionary biologists can also consider the properties of clades of lineages: so-called 'macroevolution' (Figure 14.1). Macroevolutionists tend to

Fig. 14.1 Macroevolution versus microevolution. The main picture depicts a clade evolving through time and morphospace. Species form and go extinct, and anagenesis occurs within species, and this clade varies through time in species richness and disparity, the two major macroevolutionary variables. Microevolution, in contrast, is concerned with individual species, and whether they speciate, go extinct, or change in form (magnified section).

Fig. 14.1 Macroevolution versus microevolution. The main picture depicts a clade evolving through time and morphospace. Species form and go extinct, and anagenesis occurs within species, and this clade varies through time in species richness and disparity, the two major macroevolutionary variables. Microevolution, in contrast, is concerned with individual species, and whether they speciate, go extinct, or change in form (magnified section).

concentrate not on averages but on variation and diversity. Two types of diversity, in particular, are of interest: the species richness of the clade (number of lineages) and their diversity of form (disparity). While a single species can in theory have a disparity, diversity of form among a number of species can be much greater because the absence of interbreeding among them means that evolutionary divergence can much more readily occur. Species richness is a property that is obviously only interesting in clades as opposed to individual species!

Underlying a clade's species richness are the two cladogenetic processes of speciation and extinction, the net effect of which (speciation minus extinction rate) gives rise to the net rate of diversification. Disparity is influenced not only by these cladogenetic processes, which sprouts and prunes out the different phenotypes, but also by the other evolutionary process of anagenesis, which moves the phenotypes of each lineage around. Macroevolution is of interest to ecologists simply because the currencies of interest to macroevolutionists, species richness, and diversity of form, are also currencies that ecologists measure. Thus, ecologists attempting to explain numbers of species or diversity of function frequently experience the need for macroevolutionary explanation.

We can ask a number of questions about these properties and processes affecting clades. How do they vary through time? How do they vary geographically through space? How do they vary across clades? What properties of clades and the environment affect them? To answer these questions we have the tried and tested combination of theory and data. We will discuss the contribution of theory later in the chapter. The data come from two main sources: the fossil record, and studies of extant clades. The studies of extant clades come in three main kinds. First, studies of phylogenetic tree shape enable us to derive information about cladogenetic processes. Second, studies of endangered or recently extinct species reveal underlying patterns in extinction rates. Third, studies of recent adaptive radiations allow us to infer the relative progress of diversity and disparity during a clade's evolution. It will be obvious from this discussion that not all sources of data impinge on every question. In addition, some questions have received a relative dearth of attention. In particular, most work on macroevolution has concentrated on diversity rather than disparity, as will be obvious below (Table 14.1).

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