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Evolution__3rd_Edition

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..<br />

Time (Myr)<br />

Two species/million years Three species/million years<br />

x + 1<br />

x<br />

Character<br />

The number of species per time unit<br />

is a rough measure of evolutionary<br />

rate<br />

CHAPTER 21 / Rates of <strong>Evolution</strong> 609<br />

Figure 21.11<br />

Taxonomic measurement of evolutionary rate. If a taxonomist<br />

has divided one group into two species, and another group into<br />

three species, in the same time interval, the latter group shows a<br />

50% higher taxonomic rate of evolutionary change. The diagram<br />

illustrates only the logic of the argument. In real data, there would<br />

be gaps in the lineages, and the real pattern of evolution could<br />

have been smooth or jerky, with any number of branches in<br />

addition to the lineages shown here.<br />

21.6 Taxonomic data can be used to describe the rate of<br />

evolution of higher taxonomic groups<br />

For a question such as “Do mammals evolve faster than bivalve mollusks?,” a further<br />

kind of measurement of evolutionary rate can be used. The question could be tackled<br />

by either of the methods we have discussed so far. We could calculate the evolutionary<br />

rates for individual characters, measured either metrically or in discrete states, in mammals<br />

and bivalves, and then compare them, although there is a danger the comparisons<br />

might be meaningless. Any differences in evolutionary rate may reflect only the way we<br />

measure (for example) teeth in mammals and shell shape in bivalves, and nothing real.<br />

However, there may be some purposes a comparisons with rates of nucleotide change,<br />

perhaps a for which the measurements might be useful.<br />

Another method is to use taxonomic evidence. When taxonomists divide a set of<br />

organisms into a number of species, they make their judgment according to the degree<br />

of phenetic differences among the forms. Their judgment will not usually be based on<br />

a single character, but on several characters, integrated in the taxonomist’s mind into<br />

a single dimension of taxonomic similarity. Thus if there were two separate but comparable<br />

evolutionary lineages, and in the same time interval a taxonomist divided one<br />

lineage into two species and the other into three, it would suggest that the latter lineage<br />

had evolved at a higher rate (Figure 21.11). This comparison uses a taxonomic rate of<br />

evolution.<br />

A taxonomic rate of evolution offers an abstract measure of how rapidly change is<br />

taking place in a group of species. The exact meaning of a taxonomic rate is less easy to<br />

specify than for an evolutionary rate of a single character and has a relatively imprecise<br />

meaning. It can be said in their defense that they summarize evidence from more than<br />

one character, and have greater generality. How reliable a taxonomic rate is depends on<br />

how reliable is the judgment of the taxonomist who divided the lineage up into species<br />

and genera.<br />

Taxonomic rates of evolution are expressed in two main ways. One is the number of<br />

species or genera (or taxon) per million years. Table 21.2 gives some examples from<br />

the two taxa that we have discussed in this chapter: horses and lungfish. The rates for<br />

lungfish reillustrate how that group initially had a high rate of evolution, which then<br />

slowed down so much that they became living fossils.<br />

The same data, but for a group made up of a larger number of lineages, can also be<br />

expressed as a survivorship curve (Figure 21.12). Survivorship curves are constructed by

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