Evolution__3rd_Edition

..
Time (Myr)
Two species/million years Three species/million years
x + 1
x
Character
The number of species per time unit
is a rough measure of evolutionary
rate
CHAPTER 21 / Rates of Evolution 609
Figure 21.11
Taxonomic measurement of evolutionary rate. If a taxonomist
has divided one group into two species, and another group into
three species, in the same time interval, the latter group shows a
50% higher taxonomic rate of evolutionary change. The diagram
illustrates only the logic of the argument. In real data, there would
be gaps in the lineages, and the real pattern of evolution could
have been smooth or jerky, with any number of branches in
addition to the lineages shown here.
21.6 Taxonomic data can be used to describe the rate of
evolution of higher taxonomic groups
For a question such as “Do mammals evolve faster than bivalve mollusks?,” a further
kind of measurement of evolutionary rate can be used. The question could be tackled
by either of the methods we have discussed so far. We could calculate the evolutionary
rates for individual characters, measured either metrically or in discrete states, in mammals
and bivalves, and then compare them, although there is a danger the comparisons
might be meaningless. Any differences in evolutionary rate may reflect only the way we
measure (for example) teeth in mammals and shell shape in bivalves, and nothing real.
However, there may be some purposes a comparisons with rates of nucleotide change,
perhaps a for which the measurements might be useful.
Another method is to use taxonomic evidence. When taxonomists divide a set of
organisms into a number of species, they make their judgment according to the degree
of phenetic differences among the forms. Their judgment will not usually be based on
a single character, but on several characters, integrated in the taxonomist’s mind into
a single dimension of taxonomic similarity. Thus if there were two separate but comparable
evolutionary lineages, and in the same time interval a taxonomist divided one
lineage into two species and the other into three, it would suggest that the latter lineage
had evolved at a higher rate (Figure 21.11). This comparison uses a taxonomic rate of
evolution.
A taxonomic rate of evolution offers an abstract measure of how rapidly change is
taking place in a group of species. The exact meaning of a taxonomic rate is less easy to
specify than for an evolutionary rate of a single character and has a relatively imprecise
meaning. It can be said in their defense that they summarize evidence from more than
one character, and have greater generality. How reliable a taxonomic rate is depends on
how reliable is the judgment of the taxonomist who divided the lineage up into species
and genera.
Taxonomic rates of evolution are expressed in two main ways. One is the number of
species or genera (or taxon) per million years. Table 21.2 gives some examples from
the two taxa that we have discussed in this chapter: horses and lungfish. The rates for
lungfish reillustrate how that group initially had a high rate of evolution, which then
slowed down so much that they became living fossils.
The same data, but for a group made up of a larger number of lineages, can also be
expressed as a survivorship curve (Figure 21.12). Survivorship curves are constructed by