Evolution__3rd_Edition

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Horse teeth show a representative
range of values
CHAPTER 21 / Rates of Evolution 593
longer time intervals things are not so simple. If the lineage continued to change by the
same increment (0.0001%) per year for up to 1 million years, it would have increased
by 100%. But a lineage that evolves at 1 darwin will in fact increase by 272% in 1 million
years. Therefore, the familiar units of “percent change” give reasonable results even
with logarithmic units such as darwins, but only over short time intervals. (The best
way to familiarize yourself with the meaning of darwins is to calculate a few: see the
study questions at the end of this chapter.)
The 26 ancestor–descendant species pairs and four dental characters measured
by MacFadden produced 4 × 26 = 104 estimates of evolutionary rates (Figure 21.1d).
The different tooth characters show different patterns, with height (M1MSTHT in the
figure), for instance, evolving rapidly between Parahippus and Merychippus, while the
other characters were evolving at normal rates. But the detailed pattern of the numbers
is not important here, 1 though the approximate absolute values of the rates are worth
bearing in mind.
The values in Figure 21.1 are mainly about 0.05–0.1 darwins, or about a 15–30%
change per million years. They are mainly positive, indicating that the lineage was on
average increasing in size. There are, however, negative values, as the horses in the
lineage evolutionarily shrunk as well as expanded. The values in Figure 21.1 are averages
for a lineage connecting an ancestral–descendant species pair, and do not imply
that evolution had a constant rate throughout that time. An average is not a constant,
and the rates for short periods may have been very different from the long-term average.
However, as average figures, the values in Figure 21.1 are fairly typical for the fossil
record, being neither exceptionally fast or slow. We shall see in a minute (Table 21.1
below) that the average figure for a large set of evolutionary rates in vertebrates is about
0.08, and rapidly evolving vertebrates show rates of more like 1–10 darwins over short
periods. Simpson (1953), who did more than anyone to stimulate the study of fossil
evolutionary rates, noticed that rates vary between taxa, characters, and times, and he
invented the terms bradytelic, horotelic, and tachytelic, to describe slow, typical, and
rapid evolution; horse evolution as such is horotelic.
21.1.1 How do population genetic, and fossil, evolutionary
rates compare?
Rates of evolution in the fossil record have been measured for many characters, in
many species, at many different geological times. A compilation by Gingerich (1983,
2001) included 521 different estimates, of which 409 were for the fossil record. The estimates
vary between 0 and 39 darwins in fossil lineages. The main problem of evolutionary
rates is to understand why they differ between times and taxa in the way they do.
Before we come to that problem, we can ask a more general question. Are the rates of
change seen in the fossil record consistent with the mechanisms of evolutionary change
1 The patterns mainly make sense in terms of the grinding functions of the teeth and the diets eaten by individual
horse species. Diets in turn were influenced by changes in vegetation, particularly the spread of grass,
and in climate. See Section 18.5 (p. 540).