Evolution__3rd_Edition

..
Summary
CHAPTER 21 / Rates of Evolution 611
evidence can provide another sort of measurement of evolutionary rates, along with
measurements of single characters.
21.7 Conclusion
1 Evolutionary rates of fossil characters can be
measured as simple rates of change through time;
logarithms are often taken of the character measurements
and the rate expressed in “darwins.” Evolution
in horse teeth is a classic example.
2 Rates of evolution measured in the fossil record are
slower than those produced by artificial selection in
the laboratory.
3 Evolutionary rates vary between different geological
times, taxa, and types of taxa. The science of evolutionary
rates is mainly concerned to explain the pattern of
evolutionary rates.
4 Among published measurements of evolutionary
rates, the rate and the time interval over which it was
measured are inversely related: faster evolution is seen
in shorter intervals. The reason is probably that the
direction of evolution fluctuates through time.
5 Eldredge and Gould stimulated a controversy about
evolutionary rates by their suggestion that rates have a
strict pattern (called punctuated equilibrium) where
evolution is fast at times of splitting (speciation) and
comes to a halt between splits. The opposite pattern, in
which evolution has a constant tempo, they called
phyletic gradualism.
6 It is difficult to discover the pattern of evolutionary
This chapter has introduced three methods of measuring rates of evolution. For single
characters that show continuous variation, we can measure the metrical rate of change
and express it in darwins. For characters with discrete states, we can give each state an
arbitrary score and measure the rate of change of the score. A cruder measure is provided
by the duration of species in the fossil record, because taxonomists will recognize
a higher turnover of species in groups that evolve rapidly. All three methods have their
particular uses and applications. In this chapter we have seen how paleobiologists have
used all three methods to study general evolutionary questions.
rates at, and between, speciation events because the
fossil record is incomplete.
7 There is some evidence for punctuated equilibrium,
such as Cheetham’s study of Caribbean bryozoans
from the Miocene and Pliocene, and some for phyletic
gradualism, such as Sheldon’s study of Ordovician
trilobites in Wales. No general empirical conclusion is
yet possible, though punctuated equilibrium is a well
confirmed phenomenon.
8 Evolutionary processes and rates can be examined
at all taxonomic levels from evolution within populations,
through speciation, to the origin of the higher
groups. Evolution may have characteristic mechanisms
and rates at different levels, or the same set of rates and
processes may operate equally at all levels.
9 For large changes, like from a limb into a wing,
evolutionary rates cannot be measured as a continuous
variable. The character can instead be divided into
states. The evolutionary rate can then be studied in
the rate of change between states. Westoll studied the
evolution of lungfish by this method.
10 The number of species in a lineage per million
years is a complex measure of evolutionary rate, called
a taxonomic rate of evolution. Taxonomic rates can be
expressed as survivorship curves.