Evolution__3rd_Edition

606 PART 5 / Macroevolution
Discrete character states ...
. . . can be used to measure
evolutionary rates
bryozoans, illustrate a punctuated equilibrial pattern of evolution; others, like
Sheldon’s trilobites, show a pattern of phyletic gradualism. Evolution has a range of
rates, from sudden to smooth, in real examples of fossil speciation. Punctuated
equilibrium may be somewhat commoner than phyletic gradualism, however (Erwin
& Anstey 1995; Jackson & Cheetham 1999). A future research question will be to ask
what conditions lead to more gradual evolution, and what conditions to punctuated
evolution, but at present paleontologists are still answering the prior question of what
the empirical rates of evolution are during, and between, speciation events.
21.5 Evolutionary rates can be measured for non-continuous
character changes, as illustrated by a study of “living
fossil” lungfish
The measurement of evolutionary rates in darwins is appropriate for metrical changes,
such as a character evolving to be longer or shorter; but for larger changes, such as from
a leg to a wing, this method ceases to be useful (Section 21.1). However, it is still possible
to measure rates of evolution for larger changes. The last two sections of the chapter
describe two methods. The first is a famous early study of evolutionary rates: Westoll’s
(1949) work on lungfish.
Lungfish (Dipnoi) form one of the four main divisions of fishes. They are an ancient
group dating back over 300 million years, but only six modern species exist. The modern
forms are examples of living fossils, species that have changed little from their fossil
ancestors in the distant past. They should therefore show, at least recently, slow rates of
evolution. Westoll investigated this question quantitatively. He distinguished 21 different
skeletal characters of fossil Dipnoi. For each of the 21, he distinguished a number
of character states (like the character states discussed earlier for classification and phylogenetic
inference). The 21 characters showed between three and eight different states.
Character number 11, for example, was “degree of fusion of bones along the supraorbital
canal.” Westoll distinguished five different states, namely:
4. Irregular, more or less random fusions.
3. Tendency for fusions to be in twos, especially in some parts of the canal.
2. Still stronger tendency to fusion, rarely in threes or fours in specific sections.
1. Three or four elements (K–M) generally fuse, but there are numerous irregularities.
0. Three or four elements (K–L 2 or K–M) always fuse.
(Letters like K and L 2 refer to particular, identifiable bones.) The highest state (4) is the
most primitive condition of the character, and 3, 2, 1, and 0 are successively later, more
derived states. Westoll made an analogous list of states for all 21 characters. These
character states are not the sort of metrical changes for which evolutionary rates can be
measured in darwins (Section 21.1). The fusion of two bones into one is a discrete, not
a continuous, evolutionary change.
For each fossil, Westoll calculated a total score, made up of its total for all 21 characters.
The most advanced possible lungfish, with 21 characters in the most advanced
state, would therefore have a total score of 0; the score for the most primitive possible
lungfish, which had the highest scores for all 21, would have been 100. The rate of
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