Evolution__3rd_Edition

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Time
General younging direction
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11
47
11
11
Nileids Cnemidopyge Platycalymene
43
24
4
Bergamia
6
6
19
5
41
23
CHAPTER 21 / Rates of Evolution 605
21.4.3 Ordovician trilobites show gradual evolutionary change
The extinct arthropod group of trilobites is classified by external morphological features
such as the number of pygidial ribs (the pygidium is the tail region of a trilobite’s
body). Sheldon (1987) made a rigorous biometrical study of their evolution at a site in
Wales. He measured the number of pygidial ribs in 3,458 specimens from eight generic
lineages, taken from seven stratigraphic sections. The total time period spanned by the
sections is about 3 million years.
In all eight genera, the average number of pygidial ribs increased through time, and
in all eight the evolution was gradual (Figure 21.8); a population at any one time
was usually intermediate between the samples before and after it. There is one possible
artifactual explanation for the result, and Sheldon was able to rule it out. A gradual
increase in the number of pygidial ribs would result if two populations, one with a
higher number of ribs than the other, were mixed together, with successively later
samples having increasing proportions of the high-rib-number population. Sheldon
argued this was not the case because, with rare exceptions, his samples did not show
bimodal frequency distributions, as they would if they contained a mixture of two distinct
populations. These trilobites look like a good illustration of gradual evolution.
21.4.4 Conclusion
852
27(H)
19
70
640
54
23(H)
Whittardolithus
Ogyginus
Ogygiocarella Nobiliasaphus
0 1 2 3 4 5 6 6 7 8 6 7 8 6 7 8 9 11 12 13 10 12 14 16
Figure 21.8
The gradual evolution in Sheldon’s study of Ordovician Welsh
trilobites. In eight lineages, the pattern of change is gradual
rather than punctuated. Time goes up the page (total timespan
. . . and a thorough study of
trilobites
4
6
1
10
17
4
Number of ribs
22
8
8
On the evidence so far we can conclude that both punctuated equilibrium and phyletic
gradualism are real facts about fossil evolution. Some examples, like Cheetham’s
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33
237
27
26
525
178
3 million years) and the biometrical variable (number of ribs) is
along the bottom. The numbers besides the lines are the sample
sizes. Redrawn, by permission of the publisher, from Sheldon
(1987). © 1987 Macmillan Magazines Ltd.
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176
1
14
1