Evolution__3rd_Edition

594 PART 5 / Macroevolution
Table 21.1
Gingerich’s summary of evolutionary rates. The summary is large but not complete, and is based on 521 different measurements.
Gingerich divided the measurements into four classes. The importance of the column for time intervals will become apparent in
Section 21.2.
Evolutionary rate (darwins) Time interval
Domain Sample size Range Geometric mean Range Geometric mean
I Selection experiments 8 12,000–200,000 58,700 1.5–10 yr 3.7 yr
II Colonization 104 0–79,700 370 70–300 yr 170 yr
III Post-Pleistocene 46 0.11–32.0 3.7 1,000–10,000 yr 8,200 yr
Mammalia
IV Fossil Invertebrata 363 0–26.2 0.08 8,000 yr–350 Myr 3.8 Myr
and Vertebrata
Fossil Invertebrata alone 135 0–3.7 0.07 0.3–350 Myr 7.9 Myr
Fossil Vertebrata alone 228 0–26.2 0.08 8,000 yr–98 Myr 1.6 Myr
I to IV combined 521 0–200,000 0.73 1.5 yr–350 Myr 0.2 Myr
Rates of evolution in fossils are
usually slower than in artificial
selection experiments
studied by population geneticists? Population genetics identifies two main mechanisms
of evolution, natural selection and random drift, though drift is arguably unimportant
in morphological evolution (Section 7.3, p. 165). For changes like those of tooth size in
the history of horses, we cannot confirm directly that selection was the cause. That
would require us to show that the character was inherited (that is, larger toothed horses
gave rise to larger toothed offspring than average). We should also have to show that
larger horses produced more offspring than average. That kind of study is usually
impossible with fossils.
However, we can at at least find out whether the results of research in the two areas
are consistent. We can first ask whether there is any contradiction between the rates of
evolution observed in population genetics work, such as artificial selection experiments,
and those observed in fossils. If, for example, the fossil rates are significantly
higher, it would suggest that selection alone cannot be the only cause of evolution.
Some other more rapid factor would be needed. In fact, it turns out, the rates of evolution
in artificial selection experiments are far higher than those measured in fossils.
Evolution under artificial selection has proceeded about five orders of magnitude faster
than in the fossil record (Table 21.1). We can conclude that the known mechanisms of
population genetics can comfortably accommodate the fossil observations.
Strictly speaking, this does not confirm that the fossil changes were driven by selection
and (perhaps) drift. However, it does show that the observations are consistent.
For this reason, and because no other mechanisms of evolution are known, no one
seriously doubts that the microevolutionary processes of Chapters 4–9, 14 and 15 a
even if operating indirectly (tooth sizes might increase because of selection for larger
body size, for instance) a ultimately underlie the observed rates of evolution over
geological time periods. We have no reason to think that some additional but unknown
mechanisms of evolution were at work.
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