Evolution__3rd_Edition

610 PART 5 / Macroevolution
Figure 21.12
Survivorship curves for
(a) bivalves mollusks and
(b) carnivores (Mammalia).
The curves express the numbers
of genera surviving for different
amounts of time. Note bivalve
genera tend to last longer than
carnivore genera, as is clear
from the different scales of the
x-axes in the two figures. The
average duration of a bivalve
genus is 78 million years,
and 8.1 million years for a
carnivore. Redrawn, by
permission of the publisher,
from Simpson (1953). © 1953
Columbia University Press.
Taxonomic survivorship curves
express evolutionary rates
Percentage
(a) (b)
100
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
25 50 100 150 200 250 0 5 10 15 20 25 30
Time (Myr)
Time (Myr)
Table 21.2
Taxonomic rates of evolution in mammals and lungfish. Early in their evolution,
lungfish evolved about as rapidly as mammals, but they have subsequently slowed down.
Hyracotherium–Equus is the horse lineage discussed in Section 21.1. From Simpson (1953).
Group or line Average duration of genus (Myr)
Hyracotherium–Equus 7.7
Lungfish
Devonian 7
Permo-Carboniferous 34
Mesozoic 115
taking a sample of a number (such as 100) of mammalian genera and measuring how
long each one lasts in the fossil record. (Any taxonomic level within the mammals
can be used; genera are only an example.) The survivorship curve plots the number of
genera surviving for different times. Most of the genera are still surviving after a short
time, but as time passes the members of the original sample drop out one by one. The
slope of a survivorship curve measures the rate of evolution of the group. If the group is
evolving fast, the survivorship curve falls rapidly, but is more drawn out for a slowly
changing group. (Survivorship curves are more familiar for populations of individuals
(Section 4.1, p. 72). An actuarial survivorship curve plots the survival of a sample of
individuals through time, but the same type of graph can be plotted for species and
other taxonomic groups.)
There is a similar problem in comparing taxonomic evolutionary rates between
groups as there is for single characters. A bivalve taxonomist may be a good judge of
a bivalve genus, and a mammal taxonomist of a mammal genus, but it is still difficult
to know how to interpret any differences in the rates of turnover of the two sorts of
genera. The differences in rate could just reflect some difference in the way the two
taxonomists work. Within a group, survivorship curves can have revealing features
(Section 22.7, p. 637). For now, however, we only need to know that taxonomic
Percentage 100
..