Evolution__3rd_Edition

604 PART 5 / Macroevolution
Figure 21.7
The punctuated equilibrial
pattern of evolution in
Caribbean species of the
bryozoan Metrarabdotus.
Each point is the average for
a large sample of individuals.
Note that most lineages do
not change through time;
new species appear suddenly,
without intermediates; and the
ancestral species often persists
alongside its daughter species.
Time goes up the page and
the x-axis indicates phenetic
distance. Redrawn, by
permission of the publisher,
from Cheetham (1986).
. . . as is illustrated by a thorough
study of bryozoans ...
Myr
0
5
10
15
>20
PLEISTOCENE
E PLIOCENE M L
L
MIOCENE
E M
OLIG.
NN20
NN19
NN18
NN17
NN16
NN15
NN14
NN13
NN12
NN11
NN10
NN6
NN5
NN4
NN3
NP
21–25
tenue
n. sp. 10
auriculatum
n. sp. 9
n.sp. 5
colligatum
micropora
n. sp. 1
unguicalatum
lacrymosum
Cheetham’s was a thorough study. He measured up to 46 morphological characters
per specimen in a total of about 1,000 specimens, from about 100 populations. The
results show that these bryozoans mainly evolved in the punctuated equilibrial mode
(Figure 21.7). Most of the species did not change in form over long periods of several
million years, and most of the new species appeared suddenly without intermediate
transitional populations. If there were intermediate forms, they lasted (on average) less
than 160,000 years. Moreover, in a number of cases, the ancestral species persisted
alongside its descendant species. Some species do occasionally appear to show short
periods of gradual change within a lineage, but they are sufficiently rare that they could
be interpreted as being due to sampling from a constant population. Cheetham also
tested whether the forms classified as different species might just be ecophenotypic
variants within one species. He raised members of a modern species in a range of
environments. They all grew up much the same, and recognizably as members of
one species. The punctuations, therefore, are likely to be real evolutionary events, not
ecophenotypic switches.
n.sp.6
chipolanum
n.sp. 7
n. sp. 8
n. sp. 2
n. sp. 3
kugleri
n. sp. 4
..