Page 136 - Introduction to Paleobiology and The Fossil Record

MACROEVOLUTION AND THE TREE OF LIFE 123

Sedimentary
Level of lake sequence Morphology of molluskan species

tuff beds

Time

Suregai
tuff

50 m

fall rise snalis bivalves
Figure 5.4 Fine-scale evolution in fresh-water snails and bivalves in Lake Turkana, Kenya, through the
last 4 myr. The volcanic tuff beds allow accurate dating of the sequence. Major speciation events seem
to take place at times of lake-level change: are these examples of punctuational speciation, or merely
ecophenotypic shifts? (Based on Williamson 1981.)

without gaps, abundant fossils through- colonize Lake Turkana after the lake-level
out and good dating. changes had taken place.
However, even this enormous study aroused
The problems in testing became evident controversy. Critics pointed out that the
early on, because sampling was generally sequence of sediments was not complete
not extensive enough. Williamson (1981) enough to be sure that all fossils had been
attempted to counter this problem in one of found: there were gaps of 1000 years or more,
the most enormous sampling exercises ever. and a great deal of gradual evolution could
He studied hundreds of thousands of speci- take place in that time. Second, Williamson’s
mens of snails and bivalves in sediments critics noted that the environmental stress of
deposited in the Lake Turkana area of Kenya lake-level change induced only short-term
from 1.3 to 4.5 Ma (Fig. 5.4). Lake Turkana changes in shell shape, and when the stress
lies in the East African Rift Valley, on a tec- was over, the shell shapes apparently reverted
tonically active line where the continent of to normal (Fig. 5.4). Hence, they proposed
Africa is unzipping to form two major plates. that speciation had not taken place, and that
Lake muds and sands accumulated in thick the shells had merely changed shape ecophe-
deposits as the rift opened, and volcanic ash notypically. This means that the changes hap-
(tuff) beds occur sporadically throughout the pened during the animals’ lifetimes, in response
sequence. to particular stresses as they grew in size, and
Williamson recorded changes in 19 species these changes were not genetically coded, and
lineages, and found that stasis was the normal hence were not evolutionary.
state of affairs, but that rapid morphological Most recently, in a thorough re-study of
shifts had taken place three times, two of this work, Bert van Bocxlaer and colleagues
which corresponded to substantial lake level (2008) have suggested that Williamson got it
rises (Fig. 5.4). He interpreted this as evidence wrong. They studied mollusks from several of
for the punctuated equilibrium model, arguing the African great lakes, revised the taxonomy,
that rapid environmental changes had caused and argue strongly that the three apparent
evolutionary shifts and speciation events. The speciation shifts (Fig. 5.4) are invasion events,

new species were short-lived, he argued, when flooding episodes allowed bivalves and
because the parental stock had survived in gastropods to enter the lakes from nearby
neighboring unstressed lakes, and returned to rivers. As water levels subsided, the faunas