Evolution__3rd_Edition

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Score
Change in score per
million years
(a) Modernization of a lungfish character complex
0
10
20
30
40
50
60
70
80
90
100
350 250 150 50
Age (Myr BP)
(b) Rate of evolution in lungfish
2.5
2.0
1.5
1.0
0.5
0
350 250 150
Age (Myr BP)
50
Figure 21.9
(a) Evolution in lungfish, shown as the total score for each
fossil (the text explains the scoring). Score 0 is for the most
primitive form, and 100 for the most advanced. The rate of
evolution is the slope of the graph: when the graph is flat,
Lungfish have not always evolved
slowly
CHAPTER 21 / Rates of Evolution 607
Dipterus (350 Myr BP)
Neoceratodus (modern Australian lungfish)
Protopterus (modern South African lungfish)
evolution is not happening. (b) Rate of evolution. The
graph is derived from (a) and shows the rate of change of
the score through time. Lungfish have been living fossils
since about 250 to 200 million years ago. Modified from
Westoll (1949).
change in the score measures the rate of evolution of the group. The numbers assigned
to the character states are arbitrary, but they can still be used to portray evolutionary
rates. (By the way, the lungfish in Westoll’s study are not a simple sequence of ancestors
and descendants, in the way that the horses in MacFadden’s work probably were.
Westoll’s rates are for evolution within the Dipnoi as a whole and are not rates of
change down a single evolutionary lineage; the whole groups of Dipnoi would have
contained many lineages.) Westoll’s results are shown in Figure 21.9. Dipnoi, it reveals,
have not always been “living fossils.” Around 300 million years ago they were evolving
rapidly, but since about 250 to 200 million years ago, their evolution has slowed right
down. Their description as living fossils is accurate for the modern forms.
The obvious biological question is why evolutionary change came almost to a stop in
lungfish 200 million years ago. Lungfish are not the only examples of living fossils; other
examples include the brachiopod Lingula and the horseshoe crab Limulus (Figure 21.10).
The supreme examples of living fossils are the Cyanobacteria (sometimes called “bluegreen
algae”) a 3 billion-year-old fossils look much like forms living today (Schopf
1994). There are many particular conjectures about why these groups have changed so
little, but no general theory. The question is an instance of the general question of why
there should be evolutionary stasis. Their stability may be due to stabilizing selection