Evolution__3rd_Edition

662 PART 5 / Macroevolution
Figure 23.8
Duration in the fossil record
and geographic ranges for late
Cretaceous gastropods from
North America. Species with
planktonic larvae (a) last longer
in the fossil record (i.e., they
have lower extinction rates) and
also have wider geographic
ranges than species with
direct development (b). The
extinction rate is given as the
chance that a species lineage
will go extinct per million years.
See Table 23.1 for related
results. (500 miles ≈ 800 km.)
Redrawn, by permission of the
publisher, from Jablonski &
Lutz (1983).
Planktonic development is
associated with low extinction rates
...
. . . decreasing relative diversity ...
. . . and (possibly) low speciation
rates
Percentage Percentage
40
30
(a) Planktonic development
(50 species)
Extinction rate per
million years = 0.17
40
30
Median = 1,600 km
20
20
10
10
0
0 2 4 6 8 10 12 14 16
0
0 1,000 2,000 3,000 4,000
Duration (Myr) Geographic range (km)
(b) Direct development
(50 species)
70
60
Extinction rate per
50
40
30
20
10
million years = 0.34
0
0 2 4 6 8 10 12 14 16
Duration (Myr)
Percentage Percentage
70
60
50
40
30
20
10
0
Median = 250 km
0 1,000 2,000 3,000 4,000
Geographic range (km)
What is the relation between larval type and extinction rate? Several studies have
found that species with planktonic larva have lower extinction rates (Figure 23.8). As
the figure shows, they also have wider geographic ranges. This may be the reason for
their lower extinction rate, because a species with a wider range is less vulnerable to
local circumstances. Or it may mean merely that planktonic forms have a higher chance
of being preserved as fossils than have directly developing forms, because their wider
distribution increases the chance that in one site the conditions will be right for
fossilization; the difference could then be just a bias in the fossil record.
Hansen (1978, 1983) looked at the relation between larval type and speciation rate. He
predicted that snails with direct development will speciate more rapidly than species
with planktonic larvae, because the species with non-planktonic development will be
more likely to be geographically localized and isolated, which makes allopatric speciation
easier. Planktonic development increases gene flow and makes allopatric speciation less
probable. He used this idea to explain an observed trend in snails of the early Tertiary
(Figure 23.9). The proportion of planktonically developing species declined through
the Paleocene and Eocene. The trend was not being produced by the difference in extinction
rates. The planktonically developing species, as usual, had lower extinction rates
(Figure 23.9b), and that would tend to produce the opposite trend from that observed.
Two alternatives are left. Natural selection could have been favoring direct development
within the majority of lineages. Hansen “suggested” this was not true (though he
gave no evidence). The period was a time of global cooling, which might favor direct
development, given the latitudinal trend mentioned earlier. Hansen said the decline in
planktonically developing forms preceded the global cooling; concrete evidence, however,
rather than a vague statement, would be needed to persuade a skeptic. The second
alternative is that the increase was due to a higher speciation rate of the directly developing
forms, simply because forms with lower dispersal rates are more likely to speciate.
..