Evolution__3rd_Edition

664 PART 5 / Macroevolution
Hansen’s work has since been
criticized
Sticklebacks that occupy longlasting
estuarine niches have low
extinction rates
Since Hansen’s work, Duda & Palumbi (1999) have cast doubt on one assumption of
his argument. They show that, in a group of modern snails, species with planktonic
development have repeatedly evolved from ancestral species with direct development.
For the trend in Figure 23.9 to be driven by differences in speciation rate, it is important
that the ancestor–descendant lines of species tend to retain the same mode of development.
(In technical language, heritability is required at the level of species.) Duda and
Palumbi’s result for modern species suggests that the expanding group of species with
direct development may not have been a clade with a constant mode of development.
Species with direct development may have arisen from ancestors with planktonic
development. Currently it is uncertain whether, as Hansen originally argued, the
decline in planktonically developing forms in the early Tertiary occurred because they
had a low speciation rate.
23.6.2 Differences in the persistence of ecological niches will influence
macroevolutionary patterns
In the previous section, we considered the possibility that a character (larval type)
might influence speciation, and extinction, rates. The influence, if real, is a straight
consequence of the character itself: species in which there is direct development are
more likely to split, in the process of allopatric speciation, than species in which there is
planktonic development. A second factor that can influence speciation and extinction
rates is the nature of the ecological niche occupied by species. Species that occupy
niches that last longer will have lower extinction rates than species that occupy shortlived
niches. Williams (1992) introduced this idea in terms of a concrete example a the
three-spined stickleback (Gasterosteus aculeatus).
The three-spined stickleback is a fish with a widespread distribution in coastal waters
in the northern hemisphere, on both sides of the North Atlantic and Pacific Oceans.
From these coastal waters (it appears), many populations have separately colonized the
local freshwater rivers and their tributaries inland. Some of these freshwater populations
have been studied and they show various local adaptations to the rivers they
occupy and have formed a complex set of local races or subspecies.
However, the populations that colonize the freshwater rivers are probably evolutionarily
short lived. Ecological and geographic changes, for example, may be more frequent
in these habitats. A river may dry up, or change its course or nature in such a way that
the fish are driven extinct. The main coastal niche persists for longer. Thus when a new
freshwater tributary opens up, it is usually colonized from the main coastal population
rather than another freshwater population. The populations in the coastal niche have
a low extinction rate, and probably a higher speciation rate. The populations in the
freshwater tributaries have high extinction rates, and probably low speciation rates.
The difference in extinction rates is not a straight consequence of the characters of the
organisms. The coastal and freshwater populations have evolved different adaptations.
The different adaptations are associated with, but do not directly cause, differences in the
extinction rate. (By contrast, for instance, species with asexual reproduction go extinct
at higher rates than species with sexual reproduction (Section 12.1.4, p. 318). The difference
in extinction rate is partly a consequence of sexual and asexual reproduction.)
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