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Evolution__3rd_Edition

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664 PART 5 / Macroevolution<br />

Hansen’s work has since been<br />

criticized<br />

Sticklebacks that occupy longlasting<br />

estuarine niches have low<br />

extinction rates<br />

Since Hansen’s work, Duda & Palumbi (1999) have cast doubt on one assumption of<br />

his argument. They show that, in a group of modern snails, species with planktonic<br />

development have repeatedly evolved from ancestral species with direct development.<br />

For the trend in Figure 23.9 to be driven by differences in speciation rate, it is important<br />

that the ancestor–descendant lines of species tend to retain the same mode of development.<br />

(In technical language, heritability is required at the level of species.) Duda and<br />

Palumbi’s result for modern species suggests that the expanding group of species with<br />

direct development may not have been a clade with a constant mode of development.<br />

Species with direct development may have arisen from ancestors with planktonic<br />

development. Currently it is uncertain whether, as Hansen originally argued, the<br />

decline in planktonically developing forms in the early Tertiary occurred because they<br />

had a low speciation rate.<br />

23.6.2 Differences in the persistence of ecological niches will influence<br />

macroevolutionary patterns<br />

In the previous section, we considered the possibility that a character (larval type)<br />

might influence speciation, and extinction, rates. The influence, if real, is a straight<br />

consequence of the character itself: species in which there is direct development are<br />

more likely to split, in the process of allopatric speciation, than species in which there is<br />

planktonic development. A second factor that can influence speciation and extinction<br />

rates is the nature of the ecological niche occupied by species. Species that occupy<br />

niches that last longer will have lower extinction rates than species that occupy shortlived<br />

niches. Williams (1992) introduced this idea in terms of a concrete example a the<br />

three-spined stickleback (Gasterosteus aculeatus).<br />

The three-spined stickleback is a fish with a widespread distribution in coastal waters<br />

in the northern hemisphere, on both sides of the North Atlantic and Pacific Oceans.<br />

From these coastal waters (it appears), many populations have separately colonized the<br />

local freshwater rivers and their tributaries inland. Some of these freshwater populations<br />

have been studied and they show various local adaptations to the rivers they<br />

occupy and have formed a complex set of local races or subspecies.<br />

However, the populations that colonize the freshwater rivers are probably evolutionarily<br />

short lived. Ecological and geographic changes, for example, may be more frequent<br />

in these habitats. A river may dry up, or change its course or nature in such a way that<br />

the fish are driven extinct. The main coastal niche persists for longer. Thus when a new<br />

freshwater tributary opens up, it is usually colonized from the main coastal population<br />

rather than another freshwater population. The populations in the coastal niche have<br />

a low extinction rate, and probably a higher speciation rate. The populations in the<br />

freshwater tributaries have high extinction rates, and probably low speciation rates.<br />

The difference in extinction rates is not a straight consequence of the characters of the<br />

organisms. The coastal and freshwater populations have evolved different adaptations.<br />

The different adaptations are associated with, but do not directly cause, differences in the<br />

extinction rate. (By contrast, for instance, species with asexual reproduction go extinct<br />

at higher rates than species with sexual reproduction (Section 12.1.4, p. 318). The difference<br />

in extinction rate is partly a consequence of sexual and asexual reproduction.)<br />

..

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