Evolution, 4th Edition

206 CHAPTER 8

meadow appears, for example after a fire, it is colonized by butterflies that disperse

from neighboring patches. The colonizers are not a random sample of the popula-

tion: they are strong fliers that disperse well. Flight strength is strongly heritable,

so the population that is established in a new patch has strong fliers for the next

several generations. But in later generations, the population evolves toward weaker

and weaker flight. Why? Simply because the strongest fliers disperse, leaving the

weaker fliers at home [11].

The Glanville fritillary illustrates an interesting consequence of dispersal: it

sorts individuals according to how well they disperse. As a result, dispersal can

cause evolutionary change even without the help of natural selection. A dramatic

example of this effect comes from the cane toad (Rhinella marina) [27]. The toad

was introduced to Australia from South America in 1935 and has been rapidly

expanding its range ever since. Because of the sorting effect, we would predict that

toads at the leading edge of the invasion should disperse better than those from

older, established populations. That is exactly what is seen: frogs at the leading

edge have longer legs, and frogs with longer legs disperse faster than their short-

legged conspecifics (FIGURE 8.16).

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FIGURE 8.15 Roesel's bush-cricket

(Metrioptera roeselii) has two morphs,

one with normal wings (left) and one

with highly reduced wings (right). The

morph with normal wings can disperse

long distances by flying, but has lower

fecundity because of the energy it in-

vests in the growth and maintenance of

flight muscles.

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Evolution4e_08.16.ai Date 03-16-17

Relative leg length

(^1200400600800)
Order of arrival
1.0
1.1
0.9
(A) (B)
Rate of spread (km/y)
1955–
1964
1965–
1974
1980–
1984
1945–
1954
Year
30
40
50
20
10
0
60
2001–
2005
FIGURE 8.16 When a species’ range expands, the most rapidly dispersing genotypes
automatically become more common at the range edge, causing the rate of spread
to increase. (A) During its range expansion in Australia, the first cane toads (Rhinella
marina) to arrive at a research station had the longest legs, while those that arrived later
had shorter legs. Toads with longer legs disperse faster because they jump farther.
(B) As a result of the sorting process, the advancing front of the cane toad invasion has
accelerated since the toad was introduced in 1935. (After [27].)
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