Clover is said to confer good luck to any person who finds a four-leafed
plant, but for herbivores coming across a clover is less fortunate. When white
clover (Trifolium repens) is eaten, vacuoles in its leaves break open and release
compounds that produce cyanide, a chemical that tastes bad and is highly
poisonous to herbivores (as well as to secret agents). This protects the clover,
since a taste of cyanide quickly persuades most herbivores to feed on a dif-
ferent plant. This is clearly good for clover, but leaves us with a mystery: the
genes needed to produce cyanide have been deleted in clover growing in
cold climates (FIGURE 8.1) [25]. Why should clover lose the genes that help
defend it? One hypothesis is that when the leaves freeze, the vacuoles in
the leaves break open. Plants with the genes to make cyanide poison them-
selves, while those missing those genes survive. If this explanation is correct,
then cyanide-producing vacuoles are beneficial in warm regions but suicidal
in cold ones.
One of the most common features of life on Earth is that species vary geo-
graphically. In cases like the white clover, the patterns reflect selection pres-
sures that vary with climate, and therefore in space. Other evolutionary factors
also shape the spatial variation in phenotypes. Studying that variation helps us
understand the interplay among selection, genetic drift, and the movement of
individuals. Patterns of variation in space also give insights into variation in time,
that is, evolution itself.8
These puffballs (Calostoma cinnabarina) are fungi that eject spores which disperse pas-
sively on currents of air. Interactions among dispersal, selection, and other evolutionary
factors produce patterns of spatial variation in the characteristics of almost all species.Evolution in Space
Space
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