PHylogENy: THE UNITy ANd dIvERSITy of lIfE 405
Because many phenotypic traits are genetically variable within species (see
Chapter 6), they can readily evolve by natural selection and genetic drift. It is not
surprising that size, shape, coloration, and many other traits undergo convergent
evolution (similar evolutionary changes) in diverse lineages (FIGURE 16.4). T he
same holds for mutations. Because there are only four possible states of a particular
site in a DNA sequence, exactly the same mutation will occur repeatedly over suf-
ficiently long periods of evolutionary time, and some mutations will be reversals
to the ancestral state. For this reason, a single base pair difference among species
provides little reliable evidence about their phylogeny. We require, instead, many
differences. The more derived mutations there are that are shared between two
species, the less likely it is that they all arose and were fixed twice. Furthermore,
adding taxa to the analysis can often help us detect evolutionary reversals. Sup-
pose that the only insects we included in the phylogeny in Figure 16.3 were fleas,
dragonflies, silverfishes, and bristletails. We could err by supposing that fleas are
more closely related to silverfishes or bristletails, because they all lack wings, than
they are to dragonflies. But fleas share many features with some groups of winged
insects, as shown in Figure 16.3; for example, fleas and flies both have complete
metamorphosis (larval and pupal stages). Including flies and other insects shows
us that fleas are not closely related to silverfishes, and also tells us that fleas
reverted to the wingless condition from a winged ancestor.
If a site can undergo a substitution twice, it can do so again and again, over a
sufficiently long time: it can change from A to C, then from C to T, and even back
to A. Thus the number of differences between species may be less than the number
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_1604.ai Date 11-02-2016(B)(A) (C) (E)(D) (F)FIGURE 16.4 Convergent evolution: in many plant lineages, bilaterally symmetrical
flowers have evolved from radially symmetrical ancestors. Bilateral symmetry is an ad-
aptation to pollination by bees, which are more attracted to such flowers. Examples of
three plant families with bilaterally symmetrical flowers (above) are paired with related
families (below) that retain radial symmetry, the ancestral condition: (A) Orchidaceae
(orchids) and (B) Liliaceae (lilies); (C) Fabaceae (pea family) and (D) Rosaceae (roses,
cherries); (E) Violaceae (violet family) and (F) Passifloraceae (passionflower family).16_EVOL4E_CH16.indd 405 3/22/17 1:33 PM