72 CHAPTER 3a fixed amount of available energy or nutrients, a plant species
might evolve higher seed numbers, but only by reducing the
size of its seeds or some other part of its structure. If genotypes
differ in reproductive output, one would see a negative correla-
tion between seed number and seed size: the greater the num-
ber, the lower the average size (see Chapter 11).
In some cases, it appears that adaptations have not
evolved because of a shortage of suitable mutations [2]. For
example, although some species of grasses have rapidly
evolved tolerance to heavy metals in the vicinity of mines,
other species have not. In large samples of seeds collected
from grass populations on normal soils, far from copper
mines, a small percentage of seeds produced copper-tolerant
seedlings in every one of eight species that have evolved
copper tolerance near mines, but in none of seven species
that have failed to do so [2]. These species apparently lack
the genetic variation in tolerance that would be necessary for
adaptive evolution. Lack of suitable genetic variation may
explain cases of so-called phylogenetic constraints, in which
species retain nonadaptive features or are unable to evolve
adaptive traits. It makes adaptive sense that birds such as
swans have more neck vertebrae than birds with shorter
necks. But almost all mammals have seven neck vertebrae,
including giraffes, the aquatic dugongs, and whales, despite the extreme differ-
ence in the lengths of their necks (FIGURE 3.21). Individual mice and humans
with an aberrant number of cervical vertebrae show various skeletal abnormali-
ties and a high incidence of embryonic cancer—harmful side effects that prob-
ably prevented the evolution, in other mammals, of what might otherwise have
been advantageous changes in vertebral number [15, 50].Natural Selection and the Evolution of diversity
A mechanic uses a variety of different wrenches because each is suited to a dif-
ferent task. Likewise, any characteristic of an organism is likely to be advanta-
geous under some circumstances but not others. That is, the optimal feature, the
character that maximizes fitness, depends on the context in which it functions. A
simple, even obvious, example is provided by many instances of cryptic coloration
(camouflage) in animals, whereby colors and patterns that match the background
lower the likelihood that an animal will be detected by predators. For example,
darker populations of many species of animals inhabit areas with darker rocks
than do pale populations. In a species of pocket mouse, this difference is based
on a single gene, Mc1r (see Figure 6.29).
Both on the land and in the sea, the variety of different environments organ-
isms face is immense. There are major differences in physical conditions among
geographic regions and over even short distances, in which a species may encoun-
ter different sets of prey, predators, parasites, and competitors. Different parts of
the human body are different environments for bacteria, and support very differ-
ent, diverse bacterial communities. Any of these variables may be relevant to a
particular species and impose natural selection on many of its features, so natural
selection is the ultimate cause of divergence among populations and species: it is
the source of the immense diversity of life.
Darwin, in considering why the various species descended from a common
ancestor should become different from one another, drew special attention to
the role of competition for limiting resources, such as food. He postulated thatFutuyma Kirkpatrick Evolution, 4ev
Sinauer Associates
Troutt Visual Services
Evolution4e_03.21.ai Date 11-28-2016FIGURE 3.21 Almost all mammals,
including the long-necked giraffe and
the short-necked aquatic dugong, have
seven neck vertebrae, a likely example of
a phylogenetic constraint.03_EVOL4E_CH03.indd 72 3/22/17 1:19 PM