240 ■ CHAPTER 13 Adaptation and SpeciesEVOLUTION
Another example of coevolution is the relation-
ship between hummingbirds and certain species
of flowers (Figure 13.11).
Alternatively, or in addition to hosting specific
algae, Hellberg hypothesizes that the shallow-wa-
ter and deep-water sea fans may have adapted to
the type and density of sedimentation found at
different ocean depths. Or perhaps predators are
more abundant at certain depths. Whatever the
major habitat factors driving the corals’ adapta-
tion, natural selection led to the formation of two
different coral populations tailored to the ocean
depths at which they grow.
To d ay, He l l b e r g c o n s id e r s t h e t w o p o p u-
lations different species. “What makes this
interesting is that you have populations that
could potentially interbreed, yet they main-
tain their differences in the face of each other,”
says Hellberg. “For just about any biological
question you want to ask, they are indepen-
dent entities.” The formation of new species
in the absence of geographic isolation is called
sympatric speciation (from sym, “together”;
Figure 13.12). Sympatric speciation is a
particularly important process in plants
(see “So Many Chromosomes”).
Still, a question lingers: Why don’t the two
coral species interbreed? They appear to be
capable of it, because of evidence of some geneticFlowers that coevolved with
hummingbirds produce
nectar, are colored to attract
the birds, and are shaped
specifically to fit the
hummingbirds’ bills.
Figure 13.11
Coevolution at its finest
This hummingbird’s bill fits perfectly into this
flower for easy access to the hummingbird’s
favorite food, nectar. The hummingbird feasts and
then distributes the flower’s pollen by carrying
the pollen along to its next meal. It’s a win-win
situation!Q1: Describe how coevolution, as with the
hummingbird bill and hummingbird-pollinated
flowers, is different from the kind of
evolution described in Chapters 11 and 12.Q2: Is coevolution the same thing
as convergent evolution, described in
Chapter 12? Why or why not?Q3: Do you think one species’ adapting
over time to feed specifically and extremely
successfully on another species is an
example of coevolution? Why or why not?N
ew plant species can form in a single generation as a result of
polyploidy, a condition in which an individual gains an extra full set
or two (or three) of chromosomes. Humans and most other eukaryotes
are diploid (having two sets of chromosomes), but some organisms are
triploid (three sets) or tetraploid (four sets), or have an even higher number
of chromosomes. Polyploidy is invariably fatal in people, but in many plant
species it is not lethal.Haploid (n) Diploid (2n) Triploid (3n) Tetraploid (4n)Polyploidy can occur when two different species hybridize and
produce an offspring with an odd number of chromosomes, because of
improper alignment of the chromosomes during mitosis (see Chapter
6). This increase in chromosome sets can lead to reproductive isolation
because the chromosome number in the gametes of the new polyploid
individual no longer matches the number in the gametes of either of its
parents.
Polyploidy has had a large effect on life on Earth: more than
half of all plant species alive today are descended from species
that originated by polyploidy. A few animal species also appear to
have originated by polyploidy, including several species of lizards,
salamanders, and fish.