THE EvoluTion of GEnEs And GEnomEs 361
are responsible [36]. First, an inversion is produced when a chromosome
breaks at two places and the middle segment is then reinserted back-
ward. The break points can disrupt a gene or alter its expression. Occa-
sionally this generates a beneficial mutation that causes the inversion to spread by
positive selection. Second, some inversions benefit from meiotic drive. When such
an inversion is heterozygous (that is, paired with a chromosome that lacks the
inversion), it is transmitted to the gamete more than 50 percent of the time. This
favors the inversion to spread just as if it increased survival.
Inversions can also increase in frequency because of their effects on recombi-
nation [36]. When an inversion is heterozygous, recombination is blocked in the
inverted region of the chromosome. This can bind together a favorable combina-
tion of alleles at several loci, causing the inversion to spread. The ruff (Philoma-
chus pugnax) is a sandpiper with three male morphs that use different strategies to
obtain mates (FIGURE 14.17). The morphs differ dramatically in plumage, behav-
ior, and body size. These phenotypes are determined by a chromosome inversion
that carries alleles that code for combinations of feather colors and reproductive
hormones that determine each morph’s mating strategy [39]. (One of the loci
is Mc1r, which is involved in adaptive changes to coloration in many groups of
animals—see Figure 6.29.) Inversions can also be established when they capture
alleles that are beneficial in particular environments.
Last, an inversion can spread by random genetic drift. You saw in Chapter 5
that some inversions are underdominant: they decrease fitness when heterozy-
gous. When an underdominant inversion is rare, as when it first appears, selection
acts to eliminate it from the population. But if drift causes it to reach a frequency
greater than 50 percent, then selection favors it to spread to fixation. Inversions
established this way in one population will cause interbreeding with other popula-
tions to produce heterozygote offspring with low fitness. This can generate genetic
isolation between the populations and contribute to speciation (see Chapter 9).Evolution of Genome size and Content
More than 98 percent of our genome does not code for a protein or other gene
product (see Figure 14.14). In other eukaryotes, the fraction is even larger. Many
scientists think that the noncoding part of a genome is largely “junk DNA” that
has no function that is useful to the organism. Where does all this noncoding
DNA come from? And is it really junk?Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_14.17.ai Date 12-29-2016(A)(B)Female IndependentInverted regionSatelliteGenesFIGURE 14.17 Chromosome inversions in the ruff (Philomachus pugnax) are
responsible for dramatically different male morphs. (A) The three morphs,
which differ in plumage, behavior, and body size, represent alternative mat-
ing strategies. “Independent” males are territorial, dominant, and display
to attract females; “satellite” males are nonterritorial and submissive; and
“faeder” males (which look much like the female shown here) obtain sneaky
copulations by mimicking females to avoid aggression from independent
males. (B) Satellite and faeder males are heterozygous for a 4.5-Mb chromo-
somal inversion that independent males do not carry. The inversion spans
many loci (indicated by the triangles), some of which affect plumage and sex
hormones. Recombination is suppressed between inverted and uninverted
chromosomes, which binds together alleles that determine each of the three
morph phenotypes. The inversion carried by satellite males has different
alleles than the inversion carried by faeder males, which accounts for the dif-
ferences between those two morphs. (After [39].)14_EVOL4E_CH14.indd 361 3/22/17 2:44 PM