Evolution, 4th Edition

360 CHAPTER 14

chromosomes for transmission to the egg. Whether the fused or unfused chro-

mosomes win this battle depends on which of them attaches most strongly to the

meiotic spindle of the egg and which to the spindle of a polar body. In some mouse

populations, unfused chromosomes attach more often to the egg’s spindle, caus-

ing the mouse karyotype to evolve rapidly toward 20 pairs of unfused chromo-

somes. But in other populations, changes to the machinery of cell division turn the

tables, and then meiosis favors 11 pairs of fused chromosomes. Remarkably, when

this happens it doesn’t matter which chromosomes fuse—almost every possible

pair of unfused autosomes have been fused in different mouse populations [50].

It appears that the same shifts between karyotypes made up mainly of fused or of

unfused chromosomes are playing out at a slower evolutionary tempo across all

mammals. As a result, species tend to have either most of their autosomes fused or

most unfused (see Figure 14.15).

While the details of how meiotic drive causes fusions and fissions to evolve are

a bit complex, the bigger message of this story is simple. Some of the most basic

features of the genome, including the number of chromosomes in the karyotype,

are not refined adaptations that enhance survival and reproduction. Instead, they

are the messy outcomes of competing evolutionary processes that act at different

levels of selection.

Inversions and the evolution of chromosome structure

When geneticists began to study the chromosomes of fruit flies in the 1930s, they

saw banding patterns that vary within and among species (FIGURE 14.16). Closer

study revealed that many of these differences result from chromosome inversions

(see Chapter 4). A chromosome with an inversion has the same genes as one with-

out it, but they are in a different order.

If chromosomes with and without an inversion have the same genetic con-

tent, what could cause an inversion to spread in a species? Several mechanisms

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_14.16..ai Date 12-29-2016

Chromosome arm

A B C D E

D. melanogaster

D. erecta

D. ananassae

D. pseudoobscura

D. willistoni

D. virilis

D. mojavensis

D. grimshawi

(A)

(B)

FIGURE 14.16 Chromosome rearrangements in fruit flies. (A) The

third chromosome of Drosophila pseudoobscura as seen under

a light microscope. The banding patterns, which result from

staining the DNA, are altered by chromosome rearrangements.

This species is famous for being highly polymorphic for chromo-

some inversions, which were studied intensively by the famous

evolutionary geneticist Theodosius Dobzhansky during the

mid-twentieth century. Inversion heterozygotes produce loops in

their chromosomes, as seen in this picture. (B) Differences in the

gene order among eight Drosophila species that have resulted

from rearrangements. The phylogeny for the species is shown at

left. For each species, individual genes are indicated by colored

vertical lines, and chromosomal arms are shown in different

colors. The lines connecting the genomes of adjacent species

link homologous genes. Crossing lines show that two species

differ in a chromosome inversion. A translocation can be seen in

D. pseudoobscura that moved a segment of arm A to arm D. (A,

chromosome from [15b]; B from [7].)

14_EVOL4E_CH14.indd 360 3/22/17 2:44 PM