Evolution, 4th Edition

T HE EvoluTion of GEnEs And GEnomEs 359

you start reading.) Overlapping genes are found in viruses and bacteria, and much

more rarely in eukaryotes, including humans. The selective advantage of overlap-

ping genes is debated. One hypothesis is that the overlapping arrangement allows

the genes to share their timing and level of expression. A second idea is that they

are favored because they allow the genome to be streamlined.

Chromosome Evolution

Before the rise of molecular genetics in the late twentieth century, much of what

we knew about evolutionary genetics came from studying chromosomes using the

light microscope. Species differ in their number of chromosomes and in how the

genes are arranged on them. The karyotype consists of the number and structure

of the chromosomes. How do changes in the karyotype evolve? Although many

questions are not yet answered, several patterns have emerged.

Fissions, fusions, and the evolution of chromosome number

The most common way for the number of chromosomes to change is when two

chromosomes fuse, reducing the haploid chromosome number by one, or when they

fission, increasing the number by one. Humans have 23 pairs of chromosomes, but

all of the other great apes have 24 pairs. At some point since our lineage split from

that of chimpanzees, two chromosomes fused. That mutant chromosome spread

throughout our species, resulting in the second-largest chromosome in the human

karyotype. We do not yet know what caused it to spread: it may have had a boost

from positive selection, or it may have simply drifted to fixation.

We do, however, have a good idea about how fissions and fusions evolve in

the house mouse (Mus musculus). Here karyotype evolution is on the fast track:

changes in chromosome number are evolving at rates hundreds of times faster

than in most other mammals [50]. Populations can have anywhere between 11 and

20 chromosome pairs. What accounts for this chromosomal chaos? The answer

seems to be meiotic drive and selection that has favored selfish

genes rather than the fitness of individuals [54]. During meio-

sis in females, one haploid set of chromosomes is transmitted

to the egg while the other set enters the polar body, where it

dies (FIGURE 14.15). When a female mouse is heterozygous for

a fusion, the fused chromosome competes with the two unfused

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_14.15.ai Date 02-02-2017

(A)

(B)

Polar body has

stronger spindle

Polar

body

Egg

Egg has

stronger spindle

0.25 0.5 0.75 1

Fraction of unfused chromosomes

Expected

Number of species

400

0

200

Observed

Spindle

Fused

chromosomes

Unfused

FIGURE 14.15 Female meiotic drive can favor chromosomes that chromosomes

are either fused or unfused. During meiosis, one haploid set of chro-

mosomes enters the egg while the other enters a polar body, which

dies. (A) It is hypothesized that in some populations and species, the

polar body has a stronger meiotic spindle than the egg (left). In in-

dividuals that are heterozygous for a fused chromosome and its two

unfused homologues, the unfused chromosomes are more likely

to attach to a polar body spindle, giving an advantage to the fused

chromosome. In other populations and species, the egg has a stron-

ger spindle, which favors the unfused pair of chromosomes (right).

(B) Consistent with the meiotic drive hypothesis, mammals tend to

have either most of their chromosomes fused or most unfused. The

bars show the frequencies of unfused chromosomes in the genomes

of 1170 species of mammals. The dots show the distribution ex-

pected if pairs of chromosomes fused independently of each other,

which is the null hypothesis in the absence of meiotic drive. (A after

[66]; B after [54].)

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