Evolution, 4th Edition

90 CHAPTER 4

of three DNA bases (CAG) into the huntingtin gene. Other insertions have played

important roles in adaptation.

A duplication is a mutation in which a second copy of a gene is inserted into the

genome. This process can be repeated, giving rise to a gene family with several

copies of the original locus. In some cases, the DNA sequences of the duplicate

loci diverge and lead to the evolution of new biological functions (see C hapter 14).

Less often, duplicates retain the same sequence. This outcome can result from gene

conversion, which is an unusual type of mutation in which the DNA sequence of

one duplicate in a gene family is replaced by the sequence of another.

Inversions are structural mutations that occur when a chromosome breaks in

two places and the middle segment is reinserted in the reverse orientation. Inver-

sions played an important role in the development of evolutionary genetics in the

first half of the twentieth century when it was discovered that inversions in fruit

flies (Drosophila) can be seen under the light microscope. The geneticist Theodosius

Dobzhansky, who was one of the leaders of the modern synthesis (see Chapter 1),

pioneered the use of inversions to study the evolution of genetic differences within

and between species [6]. With the advent of powerful DNA sequencing technolo-

gies in the twenty-first century, it has become clear that inversions are a common

feature in the evolution of many species. The genomes of humans and chimpanzees

differ in about 1,500 chromosome inversions that became fixed in one lineage or

the other since our last common ancestor roughly 7 million years ago (Mya) [9].

A reciprocal translocation is the exchange of chromosome segments between two

nonhomologous chromosomes. Translocation heterozygotes can have reduced fer-

tility, which contributes to the genetic isolation between some closely related species

(see Chapter 9).

Fusions are structural changes in which two nonhomologous chromosomes are

joined. Fissions are the opposite type of mutation, in which one chromosome breaks

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_04.14.ai Date 11-02-2016

(A) n = 1

(C) n = 630

(B) n = 23

FIGURE 4.14 Variation in haploid chromosome (D) n = 16,000

numbers. Clockwise from top left: a jack jumper ant

(Myrmecia pilosula), a human, a fern (Ophioglossum

reticulatum), and a ciliate (Oxytricha trifallax).

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