T HE EvoluTion of GEnEs And GEnomEs 353
Last, a duplicate gene and its parental copy can subfunctionalize so that each
carries out only some of the roles that the ancestral gene performed. In most ver-
tebrates, the same hemoglobin molecule transports oxygen in the bloodstream at
all stages of life. In mammals, however, a duplicate of the β-hemoglobin locus is
expressed in the fetus [62]. The fetal hemoglobin has evolved differences that give
it a higher affinity for oxygen than the adult hemoglobin. This enables the fetus
to strip oxygen from its mother’s blood in the placenta. A few months after birth,
expression of the fetal hemoglobin is shut off, and the adult hemoglobin takes over
the task of oxygen transport. The evolution of the specialized fetal hemoglobin by
subfunctionalization was one key to the origin of live birth in mammals.The death of a Gene
There are many ways that a gene can die. When a gene is duplicated, the new copy
is often dead on arrival. The duplicate many not include the entire gene. Even if it
does, if often lacks the regulatory elements needed to express it at the right time
and in the right places. Duplicates produced by retrotransposition face an addi-
tional challenge. Since they lack introns, there is no opportunity for posttranscrip-
tional regulation that involves splicing.
When a nonfunctioning duplicate is fixed in the population, or a functioning
gene becomes nonfunctional, the result is a genetic skeleton called a pseudogene.
The genomes of many species are littered with pseudogenes. Our own genome
has almost as many pseudogenes as functional genes [44]. Although they serve
no function, pseudogenes are useful to evolutionary biologists as a sort of natural
controlled experiment. The parental gene from which a pseudogene originated
continues to evolve under the forces of selection. But the pseudogene does not pro-
duce a functioning product, so it is freed from selection. Comparing the sequences
of pseudogenes with those of their parental genes reveals that pseudogenes typi-
cally evolve much more quickly (see F i g u r e 7.19). This shows that most mutations
to functioning genes reduce fitness and are removed from the population by puri-
fying selection (that is, selection against deleterious mutations). In contrast, muta-
tions in a pseudogene are selectively neutral and so they are free to drift to fixation.
Deletions are important in shaping the genome (see Chapter 4). Deletions are
indiscriminate: they will sometimes eliminate part of a gene, all of a gene, or even
a large piece of chromosome that carries many genes. The loss of genes by deletion
and pseudogenization contributes to the divergence in the genetic content of the
genomes of related species (see Figure 14.5).
It may seem surprising that a deletion can sometimes be beneficial. Attached to
the surface of human white blood cells is a protein called CCR5 that plays a role in
inflammatory response. Unfortunately, this protein is also a key to human immu-
nodeficiency virus (HIV) infection. The virus binds to a loop of the CCR5 protein
and then enters the cell. Some people have an allele called Δ 32 at the CCR5 locus
in which 32 bp are deleted. The missing part of the gene codes for the loop in the
protein to which the virus attaches. People who are homozygous for the deletion are
highly resistant to HIV, while heterozygotes are partly resistant. The deletion has
spread in some populations by positive selection that occurred long before HIV first
infected humans, probably because of the resistance it confers to other diseases [19].
Deletion plays a key role in gene trafficking, which is the movement of genes
to new sites in the genome. When a functioning gene duplicate is established at a
new location, its parental copy is sometimes lost by deletion. The result is that the
gene has moved to a new address in the genome. Although the probability that a
gene will move by gene trafficking is very low, over millions of years gene traf-
ficking can cause certain genes to accumulate on certain chromosomes. In fruit14_EVOL4E_CH14.indd 353 3/22/17 2:44 PM