352 CHAPTER 14A functioning duplicate that becomes fixed in a population can meet one of sev-
eral ultimate fates [37, 44] (see Figure 14.9). If the duplicate is redundant and does
not provide a fitness benefit, it will be lost by deletion or the accumulation of loss-
of-function mutations. A second fate is that a duplicate can simply retain its origi-
nal function. A locus that has been amplified this way now has two (or more) cop-
ies in the genome. They can all be favored to continue functioning when there is
selection for increased expression of the gene’s product. Many insects have evolved
resistance to pesticides this way [40]. The insects have enzymes that degrade a
broad spectrum of substrates, including insecticides. In the presence of insecticide,
duplicates of those genes are favored because they increase gene expression and so
enable the insect to detoxify the pesticide more rapidly.
A strange form of inheritance occurs in some families of amplified genes. When
duplicates occur in tandem along the chromosome, the DNA replication machinery
sometimes gets confused and aligns one copy of a gene with a paralogous copy
nea rby. Gene conversion can then happen between the two genes, causing a mutation
in one locus to be copied to the other (see Chapter 4). A mutation at one locus can
spread this way through all the paralogs in the gene family. This process is called
concerted evolution. The ribosomal RNA genes exist in many copies in eukaryote
genomes. Gene conversion between the copies keeps their sequences from diverg-
ing within species at the same time that they diverge between species [20].
A third fate that can befall a duplicated gene is neofunctionalization. Here the
duplicate evolves a novel biological function. A crystallin in the lens of the ver-
tebrate eye originated by duplication of a heat-shock gene. Another example is
found in electric fishes. All teleost fishes have two duplicates of a sodium channel
gene that is expressed in muscles. In most teleost fishes, both genes are involved
in triggering muscle contractions. But in two families of fishes (the knifefishes and
elephant-nosed fishes), one of the paralogs has independently evolved an entirely
different function (FIGURE 14.10). Here it plays a key role in firing the electric
organ, which is a unique structure that enables these fishes to sense prey and com-
municate with each other in darkness [2].
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_14.10.ai Date 12-30-2016GasterosteusScn4aa Scn4aaTetraodonPorichthysEigenmanniaIctalurusDanioPantodonOsteoglossumGnathonemusXenomystus100 MyElectricElectricFIGURE 14.10 The neofunctionalization of a so-
dium channel gene was a key to the independent
origin of electric organs in two families of fishes.
The phylogeny shows representatives of 10 families
of distantly related teleost fishes, and the pie dia-
grams show the relative expression levels in muscle
tissue of two duplicated sodium channel genes,
Scn4aa and Scn4ab, that trigger muscle contrac-
tion. In South America an electric organ evolved
in the knifefishes (represented here by Eigenman-
nia virescens), while in Africa the organ evolved in
elephant-nosed fishes (represented by Gnathone-
mus petersii). In those two families, expression of
Scn4aa has been lost completely from the muscle.
Instead, the gene is expressed in the electric or-
gan, which gives these fishes a remarkable sensory
modality that allows them to hunt and communi-
cate in darkness. (After [63]; images from [63].)14_EVOL4E_CH14.indd 352 3/22/17 2:44 PM