T HE EvoluTion of GEnEs And GEnomEs 351
useful protein seems vanishingly small. But as unlikely as that is, recent research
has uncovered new genes in organisms ranging from yeast to humans that did
indeed originate from noncoding DNA [24, 61]. These are called de novo genes,
to distinguish them from genes that are born by the much more common routes of
gene duplication and HGT. The best-studied examples are in Drosophila. Many of
their de novo genes are expressed in testis, which is consistent with other evidence
that sexual selection on males is a powerful driver of evolutionary change in many
animals. The origin of de novo genes may be as simple as a mutation at a single
base that by chance turns on transcription of a downstream stretch of DNA that
fortuitously codes for a protein that enhances fitness.
No matter how it is born, a new gene starts its life as a single copy in the popu-
lation. As with any new mutation, by far the most likely outcome is that it will be
lost by random drift or selection over the next few generations (see Chapter 7). A
very small fraction of new genes manage to spread and become fixed in the spe-
cies (FIGURE 14.9). Even those that do usually do not survive very long—they are
later destroyed by mutations, as you will see below. A very small fraction of new
genes remain functional and become permanent features of the genome. Often
these genes survive because they evolve a new adaptive function, like the RNASE1
duplicate in the douc langur.Gene families
The loci that encode hemoglobins are members of a gene family, which is a set of
loci that arose by duplication and that code for proteins that typically continue to
have similar biochemical function. Two or more genes that originated by duplication
are said to be paralogs (see Fig u re 2.14). Some gene families, such as the hemoglo-
bins, have many paralogs created by several duplication events that were widely
separated in time (see Fig ure 2.15). In other cases, the rapid growth of a gene fam-
ily can contribute to a novel adaptation. Male stickleback fishes construct nests in
which females lay their eggs. The nest is held together by a glue that the males
secrete. This glue is produced by a family of genes called spiggin that are recent
duplicates of a single gene that made mucus in the ancestor of sticklebacks [35].
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_14.09.ai Date 01-02-2017Dup Dup Del NeoSingle
copyCopy number
variationGene
familyTimeFrequencyABAAAAAAFIGURE 14.9 ates of gene duplicates. Gene duplication mutations (“Dup”) produce F
chromosomes carrying an extra copy of locus A. Copy number variation occurs when
a duplicate is polymorphic: chromosomes with different numbers of copies of locus A
are present in the population. Some duplicates that become fixed retain their original
function and serve to upregulate the total amount of gene product that is produced.
A functioning duplicate that has become fixed can later be lost by a mutation that
deletes it or that renders it a pseudogene (“Del”). A duplicate can also acquire a muta-
tion that leads to neofunctionalization or subfunctionalization (“Neo”). If this mutation
becomes fixed, a gene family is produced. (After [31].)14_EVOL4E_CH14.indd 351 3/22/17 2:44 PM