356 CHAPTER 14coefficient evolves almost as if it is selectively neutral, and so can be fixed by drift.
As a result, more evolutionary change is caused by drift and less by positive selec-
tion in humans than in D. melanogaster. Some microbes have population sizes that
are vastly larger than those of fruit flies, and evolution in those organisms is even
more strongly dominated by selection. In each generation, moreover, more benefi-
cial mutations enter a big population than a small population, which further tilts the
balance of molecular evolution away from drift and toward adaptation.
Earlier we discussed how the evolution of synonymous changes by drift often
makes a big contribution to the divergence between the DNA sequences of spe-
cies. In fact, selection also plays a role in the evolution of synonymous mutations.
The different codons that correspond to the same amino acid appear at different
frequencies in the genome, a phenomenon called codon bias. One cause of this
bias is mutation. Mutations from the DNA base G to A and from C to T are more
than twice as common as other types of mutations [26, 45]. This tends to favor the
accumulation of codons with A and T bases. A second cause of codon bias is natu-
ral selection [57]. Because they do not change the protein, synonymous mutations
were long thought to be selectively neutral. In fact, they can have minute effects
on fitness. The translation of highly expressed genes is most efficient when their
codons correspond to transfer RNAs that are common in the cytosol, so selection
favors those codons. Selection can also favor codons that produce messages that
are less prone to translation errors. The relative strengths of the forces, and so the
direction and degree of codon bias, differ among taxa. Codon bias driven by selec-
tion tends to be stronger in genes that are highly expressed and in species with
very large population sizes, such as free-living microbes. In species with smaller
population sizes (such as vertebrates), drift overwhelms whatever selection acts on
synonymous mutations, and codon bias is very weak or absent.Evolution of Gene Expression
The mosquito Aedes aegypti is the vector of Zika virus, dengue fever, yellow fever,
and chikungunya virus—diseases that together kill some 50,000 people each year.
In East Africa, there are two genetically distinct types of mosquitoes (FIGURE 14.12).
The domestic type specializes in biting humans (whom the mosquitoes infect), while
the forest type feeds on other animals. A key adaptation that enables the domesticFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_14.13.ai Date 12-29-2016DM: Photo in (A) is 300dpi at this size.0(B)(A)Relative preferenceHumanGuinea pigDomestic ForestFIGURE 14.12 The mosquito Aedes aegypti is the vector that
transmits the Zika virus, dengue fever, yellow fever, and chikun-
gunya virus. (A) In East Africa the mosquito has a domestic and a
forest form. (B) Among its adaptations to feeding on humans, the
domestic form is attracted to human body odor. This adaptation
involves changes both to the structure and the expression level
of Or4, a receptor on the mosquito’s antenna that is sensitive to
an odor distinctive to humans. The graph shows results from an
experiment in which mosquitoes could respond to the odor of
either a human or a guinea pig. Each vertical bar represents the
relative preference of a different laboratory colony of mosquitoes
that was established from a small number of mosquitoes sampled
from Rabai, Kenya. (Colonies are arranged from those that most
prefer humans to those that most prefer guinea pigs.) All colonies
of the domestic form (red bars) preferred the human odor, while
all but one of the colonies of the forest form (blue bars) preferred
the guinea pig odor. (After [47].)14_EVOL4E_CH14.indd 356 3/22/17 2:44 PM