364 CHAPTER 14coding sequence or its proper expression. A second problem for the host is that
two copies of a TE at different places in the genome can recombine, causing a
chromosomal mutation. (TEs tend to accumulate in regions of the genome that
have low recombination rates because there they are less likely to cause this prob-
lem and so be selected out of the population.) Host genomes have evolved several
ways to fight against the spread of TEs. When they succeed, the host genome is
left with the aftermath of the battle: dead TEs that can no longer spread but that
still fill up much of the genome. That is the current state of our own genome. Our
ancestors evolved ways to shut down most of the movement of TEs. Figure 14.19
shows that currently the activity of human transposons is much lower than in
the past. But occasionally a human TE is able to reproduce. When it does, genetic
diseases can result [5].
Although transposable elements are largely harmful to their hosts, they occa-
sionally produce a beneficial mutation. Chapter 5 recounts the evolution of melanic
coloration in the peppered moth (Biston betularia), which is perhaps the most
famous example of evolution observed in action. Recent research has revealed that
the mutation responsible for the melanism is a transposon whose insertion altered
the expression of a gene called cortex [29]. On a much grander scale, TEs may be
responsible for a fundamental feature of the eukaryotic genome. The molecular
machinery used to splice introns out of messenger RNA is also used by TEs. In
fact, some introns contribute to their own splicing. These facts suggest that TEs
may have been responsible for the evolutionary origin of introns, which are now
so essential to gene regulation [44]. If that hypothesis is correct, introns will be the
most spectacular example imaginable of a “bug” that was turned into a feature.Routes to the evolution of the smallest
and largest genomes
This chapter opened by looking at the dramatic variation in the sizes of different
genomes. While we understand some of the factors responsible, there is still much
uncertainty (and debate) among evolutionary biologists about how genome size
evolves.
The very smallest genomes are found in viruses (see Figure 14.18). These
genomes have very little or no noncoding DNA. They have been highly stream-
lined by the deletion of genes that are essential to life in free-living organisms, and
they reproduce by hijacking gene products produced by their hosts. Many viruses
are in a race to replicate, which favors reducing the genome to a minimal size.
The bacterium Buchnera aphidicola has evolved a small genome for very dif-
ferent reasons. It has one of the smallest genomes of any bacterium, with only
about 500 genes and 600 kb of DNA. Buchnera developed a symbiotic relation with
aphids more than 160 Mya (see Figure 13.4). Since then, it has lived entirely within
its host, and is transmitted vertically from mother aphids to their offspring. Like
viruses, Buchnera uses metabolic products provided by its hosts, which allows its
genome to do without many essential genes. Unlike viruses, replication of these
symbionts is typically limited by that of their host, so there is little or no fitness
benefit to speed the genome’s replication. The population sizes of Buchnera are
much smaller than those of most free-living prokaryotes, and they experience very
strong drift. If deletion of a gene decreases fitness but is not lethal, it can become
fixed by chance. Deletions are more frequent than duplications in bacteria, so
genome reduction is more likely than expansion. This creates a ratchetlike process
in which the genome’s size evolves downward until it teeters on the brink of its
own destruction: the minimal size necessary for survival [48].14_EVOL4E_CH14.indd 364 3/22/17 2:44 PM