>>In the earlier lectures we showed how evolution within species using population genetics really
has a lot of practical importance. For example, by its very definition these studies of natural
selection are studies of reproductive health. But what about the practical importance of
evolutionary studies between species? As a one-credit course, I wanted to keep this focused
on humans, so we won't go into the fact that we use evolution to predict evolving pathogens
like viruses and bacteria, but instead let's stay closer to human variation. Using evolutionary
models to compare human genes to other species has had a lot of practical applications. To
understand this, we first need to discuss homologous or orthologous genes. You see,
much like the fact that all life as we know it is descended from a common ancestor, all
genes as we know them are descended from common ancestral genes. But over the billions of
years of evolution, genes have duplicated and changed in a diversity of ways. Nevertheless,
there are plenty of genes that have remained relatively similar to each other, even over
long periods of time. These conserved genes often serve us very important functions, and
as a result purifying selection prevents them from diversifying much. Thus, there are genes
that serve the same function and are found among different species because the gene and
the function was in place in the common ancestor of those species. The gene has yet to change
in a significant way. We call these genes homologous, or more specifically orthologous
genes. That is, orthologous genes are genes that are similar between species because they
remained mostly unchanged since the species' shared ancestor. Evolutionary studies comparing
orthologous genes between humans and other species has had a lot of practical importance.
For one, it provides a different approach to discovering areas of the human genome that
have undergone unique natural selection when compared to other species. In other words,
these studies provide an excellent approach to find out what genes make us uniquely human.
For example, we could align orthologous genes between humans, and chimps, and gorillas to
determine which of these orthologous genes have experienced more changes in humans than
any other ape. When you do this, one gene that stands out is FOXP2. FOXP2 has known
linkages to severe language and speech disorders, and we know it is a fundamental gene involved
in the evolution of language in humans. The orthologous FOXP2 gene in songbirds has undergone
a great deal of evolution and is associated with song learning. And FOXP2 gene in bats
is associated with echolocation, in other words, bat sonar. By examining the evolution
of FOXP2 in different species, we are gaining not only a knowledge about what makes us uniquely
human but also how the FOXP2 gene works biologically, which serves as a foundation for interventions
for FOXP2 associated deficiencies. Now ideally, figuring out how genes work needs a setting
where you can control the environmental variables. But such experiments in humans would rarely
be desirable. But because the common ancestry of life retains orthologous genes, we don't
always need to look at humans to figure out how human genes work. This is where the study
of model organisms is so important. Instead of controlled experiments on humans, we can
do controlled experiments on fruit flies, or worms, or plants, or even yeast, or any
number of life forms that are easy to manage and less ethically worrisome. Some of these
experiments also take advantage of our knowledge of evolution in unique ways, including imposing
our own human-induced natural selection.
One great example of this human-induced natural selection is an experiment done with fruit
flies and alcohol. I already like where this one's going. Fruit flies have been an incredible
model organism for genetic research in general, but in this study a population of fruit flies
belonging to the genus Drosophila was placed inside of an artificial chamber. Then the
researchers gradually exposed this population of fruit flies to alcohol vapor. Most fruit
flies immediately exhibited changes to their locomotor behavior. In essence, they were
drunk. But other flies showed some resistance. The resistant flies were then bred to create
a new generation of fruit flies. The experiment is repeated, increasing the levels of alcohol
vapor. Over generations, a new population of fruit flies is obtained that's taken the
genetic potential of the earlier parent population and streamlined it to provide specific advantages
to alcohol resistance. Much like natural selection, we've created an artificial selective environment.
In fact, arguably, this artificial selective environment is natural selection, natural
selection where humans are the selective pressure. While not in a laboratory environment, the
same process is what led to the domestication of dogs from wolves, and cows from aurochs,
and transformed the silky sweet domesticated banana from a more seed-burden, tart, wild
ancestor. As for our new population of alcohol-resistant fruit flies, we can now examine their genomes
in comparison to that of the parental population to determine what areas of the genome were
placed under selective pressure. Doing so will reveal gene variants involved in alcohol
resistance. When scientists found it, they named the alcohol resistant allele "happy
hour." Now from my experience, scientists studying insects tend to have a unique sense
of humor. But the happy hour variant is a serious finding. The associated gene has strong
homology to a family of genes in mammals, including humans, called Ste20. Happy hour
was found to inhibit a growth factor in certain body tissues, and this growth factor is involved
in alcohol sensitivity. This fact led the researchers to explore similar patterns in
laboratory mice. The mouse models provided further interesting patterns. In fact, when
these growth factors were inhibited, mice lost all interest in alcohol, likely because
they no longer got the beneficial sedative effects that alcohol typically brings. The
researchers propose that this may be a pharmacological path for interventions for alcoholism. If
you no longer feel the effects of alcohol, you probably lack the incentive to drink it.
Seems pretty reasonable. So we've learned that evolution-based statistical approaches
to examining genetic variation within human populations in between humans and other species
have clear practical impact. Importantly, these studies are not secluded. Remember in
our foundations there was no clear line between macro- and microevolution. All these studies
are connected within the framework of evolutionary theory. This is why evolutionary theory is
called the cornerstone of biology. This is why Theodosius Dozamsky said, "Nothing in
biology makes sense except in the light of evolution." Evolutionary theory provides more
than just a piecemeal application. Evolution provides the foundation. It serves as the
intellectual framework to build upon, to design new tools, to resolve new problems, and to
move our understanding forward.