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[Patricia F. Brennan, PhD] We've talked before about your interest in genomic
evolution and that the the manner in which organisms change over time. And
I'd like you to talk a little bit about that now. And if you could
specifically talk a little about the challenge of understanding conservation versus mutation.
[Eugene Koonin, PhD] I have to start my response to this question with the somewhat tired aphorism
of the famous geneticist, Theodosius Dobzhansky - "Nothing in biology
makes sense except in light of evolution."
[Brennan] Okay.
[Koonin] This is oft repeated but remains nevertheless completely valid and essential.
All genomes that exists, exist on our planet, result from billions and millions of year,
billions and millions of years of evolution. It's
entirely impossible to understand how they work without taking this into account.
To put it very simply, when we compare genomes of
either closely related or distantly related organisms, we delineate conserved and less
conserved variable paths. And what we can see is that the highly conserved parts are
involved in functions that are essential for the organism, organism's survival.
Conversely, the parts that are variable
contain genes that are involved in interactions between organisms and in interactions between
organisms and the environment. For instance, we have been talking about viruses. For instance,
the numerous genes involved in defense against viruses and other pathogens.
These are also important genes, but they are not important all the time. They become important only
when the organism is assaulted by the particular pathogen. Therefore, they evolve much faster and
they they are locked in what is often called "arms race" between hosts and parasites.
Even if we are not excited about the fundamental interest of the evolutionary process,
for purely practical reasons, to understand what is important in genomes - which parts. And which
parts are involved in highly conserved functions that are common to a great variety of organisms.
And which are involved in specific ones in the process of adaptation that occurs in front of us.
We must perform evolutionary genomic analysis on a scale that is most appropriate for a given task.
[Brennan] So when you speak about conserved, this is not only sections of the gene that appear over
generations of the organism, but also that appear commonly across groups of organisms.
[Koonin] To put a bit of a perspective onto this importance or essentiality of
the evolutionary thinking for understanding life, it's useful to note that by now,
it's a firmly established plain fact that all cellular organisms - I'm not
talking about viruses - all cellular organisms that exist on this planet
originate from a single common ancestor. A so-called LUCA, Last Universal Cellular
Ancestor, that existed about 4 billion years ago. Just a little bit less maybe.
This was probably the most brilliant hypothesis in the history of science
proposed by none other than Charles Darwin. But at the time, it appeared about speculation.
But now it is a plainly established fact. So the entire history of life,
the unimaginably long history of life on our planet spanning about 4 billion years,
is permeated by persisting evolutionary relationships.
To be even a little more specific, there is about a hundred of genes that are
shared by all cellular organisms, from mammals to plants to amoeba to bacteria.
Of course, the higher we go in the evolutionary tree,
the more, the larger are the sets of conserved genes. The different bacteria,
the huge variety of existing bacteria may share altogether about 500 genes, with a few exceptions.
There is, let us say all animals share several thousand genes, seven or 8000 genes.
Whereas, if you for instance, look at humans and mice.
Hope that cannot be offensive in any sense. But these organisms share the great majority
of their genes, about 80%, which may amount to something like about 20,000.
What you call homologous genes, genes that come from a common ancestor that existed in the in
the genome of, let us say our ancestors with mice. They usually retain the same functions.
This is more complicated
in the sense that in the course of evolution, what often happens is so-called duplication of genes,
when two copies are formed. And then they diverge and acquire somewhat different functions.
So when we look at two related sequences, we have to be careful about inferring the exact
same function. But nevertheless, there is always some type of functional relationship.
[Brennan] Do they ever revert?
[Koonin] Repair.
[Brennan] Or recover early function?
[Koonin] This is an interesting question. For
somewhat complicated statistical reasons, this is highly unlikely.
[Brennan] Okay.
[Koonin] But this is not to say never ever happened,
but this is really not a major route of evolution. In order to understand why,
this is where a little theory is actually required. But it's extremely unlikely to go back.
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