MadSci Network: Evolution

Re: what of natural selection for modern man?

Date: Fri Sep 17 14:22:12 1999
Posted By: Steve Mack, Post-doc/Fellow, Molecular and Cell Biology, Roche Molecular Systems
Area of science: Evolution
ID: 935270599.Ev

I have heard this sort of question before, and while it is interesting to 
look at from a theoretical point of view, it is still largely the realm of 
science fiction. However, that doesn't mean that it isn't an interesting 
question, because alot of things that were science fiction even twenty 
years ago are science facts now. So, I have a short answer, and a long 
answer, each of which addresses the question differently.

The short answer is that evolution takes place on a very long timescale, on 
the order of millions and tens of millions of years. Therefore, the 
observation that people in so-called developed countries currently enjoy a 
very high life expectancy (reflecting a low death rate) doesn't tell us 
much about evolution. We have only had this medical technology for about a 
century, and it is unclear what will happen in the future. 

The fact that we have very good medical technology today, does not mean 
that we will have that same technology tomorrow. For example, if bacteria 
manage to develop resistances to all of our anti-bacterial drugs, then the 
life expectancy will start to drop. I think that you can see that it is 
impossible to predict what will be happening to our species in 10,000 years 
or 100,000 years, much less a million years, but those are the short end of 
the timescale that evolution works on.

In addition, the life expectancy in so called developing countries is still 
much lower than that in developed countries. For example, the current life 
expectancy in the US and Europe is about 73 years for men and 79 years for 
women. In Asia, the life expectancy is 62 years for men and 64 years for 
women.  In Africa it is about 51 years for men and 54 years for women, and 
in the 29 african countries hardest hit by AIDS, the life expectancy is 44 
years for men and 47 years for women. This is just to make the point that 
there is still a high death rate for a very large portion of our species.

For the long answer, we have to look at evolution itself, what it means, 
and how it works. It is difficult to say if we are free natural selection 
or not. By natural selection we mean that individuals of a species who are 
"unfit" do not contribute genes to the next generation. The presumption is 
that their fitness is determined by their genes, and that only those 
individuals with the "best" genes for the particular environmental 
conditions those organisms live under will reproduce, and thus the gene-
pool for that species will become enriched with "good" genes. 

Even though we can keep people alive for a long time, not every child that 
is conceived comes to full term, and some of the reason for this is natural 
selection working on "bad" genes.(I put these terms in quotes because they 
are all relative. What is unfit for one environment might be very fit for 
another, and a "bad" gene that gives you a genetic disease might also be a 
"good" gene that protects you from some sort of infection.)

In fact, the definition of evolution that I think best describes the 
process is, "a change in gene frequencies of a species over time". Most 
people seem to think that evolution is about the outward appearance of the 
organism, but much of evolution takes place only on the genetic level, 
which is why we define it in a genetic manner.

I think the assumption in your question is that if there is no natural 
selection, that there is no longer any evolution, but evolution operates by 
other mechanisms in addition to natural selection. One of these is 
mutation. Mutation is a process that ocurrs at a regular rate, and results 
in novel versions of genes. Some of these novel versions might be "good", 
and some of them might be "bad", but for the most part, they are what we 
call "neutral" and they don't have much of an effect on things. However, 
you can see that when a mutation ocurrs in a population, the gene 
frequencies in that population will change, simply because you have a new 
gene to consider.

We also have a process called genetic drift. This means that sometimes, due 
to things that happen completely by chance, the frequencies of genes will 
change over time. Not every organism in a population reproduces, and this 
is not always due to natural selection. For example, in our species, people 
sometimes decide that they do not want to have children despite the fact 
that they are descended from a long line of organisms that did want to have 
children. Or sometimes, there is a freak accident that sinks the fishing 
boat with 90% of the men from your village. Or sometimes people decide to 
leave their home population and found a colony, but they do not represent 
the full genetic diversity of their home regions. All of these things 
contribute to a random change in the gene frequencies.

Gene frequencies also change when populations merge and assimilate one 
another. That is a process which we are experiencing right now as a 
species. For the last 500 years, people have been moving all around the 
world forming hybrid populations. The gene frequencies in the new 
populations are products of genetic drift and hybridization of the old 

So, even if the life expectancy were very high for everyone, our species 
will continue to evolve, because the gene frequencies in our species are 
going to continue to change. And sometimes, when you bring togther genes 
that have been isolated in seperate populations, they can interact in new 
and interesting ways.

In the next 500 to 1000 years, the process of global hybridization is 
likely going to continue. More and more people are going to leave the 
places where their ancestors lived, places where there populations were 
relatively homogenous but also very different from the other populations 
around the globe, and form new hybrid populations that are very similar 
genetically to other populations around the globe. This is probably the 
most important change that will happen to our species in the next 1000 
years; we will all start to become alot less different; our species will 
become more homogenous, but the potential that was sequestered in discrete 
populations will be shared all around the globe.

As far as taking charge of our own evolution, we are still learning the 
basics of genetic biology. In the last 20 years, we have learned about how 
genes work and how to manipulate them in simple ways, but we are along way 
from understanding how genes work together to make organisms. We still do 
not really know what it is that makes us different from our closest 
relatives, the chimpanzees. We will need to understand the underpinnings of 
the system before we can really go about making specific, directed changes 
to ourselves. Most of the research that pertains to evolution is focused on 
understanding what has already happened, rather than actually bringing 
about new evolutionary developments.

Finally, evolution operates on the level of a species, but as individuals 
people tend to be very opinionated. I think it would be very difficult for 
our species to make a decision to evolve in a particular direction when 
everyone would have their own opinion about what that direction should be.

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