![](img014.GIF) |
To take a specific,
evolutionary perspective, genes encode information to make proteins. That’s all. Nothing
more. Now, a specific protein may provide some advantage to an organism such as a cell, in
terms of efficiency of metabolism, or absorption or excretion, or it may not. It might
provide a disadvantage to that cell. Whether a protein is advantageous, neutral or
disadvantageous is solely dependent on the interaction the cell has with its immediate
surroundings, other proteins, and so forth. In environments that confer advantage, the
cell is more likely to survive and reproduce, and hence the numbers of cells carrying that
particular gene will also increase - the frequency of the gene will increase in the
population. If the environmental conditions change, there may be a selective disadvantage
against cells carrying that gene, so the gene frequency will decline with each cell that
fails to reproduce. Useful genes are usually “conserved”, that is, they may not be
expressed, or only be partly expressed under conditions where there is no advantage.
Disadvantageous genes under one type of environment may prove to advantageous in a
different environment. A gene that alters the structure of the red blood cell membrane so
that malarial parasites are less likely to successfully develop will do two things: it
makes it more difficult for the cells to function normally, but it doesn’t kill you.
Malaria, on the other hand, can kill you, so it is to the organism’s advantage to be a
little weaker, but alive to reproduce, than to be strong, perfect and dead before reaching
puberty. So it is the interaction of a specific phenotype with the range
of selection pressures presented by environment and lifestyle that leads to the
development of specific advantageous and disadvantageous outcomes. The disadvantageous
ones we now call disease, but we have simply tended to ignore the advantageous ones, so
far.
A key point here to remember is that, from a population perspective, gene
frequencies only change over generational time, taking hundreds, if not thousands of years
to increase or decline. |