Notes

Natural selection is the process that results when the characteristics of a population
of organisms change over many generations. Th is change happens because individuals
with certain inherited traits survive specifi c local environmental conditions and,
through reproduction, pass on their alleles to their off spring. For natural selection
to occur, there must be diversity within a species. Th is is what allowed the changes
in proportions of the peppered moths. Th is is also why some populations of
Staphylococcus aureus survived the treatment of a certain antibiotic.
Selective Pressure
In those populations of Staphylococcus aureus, the individual members of the bacteria
population were selected for by their environment. Th ey survived the change in the
environment around them, which was the application of an antibiotic. Th us, they
could reproduce and pass on the genetic information that coded for resistance to that
particular antibiotic. An abiotic (non-living) environmental condition can be said
to select for certain characteristics in some individuals and select against diff erent
characteristics in other individuals. In this way, the environment exerts selective
pressure on a population. Selective pressure may result from biotic factors as well,
such as predators, parasites, and competition for resources.
As another example of selective pressure, consider a population of young trees—
in this case, a dense forest with low light, as shown in Figure 7.7. Th ose individual
trees that are able to survive in the shade will reproduce and pass on the alleles to
survive those conditions. In the next generation, the abundance of the alleles for
these conditions will increase in the population because more of the individuals will
have survived and reproduced. Over time, the tree population will be able to survive
the shady conditions. Should there be a big increase in the light levels in the trees’
environment, the trait for resisting these conditions will no longer be an advantage.
In fact, if no individuals in the population can withstand increased light levels, the
population may not survive in that environment.