Changes in allele frequency
Within a population there will be a range of expression of many characteristics: a 'spectrum of variation', often seen as a bell-shaped frequency curve characteristic of a normal distribution
As a consequence, some individuals will be more 'favoured' than others, and this will lead to selection pressures for or against certain alleles held by these individuals, and this may have the effect of changing some parts of the population in an apparently defined way.
Data from 1295 births
Human birth weights
are a case in point: It is obviously advantageous for a foetus to grow in size inside its mother's uterus, but not to grow so large that it threatens the mother's life. Small babies are less capable of survival due to inadequate development in utero
. This results in stabilising selection
, and most babies' birth weights are close to 3.5 kg.
in bacteria shows a fairly dramatic and quick change in the mean value - directional selection
In a population of bacteria there will be a number with alleles that make them potentally resistant to certain antibiotics, even if there is no antibiotic in their general area!
In fact some may be capable of producing an enzyme that inactivates a certain type of antibiotic, or they may make their cell wall in a slightly different way than most of their relatives. Sometimes this puts them under some sort of disadvantage, producing something that is not really needed.
BUT - and this is a matter of life and death for the bacteria - if they find themselves in an environment where antibiotics are in circulation, those bacteria which do not have a mechanism providing protection from that particular antibiotic are likely to die fairly quickly.
Those bacteria with a form of resistance to the antibiotic are unaffected and they continue to live and reproduce, so the frequency of antibiotic resistance alleles in the population will rise as the antibiotic-sensitive bacteria are replaced the antibiotic-resistant bacteria.