Natural selection is a simple theory because it can be understood by anybody; to misunderstand it requires special training.
Graham Bell, The Masterpiece of Nature
Interest has been expressed in a thread on selection and drift, so I thought I’d start one, and offer my own 2-cent summary of the concepts.
Evolution, as commonly understood in biology, simply involves change in a lineage. Through mechanisms of change – principally, mutation and the insertion of ‘foreign’ DNA sequence – offspring frequently contain DNA sequences that do not derive by simple copying from their parent(s). This change is inevitable and, iterated, inexorable. There is no memory, no externally-stored blueprint for organisms; the specification of a species ‘floats on the breath of the population’, as Doctor Johnson wrote of the unwritten Gaelic language. Unless there is some kind of boundary blocking all possible avenues, a continuing source of variation is sufficient to keep a lineage exploring never-before-seen genetic space ad infinitum.
Among close ecological competitors (among, for example, the genetically similar members of a species), at a given locus in a finite world one individual ancestor’s genetic sequence is headed towards being the ancestor of every instance of that locus in a future population, and all the others with which it shared a population are heading for extinction. This derives from two facts: samples are more likely to deviate from the frequencies in the wider group than match them, and the probability of fixation of an allele is equal to its current frequency. The distortions on generational sampling tend to reinforce, through to extinction of all but one variant. This same tendency underlies the ecological principle of competitive exclusion between non-interbreeding competing species.
If a particular locus is invariant in a population, fixation has already happened. An original mutation, occurring in a single ancestor, has been passed to every member of the current population. Looking forwards, the mechanism of this concentration continues to operate, and so one particular individual from the present population will become the ancestor at that locus of all members of a future population. From any given starting point, a population of N diploid individuals will take a mean 4N generations to achieve fixation of one ancestor’s copy, and the probability for any diploid locus of being that copy is 1 in 2N. This doesn’t mean that large populations cannot fix neutral alleles, however – the number of mutations occurring scales with population size, so mutations will be fixed at the same rate they are generated, completely irrespective of population size. Doubling the population gives twice as many mutations taking twice as long to fix – the result is the same number of mutations being fixed per generation.
At the point of fixation, all instances of that locus descend from the same ancestor – they coalesce upon that ancestor. The case described – where there is no variation at all at the locus, ie there is just one allele – is the baseline process, the neutral case. If there is no variation, there is nothing for Natural Selection to ‘see’. The only process in operation is random genetic Drift – even though in this instance, it effects no evolutionary change because there are no variant alleles. The change occurred with the original mutation. This latter fact leads me to prefer the view of ‘descent with modification’ over the population geneticist’s ‘change in allele frequency’. It is true that allele frequency change is also evolution, the only part over which selection and drift have a role, but as far as each lineage is concerned, the change occured at the moment of mutation. The lineage changed at that point; the population changed somewhat later, when this mutation became the norm.
Suppose we could uniquely label the locus for every member of the population, in a heritable manner. Now, we have essentially created 2N alleles. If we allow them to operate neutrally, just as when there was no variation, evolution will now occur because allele frequencies must change in the population. Because our labelling has had no effect on the neutral ancestry-fixation process, the label itself will surf to fixation on this process, while all others become extinct.
If, instead of labelling every instance, we simply labelled one, we would find that it still had the same 1 in 2N chance of becoming fixed. And this is the situation for any neutral mutation: 1 in 2N neutral mutations will become fixed; the neutral mutation simply functions as a label.
So now, having laboured the neutral case, where all is Drift, we can look to introduce a differential between alleles. If a new allele consistently performs better or worse than the existing one – meaning that it enhances or hinders the survival and/or reproduction of its bearers – then Natural Selection has come into play. It is a simple and obvious and non-tautological!) truth that a consistent increase in survival/reproduction – in fitness – will tend to favour such alleles over the purely neutral case, and render fixation more likely and speedy, while a reduction will increase the likelihood and speed of elimination.
Unlike the purely neutral case, in which population size is cancelled out, the behaviour of selectable alleles is affected by population size. In smaller populations, random factors have a greater influence than in larger ones, and hence alleles may behave as effectively neutral despite possessing an advantage which would see them selected in a larger population.
Drift does not simply disappear when you start to turn up the selective ‘heat’. Drift essentially derives from random sampling, the tendency of subsets to deviate from the distribution of the complete set, and such sampling is in effect almost all the way along the continuum of selective advantage (apart from alleles that are so strongly detrimental that they never gain a foothold). Even a favourable allele can disappear through Drift, likewise a deleterious allele can become fixed through the same mechanism. But more often, progress will go with the expectation, not against it. The large-number tendency is for genomes to become enriched in advantageous alleles and impoverished in detrimental ones. Because this process is environmentally conditioned, it allows populations to adapt to their circumstances, by purging the traits that do worst in the recent environment.
There continues to be a debate about the relative importance of Selection and Drift in evolution generally, and in driving speciation among sexual forms. Only selection can be adaptive, because it is the only component that is responsive to the environment. But they both have significant contributions to make, and cannot readily be teased apart. Both tend to reduce the variation in a population, which variation is only restored by mutation, recombination or immigration.