Back to 2. Genes and Behavior
The procss of natural selection is often characterized as "blind" and this is largely because the source of all genetic variation is largely a stochastic process. The ultimate source of genetic variation is mutations, however, segregation, and recombination provide an important stochastic mechanism for randomizing genetic variation in a population. While mutations alter the DNA by changing base pairs or blocks of base pairs, segregation and recombination do not alter the material content of DNA, but provide powerful mechanisms for mixing up the DNA during the process of sexual reprodcution.
The process of mutation is probabilistic. We describe the process in terms of the probability of a mutation occuring in an individuals during its lifetime and typical rates of mutation are between 1 in 10,000 (10E-4) and 1 in 1,000,000 (10E-6) for many organisms. Either this means a long time must pass before a mutation occurs in a population, or the population must be very large. For a mutation rate of 10E-6, in order to see an average of one mutation per generation, the population must number in excess of 10E6 or one million members. Most mutations are infact deterimental and perhaps only 1 in 1,000 is beneficial. Thus, in this population of a million we might have to wait for one thousands years for a specific genetic locus to throw us a beneficial mutation (of course there are thousands of possible loci in an organisms so the waiting time for any beneficial mutation is less). Even if a mutation arises, there is no guarantee that it will be acted upon by natural selection.
A beneficial mutations occurs within a diploid parent which has two copies of each gene. The beneficial mutation has 50% chance of being transmitted to any one offspring and a 50% chance of not being passed on. Let's consider an organism that has four progeny, then the probability of single offspring not getting the beneficial mutation is independent of the other offspring not getting the mutant allele and by the laws of probability we multiply successive independent events to compute the probability that none of the four progeny get the beneficial mutation, which is given by:
1/2 x 1/2 x 1/2 x 1/2 = 1/16.
If the unique beneficial mutation is not passed on to the progeny then more time will be required for a new beneficial mutation to arise in the population. At which, point natural selection has a chance of promoting its spread through the population.
Genes are found on chromosomes. The first random event in sexual reproduction occurs during meiosis when homologous pairs of chromosomes line up on the equalitorial plate in preparation for the cell division that reduces a diploid (2N) cell of the germ line to an haploid (N) gamete. Because different homologues will be randomly distributed between the two daughter chromosomes during this process of segregration, gametes end up with different very different chromosome complements. Given that the total number of chromosomes is C, then there are 2 raised to the power of C different gametes that could be produced from segregration of chromosomes (in humans that would be 2E-46 = XX). Segregation can produce a vast number of different gametes and is a potent mixer of variation in sexually organisms.
Recombination between homologous pairs of chromosomes during meiosis can further generate even more gamete types. The chromosome is loaded with recombinational hotspots. Any given chromosome will typically recombine with its homologue at one or two points during a given meoiotic event (recombination rate on a given chromosome is a function of chromosome length). However, the recombination points for two different meiotic events can be different. Given that gametes such as sperm are generated by millions of different meiotic events, the number of potential gamete types from a single parent is vast indeed. If one considers all possible parents in a modest-sized breeding population such as humans (5 billion), there are easily more potential recombination products than molecules in the known universe.