Genetic Variation

Genetic Variation - The Fuel for Evolution and the Cambrian Explosion

Simply put genetics is a science of difference, that is, variation in genomes. Genetic or genomic variation among individuals and in the gene pool of a population of individuals can be considered the fuel required for evolutionary adaption to proceed. Without fuel, the machine stops, at least until replenished by the accumulation of new mutations.

Our Darwinian Machine has little to produce in the absence of selective pressure for an organism to adapt in order to survive. A period of pseudo stasis ensues, where fuel stores (mutations) increase as to stores of raw materials build up.

The Origin of Species - Charles Darwin - Chapter 14 - Recapitulation and Conclusion

Darwin on Variation

DarwinThat many and grave objections may be advanced against the theory of descent with modification through natural selection, I do not deny. I have endeavoured to give to them their full force. Nothing at first can appear more difficult to believe than that the more complex organs and instincts should have been perfected not by means superior to, though analogous with, human reason, but by the accumulation of innumerable slight variations, each good for the individual possessor. Nevertheless, this difficulty, though appearing to our imagination insuperably great, cannot be considered real if we admit the following propositions, namely, -- that gradations in the perfection of any organ or instinct, which we may consider, either do now exist or could have existed, each good of its kind, -- that all organs and instincts are, in ever so slight a degree, variable, -- and, lastly, that there is a struggle for existence leading to the preservation of each profitable deviation of structure or instinct. The truth of these propositions cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what gradations many structures have been perfected, more especially amongst broken and failing groups of organic beings; but we see so many strange gradations in nature, as is proclaimed by the canon, `Natura non facit saltum,' that we ought to be extremely cautious in saying that any organ or instinct, or any whole being, could not have arrived at its present state by many graduated steps. There are, it must be admitted, cases of special difficulty on the theory of natural selection; and one of the most curious of these is the existence of two or three defined castes of workers or sterile females in the same community of ants but I have attempted to show how this difficulty can be mastered. With respect to the almost universal sterility of species when first crossed, which forms so remarkable a contrast with the almost universal fertility of varieties when crossed, I must refer the reader to the recapitulation of the facts given at the end of the eighth chapter, which seem to me conclusively to show that this sterility is no more a special endowment than is the incapacity of two trees to be grafted together, but that it is incidental on constitutional differences in the reproductive systems of the intercrossed species. We see the truth of this conclusion in the vast difference in the result, when the same two species are crossed reciprocally; that is, when one species is first used as the father and then as the mother

There are three primary sources of genetic variation:
  • Mutations are changes in the DNA. A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations.
  • Gene flow is any movement of genes from one population to another and is an important source of genetic variation.
  • Sex can introduce new gene combinations into a population. This genetic shuffling is another important source of genetic variation.

Mutations and variation:

Point MutationVariety is required for evolution to proceed. Variety is used up in the evolutionary process, but is then renewed by chemical errors that become part of copied DNA - the mutations. A mutation is a change in the nucleotide sequence of the genome of an organism, or a nonchromosomal genetic element that becomes a permanent change in the chemical structure of a gene, or actually in one of the two alleles of each gene in the genome of eukaryotic organisms. Mutations result from unrepaired damage to DNA or to RNA genomes that are nomally caused by radiation or chemical interactions), errors in replication of DNA, or from either the insertion or deletion of segments of DNA. A mutation in a parent, has a chance to passed to offspring upon mating. A mutated offspring can pass the error on in kind to its offspring.

Mutations are rare events, but once they occur they can have a range of effects on an organism, from essentally no effect, to most deleterious, to new mutated alleles past to offspring that can be favored by the darwinian machine, whereby the mutation selectively expands within a population's gene pool, enabling adaptation under selective pressure from environmental factors. Darwin called the recipiants of favored alleles favored individuals.

Sex and variation:

There are many modern theories that explain the advantage affords sex incorporate an idea originally proposed by Weismann more than 100 years ago, but they boil down to the idea that sex allows natural selection to proceed more effectively because it increases genetic variation.Genetic variation can also be caused by differing recombination of chromosomes in sexual reproduction, a process called independent assortment that occurs in eukaryotics and produces a gamete with a mixture of the organism's chromosomes; the chromosomes that result are randomly sorted from all the potential combinations of parental chromosomes. [1-3]

Gene flow and variation:

Gene flow is the transfer of alleles or genes from one population to another. Include genetic drift here.



  1. Burt, A. Perspective: sex, recombination, and the efficacy of selection--was Weismann right? Evolution. 2000 Apr;54(2):337-51.
  2. Goddard MR, Godfray HC, Burt A. Sex increases the efficacy of natural selection in experimental yeast populations. Nature. 2005 Mar 31;434(7033):636-40.
  3. Hoekstra RF. Evolutionary biology: why sex is good. Nature. 2005 Mar 31;434(7033):571-3.