Click here to
Evolution
The World of Darwin
Sources of Variation
Sources of Variation
Central to the idea of Darwinian evolution is the concept of genetic variation.

Regrettably, Darwin never understood the work of his contemporary Mendel and , therefore, never understood genetics. This unfortunate gap in his knowledge gave him great difficulties when he tried to explain how variation arose and how it was inherited.

Modern evolutionary scientists do not have this problem. Since the 1930's, Mendelian genetics, population genetics, and modern molecular genetics have revealed much about DNA, genes, alterations in genes, and the mechanism of inheritance.

Now it is possible to examine the sources of increases and decreases in variation at both the molecular and organismic levels.

Sources of Increased Variation All genetic change begins with mutation, that is, with alterations in the base sequence of a gene.
Mutation
Mutations are usually harmful and either kill the recipient or put it at a disadvantage. A few mutations are neutral in that they neither harm nor help, and a very few are even beneficial, increasing the size of a corn kernel or the milk yield of a cow, for example.

But mutations in vital genes, however, cannot usually be tolerated and will kill the individual rather than become a source of new variation. Changes in genes that will lead to a new function or a new variety, therefore, cannot occur in a vital gene.

Where then can they take place?

Duplication
Evidence at the molecular level shows that every once in a while a cell will make a mistake in the replication of its genes. By chance, a gene will be duplicated, leaving the cell with two copies instead of the usual one copy per chromosome.

So long as one of these copies remains unchanged and continues to perform its required function, the second duplicate copy can undergo random mutation without harming the cell.

Slowly, and by chance, this second gene may change enough to acquire a new function. Molecular biologists have found evidence that genes duplicate and then diverge in function by the accumulation of mutations.

At first, this divergence may simply produce variants of the same product, as in the case of human hemoglobin genes. Humans have at least three duplicates of the hemoglobin gene.

One variant is used by embryos, another by fetuses, and yet a third by adults. Eventually, variants may acquire a related but entirely different function.

A single gene codes for the production of a vial protein.


A mutation strikes this gene.
The altered gene now codes for a damaged protein, which can lead to the death of the individual carrying this mutant gene.


The single gene is accidently duplication during replication.
The individual now carries two copies of this gene in its genetic data bank
.
The original gene continues to code for its vital product.
The "spare", duplicated, gene can now be mutated, and possibly produce a different protein with a different and new function.
Figure legend: Gene Duplication and Mutation: A vital gene cannot mutate without losing activity and thus causing the possible death of the individual.

If the gene is duplicated first, however, then the second copy can undergo mutation to new and potentially beneficial forms.


Rates of Mutation There is a background rate of spontaneous mutations occurring naturally in all organisms. For example, E. coli, a haploid bacterium, mutates to streptomycin (an antibiotic) resistance at a rate of 4 x 10-10 per generation.

Fruit flies mutate spontaneously to an eyeless form 6 x 10-5 per gamete per generation.

Humans mutate to retinoblastoma (an eye tumor) at 2 x 10-5 per gamete per generation.

Some of these are recurrent mutations that occur often enough in a population to be quite predictable in their frequency. Others are so rare as to be virtually unique.

Equal expression If we assume normal mutation rates of 10-4 to 10-9 per generation for a given mutation in a single gene, it will take anywhere from 5,000 to 50,000,000 generations for the mutated gene to be equally represented with the normal gene in a population. In humans, if the mutant form is not at a disadvantage, this process may take 2 million years.

At the dawn of life, when all organisms were haploid and mutation was the only source of variation, evolution was very slow. It took only "moments" for life to begin, but billions of years for the first eukaryotic cell to evolve.

Once eukaryotic cells evolved, and sexual reproduction became widespread, however, evolution took place very rapidly. This rapid increase in the rate of evolution came about because one of the products of sexual reproduction is the creation of large amounts of variation in the offspring - every generation.

Recombination
Bacteria mainly reproduce asexually. Cells enlarge, copy their DNA, and divide into two new cells. The only source of variation, therefore, is mutation, and each mutation must accumulate with other mutations, one after another, before a new combination of genes is possible.

This takes a long time.

In eukaryotic organisms, however, gene duplication and mutation not only create new variations of a gene or genes, but during sexual reproduction, these genes are randomized into sex gametes and then recombined into new, random assortments every generation.

Recombination is the mixing of genes into new combinations and, unlike mutations, takes place each time meiosis is followed by fertilization. Because very few genes act totally alone, and most interact with many other genes when comprising a genotype, some combinations of genes will create a more fit, better adapted individual than others.

In addition, the number of possible combinations of genes brought about by recombination is very large.

If an organism has one normal and one mutant gene for each of its 10,000 genes (a reasonable number; humans may have 30,000 to 40,000 genes), there would be: -

210,000

different permutations and combinations of these genes possible in each egg or sperm. This is an impossibly large number, which essentially means that the variety of gene combinations that can be created during sexual recombination is infinite!

Although mutation is the only way of producing new variants of single genes, recombination, brought about through sexual reproduction, produces more new types of individuals much faster than mutation. In eukaryotic organisms, therefore, recombination is the greatest source of variation.

Gene Flow
If two populations of individuals belonging to the same species become separated, each separate population will suffer different mutations, and thus accumulate different combinations of genes.

Now, if an individual from one group wanders into the second group (and is accepted), genes will be exchanged between these two populations.

If the stranger carries variations not present in the second group, then the effect is the same as a mutational event. The initial effect of the stranger's genes is very similar to a burst of mutations. Variability is introduced and many new combinations are possible.


BIOdotEDU
© 2001, Professor John Blamire