Mutation and Evolution

Mutations are the raw materials of evolution.

Evolution absolutely depends on mutations because this is the only way that new alleles and new regulatory regions are created.

But this seems paradoxical because

So how can the small changes in genes caused by mutations, especially single-base substitutions ("point mutations"), lead to the large changes that distinguish one species from another?

These questions have, as yet, only tentative answers.

The Problem of Harmful Mutations

One Solution: Duplication of Genes and Genomes

Mutations that would be harmful in a single pair of genes can be tolerated if those genes have first been duplicated.

Gene duplication in a diploid organism provides a second pair of genes so that one pair can be safely mutated and tested in various combinations while the essential functions of the parent pair are kept intact.

Possible benefits:


A Second Solution: Mutations in Regulatory Regions

Not all genes are expressed in all cells. In which cells and when a given gene will be expressed is controlled by the interaction of:

A mutation that would be lethal in the protein coding region of a gene need not be if it occurs in a control region (e.g. promoters and/or enhancers) of that gene.

In fact, there is increasing evidence that mutations in control regions have played an important part in evolution. Examples:
Follow this link to more discussion of the role of changes in gene regulatory regions in the evolution of animal form.

The Problem of How Large Changes in Phenotype Can Come from Small Changes in Genotype

Selector Genes

The building of an organ requires the coordinated activity of many genes. However, these are often organized in hierarchies so that "upstream genes" regulate the activity of "downstream genes". The closer you get to the top with a mutation, the greater the changes affected downstream.

Follow these links to see examples of the influence of "master" (selector) genes on the phenotype.

The Story of Pitx1

Pitx1 is

Pitx1 is an essential gene. Mutations in the coding regions are lethal when homozygous (shown in mice).

However, mutations in noncoding regions need not be.

All vertebrates have a pelvic girdle with associated bones which make up

Pitx1 is needed by them all for the proper development of these structures (as well as the other functions of Pitx1).

In a remarkable study of three-spined sticklebacks published in the 15 April 2004 issue of Nature, Michael Shapiro, Melissa Marks, Catherine Peichel, and their colleagues report that a mutation in a noncoding region of the Pitx1 gene accounts for most of the difference in the structure of the pelvic bones of the marine stickleback and its close freshwater cousins.

The marine sticklebacks The freshwater sticklebacks

Here then is a remarkable demonstration of how a single gene mutation can not only be viable but can lead to a major change in phenotype — adaptive evolution. (The changes seem not to have produce true speciation as yet. The marine and freshwater forms can interbreed. In fact, that is how the differences in their hind limbs were found to be primarily due to the expression of Pitx1.)

A survey of 21 different populations of sticklebacks — both freshwater and marine — from different regions of North America, Europe, and Japan has revealed a pattern of consistent genetic differences that distinguish the freshwater from the marine forms. However, only 17% of the distinguishing mutations were found in exons that alter the amino acid sequence of the encoded proteins. All the rest were "silent" and most, 41% or more, of these occurred in intergenic regions. These results further demonstrate the importance of mutations in regulatory regions — promoters and enhancers — in the evolution of adaptive phenotypes.

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20 September 2012