A walk through evolution

Veni winner Bertus Beaumont is researching how spontaneous genetic mutations iaffect the way organisms look and function.

There can be no evolution without variation in the functional and external characteristics of organisms, and no variation in these properties without spontaneous changes in hereditary material; this is something upon which biologists are in agreement.   But as yet little is known about precisely how spontaneous changes in hereditary material lead to new functional and external characteristics. Leiden evolutionary and microbiologist Bertus Beaumont, winner of a Veni subsidy, will be working on changing this situation.

Evolutionary theory
In biology a distinction is made between the genetic equipment of organisms (the genotype) and the functional and external characteristics (the phenotype). The genotype is the 'recipe' for the formation of the phenotype. 'Evolutionary theory states that the phenotypical characteristics of organisms are constantly varying,' explains Beaumont. 'Fro these variants, Nature selects those individuals that have favourable characteristics which allow them to produce more offspring (natural selection). These descendents inherit the favourable characteristics and, in turn, undergo variation themselves.  Over time, the processes of variation and natural selection lead to evolutionary change.' 

Black box
The genetic variation wit which it all begins is the consequence of spontaneous  changes in genes.  These 'mistakes', also known as mutations, occur during the copying of genetic material when cells divide.  Where the mutation occurs is a matter of chance.  In what manner do organisms change as a consequence of such genetic mutations?  Beaumont: 'The fact that phenotypes change as a result of mutations has long been common knowledge, but little research has been done into whiat particular changes are possible.  This is a real instance of the 'black box'. With my research I hope to be able to open that black box, in orde to be able to find out more about the evolution of new characteristics.' 

Genotype-phenotype map
Beaumont visualises the relationship between genotype and phenotype as a three dimensional space with two parallel surfaces (the genotype-phenotype map). On one surface all the possible genotypes are depicted as points (the genotype space); on the other surface all the possible phenotypes are depicted (the phenotype space).  By connecting points on each of the two surfaces, it is possible to indicate which phenotype corresponds with a particular genotype.   

The genotype-phenotype map shows which functional and external characteristics correspond with a particular genetic profile.  It is possible for several different genotypes to lead to the same phenotype.'

Neutral networks
A typical characteristic of the genotype-phenotype map is that there is no one-on-one relationship between genotype and phenotype. Beaumont explains: 'It is possible for several genotypes to lead to the same phenotype. Spontaneous change in the genetic material of an organism will therefore not always result in different characteristics.' Different genotypes which lead to the same phenotype together form so-called neutral networks in genotype space.  Evolutionary biologists believe that these neutral networks play a significant role in the course of evolution.  In his research Beaumont will introduce mutations into genes in order to see when the phenotype changes.  'I will, as it were, take a walk past different genotypes in the genotype space. By doing this I can determine the location of the neutral networks in relation to one another in the genotype space.  Or to put it more simply: how many changes can you introduce before the characteristics of the organism change?'

Plausible evolution
What is the significance of the genotype-phenotype map for evolutionary theory? 'The structure of this map influences what is possible in evolutionary terms over a given timespan,' explains Beaumont.  He illustrates this with an example: 'The evolutionary explanation for the existence of white polar bears is based on variation in fur colour among brown bears.  Natural selection selects lighter variants because these have an advantage (camouflage) during hunting. But how likely is this evolutionary development?  The chance of white bears coming into existence as a result of spontaneous genetic changes depends on the distance between the brown genotype and the white genotype in the genotype space; the smaller the distance, the greater the likelihood of a white bear evolving.  The structure of the genotype-phenotype map therefore dictates what is plausible during evolution over a finite time.' 

Model systems

Some evolutionary changes are more likely than others.

Beaumont is researching the structure of the genotype-phenotype map using bacteria as model systems, 'small evolution experiments,' as the Veni winner calls them.  'First, I will explore the genotype space using artificially constructed genotypes. Then I will allow these genotypes to evolve in a laboratory environment that I am able to control myself.  By measuring which genetic and phenotypic changes occur, I can gain an insight into the influence of the structure of the genotype-phenotype map on the course of evolution.'

Synthetic biology
This knowledge could be applied in synthetic biology, the branch of biology which is concerned with the construction of artificial (parts of) organisms. For example, bacteria can be engineered to degrade waste products. Beaumont: 'The trick is to design the bacteria so that they do not immediately lose the desired phenotype through evolutionary processes. You have to choose the genotype such that a mutation does not directly lead to a different phenotype. This is where knowledge of the genotype-phenotype map proves useful.'

(26 June 2007/Tristan Lavender)