How Can You Test Evolvability In Experiments
During the early 1990’s scientists were more concerned about the actual resultant variation from Darwinian evolution, rather than its origin. Many scientists assumed that the origin of the variation was a product of Darwinian evolution. But recent evidence from simulations and observations of simple evolutionary systems has yielded results which prove that this is not the case (Nehaniv, 2003).
Certain traits are more likely to evolve, such as traits controlled by underlying genes. Mutations in these genes will cause an alteration of the phenotype and it is more like to become fixed (Landry et al, 2007).
The first mechanism offered to show how this process of evolvability enhances the evolutionary process was provided by Rutherford and Lindquist (1998). It came after examining the Hsp83 gene in drosophila. The Hsp83 gene creates a heat shock chaperone protein known as hsp90 found in eukaryotes. The protein has the function of activating many important proteins which have key roles such as targeting signal transducers, acting as cell cycle regulators and they are also key players in various developmental pathways for the fruit fly (Prodromou, 2000). Rutherford and Lindquist reported that mutated drosophila for the Hsp83 gene developed specific variants, in particular abnormalities occurred in developmental pathways, and this could be seen by drosophila which had abnormal body plans, eye defects and mutated bristle formation. Furthermore they noted that when two different forms of the hsp83 mutated allele were bred to together it lead to much more significant mutations which had a severe phenotype.
This suggested to the research group that although there are different variations present for the different pathways of drosophila development, the hsp90 protein prevents any variants from being expressed into a physical phenotype. This would then prevent any significant variation from occurring within the fruit fly, but when this gene is altered or cannot function, (for example through changes in temperature or other abiotic environmental stressors), these genetic variations are uncovered.
The research group came to the conclusion that this mechanism causes these variants and after numerous rounds of selection, specific parts of the trait become fixed within the drosophila as they are selected for and become heritable. This was the first proposed mechanism for evolvability.
Further work by Lindquist and true (2000) into the Saccharomyces cerevisiae prion [PSI+] provided an alternative mechanism that works to find variation which is hidden within the genome of Saccharomyces cerevisiae , but nonetheless provided a mechanism for evolvability. [PSI+] is a prion (infections agent composed of protein) for Sup35p. The [PSI+] prion is epigenetic by nature. Its role is in translation termination, meaning that it can go past stop codons and continue to read the genetic sequence (Jung, 2000). This ability of the prion to partake in epigenetic modification and translation termination means that they can find silent variations and express them by skipping stop codons and finding new sequences. By doing this the organisms can thrive under conditions which it is not used. Moreover the ability of the [PSI+] prion to alter the neutrality of variants has been proposed as the reason to why new traits can be produced through evolution.
Landry et al (2007) conducted a study to identify which regions of the gene are responsible for the sensitivity towards mutations that alter the transcriptional phenotypes. Instead of looking at the effect of mutations between species (which is considered bias due the filtering of mutations by natural selection), the research group decided to examine yeast mutant lines all linked through a common ancestor. Each yeast population was maintained at small level, so that mutations could only be filtered out through genetic drift as opposed to natural selection. The group then chose four random strains from all the populations and they set about analyzing the expression of the various genes.
2,031 genes were identified as being expressed differentially in the different strains of yeast mutants; it was these genes that were further analyzed for the actual variation of expression. They noticed that the variation correlated with a variety of different factors:
- The number of regulatory sites
- The size of the trans and cis- mutational target
- The genes affected by the variation
These results suggested that these genes are prone to evolvability because of their susceptibility to mutations and the fact that they are involved in intricate interactions. The research group also looked at genes which have a TATA box, because these genes when placed under experimental evolution conditions, are more prone to between-species and adaptive divergence. They noted that they had a higher rate of mutational variation, which is in concordance with evolvability.
