Do the environment and metabolism constrain the power of natural selection in bacteria?
Lead Research Organisation:
Newcastle University
Department Name: Sch of Engineering
Abstract
The staggering variety of the biological world is the result of 4.5 billion years of evolution. But we can only see a fraction of that variety by looking around us. To glimpse the true extent of that diversity one must sequence DNA. From the first painstaking steps in the 1980s to the terabytes of sequencing today, we now understand most diversity is microbial. Moreover, there are patterns in that diversity, some groups of bacteria and some environments, are more diverse than others. When asked to explain patterns in diversity many will invoke natural selection and niches.
However, thoughtful biologists know that this is not the whole story. There are limits to Darwinian selection that we often overlook (though Darwin himself did not). The limit we are interested in is the drift barrier. This how it works: if a mutation occurs it will only be selected for if the selective advantage, or disadvantage, it confers is greater than the power of random births and deaths in that population. A power we call drift. The larger the population the weaker the power of drift. We know the effective size of the population subject to these random births and deaths is smaller than the total number of individuals. Nevertheless, in bacteria those effective population sizes are very large: of the order of 108 or more. This means that in bacteria only mutations with selection power of greater than 10-8 will be selected for. This is the drift barrier.
But how many selectable mutations lie above the drift barrier? This is an interesting question. The number of selectable mutations depends on two things: the effective population size (which may not vary that much in bacteria) and something called the selective gradient. The selective gradient is the relative impact of a mutation on the fitness of a given bacterium. Something we know very little about, and what we do know is about Escherichia coli.
At present we implicitly assume that the selective gradient is the same as Escherichia coli in all bacteria (indeed arguably in all living things). If this was true, it would be amazing.
However, it seems plausible that the relative impact of a mutation on the fitness of an organism is affected by both that organism's metabolism and environment. If this was true then the number of selectable alleles (variations on a gene) would be constrained by metabolism and environment. This would mean that natural selection was "capped" by environmental or metabolic control of the drift barrier.
We wish to test this hypothesis using recently developed microfluidic methods. These methods (micro-mutation accumulation) measure the distribution of fitness effects (DFE) in bacteria.
If our hypothesis is upheld, and selective gradients are subject to environmental and metabolic control we will have generated new "rules of life" concerning the ability of these variables to control the number of selectable alleles, and thus microbial diversity and molecular perfection.
However, thoughtful biologists know that this is not the whole story. There are limits to Darwinian selection that we often overlook (though Darwin himself did not). The limit we are interested in is the drift barrier. This how it works: if a mutation occurs it will only be selected for if the selective advantage, or disadvantage, it confers is greater than the power of random births and deaths in that population. A power we call drift. The larger the population the weaker the power of drift. We know the effective size of the population subject to these random births and deaths is smaller than the total number of individuals. Nevertheless, in bacteria those effective population sizes are very large: of the order of 108 or more. This means that in bacteria only mutations with selection power of greater than 10-8 will be selected for. This is the drift barrier.
But how many selectable mutations lie above the drift barrier? This is an interesting question. The number of selectable mutations depends on two things: the effective population size (which may not vary that much in bacteria) and something called the selective gradient. The selective gradient is the relative impact of a mutation on the fitness of a given bacterium. Something we know very little about, and what we do know is about Escherichia coli.
At present we implicitly assume that the selective gradient is the same as Escherichia coli in all bacteria (indeed arguably in all living things). If this was true, it would be amazing.
However, it seems plausible that the relative impact of a mutation on the fitness of an organism is affected by both that organism's metabolism and environment. If this was true then the number of selectable alleles (variations on a gene) would be constrained by metabolism and environment. This would mean that natural selection was "capped" by environmental or metabolic control of the drift barrier.
We wish to test this hypothesis using recently developed microfluidic methods. These methods (micro-mutation accumulation) measure the distribution of fitness effects (DFE) in bacteria.
If our hypothesis is upheld, and selective gradients are subject to environmental and metabolic control we will have generated new "rules of life" concerning the ability of these variables to control the number of selectable alleles, and thus microbial diversity and molecular perfection.
