microbiologists watch evolution in action

rapid advances in molecular genetics are now allowing scientists to watch, and even manipulate, evolution. this may seem hard to reconcile with the idea that evolution is a very gradual, long-term process, whose effects are seen only on timescales of thousands or millions of years. the misconception here is that time governs the rate of evolution. time in the abstract is relentless and constant, moving forward endlessly. this conception of time does not take the broad range of life strategies that evolution has produced into account.

in fact it is generation time that governs rates of evolution. human beings, who tend to have several children over the course of a multi-decadal life, have relatively slow generation times. it can take millions of years for noticeable evolutionary shifts to occur in a population that grows and reproduces slowly, simply because the genetic mutations that lead to evolution are very rare, and advantageous mutations take a long time to become established in a population.

it has long been known that microbial evolution occurs rapidly- noticeable genetic shifts can be observed in a manner of days or weeks. the evolution of pathogen resistance to pesticides or medicine occurs through natural selection for a rare genetic mutation that allows survival. because microbial populations often grow exponentially and generation time can be as short as twenty minutes, rare genetic mutations can sweep through a population and become ubiquitous rapidly. this is evolution in action!

historically, experiments in microbial genetics have focused on determining the function of an existing gene. this is generally accomplished by creating a strain with a defective, mutant version of the gene of interest, and observing how its function differs from the normal gene. studying defective mutants, however, does not provide insight into how gene pools can be improved.

now scientists are growing microbial cultures whose entire genetic makeup is known, and performing experiments that test evolutionary theory. any number of questions are being asked- how do the bugs evolve in response to an environmental change? a new food source? the introduction of a genetically different strain? for example, if a microbial population that requires oxygen to breathe is suddenly placed in a low oxygen environment, will genetic shifts occur that allow the microbes to use oxygen more efficiently? this certainly seems to be the case with humans- human populations that have existed for centuries at high altitudes, where oxygen is scarce, exhibit slight alterations in genes that encode hemoglobin, the protein that binds oxygen and transports it throughout the body. controlled evolution experiments allow replication, which means that scientists can now ask how frequently a positive evolutionary outcome, such as increased oxygen efficiency, occurs.

though such experiments may only provide a simplistic illustration of evolution, the mechanisms leading to genetic changes in microbial populations are remarkably similar to the basic mechanisms governing genetic change in higher organisms, including humans. insights developed from these experiments may be a first step towards unraveling the complex chain of events that has created the extraordinary diversity of adaptive traits across all types of life.


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