Survival of the mutated

In his theory of evolution, which dates back to the 1800s, Darwin argues that living organisms evolve over time to promote survival and to produce the fittest offspring. This evolutionary model is based on two concepts: That all life on Earth is interconnected and that its diversity results from certain traits predominating over generations. Usually, these changes take place over thousands, if not millions, of years.

Given the right conditions, evolutionary change can be approximated on much shorter timescales. In a recent study, McGill researchers collaborated with an international team to model natural selection by observing wild mice for 14 months in the Nebraskan desert. Mice with light and dark coloured fur were captured and placed in enclosures with light and dark-coloured soil to investigate their evolution over time.

“My objective was to provide a complete story of adaptation, from mutation to phenotype to fitness, in a single study,” Rowan Barrett, an associate member of McGill’s Department of Biology and lead author on the study, said.

The goal of the study was to succinctly illustrate the process of adaptation, or changes at the genetic level that lead to better survival rates. Adaptations can manifest themselves as behaviours, which may allow for better evasion from predators, or as physically anatomical changes. Such adaptations are the outcome of mutations, or changes in DNA, which are demonstrated by environmentally-sensitive characteristics observable with the naked eye.

While there is a growing number of gene-focused studies that identify the role of  DNA in changing characteristics, Barrett explained that the scientific community knows little about the ecological or genetic factors that prompt the evolutionary changes to begin with. In contrast, his research combined a large-scale field study with laboratory-based tests in order to examine both the ecological and genetic factors that drive adaptation in wild mice.

Throughout the study, both light and dark mice were found to adapt to their new enclosure regardless of whether or not they were originally captured from light or dark regions. Mice placed in darker enclosures overall had a greater survival rate, as did mice placed in enclosures matching their original habitat. Mice placed in light enclosures evolved to have lighter-coloured fur than their ancestors while mice placed in dark enclosures evolved to have fur almost twice as dark.

Researchers found a mutation that deletes part of the Agouti gene decreases pigment molecules, changing fur colour. While the frequency of the deletion seemed to be the same between light and dark mice at the beginning of the experiment, the rate increased for mice in light enclosures and decreased for those in dark enclosures by the end of the study. These findings demonstrate a strong propensity for genetic selection.

Ultimately, the study is an example of evolution in an accelerated time frame. Although the mice are similar, there are variations among them that ensure longevity in a world dictated by the rule that the fittest survive.

The study implemented an experimental setup that effectively imitated the complexities of the evolutionary environment.

“Working outside the lab is difficult because you lose control of a lot of factors,” Barrett said. “You have to adapt to changing circumstances and unexpected problems.”

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