How is humanity still alive? How is most complex life still going in the first place? Biologically speaking, the number of genetic mutations we have with every generation that results in a negative effect should have wiped us all out.

Simple Processes

Natural selection as a random process and the error rate in our own genome duplication, about 70 new mutations per breeding event even with genetic repair mechanisms, should have killed more often than it clearly does. The fact that that extinction has not occurred implies that there is more going on in our passed on genomes than we have previously understood.

Perhaps it is a simple mechanism that just has been overlooked. A quirk of evolution and genetics that results in dangerous mutations not spreading harmfully throughout a population. Many such broad questions have turned out to have basic solutions such as this.

A Fly To Impress

Researchers at the Harvard Medical School and the University Medical Center Utrecht in the Netherlands decided to work together to determine the answer to this question. It is an answer that should apply to all complex life forms that experience high rates of deleterious mutations. Thus, finding a solution in a test species should mean that it is the same reason in humans as well.

Due to this, they decided to work with fruit flies (Drosophila melanogaster), a well understood model organism, The goal was to determine if deleterious mutations affect an organism’s fitness (its ability to survive) independently of each other or if they worked together to compound the negative effects.

The answer to this would go a long way toward determining whether these mutations should build up over time and kill off a population, thus putting us back at square one on why this isn’t happening, or whether they are deadly enough that they essentially neuter themselves from being able to spread to future generations. The effects of mutations on an organism’s chances is referred to as the “mutation burden”.

Measuring Fitness

It is difficult to objectively measure things like the effects on fitness unless one is following a large population in the wild. Which isn’t particularly feasible at the best of times. And that’s where genomic modeling comes into play. While not as perfectly accurate as a test in the wild would be, modeling still enables a proper look into the effects on a population, especially when dealing with simple direct changes to the genetic code.

Using genetic sequencing, they collected whole genomes from multiple different fly specimens and subspecies of fruit fly to make sure that the effects were seen in all of them. The focus was on loss of function (LOF) mutations that inhibits the ability of a gene to be properly translated into a working protein.

What they found was surprising, but also expected as the only possible explanation for why deleterious mutations do not build up harmfully in a population. These LOF mutations were much more under-dispersed throughout the genome than expected, with fewer of them existing and the ones that did exist being farther apart than expected.

Natural Protection

The reason for this is simple. Rather than these mutations affecting fitness independently, they do synergistically impact each other, with an individual that has more deleterious mutations having a greater and greater negative impact on their fitness with each addition LOF mutation.

In a manner of speaking, it is a mathematical failsafe built in natural selection that causes individuals with a higher amount of LOF mutations to have a much lower likelihood of breeding and passing on their genome. The effects on fitness are increased dramatically more and more in order to ensure this outcome. Thus, higher levels of damaging mutations are selected against naturally.

This result is also a method of showing why sexual reproduction is more successful in protecting against extinction events than asexual reproduction. Because the latter has a lower capability of purging negative mutations from their genomes, while the former naturally prevents a more damaged genome individual from reproducing.

The Strength of Repetition

Thus, evolution and formation of sexual reproduction is once again confirmed to work as expected. Maybe that’s a bit boring of a result from such an experiment, but confirming yet again that the fundamental tenets of evolution are true is still worthwhile science nonetheless.

Press Article Link

Study Link

Open Access Study Link

Photo CCs: A fly’s first breath 1 from Wikimedia Commons

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