Mouse Utopia, Mutational Meltdown, Extinction by Natural Selection

The National Institute of Mental Health (NIMH) of the National Institutes of Health (NIH) sponsored the work of John Calhoun on social behaviors. Here were the results of one of his experiments:

On July 9th, 1968, eight white mice were placed into a strange box at the National Institute of Health in Bethesda, Maryland. Maybe “box” isn’t the right word for it; the space was more like a room, known as Universe 25, about the size of a small storage unit. The mice themselves were bright and healthy, hand-picked from the institute’s breeding stock. They were given the run of the place, which had everything they might need: food, water, climate control, hundreds of nesting boxes to choose from, and a lush floor of shredded paper and ground corn cob.

This is a far cry from a wild mouse’s life—no cats, no traps, no long winters. It’s even better than your average lab mouse’s, which is constantly interrupted by white-coated humans with scalpels or syringes. The residents of Universe 25 were mostly left alone, save for one man who would peer at them from above, and his team of similarly interested assistants. They must have thought they were the luckiest mice in the world. They couldn’t have known the truth: that within a few years, they and their descendants would all be dead.

As new generations reached adulthood, many couldn’t find mates, or places in the social order—the mouse equivalent of a spouse and a job. Spinster females retreated to high-up nesting boxes, where they lived alone, far from the family neighborhoods. Washed-up males gathered in the center of the Universe, near the food, where they fretted, languished, and attacked each other. Meanwhile, overextended mouse moms and dads began moving nests constantly to avoid their unsavory neighbors. They also took their stress out on their babies, kicking them out of the nest too early, or even losing them during moves.

Population growth slowed way down again. Most of the adolescent mice retreated even further from societal expectations, spending all their time eating, drinking, sleeping and grooming, and refusing to fight or to even attempt to mate. (These individuals were forever changed—when Calhoun’s colleague attempted to transplant some of them to more normal situations, they didn’t remember how to do anything.) In May of 1970, just under 2 years into the study, the last baby was born, and the population entered a swan dive of perpetual senescence. It’s unclear exactly when the last resident of Universe 25 perished, but it was probably sometime in 1973.

The rest of the article is here:

Here is a video of some the experiments with footage of the mice after they injured each other:

The Mouse Utopia experiment shows a population can self-extinct itself without any mutational meltdown under utopian conditions.

But there were some experiments with yeast that showed a population can self-extinct itself through mutational meltdown also under supposed utopian conditions.

So much for survival of the fittest.

In small or repeatedly bottlenecked populations, mutations are expected to accumulate by genetic drift, causing fitness declines. In mutational meltdown models, such fitness declines further reduce population size, thus accelerating additional mutation accumulation and leading to extinction. Because the rate of mutation accumulation is determined partly by the mutation rate, the risk and rate of meltdown are predicted to increase with increasing mutation rate. We established 12 replicate populations of Saccharomyces cerevisiae from each of two isogenic strains whose genomewide mutation rates differ by approximately two orders of magnitude. Each population was transferred daily by a fixed dilution that resulted in an effective population size near 250. Fitness declines that reduce growth rates were expected to reduce the numbers of cells transferred after dilution, thus reducing population size and leading to mutational meltdown. Through 175 daily transfers and approximately 2900 generations, two extinctions occurred, both in populations with elevated mutation rates. For one of these populations there is direct evidence that extinction resulted from mutational meltdown: Extinction immediately followed a major fitness decline, and it recurred consistently in replicate populations reestablished from a sample frozen after this fitness decline, but not in populations founded from a predecline sample. Wild-type populations showed no trend to decrease in size and, on average, they increased in fitness.

And finally E.O. Wilson (as reported by Niles Eldridge) provides estimates on what happens to species under increased selection pressure of habitat removal and destruction. This shows how one species (human) can cause extinction of other species:

E.O. Wilson estimated that Earth is currently losing something on the order of 30,000 species per year — which breaks down to the even more daunting statistic of some three species per hour.
Some biologists have begun to feel that this biodiversity crisis — this “Sixth Extinction” — is even more severe, and more
imminent, than Wilson had supposed.

Some will take exception that I characterize this sort of extinction as “natural selection”, to which I respond, “well shouldn’t a theory that uses the label of ‘natural’ actually model what happens in nature? Where is the net origin of species by means of natural selection going on. I see a net elimination of species. Shouldn’t population genetics model this scenario since it is so ubiquitous and real? So much for Genetic Algorithms modelling the real world.”

The point of all this is that the label of “natural” selection isn’t necessarily accurate to the extent it implies ever increasing fitness. If the population goes extinct (for whatever reason), fitness won’t increase. There is no inherent reason the idealized models in population genetics (where fitness keeps increasing) will actually be realized in nature. Those models always work except when they don’t. And not to mention, even in real world scenarios where fitness is ever increasing, unless there is something like HGT or plasmid exchange, the fitness increase is usually due to loss of function (reductive evolution) not gain of function — the so called “survival of the sickest” scenario.

If the speculated models of Darwin’s “natural” selection are so ad hoc in the way they are used to explain what happens in the lab and field, if Lenski’s experiments of fitness improvement in simple creatures take so much more attention than the numerous examples where fitness in more complex organisms goes to zero (aka extinction), then the success of Darwin’s theory seems a bit more the result of sampling bias, confirmation bias, and imagination than what actual real time data may justify. Until these issues are resolved, it’s really premature for someone like Dawkins or other ultra Darwinians to argue “natural” selection is a universal acid because what is labeled as “natural” may actually be imaginary, not real.

[I extend thanks to the TSZ admins and moderators for hosting this essay.]

52 thoughts on “Mouse Utopia, Mutational Meltdown, Extinction by Natural Selection

  1. stcordova: …But that is not proof that selection was what causes diversity!…

    Who the HELL ever said that selection, by itself, causes diversity?
    That’s a dumb strawman, Sal.

    Mutation causes diversity, selection prunes the branches.
    Mutation is the chance, selection is the necessity.

    By the way, when you are done gushing over the beauty and majesty of peacocks and polymerases, you should balance your perspective by looking at some less “godly” biology:

  2. FairWitness:

    Who the HELL ever said that selection, by itself, causes diversity?

    Well, that’s not too much of stretch if one reads Darwin:

    “Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

    So from “the war of nature, from famine and death” animals are being created. But the war of man on the environment seems to be killing off animals. So how then does this make them. Famine and death logically create famine and death, not animals!

    from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows” NOT! Contradicted by evidence. Famine and death, cause famine and death, and occasional extinction of species, not creation of them. EO Wilson et all have show how nature refutes the falsely advertised label of Darwin’s “natural” selection.

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