For many, evolution boils down to a simple phrase: survival of the fittest. Within a population, individuals vary in their ability to survive and reproduce, and the winners of this competition shape the next generation.
"Everywhere we look, outcomes across populations are unequal," says Matthew Zipple, an evolutionary biologist at Cornell University. "In natural populations, there's a huge amount of variation in starting position in terms of an individual's environment and its genetics."
Biologists usually focus on the genetic and environmental causes of this variation, but sometimes, organisms just get lucky.
A male ram battling for a female's attention might face a rival that accidentally slips on a loose rock and tumbles to its death. A foraging bird may happen upon a bonanza of food before others, just by chance.
Such lucky breaks, which an individual can neither predict nor control, could at least partly explain why some individuals succeed and others fail, says Zipple. That made him wonder, "what is the role of luck, and what is the role of competition, in leading to those inequalities in adulthood."
Answering that question is tricky, since the potential ingredients for an individual's success — genes, environment and luck — vary so much in natural populations. So, Zipple and his colleagues got creative.
"We wanted to know what if we create a society where everyone starts out with the same genetics, has access to the same resources in the same environment in early life," says Zipple. "When those individuals grow up into adults, do they look different? Do we see that inequality developing?"
Creating an equal society of mice
The team created this society using around a hundred genetically identical mice. Groups of about 26 two-week-old mice and their mothers were placed in outdoor enclosures in groups that mimicked their natural environment, but had identical "resource zones" with food and shelter available to all. The team tracked the mice over 46 days, measuring how they behaved and various aspects of fitness, like weight.
There was one crucial variable in the setup. In this strain of mouse, males compete with each to form territories and access food, while females don't, says Zipple. "We have this really useful system where we have one sex experience and really intense competition and the other sex does not."
With genetics and environment held largely equal, what's left are "these really micro-contingent experiences," says Zipple.
For example, an individual male mouse might just so happen to win a fight with its identical twin over food. That lucky break would help it become bigger than its twin, setting it up to win the next fight.
Those contingent experiences mattered a lot more for males than females, the researchers reported last week in the journal Science, suggesting that competition magnifies the importance of luck.
For males, "they pretty early on start to diverge into really high quality and low quality males, or males that are gaining access to resources and males that are being excluded from resources," says Michael Sheehan, a biologist at Cornell University and senior author of the study. "We don't see that pattern pan out for the females. They all kind of stay about the same quality the whole time."
Sheehan emphasizes that these results don't mean females aren't getting lucky, too. "They still have good days and bad days," he says. But those good or bad days "don't rack up over time in quite the same way," he says. By the end of the experiment, "lucky" male mice controlled more territory and encountered about five times as many females as lower-quality males, the study found.
The importance of getting lucky
The researchers don't know what kinds of contingent events set male mice on diverging paths. But the study demonstrates how competition raises the stakes of getting lucky early in life, says Robin Snyder, a theoretical ecologist at Case Western Reserve University who wasn't involved in the research.
"As you dial up competition, it's amplifying the effect of these small moments of coin flips — did you get it? Did you not get it?" she says. "Did you manage to gobble up all the resources? And then that allows you to continue to gobble up all the resources."
It's hard to say exactly what these results mean outside of the context of this experiment. But to Snyder, who studies how chance events influence organisms' reproductive success, the study adds to a growing body of work in this area.
"What we're finding is that even if you have something special about you, something that's lasting — you're particularly vigorous or have great genes — that is a necessary but not sufficient trait to have to be exceptionally successful," she says. "You also have to be lucky."
That can be true for humans, too.
Social scientists sometimes point to the the idea that individuals who start succeeding early tend to keep succeeding, as a contributor to inequality. Of course, something like talent or merit can contribute to that early success. But the study shows luck can be a major factor, too, says Zipple.
"We have individuals who start out with identical genetics, identical, whatever you might call talent, identical resource access and they still end up in very different outcomes in adulthood."
That has implications for the study of evolution, he says, and should prompt biologists to consider non-genetic causes for an individual's success, especially when competition is high. It also has moral implications, Zipple says.
"What these results really emphasize is that just because an individual ends up with less resources than others, that doesn't mean that they're to blame for that outcome," he says. "That really makes it incumbent on the rest of us to decide collectively how we respond to that reality."
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