For numerous individuals, the concept of evolution can be summarized by the phrase: survival of the fittest. In any given population, there are differences among individuals in terms of their survival and reproductive capabilities, and those who succeed in this struggle influence the characteristics of the subsequent 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 vying for the attention of a female may encounter a competitor who inadvertently loses its footing on a loose stone and falls to its demise. Meanwhile, a scavenging bird might stumble across an abundant food source before its peers, purely by luck.

According to Zipple, the fortunate occurrences that individuals cannot foresee or influence may help clarify why certain people achieve success while others do not. This contemplation led him to question, "How do luck and competition contribute to the disparities observed in adulthood?"

Responding to that question is complicated, as the possible factors contributing to a person's success — such as genetics, surroundings, and chance — differ significantly across natural populations. Therefore, Zipple and his team decided to think outside the box.

"We were curious about the outcome of establishing a society where every individual begins with identical genetics and has equal access to resources within the same environment during their formative years," explains Zipple. "As these individuals mature into adulthood, will there be noticeable differences among them? Will we observe the emergence of inequality?"

Establishing a fair community for mice.

The researchers established this community by utilizing approximately one hundred genetically uniform mice. They organized groups of roughly 26 two-week-old mice along with their mothers into outdoor enclosures designed to replicate their natural habitat, while ensuring that all groups had uniform "resource zones" equipped with food and shelter. Over a span of 46 days, the team monitored the mice, assessing their behavior and different fitness parameters, such as weight.

In this experimental setup, a key variable stood out. According to Zipple, male mice of this strain engage in fierce competition to establish territories and secure food, whereas females do not participate in such rivalry. "This creates an advantageous system where one sex is involved in intense competition while the other remains unaffected," he explains.

According to Zipple, when genetics and environment are considered mostly equal, what remains are "these highly specific and contingent experiences."

For instance, a male mouse could unexpectedly triumph in a battle against its identical twin for food. This fortunate outcome would allow it to grow larger than its sibling, positioning it for success in future confrontations.

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 points out that these findings do not imply that females aren't experiencing their share of luck. "They also have their ups and downs," he explains. However, those fluctuations in fortune "don't accumulate over time in the same manner," he adds. The study revealed that by the conclusion of the experiment, "lucky" male mice had dominion over a larger area and came across roughly five times more females compared to their less fortunate counterparts.

The significance of experiencing good fortune

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 competition intensifies, the impact of those fleeting moments of chance becomes even more pronounced — did you succeed? Or did you miss out?" she explains. "Were you able to seize all the available resources? If so, that positions you to keep acquiring even more resources."

Determining the precise implications of these results is challenging without considering the context of the experiment. However, for Snyder, who researches the impact of random events on the reproductive success of organisms, this study contributes to an expanding collection of research in this field.

"According to her, our research shows that while possessing unique qualities, such as strong vitality or excellent genetics, is essential, it alone does not guarantee outstanding success. 'You also need a bit of luck on your side,' she emphasizes."

That can also apply to humans.

Researchers in the social sciences often highlight the notion that those who achieve success early in life are likely to continue experiencing success, which can exacerbate inequality. While factors such as talent and merit certainly play a role in this initial achievement, Zipple notes that luck can also significantly influence outcomes, according to the study.

"There are individuals who begin with the same genetic makeup, the same level of talent, and equal access to resources, yet they ultimately find themselves in vastly different situations as adults."

He points out that this has consequences for evolutionary research and encourages biologists to explore non-genetic factors that could influence an individual's success, particularly in competitive environments. Additionally, Zipple notes that there are ethical considerations to take into account.

"What these findings truly highlight is that having fewer resources than others does not imply that the individual is at fault for their situation," he explains. "This places the responsibility on all of us to come together and determine how we will address this reality."