One of the key concepts in evolution is fitness. You’ve probably even read something about how evolution is “survival of the fittest”. But what does that mean? Like many terms, fitness has a specific meaning in science that’s somewhat different from what it means in everyday life.
Walls of text get boring fast, so time for some pictures:
This is a picture of Michael Phelps, an Olympic swimmer. Since we most often use “fit” to mean some combination of healthy, strong, fast, capable of sustained physical exertion, etc, Olympic swimmers seem like a good example. In the typical sense of the word, he is a fantastically fit individual; he has more gold medals in the Olympics than anyone else in history has total Olympic medals.
In evolution, however, fitness refers to the ability to copies of ones’ genetic variants into future generations. In humans, this is most often done through having children (and thus hopefully eventually grandchildren), known as direct fitness. Alternately, people could provide help and resources to their close relatives, enabling their kin to have more children than they otherwise would; this concept is known as inclusive fitness. Because you share a lot of your genetic variants with your siblings, you can help propagate your own variants by helping your parents have more kids. Alternately, you could try helping those siblings have more kids, since your nieces and nephews will themselves contain a substantial percentage of your variants. The “fittest” individual is not necessarily the strongest, fastest, or hardiest; it’s the individual best able to get copies of its genetic variants into future generations.
In the case of Mr. Phelps above, to the best of our public knowledge, he has no genetic offspring, nor have his actions allowed his relatives to have more kids than they otherwise would have, so he currently has an evolutionary fitness of 0. So, in a very specific sense, a number of you reading this post are more fit than an Olympic champion swimmer.
Trying to apply this concept to people gets tricky for a number of reasons. One of those problems is people have long life spans, and relatively long generation times. In the case of Phelps above, he’s only 29; it’s certainly possible that he will still have children of his own at some point. Fitness changes over the course of a lifetime, but evolutionary processes depend heavily on lifetime total fitness. If we want to study evolution experimentally, it’s helpful to work with organisms that reproduce quickly, so we can look across an entire lifetime. In the Devolab, we work with digital organisms, which can reproduce extremely. In experimental evolution labs in biology, like the Lenski lab where I’m doing my PhD research, we most often work with microbes.
There are a lot of advantages of working with these types of microbes and computer programs, but for the purposes of measuring fitness, one of the key features is that we can consider multiple generations when measuring fitness. Measuring across generations is important because looking at just a single generation can make a measurement misleading. Since this is going on the internet, I’m going to illustrate the importance of looking across generations with kittens.
Let’s imagine two hypothetical mother cats. Fluffy has a litter of just 2 kittens (left); Mittens has a litter of 6 (right). That means Mittens has a higher fitness, right? Well, maybe. In this generation, she seems to. But 6 kittens is a lot for Mittens to take care of. All of them require milk, and that means Mittens has to spend a very large amount of time hunting. Even then, she has more kittens than teats, so her kittens are competing with each other for access to that milk. Fluffy, with just 2 kittens, has an easier time supplying food to her brood. She can therefore spend more time back with her offspring, guarding them, keeping them warm, and socializing them. Fluffy’s 2 kittens grow up into cats in very good condition; they will have very little trouble finding mates and creating grandkittens for Fluffy. Mittens’ kittens, on the other paw, grow up a little more sick, a little underfed. They end up smaller as adults, and have a harder time competing for resources with other cats. Perhaps not all of them make it to adulthood; even those who do may have a hard time attracting mates. Mittens’ very large litter could well net her fewer grandkittens than Fluffy’s much smaller litter. And, if this is the case, Fluffy really is the more fit cat. Her genetic variants will spread through the population, and future generations of cats will be more like Fluffy than like Mittens.
Thus fitness is a measure of long-term reproductive success, not just how many children something has.