What is this “fitness” the evolutionary biologists often talk about? When something good occurs to a living being, it gains fitness — like if it works out at the gym. Indeed, the meaning isn’t far from that, because going to the gym undoubtedly can improve fitness. Or lose it if you don’t know what you’re doing and you injure yourself. But what is it?
Loosely defined, fitness is the probability of raising more offspring in the future (or eventually). That is in line with the common but fallacious notion that the primary goal of all living beings is to survive and reproduce.
Evolutionary success is indeed measured in the number of offspring or gene copies an organism leaves behind. Fitness per se, however, does not mean having more offspring; it simply means having an increased probability of having more offspring. The two aren’t the same things.
When you do things that increase the probability of raising more offspring (go on a date, sign up kids to karate lessons, or survive a lion attack), you gain fitness. On the contrary, if you do things that reduce this probability (sleep in front of the TV, use condoms, or die), you lose fitness.
Note: human examples are useful to illustrate the picture. However, we live quite un-naturally, and activities that in nature would deplete our fitness (such as sitting on our asses and not looking for food or sex) cost little to nothing. For wild animals, time is a much more valuable asset than it is for us.
Fitness is a currency of evolutionary success
You can view fitness as a form of evolutionary currency that living beings can receive or spend. Besides, not all gains or expenses are of equal value. For example, a bird that spends time looking around for predators loses some fitness. Why? Because he could have used that time to eat. The amount of fitness loss, however, is nowhere near that if a cat catches him.
Therefore, from the bird’s perspective, it is useful to sacrifice a small amount of fitness at a constant rate for vigilance. That is because once he spots a predator early enough to escape, he will, metaphorically speaking, hit the evolutionary jackpot.
An event that can cost or win fitness can be anything from a physiological response to a decision made by an animal. This also applies to all living organisms, from bacteria and yeasts to plants and animals, and, to maximize fitness, one requires nothing near deliberate action.
This maximization is shaped by natural selection. Regardless of whether it is conscious or not, under-maximization will not perform as well as maximization. A plant that germinates seeds at the wrong moment will reproduce less and have less fitness than other plants that, by chance, time their germination better. Over time, more and more plants in the world will be able to time germination optimally.
Even more, to maximize fitness, plants have another trick up their sleeves. Many plants germinate their seed throughout the season. Since the optimal time is unpredictable, this is a good strategy. At first, fitness will decrease because the plant will grow a lot more seeds than needed, which requires energy and building materials. However, it is a clever move as it ensures that the seeds are ready at all possible times, and regardless of whether spring comes early or late in the season, a fraction of the seeds will be ready at the right time. Every year.
Thus, even if some fitness is lost, this helps to gain a lot more in the end. Investing your current fitness into your future fitness is a common theme across all living beings. And that is not all there is. A living being can also invest in its relatives’ fitness, as well.
Inclusive fitness is the fitness of you and your relatives
I said earlier that fitness is the probability of raising more offspring. That is a small deviation from the truth because fitness is actually the probability of leaving more copies of your genes. Isn’t that the same thing?
It isn’t, because your offspring are not the only those who carry your gene copies. Your sibling carries half of your gene copies, and your cousins bear one eight of your genes. You can learn more about the degree of your genetic similarity with your relatives here.
If you calculated your fitness on a spreadsheet, you’d have to write your own fitness balance, along with your relatives’ fitness balances. You can account, for example, one half of your sibling’s fitness gains and one eighth of your cousins’ fitness changes as your own. Once you’ve added them all up, what you come up with eventually is inclusive fitness.
Inclusive fitness is your fitness and the fitness of your relatives (multiplied by a percentage of relatedness) combined. This means that if my cousin is in grave danger, it is a sensible thing to help. Especially if not much is asked from me.
In that case, I will lose only a little of my own fitness, but my cousin will improve his fitness a lot. If he wins more than eight times than I lose, I can say that my balance from that action is a net positive.
Inclusive fitness is an important part of the concept of kin selection in evolutionary biology. You may be amazed by why a honey bee is willing to sting you even if she dies soon after. This is because losing her fitness can help thousands of her relatives thrive. You can learn about kin selection here.