Birds Don't Go To The Gym

Listen. I love the gym as much as the next person. And by "I love the gym", I mean I hate the gym. I do not do the sports-ball, I do not enjoy repetitive body torture, and the only place I run is to the dinner table. I like to think that Charles Darwin was of a similar mind. 

Let's not spend too long on that shoddy metaphor, and instead focus on what I mean by it. Darwin it rather famous for his unorthodox use of the word "fitness". In high school biology classes, most of us are probably guilty of taking this word the wrong way. I, for one, really enjoyed picturing buff-as-hell finches. Like, absolutely swole. Just ripped. 

https://www.reddit.com/r/birdswitharms/comments/9p08l1/the_mighty_muscle_bird_got_an_upgrade/

If I had to see it, so do you

But finches don't go to the gym (thank GOD), and Darwin wasn't implying that they do. Darwin's version of "fitness" doesn't care how much you can bench (sorry, frat boys) - it cares about how much you have sex (...never mind). 

Okay, immature jokes aside, Darwin's version of fitness really does care about reproductive success. In fact, that's what it's meant to describe! Biological fitness describes how good an organism is at surviving and passing down its genes to the next generation. Obviously this takes thousands of different forms - from being strong enough to fend off competitors to being fast enough to escape predators to being drought-tolerant enough to survive the summer. 

So...fitness is less an episode of America Ninja Warrior and more like an epic crossover of Man vs Wild and the Bachelor. 

....damn, now I kinda want to watch an epic crossover of Man vs Wild and the Bachelor....

Anyways. That's our definition of fitness. Actually measuring it, though, that can be a lot harder. Trying to measure fitness is like trying to measure how good of a kisser someone is - it's entirely subjective to the situation. You see, fitness isn't the same everywhere and everywhen. It's dependant on the environment than an organism is in. 

That's...confusing. Yeah, I'm confused. Example time!

Let's try to measure the fitness of...moths. How do we feel about moths? I like moths. Let's do moths. 

OKAY!

https://www.icr.org/i/wide/peppered_moth_wide.jpg

Meet the peppered moth, also known as Biston betularia. Recognize it? I'm not surprised; their adaptation story is a staple in most biology textbooks. But in case you haven't heard it, I'll do a quick recap:

As you can see, this moth has two main forms (called morphs): a natural white morph (right) and a dark mutant morph (left). Around the industrial revolution, the mutant form became a lot more common, to the point where the normal white forms were hardly seen at all. This is because all the ash from the factories turned the local trees from white to black. White moths used to be "more fit", because they were better camouflaged against the white trees. But when the trees turned black, the black moths became "more fit" because they matched the new environment. 

See what I mean about the importance of environment? The darker moths weren't stronger or faster or better at flying away from birds. They also didn't do anything to become better at those things. It was their environment that changed the rules, reversing which color of moth was suddenly better at surviving and reproducing. And since more fit equates to better at surviving and reproducing, it was a sudden shift in which moth type was more "fit". 

But how do you quantify this? How do you assign a "fitness score" for a dark moth versus a white moth? Well, you could measure how many of each color survive. But that's more a measure of their trait (color) than their fitness as individuals. My best guess would be to measure their reproductive success. 

Let's take a population of hypothetical moths - some dark and some light. And then we chuck 'em in an outdoor enclosure where they can be eaten by birds. Then we wait, and watch, and eventually count how many eggs each moth produces. Then we wait for those eggs to grow up, and measure how many survive into adulthood and produce their own eggs. The number of moths that survive and reproduce could be used as a direct measure of their parent's fitness, since it was their parents who gave them the genes to survive. 

Theoretically, an experiment like this would show us which color is "more fit" for the environment we put them in. If we chuck 'em in an outdoor enclosure full of white trees, we're probably gonna see that the white moths produce more successful offspring. If we chuck 'em in an enclosure full of black trees, we're probably gonna see that the black moths produce more successful offspring. 

...I wonder what would happen if we chucked 'em in an enclosure full of brown trees......

Anywho. Unlike many of my other blog posts, I am not confident on my answers in this one. But it seems logical to me that if fitness = how much of your genetic material is in the next generation, then you can measure fitness by measuring how much of the next generation has your genetic material.

But, I digress. I'm gonna go google Man vs Wild and the Bachelor crossovers. And try to scrub muscle bird out of my brain.