On the Evolution of the House Fly

“Do you feel safe and protected?”

I often ask this question to my wife when I have achieved some manly feat of heroism, like fixing a problem with her iPad. It’s a joke, referring back to prehistoric times when a man’s suitability as a mate was directly linked to his ability to protect his family from dangerous threats like a mountain lion or rival tribe.

Nowadays, we are lucky enough to not have to worry much about wild beasts, so how do I continue to prove my worth and value? Naturally, by conquering the next most dangerous threat to human existence – technology not working as it should. This skill is in somewhat short supply, so as long as I can keep the technology in line, I am proving my worth.

“Yes, I feel safe and protected,” she will respond, indicating that her iPad is now working correctly.

However, there is one area where the traditional measure of heroism still plays out in our family: house flies. It’s summer, we have little kids, and it’s not uncommon for them to walk in leaving the screen door wide open (I’ve also been known to accidentally leave the screen door partway open every once in a while, but that is another matter). Inevitably, a large fly will find its way in before the door is shut and then proceeds to buzz around the house, driving everyone crazy – especially my wife.

It is my job to go chasing the fly around with a fly swatter, waiting for it to sit still long enough to swat it. It usually takes me a couple of tries, but when I succeed, I call out, “I got it! Do you feel safe and protected?”

“Yes,” she will answer. I feel proud.

The evolution of the house fly has always puzzled me. Why haven’t flies evolved to not fly into houses? It doesn’t make sense.

My father is a professor of evolutionary biology, and so I grew up learning all kinds of esoteric facts about biology and evolution, including dinosaurs, humans, plants, and whatnot. I’m always trying to show my kids how closely we are related to the gorillas in the zoo and point out the differences in the structure of our hands and the similarity of our ears and skulls.

I also will periodically torment my father with evolutionary questions that make no sense to me. For example, how did some animals evolve the defense mechanism of being poisonous when eaten? The point of evolution is that it’s a trait that makes you more likely to survive and pass on your genes to subsequent generations, causing them to become dominant over time. If you are eaten, you are no longer able to pass on your genes. And the predator that learns not to eat you will apply this to *all* the animals of your species, not just the ones that have the poison gene. So how would it become dominant when everyone benefits? My father has never provided a satisfying answer to this question.

And that brings me to house flies. In theory, if a fly goes into a house, it is going to get trapped there and killed. According to the theory of evolution, flies that do not go into houses should have an advantage by not dying. As a result, they should be more likely to breed, and so flies should have evolved to not go into houses and torment us. Right?

Then, last night, I realized the solution.

My wife said to me, “I don’t feel safe an protected.”

A fly had gotten in and was buzzing around, and despite repeated attempts to track it down, I hadn’t managed to kill it yet.

She was just gently teasing me, but it suddenly occurred to me… I’m much better at fixing iPads than I am at killing flies. Perhaps the problem is that the flies actually have a *better* chance at surviving inside our homes than outside where there are spiders and bats and frogs.

If I’m going to keep my family safe and protected – and steer the evolution of the fly to keep them out of the house – I’d better start improving my aim.


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