Flies have a short lifespan, therefore evolution should technically happen over a shorter period of time (years).

Flies die all the time from getting hit by cars on the motorway.

Those flies that don't go near the motorways have a greater chance of reproducing.

If this was an advantage (motorway avoidance = not dying), shouldn't more and more flies be avoiding the motorway?

Cars have been around since, roughly ~1950. That's 60 years.

Why don't flies avoid the motorway?

  • $\begingroup$ Again, this kind of question is down-voted? As with this question there is an answer! In fact this is a semi-duplicate? $\endgroup$ – Luke Aug 17 '12 at 17:15
  • $\begingroup$ I agree and upvoted, though it might be nice if we had an FAQ as this sort of question pops up regularly... evolution and biology is hard to get an entree. $\endgroup$ – shigeta Aug 17 '12 at 17:51

"in the long run we're all dead men" - John Keynes

First is the fact that flies have adapted their way around cars long long ago by giving up longevity for faster breeding; flies are cheap and they just don't care if they get hit by a car. The average flying insect lays large clutches of eggs - hundreds at a go is common. See this reference on butterflies as an example. In table 2 the potential fecundity (FEC) is I believe the number of eggs laid in a lifetime. The smallest number is 53, the largest is 1500, the average is hundreds. As long as one or two fly from each lifetime of breeding survives, the species holds on just fine, more and there is a population explosion.

Another side of the question is:

It would still be an advantage to be smart and stay off the road wouldn't it? If they adapt so quickly why doesn't a mutant braniac fly come along that can see the cars and in short order all flies would have this trait... its just selection too, right?

The answer there is that not everything is available to adaption. Flies could become that smart, heck maybe even smarter than us, but they have lots of pressure on them from many things they could do well, and their system is already doing many things very well enough for them to survive as they are without making such a large change that might require some short term disadvantages that prevent them from becoming entirely different animals. The essay "why pigs don't have wings" is a pretty good reading for a longer more detailed answer.

Where I live its temperate and there is a good amount of water- lots of food and hospitable for all kinds of plants and animals can thrive here. On the hillsides where updraft currents blow flying insects. The spiders 'figured this out' and the bushes and trees are often covered with cobwebs on this side of a house. The flies don't avoid the wind just because so many of them are eaten - they would not themselves be able to mate or eat.
Note that the spiders haven't become 'smart' - just lucky to have landed in that area. But then there are many many spiders there that they probably don't get a lot more food than a spider living on the other side of the house who has the only web for a space of several cubic feet.

Everything balances itself out in a competitive landscape because there are so many other living things ready to take advantage of a break like this. Cars just aren't enough to budge the other things the flies have to watch out for. For more reading about how competition affects evolution read about the Red Queen and the Tangled bank.


Flies can and do evolve quickly thanks to a short generational time. However, your question seems to stem from a misunderstanding of what evolution entails.

Evolution occurs when the genome mutates. Why it mutates isn't particularly relevant, but a gene (or genes) mutates and as a result of that mutation a characteristic of the organism changes. It doesn't have to be a big change. Maybe a single protein somewhere operates 2% less efficiently, giving practically no outward signs of evolution. Sometimes it can be big - changes in coloration, changes in sensory perception, lifespan, etc.

In order for mutations (beneficial or detrimental) to have a significant effect across an entire population, the population itself has to be fairly small or the mutation has to give a large advantage over the old version.

Flies are not avoiding motorways because a mutation that specifically codes "AVOID CARS" is going to be very unlikely given that it would you would first have to encode what a "CAR" is in order to avoid it. That probably won't happen because on the time scale that evolution usually acts over very large populations (which flies have) we won't be driving cars by the time necessary for deaths due to cars has a serious effect.

Flies have huge populations and breed extremely quickly, so even if a few million are killed by cars, it doesn't put a dent in their population numbers. There's no reason for the change to occur. Combined with the extreme difficulty of actually encoding a gene specific to cars, the odds are it just won't happen in any realistic time-frame.

  • $\begingroup$ Wouldn't it be possible for the genome to mutate so flies avoid darker areas (i.e. black roads (not straight away but darker through generations)). These flies would be more successful. But I agree with your second point even if a few million are killed by cars, it doesn't put a dent in their population numbers $\endgroup$ – Blundell Aug 20 '12 at 7:45
  • $\begingroup$ Sure, it's possible for the genome to mutate that way, but if you're just avoiding 'dark areas' then you're also avoiding natural 'dark areas' and not avoiding all of the light-colored roads made of concrete. What's possible isn't so much the crux - there are very complex occurrences in the genome, but unless there's a big impact it's just not favored. $\endgroup$ – MCM Aug 20 '12 at 14:21
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    $\begingroup$ I second @Blundell here. There is no need for a concept of CAR in order to have a behavioural change that results in CAR AVOIDANCE. I think this is a misunderstanding of how evolution works (which I encounter in laymen very often) and downvoted this answer. $\endgroup$ – luispedro Aug 20 '12 at 15:36
  • $\begingroup$ @luispedro: the essence of the answer does not change. You may not need to encode car, you may not need to encode car avoidance... the fact is: being smashed by cars is not really a big problem for flies, as the proportion of flies that dies like that is most probably very small. Actually, encoding for "avoid big objects that move" may even be negative, because, for instance, flies love to go on wild stinky animals... :) $\endgroup$ – nico Aug 20 '12 at 16:07
  • $\begingroup$ @nico: Yes, the "irrelevancy" argument might be sufficient. That does not justify using a very wrong argument about how evolution could not generate a CAR AVOIDANCE behaviour. If that paragraph is removed, I will remove the downvote. $\endgroup$ – luispedro Aug 20 '12 at 17:51

I also got an answer via email that I think is worth sharing.

Mrs Price:

I'm not answering officially as I’m probably wrong but…

The main point mentioned is that because flies have such a short lifespan, so they should evolve faster than elephants or other slow growing species. While this is true, and there are many examples of r (ruderal) species evolving rapidly* evolution is not a predictable or sensible process, and there are many other factors that contribute to evolution other than a short generation time.

One main question to consider is… how many flies are there? And how much of that population is affected by the road fatalities? Whilst there are over 23,000 insect species in the UK, your question specifically concerned flies so… An average fly lives for around 35 days and can lay roughly 750 eggs in her lifetime, so fly numbers are bloody huge! Although a small proportion of flies may be killed by a car, the proportion will be so small as to make no impact to the overall population; therefore the driver for evolution is not strong enough to have an impact on the species as a whole.

Another reason why motorway deaths may not have a significant impact on the fly population is due to when the flies breed. Even if motorways killed 90% of all flies, the fact that many insects breed hours after hatching from their egg means that the population is still able to reproduce. Killing something after it has bred is again not a driver for evolution, it would have to be a very strong force that kills a high number of individuals before they have chance to breed.

In summary, motorway deaths affect such a small amount of individuals that it does not have a significant impact upon their populations, I imagine that many many more of them get killed in spider webs each day, however the idiots are still flying into those too!

Hope this helps :)

  • *The peppered moth changed colour from mottled to black due to the population from the industrial revolution and then to white once the air pollution had cleared as the ones that stood out had such a high predation rate

  • *in the last 50 years some fish species have evolved a much smaller body size due to the advantage of escaping through a net and being able to carry on breeding


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