# Why hasn't roadkill been selected against by now? **URL:** \ **Category:** Factual Questions\ **Created:** [October 12, 2017, 6:07pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668 "2017-10-12T18:07:57Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 12, 2017, 10:31pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/21 "2017-10-12T22:31:01Z")
> [@](#): > > The “selection effect,” in this case, humans in cars, has to cause the death of a majority of the species before it can procreate. Not at all. If, for instance, you had a dominant trait is always lethal before sexual maturity, then that trait will be completely eliminated within a single generation, even if it’s very rare (and hence the number of individuals killed by it is a very small fraction of the total population). You only need to kill a large number if you’re trying to eliminate a widespread trait. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [October 12, 2017, 10:34pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/22 "2017-10-12T22:34:51Z")
> [@igor\_frankensteen](#): > > The “selection effect,” in this case, humans in cars, has to cause the death of a majority of the species before it can procreate.) Not at all. There just has to be a differential in the number of offspring produced by individuals that cross roads vs those that avoid crossing roads. Road-crossing individuals could have a higher reproductive rate because they obtain access to more food than those that don’t, which enables them to have enough offspring to compensate for the fact that they have a somewhat higher mortality rate by being hit by cars. Selection doesn’t have to be anywhere near as drastic as killing a majority of the population to have a strong effect over time. It’s just that in this case the number of individuals killed isn’t large enough to have a strong enough selective effect in a short time. Now if you want to see strong selective effects in a short time you generally do have to practice such extreme selection as you describe. This is what humans do in selectively breeding animals and plants: the individuals selected to breed from comprise only a tiny part of the population, and such severe selection is imposed for many generations. But selection in nature is rarely this strong. ---
**Author:** ![Ignotus](https://avatars.discourse-cdn.com/v4/letter/i/ecc23a/32.png) [@Ignotus](https://boards.straightdope.com/u/Ignotus)\ **Post date:** [October 12, 2017, 11:10pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/23 "2017-10-12T23:10:11Z")
> [@Filbert](#): > > Incidentally, elephants also seem to recognise that cars stay on roads, though I admit my observations of wild elephants is a bit more limited. And cars (or, rather, their drivers) presumably have a lot more incentive to avoid collisions with elephants than with corvids or pheasants. 😉 ---
**Author:** ![Trancephalic](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/trancephalic/32/380_2.png) [@Trancephalic](https://boards.straightdope.com/u/Trancephalic)\ **Post date:** [October 12, 2017, 11:11pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/24 "2017-10-12T23:11:26Z")
I think we’re discounting the animals run over on purpose. There are no shortage of sadists who delight in running over easily avoidable critters like turtles. Just saw a dead one today that was squished on the shoulder where no vehicle is supposed to go (and there are usually telltale signs when an animal was moved there posthumously). ---
**Author:** ![BobBitchin](https://avatars.discourse-cdn.com/v4/letter/b/7ea924/32.png) [@BobBitchin](https://boards.straightdope.com/u/BobBitchin)\ **Post date:** [October 12, 2017, 11:53pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/25 "2017-10-12T23:53:41Z")
I think the time factor is key, but also maybe the imperative to roam may be stronger or more beneficial than the risk of crossing roads. ---
**Author:** ![Acsenray](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/acsenray/32/4519_2.png) [@Acsenray](https://boards.straightdope.com/u/Acsenray)\ **Post date:** [October 13, 2017, 12:19am UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/26 "2017-10-13T00:19:13Z")
Also, when a road is built through an area, accompanying the road are many other factors _besides being hit by a car_ related to human activity (or even the road itself) that may be killing off local wildlife. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 13, 2017, 1:04am UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/27 "2017-10-13T01:04:54Z")
[Previous thread](http://boards.straightdope.com/sdmb/showthread.php?t=465643) on this topic. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [October 13, 2017, 1:26am UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/28 "2017-10-13T01:26:01Z")
> [@Chronos](#): > > Is that by number or by mass? By number, squirrels are probably more common than deer or raccoon; they’re just not as noticeable, and they’ll be cleared away more quickly by scavengers. That’s by “How often I notice the species of a dead animal as I’m driving by”. The ones I don’t see I don’t see. However, two more dead raccoons today, so I’m bumping raccoons up from 40% to 50%. ---
**Author:** ![Tired\_and\_Cranky](https://avatars.discourse-cdn.com/v4/letter/t/a87d85/32.png) [@Tired\_and\_Cranky](https://boards.straightdope.com/u/Tired_and_Cranky)\ **Post date:** [October 13, 2017, 2:35am UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/29 "2017-10-13T02:35:36Z")
The OP asks a good question, even if it’s not perfectly original. I recall watching a documentary about raccoons in Toronto that suggested city raccoons were smarter than country raccoons though I don’t remember how they estimated raccoon intelligence. The documentary also noted that city raccoons rarely crossed busy streets. That suggests raccoons’ urban environment was selecting some trait (perhaps intelligence) that helps them survive traffic better. So maybe traffic is contributing to the evolution of smarter raccoons that recognize busy streets and learn to avoid crossing them. ETA: I think this was the documentary: [https://www.thestar.com/news/gta/2011/02/11/documentary\_reveals\_the\_secret\_lives\_of\_torontos\_raccoons.html](https://www.thestar.com/news/gta/2011/02/11/documentary_reveals_the_secret_lives_of_torontos_raccoons.html) ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [October 13, 2017, 3:16am UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/30 "2017-10-13T03:16:31Z")
> [@Tired\_and\_Cranky](#): > > The OP asks a good question, even if it’s not perfectly original. I recall watching a documentary about raccoons in Toronto that suggested city raccoons were smarter than country raccoons though I don’t remember how they estimated raccoon intelligence. The documentary also noted that city raccoons rarely crossed busy streets. That suggests raccoons’ urban environment was selecting some trait (perhaps intelligence) that helps them survive traffic better. So maybe traffic is contributing to the evolution of smarter raccoons that recognize busy streets and learn to avoid crossing them. > > ETA: I think this was the documentary: [Documentary reveals the secret lives of Toronto’s raccoons](https://www.thestar.com/news/gta/2011/02/11/documentary_reveals_the_secret_lives_of_torontos_raccoons.html) Moderator Note The above post was posted to an old thread from 2008 on the same subject. I have moved it to the current thread. Colibri General Questions Moderator ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [October 13, 2017, 4:40am UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/31 "2017-10-13T04:40:39Z")
> [@igor\_frankensteen](#): > > The way Natural Selection works, is simple, but an amazing number of people get it wrong. > > The “selection effect,” in this case, humans in cars, has to cause the death of a majority of the species before it can procreate. That’s it. And cars do not accomplish that, because crossing roads is not necessary for any species to maintain itself. > > Killing “a whole lot” of a given creature only matters if the kill rate is higher than the replacement rate, over a long period of time. > > Dear and squirrels obviously procreate a lot faster than the number who die under our tires. > > The species that we’ve driven to extinction, or near extinction, were all either purposely hunted to try to get them all (wolves, tigers, bison and the like), or accidentally wiped out while we were doing something else (condors, honey bees, etc) No, species are the result of natural selection not the cause. Evolution is about genes. Genes have no aspirations or morality or consciousness or goals. They’re just molecules that produce copies of themselves. It’s a mechanical process. Genes influence the behavior of the organisms in which they are located. This genetic influence may change the likelihood of the organism reproducing, which provides a new location for the genes. Genes that are useful for producing new locations like this become more common. Genes that are detrimental to producing new locations become less common. But the genes don’t know or care if they’re common or uncommon. Like I said, they’re just molecules. Everything a gene does is just a chemical reaction. Evolution occurs when something changes. It might be the gene or it might be the environment. A new gene might influence new behavior that is more useful than the behavior produced by existing genes. Or a change in the environment might make existing behavior more likely to cause a organism to reproduce than it used to be. In either event the gene becomes more common. Or the opposite effect can happen. A new gene might produce detrimental effects or an environmental change might make a genetic effect less useful than it was. And then the gene will become less common and might even disappear entirely. But again, the genes don’t know or care. There’s no greater plan at work. It’s just random chemical reactions. That’s why evolution can take so incredibly long to produce any noticeable changes. ---
**Author:** ![Tired\_and\_Cranky](https://avatars.discourse-cdn.com/v4/letter/t/a87d85/32.png) [@Tired\_and\_Cranky](https://boards.straightdope.com/u/Tired_and_Cranky)\ **Post date:** [October 13, 2017, 10:43am UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/32 "2017-10-13T10:43:42Z")
Thanks for moving my post, Colibri. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 13, 2017, 11:55am UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/33 "2017-10-13T11:55:32Z")
For what it’s worth, prey species do not respond to a perceived predator threat from a known direction by running laterally, since that reduces the component of their velocity that’s heading away from the predator, and allows the predator to cut the corner diagonally to intercept them. The optimal response (and their evolved instinct) is to run directly away from the direction from which the predator is approaching, and dodge sideways only at the very last minute if the predator closes on them. Thus, if an approaching car is seen as a predator threat, the animal will tend initially to run _along_ the road directly away from the car. I have certainly seen sheep in the U.K. behave this when when a car approaches on a road across open moorland. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 13, 2017, 12:23pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/34 "2017-10-13T12:23:55Z")
> [@igor\_frankensteen](#): > > The way Natural Selection works, is simple, but an amazing number of people get it wrong. > > The “selection effect,” in this case, humans in cars, has to cause the death of a majority of the species before it can procreate. That’s it. And cars do not accomplish that, because crossing roads is not necessary for any species to maintain itself. > > Killing “a whole lot” of a given creature only matters if the kill rate is higher than the replacement rate, over a long period of time… I don’t know what you mean by “replacement rate” but that is not what matters. It’s the difference in [fitness](https://en.wikipedia.org/wiki/Fitness_(biology)) between genotypes/phenotypes. In other words, the proportion of genes contributed to the next generation by each genotype. The first requirement for natural selection to take place is that variation in genotype & relevant phenotype _must exist_ in the population. In this case, there must exist at least two genotypes in the population, one of which has a phenotype “more likely to cross roads” and the other “less likely to cross roads” (or “worse/better car avoidance reaction”). It may be that this variation in genetically-determined behavior simply does not exist. There is a valid underlying point that the mechanism of evolution is differential rates of death among genotypes/phenotypes - and that _rapid_ evolution in response to an environmental change requires strong selection pressure, meaning a lot more death by new means. Depending on the manner of reproduction, introducing a new strong selection pressure may result in a population decline as the less fit genotype is killed off. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 13, 2017, 1:05pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/35 "2017-10-13T13:05:36Z")
One bit I do wonder about, is that cars aren’t the first threat of their type. Plenty of prey animals have evolved different responses to different threats: A rabbit, for instance, will run from a fox, but hide from a hawk. But there are naturally-occurring threats that behave in about the same way as a car: Coming right towards you, but just coincidentally, and harmful if they hit you but won’t try to chase if you get out of the way. Consider a stampeding herd, for instance. You’d think that animals would have evolved an appropriate response to a stampede, in the eons that they’ve lived with large herd animals. And once you already have that behavior established, adapting to cars would only require slotting cars into the appropriate category, which should be much easier than developing an entirely new response. ---
**Author:** ![Monimonika](https://avatars.discourse-cdn.com/v4/letter/m/85e7bf/32.png) [@Monimonika](https://boards.straightdope.com/u/Monimonika)\ **Post date:** [October 13, 2017, 1:19pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/36 "2017-10-13T13:19:04Z")
Apparently there is research that indicated prey animals (such as moose) congregate near roads in order to avoid bears. [http://www.nature.com/news/2007/071010/full/news.2007.155.html](http://www.nature.com/news/2007/071010/full/news.2007.155.html) > [@](#): > > Between 1995 and 2004, Berger monitored births from 25 female moose in and around Grand Teton National Park, inside the Yellowstone ecosystem. He reports in Biology Letters1 that pregnant moose moved an average of 122 metres closer to roads each year of their birthing period, bringing them from 1,000 metres to 380 metres of the road; non-pregnant moose, and those in bear-free areas, did not move. > > Berger suggests that the pregnant moose move because their calves are heavily predated on by bears, and the bears keep away from roads. “It would seem that moose mothers are using the fear that bears have of roads to protect their young when they are most vulnerable,” says Berger. So, it may be that roadkills of certain types of wild animal happen because the roads are considered relatively “safer”. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [October 13, 2017, 2:08pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/37 "2017-10-13T14:08:00Z")
For the smaller fur-bearing mammals, many cross the roads underneath in the culverts … go out into the country and find a culvert, look for the animal trails leading in and out … usually these trails are very easy to spot … this is a prime location to set a foothold trap for the “mixed bag” trap line, super easy access by car, generally a nice assortment of catch … the main downside is the catch and traps make for easy stealing and there’s always the danger of catching the occasional skunk … ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [October 13, 2017, 2:13pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/38 "2017-10-13T14:13:33Z")
> [@Riemann](#): > > There is a valid underlying point that the mechanism of evolution is differential rates of death among genotypes/phenotypes - and that _rapid_ evolution in response to an environmental change requires strong selection pressure, meaning a lot more death by new means. Depending on the manner of reproduction, introducing a new strong selection pressure may result in a population decline as the less fit genotype is killed off. To nitpick, as I said above the mechanism is not necessarily the rate of death, but differential reproductive rates between genotypes. Obviously if an animal is killed before it has realized its reproductive potential it will have a lower reproductive rate than other individuals, all other things being equal. But it is entirely possible for a genotype to have a higher death rate and _also_ a higher reproductive rate, if it is able to produce more offspring before dying. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 13, 2017, 2:20pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/39 "2017-10-13T14:20:56Z")
> [@Colibri](#): > > To nitpick, as I said above the mechanism is not necessarily the rate of death, but differential reproductive rates between genotypes. Obviously if an animal is killed before it has realized its reproductive potential it will have a lower reproductive rate than other individuals, all other things being equal. But it is entirely possible for a genotype to have a higher death rate and _also_ a higher reproductive rate, if it is able to produce more offspring before dying. Well, if you’re going to nitpick, don’t quote me selectively, omitting the part where I defined “fitness” more generally. In the case of roadkill, the mechanism would obviously be differential rate of death, not variation in reproductive rate, and it would be unlikely for any _strong_ selection pressure causing _rapid_ evolution to operate otherwise. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [October 13, 2017, 2:30pm UTC](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668/40 "2017-10-13T14:30:23Z")
> [@Riemann](#): > > In the case of roadkill, the mechanism would obviously be differential rate of death, not variation in reproductive rate, and it would be unlikely for any _strong_ selection pressure causing _rapid_ evolution to operate otherwise. Well, no. As I’ve pointed out above, animals that cross roads could well have a higher reproductive rate because they obtain access to more food and resources even if they have a higher death rate from cars. Focusing just on the death rate misses the point. Animals would not only have to have a higher death rate, they would also have to not gain a compensating advantage from crossing roads. In species where males compete for mates, males may have a higher death rate but the winners have a much higher reproductive rate. [Previous page](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668.md?page=1) [Next page](https://boards.straightdope.com/t/why-hasnt-roadkill-been-selected-against-by-now/798668.md?page=3)