By Dr. Debbie Rook
“Natural Selection” is a complicated and intricate biological concept that often trips up the most educated and well-read among us. To get to that, we’re first going to start with something we all can relate to and understand- specifically, breeding or “artificial selection”. This means specifically that people are the selecting what traits are passed on to the next generation, instead of nature.
Think of a dog, any dog. You may know a chihuahua down the street or your great aunt’s mastiff. Dogs come in amazingly different varieties, different sizes, weights, strengths, hair, tails, ears, noses… you name it! You probably have heard that the dog came from wild wolves, which even look like a few breeds (huskies and malamutes, mostly). But how did we get there? How is that even possible to get so much change in just a couple of thousand years?
Artificial selection is the answer. Back in the days long ago (around 130,000 years ago), there were likely wolves that hung out around farmers or nomadic tribes because there was easy food to get- whether it be cattle or other farm animals, the rabbits that came to eat crops, or simply leftover scraps that were either discarded or given to the wolves willingly. Over time, the wolves that had a nicer temperament (less biting, better smiles), would be given more scraps (think if a stray dog came up to you at a park- would you share your ham sandwich if it was growling at you or cuddled next to your side?). This was the beginning of a beautiful relationship between the more tame of the wolves and the humans. Now, how was this really selecting? The humans were not breeding these dogs yet, nor were the animals even living with them, so how could that change the population? Simple- food. The nicer dogs are more likely to get scraps from the humans, and therefore more likely to survive the winter and reproduce, while the mean wolves got no scraps and had to fend for themselves. Slowly over time, the scrap-grabbing would change into cohabitation of tame wolves and humans, which would allow those dogs to have even more offspring, because they were being actively cared for by the humans. And that’s how you get dogs! Bring a nice wolf into your house and in a couple (hundred) generations you’ll have yourself a dog (but don’t try this at home…).
So that’s great, a slow change in the population over thousands of years got us from nasty predator wolves to tame live-in dogs. But how did we go from wolf-like dogs (big, sharp teeth, long noses, pointy ears) to all the different kinds today (big/small, variable teeth, long/short/pug noses, pointy/droopy ears)? That is where serious artificial selection comes in, breeding.
Say you are a farmer and need a dog to look after your flock of sheep. You start with the basic wolf-dog and you select for traits that you want. Specifically, you want a dog that is kind to you and your sheep but will scare away other animals. So you take all your dogs and find the ones that meet those criteria best. They won’t be perfect yet, maybe it will sometimes snap at you or a sheep, but otherwise just likes to chase off coyotes. You mate the two together that have the best traits. Those offspring then will have a smaller range of these traits closer to your ideal. It’s possible that in one generation you will have succeeded with at least one of the dogs, but if not you just try again with the next breeding cycle. This also works if you need a dog to pull a sled, or find foxes, or cuddle with your kids, or even carry in your purse. Slowly, over a few generations you can get a lot of change in these animals.
I’ll give you one more example because I think it is so cool. Bull terriers. Bull terriers are known for their noses that are shorter and angled downwards. Here is one:
This is a very dramatic feature, so you’d think that it would take hundreds or years to get a nose like that from a regular looking dog. BUT…
Here is a bull terrier from 1915.
And 1918 (for good measure).
You can see that those noses have changed a lot in the last hundred years. The 1918 picture also shows you a little how this works. If you look at the four dogs, they are all bull terriers and likely related, but there is variation (differences) between the noses. Specifically, the second from the left has a nose that is slightly more downturned than his siblings. If you were trying to make the modern bull terrier, he is the dog you would want to mate to get the next generation.
So you can see that in a few decades you can get dramatic changes in breeds of dog, all by having humans select traits that they like the most. Selective breeding has brought about the multitude of types of dogs that you see today. Hopefully you have a better understanding of this now that you’ve seen it in action!
Next time: nature takes a crack at selecting for different animals, incredible variation, and adaptation occurs.