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Natural Selection

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Natural selection is a theory conceived by Charles Darwin that states that in a population, organisms with a genetic trait that increases the chance of having offspring will pass on their genes to the next generation more than those without it. Natural selection ultimately leads to evolution after many successive generations. Mutation, migration and genetic drift are some factors that significant impact heritable traits in a group of organisms and potentially influence natural selection.

One of the major forces that affects group of organisms evolution over time is this idea that Charles Darwin came up with called natural selection. And the idea of natural section is that in a population a group of organisms, some organisms who happen to have a genetic trait, if that genetic trait increases their chances of living long enough to have offspring then they will and they will pass on more of their genes to the next generation than those organisms who don't have that genetic trait.

Now this leads into a discussion of something called reproductive fitness. Reproductive fitness is how scientists measure an organisms relative ability compared to the other organisms in that population. It's their relative ability to survive and leave offspring in the next generation. So what are some examples of natural selection? Well with bacteria we use antibiotics to kill them now when antibiotics were first discovered and used penicillin was considered nuclear weapon in a doctors tool box. If somebody had a bacterial infection penicillin killed them all, and it was considered the wonder drug. People figured they could be used to kill or cure anything, well nowadays penicillin is considered just a little pop gun in the battle against bacteria.

Most bacteria in our day if you get a disease and you're given penicillin there's a really good chance that, that bacteria is going to live. Why do that happen? Well that's because initially there were very few of any bacteria that were resistant to penicillin but has it started being used in the environment, those bacteria that had a slight resistance to it survived a little bit longer than those without it and they had more offspring. Next generation there's a few more of these antibiotic resistant bacteria and every generation since then we've been attacking them with the same penicillin and we killed off all the ones who were vulnerable leaving just the resistance ones.

In some cases there're some bacteria that are essentially immune to every antibiotic we have. Now an interesting thing has been discovered there was a country in Africa where most if not all of the bacteria were completely immune to penicillin. So they just simply stopped using it, after several decades are gone by some doctors had somebody who is sick and they had nothing else and so they said hey let's try some penicillin and they gave it to him and wiped them out, wiped out the bacteria the guy was cured and they wondered what why? Well remember to make these resistances the bacteria had to spend energy to build the defenses, well without penicillin in the environment building a defense against something that doesn't exist is a waste of energy. So if you had lost their resistance had the advantage and so in that environment the best reproductively fit bacteria were ones who weren't wasting energy making defenses against antibiotic that was not present in the environment. So now you can understand why scientists are so intrigued by this idea because if we can use it, we can perhaps better protect ourselves against bacteria and viruses and other things in the environment.

What are some of the requirements in natural selection? Well one you need to have more offspring being born than can survive to adulthood. Two you need to have competition for resources, now resources can be kind of a nebulous term, because it can refer to meat, it can refer to food, it can refer to light or water all sorts of things. You need to have genetic variation or diversity within the population, so you need to have not a group of individuals who are all pretty much the same but you need to have diversity for natural selection to work upon. For example there's a kind of lizard where they're all females and the females will lay eggs but those eggs don't need males to fertilize them they can grow into yet another lizard.

The problem is this means that all the eggs laid by the lizard are identical to the mother lizard and all of her grand children will be identical. So now this lizard can if it lands on an island quickly populate that island and take it over unfortunately there's a mite a kind of little tiny insect parasite that can attack thasusceptiblet lizard and if it gets a foot hold into that lizard population they are succeptible and they all start to die very quickly because there's no variation to help them develop a resistance. Finally some of these varieties need to have an increased chance of surviving long enough in order to reproduce. So you need to have variety but some of that varieties need to be actually helpful.

Here's an example in spiders of how natural selection will reward behaviors that initially we think of as being kind of silly or foolish. There's a kind of a spider where the male spider will engage in courtship rituals to encourage a female to mate with him, she eventually will agree and he mounts up and starts to mate. And then he does something really freaky, he does this weird back flip and starts to feed her his body while he's still mating. You maybe thinking why do that? Well it turns out this is a way for him to provide more resources for his future offspring because spiders don't have pockets so how can he give food to help his babies? By feeding his body to his wife and why does she sit still in order for him to mate? Because she knows she's going to get a gift of food and he continues to mate while he is feeding her his body.

That's an advantage because she's going to sit there and let him continue mating because she is in the middle of eating. So it works out for both of them and that's why that weird strategy wound up becoming more common in that spider species until now all of that spider species I think it's boxer spiders follow that particular strategy. So natural selection is a pretty simple idea and once you get it, it'll be hard to understand how you didn't understand it in the beginning, it's just if you ever do something that helps you survive long enough to have babies then you will have babies. If you don't have that something then you die no babies once you're dead because it's really hard to have offspring once you're a member of the dead.