If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:4:34

Video transcript

So natural selection isn't the only force that drives evolution. And there are a few alternative forms of selection that also contribute. But before we look at those, let's go over what natural selection is in the first place. And it's the idea that if you have a member of a population that has a special advantageous genetic trait, then that individual is more likely to live to an age where it can reproduce and pass on that special trait to their offspring. Also remember that natural selection selects for individuals with high fitness. And fitness is a measure of an organism's total ability to pass on their genes to their offspring. And it's a combination of an organism's ability to survive to an age where it can reproduce, but also how well that organism can reproduce once it gets to that age. Also remember that populations will evolve by natural selection and not individual members of those populations. So what are natural selection's alternatives? Well, we're going to talk about two today, group selection and also artificial selection. So let's start with group selection. And this is the idea that genetic traits that benefit the population or group as a whole will still be selected for even if they don't directly actually increase the fitness of the individual with the trait. Words like altruism and martyrdom come to mind. And traits that relate to these ideas are what we're talking about when we think of group selection. And these traits can still be selected for because entire populations evolve, not just individual members. So let's look at an example. Let's say a female human has children, and her children have children of their own. Why is it that this female grandmother is able to survive after she becomes unable to have children of her own, let's say when she's already gone through menopause, which is when the female reproductive system shuts down? Any traits that would allow a human to live past this age couldn't be selected for by natural selection since by the time those traits manifested, the person would have already lost their ability to reproduce. Well, it turns out that grandparents play a distinct role in taking care of their grandchildren. And since their care increases the survival rate and thus the fitness of their grandchildren, this helps the group as a whole. And those traits that benefit survival into old age can then still be selected for by group selection. So natural selection will typically look for traits that help a survival until the age where reproduction is possible. But group selection accounts for all those other traits that might help with survival after reproduction is no longer possible. So we talked about the first alternative to natural selection. And this was group selection. But what about artificial selection? Well, in order to find what artificial selection is, let's take a step back and look at natural selection one more time. Remember that if we have a strong individual who is more likely to survive because of his or her strength, then that trait is said to be passed on to offspring more frequently than another trait. And this selection is said to be natural because it all has to do with the idea that the stronger person has a greater probability of surviving than someone else, let's say a 75% chance instead of a 50% chance of surviving. There's no outside individual who's deciding and selecting for which traits are better than others. It all happens naturally. But that's exactly the difference between natural and artificial selection. In fact, some people call artificial selection unnatural selection. And let's explain this by jumping right in with an example. If you have a farmer growing tomatoes, and some tomatoes grow bigger than others, then that farmer can literally select and choose which tomato seeds he uses to plant tomatoes next year. He's artificially selecting tomatoes for those that have a trait which makes them grow more fruit. And that's just one of the many traits of the tomato. Another great example of artificial selection is when scientists in the lab look at a tomato's DNA and again select for specific genes that make the tomato grow larger in order to give the farmer more fruit. And both of these examples, since there's an outside being selecting which traits are desired, we say that the tomatoes are undergoing artificial selection, instead of natural selection, because it doesn't occur naturally. It's not just a matter of probability. So what did we learn? Well, first we learned that natural selection is not the only force driving evolution. We have group selection, which is the idea that traits benefiting the group over the individual with the trait can still be selected for. And we also talked about artificial selection, which is where an outside individual can literally choose which traits in a given population will be passed on, instead of that selection occurring naturally.