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- [Voiceover] Mating is the pairing of opposite sex organisms for the purpose of reproduction, and the subsequent propagation of genetic material. And this includes the actual act of mating, but it can also refer to all of the other behaviors that could be associated with this process. This could include things like elaborate mating dances, and a good example of this is the superb bird of paradise, and yes, that is its real name. When the male superb bird of paradise wants to attract a female, he does kind of a complicated dance that involves him bouncing around, and then fluffing out his feathers in such a way that it kind of looks like a face, and I have a drawing of what it looks like here, but this is something that you should go on YouTube and watch a clip of, because it is really pretty fantastic and ridiculous. Other behaviors that would be included under mating behaviors would be things that happen after mating. Things like nest-building, or feeding the young. In terms of searching for a mate, animals use many different mating strategies. The first one is random mating, which would describe a situation where all individuals within a species are potential partners, meaning that they all are equally likely to mate with each other. And so, random mating is not influenced by environment, or heredity, or any kind of behavioral or a social limitation. And this can be a pretty good strategy, because it ensures a large amount of genetic diversity. There are also a number of non-random mating strategies, or cases where each individual is not equally likely to be chosen as a mate. For example, assortative mating is a strategy where individuals with certain similarities, either in genotypes or phenotypes, or genes or physical appearance, tend to mate with each other at a higher frequency. For example, large animals tend to mate with large animals, and small animals tend to mate with other small animals. And while this can generally be seen as a pretty good mating strategy, the mating of two individuals who are too genetically similar to each other, which is also known as in-breeding, tends to weaken a population overall, because it can increase the likelihood that harmful recessive traits will be passed along to offspring. Disassortative mating, or non-assortative mating, is the opposite of assortative mating. So, with assortative mating, individuals with similar traits were more likely to mate. In contrast, non-assortative mating describes a situation where individuals with different, or diverse, traits mate at a higher frequency than we would see with random mating. And you might be wondering which of these strategies is better. And that's actually kind of an odd question, but in general, I would say that scientists would point to assortative mating, because despite the dangers of in-breeding, it can generally help to increase the inclusive fitness of an organism. And I might as well point out that this concept, inclusive fitness, is one that I struggled with when I was in college, and so I'm going to try to break it down as well as I can. The inclusive fitness of an organism concerns the number of offspring an animal has, how they support them, and how their offspring could support each other. So typically, we think about fitness on an individual level, that an individual creature, on some level, wants to be able to reproduce, and pass on his or her genes. But inclusive fitness is trying to think about this on a slightly larger level. It points out that because close relatives of an individual tend to have similar genes, it would be evolutionarily advantageous for an animal to promote the reproduction and survival of closely related individuals, as well as him or herself, meaning that it is not only our individual genes, but also highly related genes, that it would be advantageous to promote. And that's what we mean when we talk about inclusive fitness. And I think that this concept can help us solve some of the problems that people tend to have with evolution. When people talk about evolution, they tend to focus on things like survival of the fittest, which, if taken literally, would predict that animals, including humans, might be predisposed to act selfishly, to do whatever would be necessary to live the longest, and reproduce the most. But of course, most people don't really act like that. They're kind to other people. They help others. And this is actually what inclusive fitness accounts for, because it predicts that we will behave helpfully and altruistically towards those with genes similar to our own, and that is exactly what we see. Studies about human altruism show that people are more likely to behave altruistically towards people who share the same last name with them, which is a modern cue of possible relatedness, and therefore, shared genetic material.