What Is Assortative Mating?

Instructor: Sharon Linde
Assortative mating is a scientific term that describes how some animals choose reproductive partners in a non-random way. This lesson defines this mating pattern and gives some examples.

What is Assortative Mating?

Emily has loved cats since before she could walk. She has always had at least one as a pet, has fostered many more for rescue shelters, and has worked as a volunteer for her local veterinarian. Now that she is an adult, she's decided to breed purebred cats. Although she likes all breeds of cats, her favorites are Siamese cats, so she starts her new business with a male and a female of that breed. She knows that the most sought after Siamese cats have a specific appearance: blue eyes, a light colored body, and black 'points' - color on the ears, face, tail-tip, and lower legs. She chooses her first two cats with this knowledge in mind and buys the best examples that her small budget allows for.

Emily may not know it, but she is choosing an assortative mating pattern for her cats' future. The official term for this practice is 'non-random mating selection', but a more familiar term is 'selective breeding'. Emily is selecting the physical traits that she wants to enhance in the next generation of her cats.

Effects of Assortative Mating

The results of assortative mating, as you would expect, are a change in the distribution of the gene pool for the population. If the preferred mating partner for all Siamese cats is a specific color of blue eyes, then the next generation of Siamese cats is likely to have a higher occurrence of that shade of blue.

Let's take a look at how this works.

From Darwin to Hardy-Weinberg

Charles Darwin was a famous biologist who came up with the theory of natural selection to explain evolution. Darwin also theorized that wild animals followed assortative mating patterns, and claimed that this had a large effect on evolution as well.

It took more than 50 years, but the scientific community eventually agreed with Darwin's theory on assortative mating. In 1908 G.H. Hardy and Wilhelm Weinberg, working independently of each other, came up with a genetic equilibrium equation that proved Darwin's original theory. They were able to show that the genetic variation would be unchanged in an animal population if animals mated randomly, and predicted the relative frequency of physical characteristics if random mating was occurring. Subsequent measurements of actual animal populations showed that these predicted frequencies were often not matched - meaning that wild animals were selecting mates in a non-random fashion.

Positive and Negative Assortative Mating

Experiments conducted after Hardy and Weinberg published their work showed both positive assortative mating and negative assortative mating. Emily and her cats have already demonstrated what positive assortative mating is - breeding on the basis of similar characteristics. Emily selected both Siamese cat parents to have blue eyes. If she had instead chosen one blue-eyed cat and one with brown eyes, that would have been an example of negative assortative mating - breeding on the basis of dissimilar characteristics. Negative assortative mating doesn't make sense for cat breeders, and appears to be rare in wild animal populations.

Other Examples of Assortative Mating

Besides the work of animal breeders, what other examples of selective breeding do you know of?

Well, farmers have been breeding plant varieties by selecting for specific characteristics for thousands of years - this is how many varieties of fruits and vegetables came to exist. Heirloom tomatoes, for example, were selectively bred based on taste, not on how they looked. Hybrid tomatoes were then chosen to look good in supermarkets - they are a nice red color, larger, and round. However, many people think hybrid tomatoes don't taste nearly as good as their heirloom ancestors.

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