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(light music) - [Narrator] We've seen that genes can vary. Most clearly, they differ among the major groups of organisms. That's evident because genes lead to body forms or phenotypes, and phenotypes of these different organisms can vary. Everything from a tree to a sand dollar, and beyond. The genes can also vary among closely related species. Even if you look at the hundreds of species of beetles in a single beetle genus, each of those species has its own genetic make-up. But here, we want to talk about how genetic diversity also occurs among distinct populations of a single species. Or among the individuals within a species. Think of the hundreds of different breeds of dogs, or the numerous varieties of roses. Those all represent genetic diversity within the single species of dogs, and the single species of roses. Dogs and roses, sounds like a rock group. Each individual is a little bit like a ship loaded with crew members. Some crew members are at the helm, some care for the engines, and some are swabbing the decks. But no two ships have exactly the same crew. Even though the ships might look identical from a distance, each crew adds subtle differences in the directions that their ship might take, how well the engines are running, or even how clean the decks are. Put all those ships into a big fleet, and you have a lot of different crew members. You have a labor pool. Just like those ships, each organism is unique internally, but each organism is unique in its genetic information. If we consider all the individuals in the population, we have a lot of different genes. Collectively, these are known as the gene pool, the complete set of genetic information within a population of a given species, or within the species itself. The larger the gene pool, the greater the genetic diversity. The higher the chances are that some members of the population will survive or even flourish in times of environmental change and challenges. That's because some of the individuals are gonna have traits that make them resilient in the face of changes in the environment, more resistant to disease, and more able to survive changes in climate, and so on. We've already seen that this is due to the way natural selection works, selecting for fitter individuals that pass on their beneficial characteristics. But now we're gonna tie that concept to the size of the population. Basically, the bigger the population, the more likely there will be individuals with some unique combination of genes that will allow their survival. But what happens when the gene pool is much smaller, in other words, when the gene pool is shallow. In the wild, in a small isolated population of organisms, the choice of mates with whom to breed tends to be restricted to closely related members of the same population, possibly siblings or even cousins. So the genetic make-up of the individuals becomes more and more uniform. And what's worse, flaws or disabilities that those individuals might be carrying in their genetic information become expressed or appear in the population more frequently, and this is known as inbreeding. Evidence shows that maintaining not only large numbers of species in ecosystems, but large numbers of individuals within populations of those species is important to preserving biodiversity overall. Lots of different studies have shown that extinction, permanent loss of a species, is preceded by a drop in genetic diversity, a decrease in the gene pool, within the threatened species. From the species' point of view, decreasing genetic diversity is a sign of trouble ahead. Degradation of habitats can cause a decrease in population size and promote inbreeding. When the population size of a species is reduced to the point that it almost goes extinct, the species is said to have gone through a genetic bottleneck. American bison are a classic example of such a bottleneck. The drop in the bison population reduced their genetic diversity, which makes them more vulnerable to environmental changes and diseases. Just as we cannot get back a species that's been lost to extinction, it's very difficult, and in most cases impossible, to get back the genes that are lost when that species goes extinct, or when individuals carrying unique genetic combinations die. As humans encroach on wild habitats, populations consequently become smaller, and so does the gene pool. This represents a loss of options for a population to respond to stresses, whether those are natural stresses, or stresses caused by humans. Ecosystem services arguments go straight to this idea of conserving genetic diversity. Genes control the production of substances that we use in medicine and food and even as energy sources. Preserving genetic diversity increases the likelihood that new substances can be found among wild populations, and that the supplies of useful substances we already have can be maintained. The erosion of genetic diversity in agriculturally important species can come in two forms. One is due to artificial selection for special traits that humans find desirable, to the exclusion of other traits. The other source of erosion is that we've focused our dependence on only a few organisms. For example, only about 100 or so species of plants account for 90% of our food crops, and only three different species, corn, rice, and wheat account for something close to 70% of the calories consumed by humankind. And these include 50% of the plant proteins we eat. What if some disease attacks any of these basic sources of our own nutrition, and those plants have no genetic resistance? I hate to think about the dire effects the lack of genetic diversity would have on our own survival. (light music)