The genetic diversity of populations is critical to conservation. Mexican gray wolves living in the mountains of southern Arizona and New Mexico represent a regional population that is genetically distinct from the wolves in Yellowstone. These genetic differences contribute to the overall genetic diversity of gray wolves worldwide. We define genetic diversity as the sum of the different genes and gene combinations found within a single population of a species, as well as across populations of the same species. Genes are stretches of DNA that direct the growth, development, and functioning of organisms (Figure 3.1). For example, some genes code for proteins that function as structural molecules, making up tendons or ligaments, whereas proteins, called enzymes, facilitate specific chemical reactions. For instance, the enzyme lactase speeds up the digestion of the lactose sugar in milk.

We can see abundant evidence of genetic diversity in facial differences among individual people. These differences allow us to instantly recognize friends and acquaintances in a crowd. Such facial variation can also be seen in the facial markings of Mexican gray wolves (Figure 3.2). However, most genetic variation—such as the differences in DNA that cause disease resistance, or tolerance of high or low temperatures—is invisible to casual inspection and is best studied using DNA sequencing.

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Conservation biologists have demonstrated that genetic variation, or genetic diversity, increases the probability that populations will survive in the face of environmental challenges. Consider, for example, genetic variation in the maximum temperature a fish can withstand without dying, called lethal maximum temperature. As Figure 3.3 shows, a population with low genetic variation may not include any individuals capable of surviving temperatures reached during an unusual heat wave. As a result, the population dies out entirely. Meanwhile, some individuals in another population with higher genetic variation do survive. Genetic diversity not only helps ensure survival in the face of environmental challenges, but it also enables a population to change and adapt to new environmental circumstances—an important factor if species are to survive on a planet with a human population changing the environment as it grows.

Genetic diversity has been crucial to domestication, the deliberate change of a wild animal or plant species to better meet the needs of humans. Humans have produced the many existing varieties of food crops and beautiful flowering plants and animal breeds, such as dairy cows and racehorses, by selectively breeding certain individuals within an ancestral wild population. The individuals chosen for breeding during domestication were those that possessed certain desired characteristics, such as the ability to produce large amounts of milk or the ability to run fast.

Domestic dogs descended from wolves, and their breeds have been refined for particular abilities, such as herding livestock, hunting game, or guarding property. The Border collie, for instance, is a specialist at herding sheep (Figure 3.4). Over generations, shepherds have kept and bred only those Border collies with characteristics helpful in controlling flocks of sheep without injuring them, including some of the hunting behaviors of its wolf ancestors like stalking, chasing, and staring down a prey animal. Meanwhile, a Border collie that shows behaviors associated with immobilizing and killing prey—behaviors essential to wild wolves—is not kept as a working dog, nor is it bred (Figure 3.5). This kind of selective breeding is called artificial selection—wherein humans “select” which individuals will reproduce to form future generations.

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In the plant world, sunflowers are grown worldwide as a source of oil and seeds. The wild ancestor of the domestic sunflower, Helianthus annuus, is native to North America, where it was domesticated and grown as a crop by Native Americans more than 4,000 years ago. Native Americans, and later Russian plant breeders, selectively bred the sunflowers that had the largest seeds and inflorescences (a tight cluster of small flowers) with other sunflowers that had large inflorescences and seeds. Over time, this resulted in a plant with a giant flowering head, bearing large seeds (Figure 3.6).

Charles Darwin, the famed 19th-century naturalist, recognized a parallel between changes in populations produced by plant and animal breeders and changes in natural populations. He proposed that as individuals are born into a population, the characteristics of some individuals better fit the requirements of the environment than others. Those individuals with the more favorable traits survive and reproduce at a higher rate. Consequently, the traits more closely matching environmental requirements increase in frequency in the population—more individuals in the next generation possess the favorable traits compared with the number that possessed the traits in the parental generation. Darwin called this process natural selection.

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A vivid example of how natural selection works can be found in the American chestnut trees that once grew in abundance in the forests of eastern North America. Individual chestnut trees grew to heights of over 30 meters (100 feet) with trunk diameters of 1 to 2 meters (3 to 6 feet) (Figure 3.7). Then around 1900, the blight fungus (Cryphonectria parasitica) arrived in North America, carried by imported Asian chestnut trees, which were resistant to the fungus.

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In less than 50 years, chestnut blight swept across the American chestnut’s entire natural range, killing nearly 4 billion trees. In most circumstances, only the root systems survived, but here and there large American chestnut trees survived because they either grew in soil and climate conditions unfavorable to the fungus or they possessed genes that gave them some resistance to the blight fungus. In the small population of surviving trees, many have blight-resistant genes. In other words, the genes that give resistance to the blight fungus have increased in frequency in the population.

The increase in frequency of resistance genes relative to susceptible genes in the population is a good example of evolution by natural selection (Figure 3.8). In general, we define evolution as a change in the genetic makeup of a population as a consequence of one of several different processes, including natural selection and selective breeding.