The ability of a population to adapt is a reflection of its tolerance limits, which largely depend on genetic diversity—different individuals having different alleles. A population that is highly diverse (has individuals with many different traits) is likely to have wider tolerance limits (see Chapter 8), which increases the population’s potential to adapt to changes. This means it is more likely that some individuals will exist that can withstand (or even thrive in) the changes and that the population as a whole will survive. If a change occurs that produces a condition outside the range where individuals can survive and reproduce (for instance, the climate becomes too warm or too dry), the population will die out. Similarly, if a new challenge is presented, such as the introduction of a new predator or competitor, the survival of the population will depend on whether there are any individuals in the population who can effectively deal with the new species. If the snakes on Guam were indeed responsible for killing off the birds, any birds that happened to have effective snake-avoidance behaviors would have had a greater chance of survival.

Two main sources of variation can increase genetic diversity in a population. The ultimate source of new variability is genetic mutation, a change in the DNA sequence in the sex cells that alters a gene, sometimes to the extent that it produces a new protein and, possibly, a new trait. Mutations are rare, but because DNA replication and repair occurs all the time, rare events do happen. When a mutation produces traits that are beneficial, they can quickly be passed on to the next generation, allowing the population to evolve to be better adapted to its environment. A second source of genetic variety occurs as eggs and sperm are made: Genetic recombination shuffles alleles around and sometimes produces individuals with new traits when a sperm fertilizes an egg.

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The value of this genetic diversity is illustrated today in the example of the rock pocket mouse of the American Southwest desert. Animals of this species have coats that are either light tan or a darker color. It turns out that coat color corresponds to a population’s environment: Areas of light-colored rock contain populations with mostly tan mice, whereas darker mice inhabit black lava rock regions. Research by Hopi Hoekstra and colleagues at the University of Arizona has shown that coat color is determined by a single gene that comes in two different alleles. The dominant allele is designated by the uppercase letter D; the recessive allele is designated by the lowercase letter d. All individuals have two copies of the gene, and the color of their coat is determined by which two alleles they possess. Darker mice have at least one dominant allele (DD or Dd). Tan mice possess two recessive alleles (dd).

It is likely that coat color provides camouflage and protection from visual hunters, but only if the mouse is on a background of the same color. A study on deer mice (a similar species) showed that predatory owls are more successful at capturing mice on a contrasting background. This gives support to the conclusion that coat color is adaptive as camouflage and therefore is responsive to natural selection. INFOGRAPHIC 11.2

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A special type of natural selection is known as coevolution. In coevolution, two species each provide the selective pressure that determines which of the other’s traits is favored by natural selection. Predator and prey species usually evolve together, each exerting selective pressures that shape the other. As predators get better at catching prey, the only prey to survive are those a little better at escaping, and it is those individuals that reproduce and populate the next generation. This game of one-upmanship continues generation after generation, with each species affecting the differential survival and reproductive success of the other. The result can be a predator extremely well equipped to capture prey and prey extremely well equipped to escape. INFOGRAPHIC 11.3

If the birds on Guam were indeed eradicated by the invasive snake species, it was because the speed at which the eradication happened prevented the bird populations from potentially coevolving survival strategies to deal with the new snake population. The brown tree snake was already well adapted to preying on birds. But Guam’s bird populations had never faced such a predator and had no natural defenses. It was an unfair fight. In fact, that the birds on Guam disappeared so quickly made it difficult to tease out the cause of their extinction at all. Savidge had to work backward to solve the mystery, putting the missing pieces together experiment by experiment. Her first step was to compare whether the distribution of the snakes on Guam matched up with the areas where birds had disappeared.

Based on what the residents told her, and what historical records like newspaper clippings reported, Savidge found that birds had begun to disappear first from southern Guam and that their disappearances matched up perfectly with when the brown tree snakes had begun populating the area. Ritidian, an area on the very northernmost tip of Guam, on the other hand, was the last area to have lost its birds, and it was also the last to be colonized by the snakes. “I found a close correlation between the bird decline and the expansion of brown tree snakes around the island,” she says, which suggested to her that brown tree snakes really might be the culprit. INFOGRAPHIC 11.4

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Some of Guam’s bird species went extinct sooner than others. For instance, the endemic bridled white-eye, the gregarious bird species that was extinguished first, happens to be very small, raising the possibility that the small size of these birds might have put them at a disadvantage. Larger species like flycatchers survived longer, though they, too, eventually disappeared. Other bird species experienced extirpation; the Guam rail, for instance, is gone from Guam, but other populations still live on the nearby island of Rota.