Evolution

D

Sexual Selection

Sexual selection operates on factors that contribute to an organism’s mating success. In many animals, sexual attractiveness is an important component of selection because it increases the likelihood of mating. Sexual selection rarely affects females, because the duration of pregnancy and infant care limits the number of babies they can have. Males, on the other hand, have few limitations on the number of offspring they can father, and a male who produces many offspring has a high level of evolutionary fitness. Males of many species, then, must compete with other males to mate with females. Some males win females’ attention more often than others and, as a result, pass their genes to more offspring.

In many species, sexual selection results in males with elaborate features. Many male birds, such as peacocks, have colorful and showy plumage. Male fiddler crabs have one greatly enlarged claw, and large skin flaps frame the face of the male frilled lizards. In some species, males perform elaborate courtship dances designed to demonstrate their virility and physical fitness.

Many such traits are a liability to survival, making them counter to the principles of natural selection. For instance, bright coloration and elaborate courtship dances draw the attention of predators. The fiddler crab’s large claw is cumbersome, as are the frilled lizard’s skin flaps. The huge tail feathers of the male peacock give it an awkward, bumbling gait. All of these features undoubtedly slow the animals down, making them less capable of evading predators or securing prey. Nevertheless, the increased reproductive success these showy characteristics instill makes them worth the risk.

IV

Genetic Drift

Natural selection is not the only force that changes the ratio of alleles present in a population. Sometimes the frequency of particular alleles may be altered drastically by chance alone. This phenomenon, known as genetic drift, can cause the loss of an allele in a population, even if the allele leads to greater evolutionary fitness. Conversely, genetic drift can cause an allele to become fixed in a population—that is, the allele can be found in every member of the population, even if the allele decreases fitness.

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Although any population can fall victim to genetic drift, small populations are more vulnerable than larger populations. Imagine that a particular allele is present in 25 percent of a population of worms. If a flood occurs and randomly eliminates half of the population, the laws of probability predict that approximately 25 percent of the surviving population will carry the allele. In a population of 120,000 worms, this means that about 15,000 of the surviving 60,000 worms will carry the allele. Even if, by chance, the flood claimed the lives of an additional 10 percent of the carriers, thousands of copies of the allele would still remain in the population. But in a population of only 12 worms, the laws of probability predict that only 1.5 of the surviving 6 worms would carry the allele. If, by chance, the flood claimed more of the carriers of the allele than the noncarriers, the allele could be eliminated.

The hypothetical flood created what is called a population bottleneck. It reduced the genetic variation in the smaller population such that, even if the group’s number again reached 12 members, its genetic diversity might very well be lower than the genetic diversity of the original population. All of the descendants came from just a few surviving individuals, who carried just a fraction of the alleles present in the former population. Likewise, when a few individuals leave a large population and establish a new one, they bring only a fraction of the genetic diversity of the original population with them. Any descendants of the founding members face the possibility of a drastically reduced genetic diversity. An example of this principle, known as the founder effect, is evident in the Amish community in Pennsylvania. All of the people in this community are descendants of about 200 individuals who established the community after leaving Europe in the early 1700s. One of these founders carried an unusual allele that causes a rare kind of dwarfism. As a result, in the Pennsylvania Amish community today the frequency of this rare allele is 1 in 14 individuals. In the general population this allele appears in 1 in 1,000 individuals.

V

Origin of New Species

The forces of natural selection and genetic drift continuously influence and change the characteristics of a population. However, most often these forces are not sufficient to create an entirely new species. Different species arise when, for one reason or another, members of a population cease to interbreed. When something prevents populations from mating, they are said to be reproductively isolated from one another. Two reproductively isolated populations cannot randomly exchange genetic material with each other, and as a result, the groups diverge as they evolve independently of one another. In this process, called speciation, the members of each group become so different that they can no longer successfully interbreed. At this point, a new species has formed.

Interbreeding normally continues if there is nothing to stop it. Anything that hinders interbreeding is called an isolating mechanism. Geographic barriers isolate populations, leading to the formation of entirely new species in a process called allopatric speciation. Less frequently, mutations or subtle changes in behavior prevent individuals living in close proximity from reproducing. This may lead to sympatric speciation, in which two distinct subgroups of a population cease exchanging genetic material and evolve into two or more distinct species.

A

Allopatric Speciation

When a barrier, such as a stretch of sea or a mountain range, separates different populations of a particular species, the populations may no longer be capable of crossing the barrier to interbreed. Speciation caused by geographic isolating mechanisms, or allopatric speciation, is evident in the many different populations of pupfish that live in the Death Valley region of California and Nevada. About 50,000 years ago this region had a damp, rainy climate and was peppered by lakes and ponds connected by streams and rivers. Over time, rainfall decreased significantly, and by about 4,000 years ago, this region was a desert. The interconnected lakes and streams dried up, and in their place remained a series of small, isolated stream-fed ponds. Each pond is home to a different species of pupfish, specially adapted to its pond’s unique temperature and mineral composition. Biologists speculate that all of these species of pupfish descended from a single species that inhabited the interconnected lakes and streams of the region about 50,000 years ago. As the lakes and streams dried up, the dry ground that separated them became a geographical isolating mechanism that prevented the individual populations from interbreeding. Consequently, the many pupfish populations evolved independently.

B

Sympatric Speciation

In sympatric speciation, isolating mechanisms may be triggered by differences in habitat, sexual reproduction, or heredity. Similar plants may fail to breed together because their flowering seasons are different. Many different types of rain forest orchids, for example, cannot interbreed because they flower on different days. Some animals mate only if they recognize characteristic color patterns or scents of their own group. Other organisms, particularly birds, are stimulated to breed only after witnessing a song, display, or other courtship ritual that is characteristic in their group (see Animal Courtship and Mating).

Sometimes two subpopulations of the same species do not produce offspring with one another, even though they come into breeding contact. This may be due, for example, to reproductive incongruities between two subpopulations that cause embryos to die before development and birth. In other instances, if viable offspring are produced, reproductive isolation is still maintained because the offspring are sterile. For example, asses and horses are capable of mating, but their offspring are usually sterile. Both types of reproductive dysfunction occur when the hereditary factors of the two groups have become incompatible in some way and cannot combine to produce normal offspring.

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