CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

 

13. Evolution and Natural Selection

 

13.4. Common Misunderstandings About Natural Selection

 

There are several common misinterpretations about the process of natural selection. The first involves the phrase “survival of the fittest.” Individual survival is certainly important, because those that do not survive will not reproduce. However, the more important factor is the number of descendants an organism leaves. An organism that has survived for hundreds of years but has not reproduced has not contributed any of its genes to the next generation and, so, has been selected against. Therefore, the key to being the fittest is not survival alone but, rather, survival and reproduction of the more fit organisms.

Second, the phrase “struggle for life” in the title of Darwin’s book does not necessarily refer to open conflict and fighting. It is usually much more subtle than that. When a resource, such as nesting material, water, sunlight, or food, is in short supply, some individuals survive and reproduce more effectively than others. For example, many kinds of birds require holes in trees as nesting places (figure 13.3). If these are in short supply, some birds are fortunate and find a top- quality nesting site, others occupy less suitable holes, and some do not find any. There may or may not be fighting for the possession of a site. If a site is already occupied, a bird may simply fly away and look for other suitable but less valuable sites. Those that successfully occupy good nesting sites will be much more successful in raising young than will those that must occupy poor sites or those that do not find any.

 

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FIGURE 13.3. Tree Holes as Nesting Sites

Many kinds of birds, such as this Gilded Flicker (Colaptes chrysoides), nest in holes in trees or other plants such as this Saguaro cactus. If such nesting sites are not available, they may not be able to breed. Many people build birdhouses that provide artificial tree holes to encourage birds to nest near their homes.

 

Similarly, on a forest floor where there is little sunlight, some small plants may grow fast and obtain light while shading out plants that grow more slowly. The struggle for life in this instance involves a subtle difference in the rate at which the plants grow. But the plants are, indeed, engaged in a struggle, and a superior growth rate is the weapon for survival.

A third common misunderstanding involves the significance of phenotypic characteristics that are gained during the life of an organism but are not genetically determined. Although such acquired characteristics may be important to an individual’s success, they are not genetically determined and cannot be passed on to future generations through sexual reproduction. Therefore, acquired characteristics are not important to the processes of natural selection. Consider an excellent golfer’s skill. Although he or she may have inherited the physical characteristics of good eyesight, strength, and muscular coordination that are beneficial to a golfer, the ability to play a good round of golf is acquired through practice, not through genes. An excellent golfer’s offspring will not automatically be excellent golfers. They might inherit some of the genetically determined physical characteristics necessary to become excellent golfers, however (figure 13.4).

 

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FIGURE 13.4. Acquired Characteristics

The ability to play an outstanding game of golf is learned through long hours of practice. The golf skills acquired by practice cannot be passed on genetically to a person’s offspring.

 

Humans desire a specific set of characteristics in our domesticated animals. For example, the standard for the breed of dog known as boxers is for them to have short tails. However, the alleles for short tails are rare in this breed. Consequently, their tails are amputated—a procedure called docking. Similarly, most lambs’ tails are amputated. These acquired characteristics are not passed on to the next generation. Removing the tails of these animals does not remove the genetic information for tail production from their genomes, and each generation of puppies and lambs is born with long tails.

A fourth common misconception involves understanding the relationship between the mechanism of natural selection and the outcomes of the selection process. Although the effects of natural selection appear at the population level, the actual selecting events take place, one at a time, at the level of the individual organism.

 

13.4. CONCEPT REVIEW

6. Why are acquired characteristics of little interest to evolutionary biologists?

7. In what way are the phrases “survival of the fittest” and “struggle for existence” correct? In what ways are they misleading?