CONCEPTS IN BIOLOGY
PART IV. EVOLUTION AND ECOLOGY
12. Diversity Within Species and Population Genetics
12.6. Is It a Species or Not? The Evidence
Scientists must rely on a variety of ways to identify species, because they cannot test every individual by breeding it with another to see if those individuals will have fertile offspring. Furthermore, many kinds of organisms reproduce primarily by asexual means. Because organisms that reproduce exclusively by asexual methods do not exchange genes with any other individuals, they do not fit the biological species definition very well. The standards used to identify various species include differences in morphology, behavior, metabolism, and genes.
Morphological characteristics are commonly used to differentiate species. Scientists compare the physical features of living and fossilized organisms when trying to identify a species. The idea that organisms can be classified as a species based on their structural characteristics is called the morphological species concept. The members of a species usually look alike, and these similarities are useful but not foolproof ways to distinguish among species. For example, the males and females of many birds look different from one another yet are the same species (figure 12.13).
FIGURE 12.13. Sexual Differences Within a Species
Male and female mallard ducks show strikingly different physical features, yet they are of the same species.
Many plants have color variations or differences in leaf shape that cause them to look quite different, although they are members of the same species. Within the species, the eastern gray squirrel has black members that many people assume to be a different species because they are so different in color. A good example of the genetic diversity within a species is demonstrated by the various breeds of dogs. A Saint Bernard does not look very much like a Basset hound; however, they are members of the same species (figure 12.14).
FIGURE 12.14. Genetic Diversity in Dogs
Although these four breeds of dogs look quite different, they all have the same number of chromosomes and are capable of interbreeding. Therefore, they are considered to be members of the same species. (a) Saint Bernard, (b) Australian Shepherd, (c)Basset hound, (d) Australian dingo. Because the extremes of these breeds rarely interbreed naturally, the question is “How long will it be before they are no longer the same species?”
It is often difficult to distinguish among species based on morphology. For example, to most people all mosquitoes look alike, but there are many species. And, although most people think all zebras are members of the same species, there are actually three species of zebras. In some cases, experts must use detailed morphology traits, such as the vein structure in the wings of insects, to identify species.
Differences in behavior are also useful in identifying species. Some species of birds are very similar structurally but can be easily identified by differences in the nature of their songs. Because it is often difficult to distinguish among bacteria, fungi, and other microorganisms on the basis of structure, metabolic differences that result in the presence or absence of specific chemicals within the organism are often used to help distinguish among species. The use of genetic differences— the analysis of DNA—is an even more precise way of distinguishing one species from another. Technology has allowed genes in many organisms to be sequenced and comparisons to be made. In some situations, this line of evidence has revealed remarkable differences between organisms once considered to be of the same species. In other cases, it has shown extremely similar sequences in what are considered to be very different species.
Because scientists use many standards—morphological, behavioral, metabolic, and genetic differences—to identify various species and none of these standards is perfect, situations frequently exist wherein individuals of two recognized species interbreed to a certain degree. For example, dogs, coyotes, and wolves have long been considered separate species. Differences in behavior and social systems tend to prevent mating among these three species. Wolves compete with coyotes and kill them when they are encountered. However, natural dog-coyote, wolf-coyote, and wolf-dog hybrids occur and the young are fertile (How Science Works 12.1). In fact, people have purposely encouraged mating between dogs and wolves for a variety of reasons. It has been demonstrated that dogs are descendants of wolves that were domesticated, so it should not be surprising that mating between wolves and dogs is easy to accomplish.
The species concept is an attempt to define groups of organisms that are reproductively isolated and, therefore, constitute a distinct unit of evolution. Some species are completely isolated from other, closely related species and do fit the definition well; some have occasional exchanges of genetic material between species and do not fit the definition as well; and some groups interbreed so much that they must be considered distinct populations of the same species. In this book, we will use the term species as a population of reproductively isolated individuals, complete with the flaws and shortcomings of this definition, because it is a useful way to identify groups of organisms that have great genetic similarity and maintain a certain degree of genetic separateness from all similar organisms.
There is one other thing you need to be careful about when using the word species. It is both a singular and plural word, so you can talk about a single species or several species. The only way to tell how the word is being used is by assessing the context of the sentence.
12.6. CONCEPT REVIEW
16. List four techniques used to distinguish one species from another.
17. Explain why the terms reproductively isolated and species are related.