What Are Species

The word species means, literally, "kinds." But what do we mean by "kinds?" Someone who is knowledgeable about a group of organisms, such as orchids or lizards, usually can distinguish the different species found in a particular area simply by examining their visible features. Standard field guides to birds, mammals, insects, and flowers are possible only because most species are cohesive units that change little in appearance over large geographic distances. We can easily recognize male red-winged blackbirds from New York and from California, for example, as members of the same species (Figure 24.2a).

But not all members of a species look that much alike. For example, males, females, and young individuals may not resemble one another closely (Figure 24.2b). How do we decide whether similar but easily distinguished individuals should be assigned to different species or regarded as members of the same species?

The concept that has guided these decisions for a long time is genetic integration. If individuals of a population mate with one another, but not with individuals of other populations, they constitute a distinct group within which genes recombine; that is, they are independent evolutionary units. These independent evolutionary units are usually called species.

More than 200 years ago, the Swedish biologist Carolus Linnaeus, who originated the system of naming organisms that we use today, described hundreds of species. Because he knew nothing about the mating patterns of the organisms he was naming, Linnaeus classified them on the basis of their appearance; that is, he used a morphological species concept. Many of the organisms that he classified as species by their appearance are indeed independent evolutionary units. Their members look alike because they share many of the alleles that code for their body structures. In many groups of organisms for which genetic data are unavailable, species are still recognized by their morphological traits.

In 1940, Ernst Mayr proposed a definition of species that has been used by many biologists since that time. His definition, known as the biological species concept, says, "Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." The words "actually or potentially" assert that, even if some members of a species live in different places and hence are unable to mate, they should not be placed in separate species if they would be likely to mate if they were together. The word "natural" is also an important part of Mayr's definition because only in nature does the exchange of genes among individuals from different populations influence evolutionary processes. The interbreeding of such individuals in captivity does not, because the resulting offspring typically spend their lives in captivity without interacting with the wild population. Since genetic integration through interbreeding maintains integrated evolutionary units, the biological species concept, although it does not apply to organisms that reproduce asexually, continues to be used by most evolutionary biologists.

Deciding whether two populations constitute different species may be difficult because speciation is often a gradual process (Figure 24.3). Once a population becomes separated into two or more populations, the daughter populations may evolve independently for a long time before they become re-

Agelaius phoeniceus (male, NY)

Agelaius phoeniceus (male, CA)

24.2 Members of the Same Species Look Alike—or Not (a) Both of these male red-winged blackbirds are obviously members of the same species, even though one is from the eastern United States and the other is from California. (b) Because red-winged blackbirds are sexually dimorphic (see Chapter 23), the female of the species appears quite different from the male.

Agelaius phoeniceus (female)

Agelaius phoeniceus (male, NY)

Agelaius phoeniceus (male, CA)

24.2 Members of the Same Species Look Alike—or Not (a) Both of these male red-winged blackbirds are obviously members of the same species, even though one is from the eastern United States and the other is from California. (b) Because red-winged blackbirds are sexually dimorphic (see Chapter 23), the female of the species appears quite different from the male.

productively incompatible. Alternatively, they may become reproductively incompatible before they evolve any noticeable morphological differences. But how do these differences between populations come about?

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