CONCEPTS IN BIOLOGY
PART IV. EVOLUTION AND ECOLOGY
14. The Formation of Species and Evolutionary Change
14.2. How New Species Originate
Speciation is the process of generating new species. When biologists look at the evolutionary history of living things, they see that new species have arisen continuously for as long as life has been on Earth. Fossils are often used as evidence of the past evolution of organisms. A fossil is any evidence of an organism of a past geologic age, such as a preserved skeleton or body imprint. The fossil record shows that huge numbers of new species have originated and that most species have gone extinct. Two mechanisms are probably responsible for the vast majority of speciation events: geographic isolation, and polyploidy (instant speciation).
Speciation by Geographic Isolation
Geographic isolation occurs when a portion of a species becomes totally cut off from the rest of the gene pool by geographic distance. In order for geographic isolation to lead to speciation, the following steps are necessary: (1) population isolation of a subpopulation of a species; (2) genetic divergence—that is, a change in the allele frequencies of the isolated subpopulation compared to the rest of the species; and (3) reproductive isolation of the new species from its parent species.
There are at least three ways that populations can become geographically isolated (figure 14.2). First, the colonization of a distant area by one or only a few individuals can lead to the establishment of a population far from the center of their home population. If these colonies are so far from their home populations that there is no gene flow between them, they are genetically isolated.
FIGURE 14.2. Geographically Isolated Populations
(a) The colonization of distant areas by one or a few individuals can cut off a population far from the center of their home population. (b) Barriers to movement can split an ancestral population into two isolated groups. (c) Extinction of intermediate populations can leave the remaining populations reproductively isolated from one another.
Second, speciation occurs if a geographic barrier totally isolates a subpopulation from the rest of the species. The uplifting of mountains, the rerouting of rivers, and the formation of deserts may separate one portion of a gene pool from another. For example, two kinds of squirrels are found on opposite sides of the Grand Canyon. The canyon is a barrier that prevents interbreeding among members of the two populations. Some people consider the two types of squirrels to be separate species; others consider them to be different, isolated sub-populations of the same species. Even small changes can cause geographic isolation in species that have little ability to move. A fallen tree, a plowed field, or even a new freeway may effectively isolate populations within such species. Snails in two valleys separated by a high ridge have been found to be closely related but different species. The snails cannot get from one valley to the next because of the height and climatic differences presented by the ridge.
Third, the extinction of intermediate populations can leave the remaining populations reproductively isolated from one another for periods that are long enough for them to develop into separate species. For example, the range of an organism is the geographic area over which a species can be found. As a species expands its range, some intermediate populations may go extinct so that portions of the original population become separated from the rest. Thus, many species are made up of several, smaller populations that display characteristics significantly different from those of other local populations. Many of these differences are adaptations to local environmental conditions. If the smaller subpopulation in the middle becomes extinct, the distance between the more extreme isolated populations may be too great for gene flow to occur.
Genetic divergence is necessary for new species to develop. Differences in environments and natural selection play very important roles in the process of forming new species. Following separation from the main portion of the gene pool by geographic isolation, the organisms within a small, local population are likely to experience different environmental conditions. If, for example, a mountain range has separated a species into two populations, one population may receive more rain or more sunlight than the other. These environmental differences act as natural selecting agents on the two gene pools and, over a long period of time, account for different genetic combinations in the two places. Furthermore, different mutations may occur in the two isolated populations, and each may generate unique combinations of genes as a result of sexual reproduction. This is particularly true if one of the populations is very small. As a result, the two populations may show differences in color, height, enzyme production, time of seed germination, or many other genetic characteristics. Over a long period of time, the genetic differences that accumulate may result in subspecies that are significantly modified structurally, physiologically, or behaviorally. In some cases the changes may be so great that new species result.
Reproductive isolation, or genetic isolation, has occurred if the genetic differences between the two populations have become so great that reproduction cannot occur between members of the two populations if they are brought together. At this point, speciation has occurred. In other words, the process of speciation can begin with the geographic isolation of a portion of the species, but new species are generated only if isolated populations become separate from one another genetically and gene flow is not reestablished if the geographic barrier is removed.
Polyploidy: Instant Speciation
Another important mechanism known to generate new species is polyploidy. Polyploidy is a condition of having multiple sets of chromosomes, rather than the normal haploid or diploid number. The increase in the number of chromosomes can result from abnormal mitosis or meiosis in which the chromosomes do not separate properly. For example, if a cell had the normal diploid chromosome number of 6 (2n = 6), and the cell went through mitosis but did not divide into two cells, it would then contain 12 chromosomes. It is also possible that a new polyploid species could result from crosses between two species followed by a doubling of the chromosome number (figure 14.3). Because the number of chromosomes of the polyploidy is different from that of the parent, successful reproduction between the polyploid and the parent is usually not possible. This is because meiosis would result in gametes that had chromosome numbers different from those of the original parent organism. In one step, the polyploid could be isolated reproductively from its original species.
FIGURE 14.3. Polyploidy
(a) The chromosome number of this diploid cell is 2n = 12. (b) What happens when it undergoes polyploidy and doubles its chromosome count—that is, 4n = 24. Polyploidy has been found in many kinds of plants including grasses and ferns. (c) Hibiscus plants (Hibiscus rosa-sinensis), and (d) many varieties of goldfish (Carassius auratus).
A single polyploid plant does not constitute a new species. However, because most plants can reproduce asexually, they can create an entire population of organisms that have the same polyploid chromosome number. The members of this population would probably be able to undergo normal meiosis and would be capable of sexual reproduction among themselves. In effect, a new species can be created within a couple of generations. Some groups of plants, such as the grasses, may have 50% of their species produced as a result of polyploidy. Many economically important species are polyploids. Cotton, potatoes, sugarcane, broccoli, wheat, and many garden flowers are examples. Although it is rare in animals, polyploidy is found in some insects, fishes, amphibians, and reptiles. Certain lizards have only female individuals and lay eggs, which develop into additional females. Various species of these lizards appear to have developed by polyploidy. To date, the only mammal found to be polyploid is a rat (Tympanoctomys barrerae) found in Argentina and is tetraploid, 4n = 204.
Other Speciation Mechanisms
Speciation can also occur without geographic isolation or polyploidy. Any process that can result in the reproductive isolation of a portion of a species can lead to the possibility of speciation. For example, within populations, some individuals may breed or flower at a somewhat different time of the year. If the difference in reproductive time is genetically based, different breeding populations could be established that could eventually lead to speciation. Among animals, variations in the genetically determined behaviors related to courtship and mating could effectively separate one species into two or more separate breeding populations. In plants, genetically determined incompatibility of the pollen of one population of flowering plants with the flowers of other populations of the same species could lead to separate species. Although there are many examples of these kinds of specia- tion mechanisms, geographic isolation and polyploidy are considered the primary mechanisms for speciation.
14.2. CONCEPT REVIEW
3. How does speciation differ from the formation of subspecies?
4. Can you always tell by looking at two organisms whether or not they belong to the same species?
5. Why is geographic isolation important in the process of speciation?
6. How does a polyploid organism differ from a haploid or diploid organism?
7. List the series of events necessary for speciation to occur.