Hybrid zones (zoology)

A hybrid is an organism that has been produced from the mating of two dissimilar species. A hybrid zone is a region where hybrids meet, cross-fertilize, and produce offspring of mixed-ancestry. Gene flow among populations tends to increase the similarity of characters among all the demes, or local populations, of a species. Natural selection has the opposite effect. It tends to make every deme uniquely specialized for its specific habitat. Clines are one possible result of these two opposing forces. A cline is a phenomenon where a genetic variation occurs caused by a difference in geographical habitat. Each species is continuously adjusting its gene pool to ensure that it survives in a continuously changing environment.

Comparing the characteristics of a single species’ demes usually reveals that they are not identical. The greater the distance between the demes, the greater the differences. The grass frogs in Wisconsin are more distinct from the grass frogs in Texas than they differ from those in Michigan. On average, the song sparrows of Alaska are heavier and have darker coloration than those in California. These phenomena, where a single character shows a gradient of change across a geographical area, are called clines.

North-South Clines

Many birds and mammals exhibit north-south clines in average body size and weight. This is where they are larger and heavier in the colder climates of the north and smaller and lighter in warmer climates to the south. Similarly, many mammalian species show north-south clines in the sizes of body extremities such as tails and ears, as these parts are smaller in northern demes and larger in southern demes. An increase in average body size with increasing cold is such a common observation that it has been codified as Bergmann’s rule. Allen's rule is the tendency toward shorter and smaller extremities in colder climates and longer and larger ones in warmer climates. The trend toward lighter colors in southern climates and darker shades in northern climates has been designated Gloger’s rule. The zebra, for example, shows a cline in the amount of striping on the legs. The northernmost races are fully leg-striped, and the striping diminishes toward the southern latitudes of Africa. This appears to be an example of Gloger’s rule. Another example of a cline that does not fit any of the biogeographical rules mentioned is the number of eggs laid per reproductive effort, or the clutch size, of the European robin. This number is larger in northern Europe than it is for the same species in northern Africa. Other birds, like crossbills and ravens, which have a wide distribution in the Holarctic realm, show a larger cline at lower latitudes. The manifestation of such clines in clutch size results from the interplay of two different reproductive strategies that may give a species a competitive advantage in an environment. The stability of the environment is what elicits the appropriate strategy.

In unstable environments, such as those in the temperate zone, there may be sudden variations in weather and extremes between seasons. A species needs to reproduce rapidly and build its numbers quickly to take advantage of the favorable warm seasons to ensure survival during the harsh, unfavorable winter conditions. This strategy is known as the “r strategy,“ where the r denotes the rate of increase. In the tropics, the climate is more equitable throughout the year. The environment, however, can only support a limited number of individuals. This number is called the ”carrying capacity.” When this threshold is reached, competition for resources increases, and the reproductive effort is reduced to maintain the population at the carrying capacity. This is called the “K strategy,” with K standing for carrying capacity.

With birds, clutch size tends to be inversely proportional to the climatic stability of the habitat. In temperate climates, more energy is directed to increase the reproductive rate. In the tropics, the carrying capacity is more important, resulting in a reduced reproductive rate. In the apparent contradiction of the crossbills and ravens, the harshness of the habitat at higher latitudes may be limiting the resources available for successfully fledging a larger number of young.

Grass Frog Clines

The cline exhibited by the common frog or grass frog (Ranatemporaria) is one of the best-known examples of this phenomenon. This frog has the greatest range, occupies the widest array of habitats, and possesses the greatest amount of morphological variability of any frog species. This variability and adaptation are not haphazard. The species includes a number of temperature-adapted demes, varying from north to south. These adaptations involve the departmental processes from egg to larva. The northernmost demes have larger eggs that develop faster at lower temperatures than those of the southernmost demes. These physiological differences are so marked that mating between individuals from extreme ends of the cline results in abnormal larvae or offspring that are inviable (cannot survive) even at an average temperature for the cline region. Leopard frogs from Vermont can interbreed readily with ones from New Jersey. Those in New Jersey can hybridize readily with those in the Carolinas and those in turn with those in Georgia. Yet the hybrids of Vermont demes and Florida demes are usually abnormal and inviable. Thus, it appears that the Vermont gene pool has been selected for a development rate corresponding to a lower environmental temperature. The gene pool of the Florida race has a slower development rate at a higher average temperature. The mixture of the genetic makeup of the northern and southern races is so discordant that it fails to regulate characteristic rates of development at any sublethal temperature, so the resulting embryo dies before it becomes a tadpole.

There are two primary reasons why characters within a species may show clinal variation. First, if gene flow occurs between nearby demes of a population, the gene pools of demes that are close to one another will share more alleles than the gene pools of populations that are far apart. Second, environmental factors, such as annual climate, vary along gradients that can be defined longitudinally, latitudinally, or altitudinally. Because these environmental components act as selective pressures, the phenotypic characters that are best adapted to such pressures will also vary in a gradient. Because of this climate component of clinal variation, scientists have proposed its use as a measure and monitoring tool concerning the interaction of climatic variables and organisms as their environment changes due to climate change.

Hybridization

Hybridization is the process where individuals of different species produce offspring. A hybrid zone is an area occupied by interbreeding species. Partial species can and do develop on the way to becoming new species as products of hybridization. Natural hybridization and gene flow can take place between biological species no matter how sterile most of the hybrid offspring may be. As long as the mechanisms that prevent the free exchange of genes between populations can be penetrated, there is the potential for a new species to develop. Because the parental species has a tendency to be replaced by the hybrid types if natural selection favors them, hybridization can be a threat to the integrity of the parental species as a distinct entity.

Hybridization between different species leads to various and unpredictable results. Any time that hybridization occurs, the isolation mechanisms of populations are overcome, forming bridging populations. Such connecting demes of hybrid origin fall into one of two general categories: hybrid swarms or introgressive demes. The formation of these types of demes reverses the process of speciation and changes the formerly distinct species into a complex mixture of highly variable individuals that are the products of the segregation and independent assortment of traits. This is the primary advantage of sexual reproduction: to produce variation in the population that is acted upon by natural selection over time. It cannot be overemphasized that hybrid swarms and introgressive demes are highly variable.

The environmental conditions that contour animal communities have endured for a very long time. In long-lived communities, every available niche has been filled by well-adapted species. When populations with new adaptive characteristics occur, there is no niche for them to occupy, so they usually die out. In contrast, when such communities are disturbed, the parity among their component species is upset, which gives new variants an opportunity to become established.

Hybrid swarms can be observed in nature by the careful investigator. The hybrid swarm forms in a disturbed habitat, where hybrid individuals backcross with the parental types to form a third population. This results in the migration of the genes of one population into the other. Such a population is designated an introgressive population. The progeny of such populations resembles the parent species, but the variations are in the direction of one parental species or the other. If introgression is extensive enough, it may eradicate the morphological and ecological distinctions of the parental types. The parental types become rarer and rarer until they are no longer the representatives of the species.

There appear to be three reasons that first-generation hybrids occurring naturally are more likely to form offspring by backcrossing to one of the parental species than by mating with each other. First, the hybrids are always rarer than their parents. Second, the parental individuals are so much more fertile than the hybrids that many more parental gametes are available than hybrid ones. Finally, backcross progeny, since they contain primarily parentally derived genes, are more likely to be well adapted to the habitat in which they originated than purely hybrid individuals.

Introgression

The most likely result of hybridization is backcrossing to one of the parental species. A backcross occurs when a hybrid parent mates with a species that is genetically similar to itself. Genotypes containing the most parental genes usually have the selective advantage, and the fact that they contain a few chromosomal segments from another species gives them unique characteristics that may also be advantageous. This sequence of events—hybridization, backcrossing, and stabilization of backcross types—is known as introgression. Hybrid swarms are interesting phenomena, but they are unlikely to be of evolutionary significance except through introgression.

There are many examples of introgression among plants, but examples of introgression in animals are not common. Those that have been demonstrated are usually associated with the domestication of livestock. In the Himalayan region of Asia, there exists a relative of cattle, the yak, which is also domesticated. Many of the herds of cattle found along the western edge of the Himalayas in central Asia contain characteristics that clearly are derived from the gene pool of the yak. Many of these characteristics are manifested as adaptations to the harsh climatic conditions in this region.

In western Canada, there has been a modest introgression of the genes of the American bison into the gene pool of strains of range cattle. The bisonlike characters incorporated into beef cattle created a new breed called the beefalo, which exhibits such characteristics as greater body musculature, lower fat content of the flesh, and great efficiency in the utilization of range forage. A beefalo steer is ready for market in only eight months, while the same live weight is not obtained in the standard beef breed until eighteen months.

These examples illustrate the concept that, as an evolutionary force, introgression is rather insignificant in natural biomes. It is almost always in the wake of human activity or the activities of domesticated animals that the process of introgression can and does result in new combinations of gene pools from different species.

Overall, hybrid zones hold important guidance for scientists working to monitor and limit climate change and its impacts. Using geographical cline analysis, scientists can understand the changes in species populations, their previous and new hybrid zones, and the physiological changes they undergo as a result of their environment. For example, scientists documented a shift in range boundaries of the Carolina chickadee and black-capped chickadee, sending both species in opposite directions into new ranges. As these shifts in hybrid zones occur, scientists will likely learn more about how species maintain their boundaries.

Principal Terms

Cline: a gradual, continuous variation from one population of a species to the next that is related to differences in geography

Deme: a local unit of the population of any one species

Gene: the unit of molecular information, a portion of a deoxyribonucleic acid (DNA) molecule that codes for some product, such as a protein, that governs inherited traits

Gene flow: the movement of genes from one part of a population to another part, or from one population to another, via gametes

Gene pool: the sum total of all the genes of all the individuals in a population

Hybrid: the offspring of a mating between genetically differing individuals

Introgression: the assimilation of the genes of one species into the gene pool of another by successful hybridization

Population: the members of a species that live in the same geographical area

Species: a group of similar organisms whose members can reproduce with one another to produce fertile offspring

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