Many birds and mammals exhibit north-south clines in average body size and weight, being larger and heavier in the colder climate farther north and smaller and lighter in warmer climates to the south. In the same way, many mammalian species show north-south clines in the sizes of body extremities such as tails and ears, these parts being smaller in northern demes and larger in southern demes. Increase in average body size with increasing cold is such a common observa tion that it has been codified as Bergmann's rule. The tendency toward shorter and smaller extremities in colder climates and longer and larger ones in warmer climates is known as Allen's rule. 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, which does not fit any of the biogeographical rules mentioned, is the number of eggs laid per reproductive effort (the clutch size) by the European robin: This number is larger in northern Europe than it is for the same species in northern Africa. Other birds, such as the crossbill and raven, which have wide distribution in the Holarctic realm, show a clutch-size cline that reveals a larger clutch size in lower latitudes. The manifestation of such clines in clutch size is a consequence of the interplay of two different reproductive strategies that may give a species a competitive advantage in a given environment. The stability of the environment is what elicits the appropriate strategy.
In unstable environments, such as those in the temperate zone, where there may occur 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 of the species during the harsh, unfavorable conditions of winter. This strategy is known as r strategy (r stands for the rate of increase). In the tropics, the climate is more equable throughout the year. The environment, however, can only support a limited number of individuals throughout the year. This number is called the carrying capacity. When carrying capacity is reached, competition for resources increases, and the reproductive effort is reduced to maintain the population at the carrying capacity. This is called K strategy, with K standing for carrying capacity.
In 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, it may be the harshness of the habitat at higher latitudes that limits the resources available for successfully fledging a larger number of young.
The cline exhibited by the common grass frog is one of the best known of all the examples of this phenomenon. It 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 matings between individuals from the extreme ends of the cline result in abnormal larvae or offspring that are inviable (cannot survive) even at a temperature that is average 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 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 rate of development that corresponds to a lower environmental temperature. The gene pool of the Florida race has a rate of development that is slower 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 pop ulation, 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.
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