Adaptive describes the characteristic of living organisms to survive and reproduce within the limits of a particular environment.
If they are not adapted to that particular environment they will not survive there. Each species is adapted to a somewhat different set of environmental conditions from every other species.
Thus adaptation to environment is the most common remarkable quality of living organisms. The real cause of it all lies in the germ-plasm itself, although it is manifest only in the somatoplasm of the animal.
In some unrelated groups similar adaptations occur, which has given rise either to parallelism or to convergence, so that animals of remote stock look and act alike; such creatures has been called homoplastic. Convergence is manifest in features concerned with food-getting, locomotion, offence and defense.
The animals, in contrast to convergence, of the same or closely related stocks have shown divergence and has given rise to such varied adaptations that they appear very different. The ideas of divergence under new and strange conditions is not new but was recognized by Lamarck. Darwin used the term “divergence”. Osborn has given these results in the form of a law which he called adaptive radiation.
According to Osborn, “Each isolated region, if large and sufficiently varied in its topography, soil, climate, and vegetation will give rise to a divesified fauna.” The larger the region and the more diverse the condition, the greater the variety of mammals which will result.
This divergent evolution or adaptative radiation is beautifully examplified in the mammalian teeth and limbs. The type of dentition of mammals is heterodont i.e. there are several types of teeth, each with different function.
The adaptations may be primary or direct if they lie in the original f adaptation, outward along any of four radii. When the adaptation is ° return direction it is called secondary or reversed adaptation. The reverse adaptation is shown by some flightless birds.
There is thus reversal of the direction of evolution, but this does not actually contradict the law of irreversible of evolution, which says that an organ once lost can never be regained and that a specialized form can never again become generalized. For example marsupials are nearly all arboreal today.
One striking feature is a grasping great toe or hallux on the hind being off-set, opposes usually the fourth digit thus forming a prehensile organ whose grasp of a branch it is difficult to loosen.
On the other hand terrestrial Kangaroo have become adapted for speed over the land and among other things have entirely lost this grasping hallux. Dendrolagus, a tree-kangaroo, which is very much at home in its arboreal retreat having no grasping greast, toes, it depends upon its claws and broadend soles for security.
The hallux would undoubtedly be better. Dendrolagus, while arboreal, shows in the loss of this useful member a terrestrial ancestry, and back of that, in common with all marsupials, an arboreal one. Hence for it the course of adaptation has been reversed for, however, useful the grasping hallux might be in present environment, its loss during the terrestrial hallux might be in present environment, its loss during the terrestrial career of ancestors is irrevocable. While the atrophied organ therefore is lost forever, its old- time function may be secondarily acquired by other organs should the need arise.
Other example is in aquatic reptiles-Ichthyosaurs or aquatic mammals like whales. Although arose from fish ancestory but during subsequent terrestrial life of their progenitor lost the water respiring gills and as such the whale has to come to the surface for breathing but the loss of gill-breathing is irrevocable. Although they are marvellously adopted for quatic mode of life but owing to the law of irreversibility, their adaptation can never be perfected.