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1. ADAPTIVE RADIATION
Definition
The term adaptive radiation has been coined by H.F. Osborn (1902) for explaining evolution, from a single ancestor,of a number
of descendants with a great variety of adaptations to different niches.
The production of several ecologically diverse species from a common ancestralspecies is called adaptive radiation, especially
when many disparate species arise within a short interval of geological time
More properly stated the phenomenon of adaptive radiation is the diversification of a dominant evolutionary group into a large
number of subsidiary types adapted to more restrictive modes of life (different adaptive zones) within the range of the larger
group.
According to George Gaylord Simpson (1940, 1953), adaptive radiation is the rapid proliferation of new taxa (species) from a
single ancestral group. Certain authors of evolution biology such as Savage (1969), Stanley (1979) and Volpe (1985) have used an
entirely new term macroevolution for the Osborn’s law of adaptive radiation.
How can we identify adaptive radiation]
Four features can be used to identify an adaptive radiation
1. A common ancestry of component species:specifically a recent ancestry.Note that this is not the same as
a monophyly in which all descendants ofa common ancestorare included.
2. A phenotype-environment correlation: a significant association between environments and the morphological and
physiological traits used to exploit those environments.
3. Trait utility: the performance or fitness advantages oftrait values in their corresponding environments.
4. Rapid speciation: presence of one or more bursts in the emergence of new species around the time that ecological and
phenotypic divergence is underway.
Examples of Adaptive Radiation
1. Adaptive radiationin Darwin’s finches. These birds belong to largest family of birds, the Fringillidae and live in Islas
Encantadas (Galapagos Islands) in the Pacific Ocean.
The ancestors ofDarwin’s finches were early migrants to the Galapagos Islands .These early colonists have given rise to 14
distinct species,each well adapted to a specific niche. Thirteen of these species occur in the Galapagos, one is found in the small
isolated Cocos Island, northeast ofGalapagos. Not all 13 species are found on each island. These 14 species belong to 4 gene ra: 1.
Geospiza; 2. Camarhynchus; 3. Certhidea; and 4. Pinaroloxias.
The most conspicuous difference among the species are in the sizes and shapes ofthe beak, which are correlated with marked
differences in feeding habits.Six of the species are ground finches, with heavy beaks specialized for crushing of seeds.Each
species,however, is specialized in feeding on a seed of a certain size. The small-beaked finch feeds on small grass seeds,whereas
large-beaked finch eats large, hard fruit.
The original ancestral stockof finches on the Galapagos diverged along several different paths.The pattern of divergence is
reflected in the biologist’s scheme of classification of these birds. All the finches are related to one another, but the various
species of ground finches evidently are more related by descent to one anotherthan to the members of th e tree-finch assemblage.
As a measure of evolutionary affinities, the ground finches are grouped togetherin one genus (Geospiza) and the tree finches are
clustered in another genus (Camarhynchus).
Darwin’s finches like adaptive radiations within ecologic islands of any sort are called ecoinsular radiations. Darwin’s finches
provide circumstantial evidence for the origin of a new species by means of geographical isolation.
2. Adaptive radiationin penguins. G.G. Simpson (1953) has cited evidence that when a group of organisms enters a new
adaptive zone previously unoccupied by the group, there may be rapid bursts ofspeciation and adaptive divergence into a variety
of ecological niches.There are three zones for penguins:(1) aerial flight, (2) aerial and submarine flight and (3) submarine flight.
These zones were sequentially invaded by penguins in their evolution and they are now extinct in the aerial flight zone or su bzone.
3. Adaptive radiationin reptiles. The class Reptilia first appeared in the fossil record in Pennsylvanian times (250
million years ago). The ancestral reptilian stock initiated one of the most spectacularadaptive radiations in life’s history . Adaptive
radiation of the reptiles occurred between Permian and Cretaceous times, and living reptiles are derived from Cretaceous
ancestors.The Mesozoic era, during which the reptiles thrived, is often referred to as the “Age of Reptiles”.
The initial success ofthe reptiles shoot from a mega-evolutionary shift from aquatic to completely terrestrial development, i.e., the
cleidoic or amniotic eggs of reptiles, like bird eggs,do not need to be immersed in water to survive. The basic stockof rep tiles
were the Cotylosauria (stem reptiles) from which a variety of reptiles were blossomed.The dinosaurs were by far the most awe
inspiring and famous. They ruled over the lands until the close of the Mesozoic era before suffering extinction. Dinosaurs were
remarkably diverse; they varied in size, bodily form and habits. Some of the dinosaurs were carnivorous, such as the gigantic
Tyrannosaurus,whereas others were vegetarians, such as elephantine Brontosaurus.Some dinosaurs strode on two feet; others had
reverted to four. The exceedingly long necks of certain dinosaurs were adaptations for feeding on the foliage of tall coniferous
trees.

2. The dinosaurs were descended from the thecodonts—slender,fast-running lizard-like creatures.The thecodonts gave rise also to
bizarre reptiles that took to the air, the pterosaurs.These "dragons ofthe air" possessed highly expansive wings and
disproportionately short bodies. The winged pterosaurs extincted before the end of Mesozoic era.
Anotherindependent branch of the thecodonts led to exceptionally more successfulflyers, the birds. The origin of birds from
reptiles is revealed by the celebrated Archaeopteryx, a Jurassic form. The feathered creatures possessed a slenderlizard -like tail
and a scaly head equipped with reptilian teeth.
Certain mesozoic reptiles returned to water. The streamlined, dolphin-like ichthyosaurs and longnecked, short-bodied plesiosaurs
were marine, fish-eating reptiles. The ichthyosaurs were expert swimmers; their limbs were fin-like and their tails were forked.
The plesiosaurs were efficient predators,capable of swinging their heads 40 feet from side to side and seizing fish in their long,
sharp teeth. These aquatic reptiles breathed by means of lungs; they did not redevelop the gills of their very distant fish ancestors.
Indeed, it is obvious that a structure once lost in the long course of evolution cannot be regained. This is the doctrine of
irreversibility of evolution, or Dollo’s Law, after Louis Dollo.
Among the early reptiles were the pelycosaurs (e.g., Dimetrodon), notable for their peculiar sail-like extensions of the back. The
function of the gaudy sail is unknown, but it should not be thought that this structuralfeature was merely ornamental or use less.It
appears that the sail of pelycosaurs was a functional device to achieve some degree of heat regulation. Pelycosaurs gave rise to an
important group of reptiles, the therapsids.The reptilian dynasty collapsed before the close of the Mesozoic era. Of the vas t host
of Mesozoic reptiles, relatively few have survived to modern times; the ones that have include the lizards, snakes,crocodiles and
turtles.
4. Adaptive radiationin eutherian mammals. The eutherian (placental) mammals provide anotherclassical
example of the process ofadaptive radiation. In this case, from a primitive, insect-eating (insectivorous), short-legged,rat-like
terrestrial creature that walked with the soles of its feet flat on the ground have evolved all the present day types of mammals.
Thus,in respect to limb structure,among eutherian mammals, adaptive radiation occurred in the following five different lines:
1…Arboreal. One evolutionary line radiates to form arboreal forms which have adapted limbs for life in trees, e.g.,
squirrels, sloth,monkeys, etc. The legs which are adapted for climbing are called scansorial
2….Volant or aerial. Anotherline leads to aerial flight terrestrial aquatic aerial and submarine flight penguins Recent
Miocene Arboreal forms such as monkeys have adapted limbs for life in trees. Aerial or volant, representing mammals adapted for
flight (e.g., bats).Somewhere along this line we can place the gliding mammals such as “flying squirrel”. The arboreal and aerial
forms not arose independently from the terrestrial forms, but perhaps through semi-arboreal or climbing ancestor.
3….Cursorial. Third line of radiation gives rise to cursorial forms such as horses and antelopes.They have developed
limbs suitable to rapid movement. The cursorial mammals have the following three types of adaptation in their foot -postures,such
as
1. Plantigrade,i.e., walking with whole sole of the foot touching the ground, e.g., bears and primates including human
beings;
2. Digitigrade,i.e., digits touch the ground and are provided with pads on their ventral side which absorb the shockand
help in making stealthy approach towards the prey, e.g., lion, tiger, leopard, cat, wolf and dog; and
3. Unguligrade,i.e., animals walk and run on the tips of their fingers and toes which are shielded by hoofs,e.g., artiodactyls
or double-hoofed animals such as cattle, sheep and buffaloes and perissodactyls orsingle-hoofed animals such as horses,
asses and zebras.
4….Fossorial. Fourth line of radiation formed the burrowing mammals, the fossorial mammals. Some of the fossorial
mammals, such as moles, have modified their forelimbs for digging but they are poorly adapted for locomotion on the ground.
While other fossorials such as pocket gophers and badgers are expert diggers but they can also move readily on the surface of
ground.
5….Aquatic. Fifth line of radiation leads to the aquatic mammals. Whales and porpoises having limbs strongly
adapted for aquatic life, but they cannot move about on land. While seals,sea lions and walruses have also strongly modified
limbs for aquatic life but they are also able to move about on land. The third group includes accomplished swimmers such as
otters and polar bears which are equally at home in water or on land. Lastly, animal’s legs modified for walking are called
ambulatory and those that are adapted for jumping are called saltatory.
5. Adaptive radiationin the marsupial (meta-therian) mammals. A remarkable example of adaptive radiation in animals is
the tremendous diversity of marsupials in Australia. In many ways they parallel the adaptations ofplacental mammals in the rest
of the world, affording striking examples of parallel evolution. Thus, in the absence of placental mammals, marsupials developed
grazing forms (some kangaroos), burrowing forms (marsupial moles); forms resembling tree and flying squirrels (flying
phalangers,pigmy gliders), forms resembling arboreal eutherians (teddy bear or koala and tree kangaroo); forms containing
rodent-like dentition (wombats, marsupial mouse), rabbit-like forms (hare wallabies), wolf-like carnivores (Tasmania wolves),
badger-like carnivores (Tasmania devil) and anteating carnivores (banded - anteater).