The
evidences in favour of evolution can be obtained from
*Evidences
from comparative morphology and anatomy
*Evidences
from fossils or palaeontology
*Evidences
from embryology
*Evidences
from physiology and biochemistry
*Evidences
from biogeography
*
Evidences from Comparative Anatomy and Morphology:
The comparative study of morphology and anatomy of
different organisms give evidences in favour of evolution. This can be obtained
from:
*Homologus
organs
*Analogous
organs
*Vestigial
organs
*Connecting
links
*Atavism
*Organ system
Richard Owen (1804-1892) introduced the term homologous.
The organs which have common origin and same fundamental structure but may have
similar or different appearance due to difference in functions are called
homologous organs. These organs follow the same basic plan of organisation
during their development. But in the adult condition, these organs are modified
to perform different functions as an adaptation to different environments. The
homologous structures are a result of divergent evolution. Homology indicates
common ancestry.
Examples:
(a) The forelimbs of man, cheetah, whale and bat have the same basic structural plan. In each
case the forelimb consists of humerus,
radio-ulna, carpals, metacarpals and digits. The skeletal parts of the forelimbs of all these vertebrates are similar in structure and arrangement.
But the forelimbs of these animals have different shapes and functions. In man
they are used for grasping, in cheetah for running, in whale for swimming and
in bat for
(b) Structural homology is also seen in the skeleton,
heart, blood vessels, brain, nerves, muscles and excretory system of different
vertebrates.
 |
| Brain of vertebrates |
(c) Another example of homologous organs is of different types of mouth parts of some insects. The mouth parts of cockroach, honey bee, mosquito
and butterfly have the same fundamental plan. In each of these insects the
mouth parts comprise labrum, a pair of mandibles and two pairs of maxillae, but
they have different functions to perform, keeping in view their different
feeding habits. The mouth parts in cockroach are adapted for biting and
chewing. In honey-bee for chewing and lapping, in mosquito for piercing and
sucking, in house-fly for sponging and in butterfly for siphoning.
(d) In plants, the homologous organs may be a thorn of Bougainvillea or a tendril of Curcurbita, both arising in the axillary position. Leaves of higher plants arise from nodes, possess axillary buds and produce a gap in the vascular supply of the stem. In form, they may be sessile (e.g., Zinnia) or petiolate (e.g., Pipal), simple (e.g., Mango) or compound (e.g., Cassia), reduced to scales (e.g.. Asparagus) modified into spines (e.g., Barberry) for protection and tendrils (e.g., Lathyrus aphaca) for climbing. The modifications indicated the evolution of the organ to suit different functions.
(e) Homology is also seen amongst the molecules. This is
called molecular homology. For example, the proteins found in the blood of man
and ape are similar. The phylogeny of an organism can be traced by using the
base sequence in nucleic acids and amino acid sequence of proteins in related
organisms.
*
Analogous Organs:
The organs which have similar in appearance and functions
but are different in their structural details and origin are called analogous
organs. The analogous structures are the result of convergent evolution.
Examples:
(a) The wings of an insect are analogous to wings of a bird. It is due to the fact that the basic structure of the wings of the insects is different from the wings of bird. However, their function is similar.
(c) Stings of honey bee and scorpion are analogous
structures. The sting of honey bee is a modification of its ovipositor
(structure that helps in egg laying) while that of scorpion is modified last
abdominal segment. Stings of both arthropods perform similar function.
(d) Eyes of Octopus and eye of cat.
The presence of analogous organs indicates a similar
adaptation by unrelated groups through modification or evolution of different
parts. It is called convergent evolution.
*Vestigial Organs:
The organs which are present in reduced form and do not
perform any function in the body but correspond to the fully developed
functional organs of related animals are called vestigial organs. They are believed
to be remnants of organs which were complete and functional in their ancestors.
They provide support for common ancestry.
Examples:
(a) Vestigial Organs in Human Body:
Human body has been described to possess about 90
vestigial organs. Some of these are nictitating (plica semilunaris) membrane,
auricular muscles (muscles of pinna), segmental muscles of abdomen, panniculus
camosis (subcutaneous muscles), vermiform appendix, caudal vertebrae (also
called coccyx or tail bone), third molars (wisdom teeth), hair on body, and
nipples in male.
(b) Vestigial Organs in Animals:
Important examples are vestiges of hind limbs and pelvic
girdles of pythons (which show that
snakes originally evolved from four-footed ancestors), wings of flightless
birds such as Ostrich, Emu, Kiwi, Rhea etc, splint bones in feet of horse and
brow spot in frog’s head (a vestige of 3rd eye).
(c) Vestigial Organs in Plants:
One or more staminodes (vestigial stamens) occur in the
flowers of several plants belonging to Labiatae, Scrophulariaceae,
Casesalpinioideae, Cucur-bitaceae, etc. Non-fictional pistils called
pistilloides occur in the male flowers of cucurbitaceae.
In the ray florets of Sunflower, the stamens are absent
while the pistil is rudimentary with small functionless stigma and ovule-less
ovary. Leaves are reduced to scales in Cuscuta, Orobanche, Asparagus, Ruscus
and a number of other plants.
* Connecting Links:
The organisms which possess the characters of two different
groups are called connecting links. They show the phylogenetic relationship
between organisms. Following are some important examples of connecting links.
Examples:
(a) Euglena is a chlorophyll containing
green protozoan that forms connecting link between the animals and plants.
(b) Proterospongia
is a colonial protozoan. It consists of flagellated and collared individuals
that resemble choanocytes (collar cells) of sponges. Thus, it is a link between
Protozoa and Porifera.
 |
| Proterosporengia |
(c) Neopilina is a connecting link between Annelida and
Mollusca. It resembles molluscs as it possesses a shell, a mantle and a large
muscular foot. Its annelid characters are presence of segmentally arranged
gills, nephridia and muscles and a trochophore-like larval stage.
 |
| Neopilina |
(d) Peripatus , an arthropod, is a connecting link between annelida and arthropoda. Its arthropod characters include haemocoel, tracheae as respiratory organs and tubular heart with ostia. The annelid characters exhibited are the worm-like body, structure of the eyes, unjointed legs, presence of segmental nephridia, soft cuticle and continuous muscle layers in the body wall.
 |
| Peripatus |
(e) Balanoglossus
is a hemichordate (non-chordate) and is a connecting link between
non-chordates and chordates.
(f) The lung
fishes, e.g., Protopterus (African lung fish), Lepidosiren (South American lung
fish) and Neoceratodus (Australian lung fish) may be considered the connecting
links between the fishes and amphibians. The lung fishes have all the
characters of a typical fish, but they are capable of respiring through lungs
and possess a three chambered heart.
 |
| African lung fish (Protopterus) |
(g) Latimeria
(Coelacanth fish) is considered a connecting link between fish and
amphibians.
 |
| Latimeria |
(h) Chimaera is a connecting link between cartilaginous
fishes and bony fishes.
 |
| Chimaera |
 |
| Sphenodon |
(j) Egg-laying
mammals (e.g., Ornithorhychus. Duck-billed platypus and Tachyglossus or
Echidna or Spiny ant eater) bear hair and mammary glands, but also possess some
of the reptilian characters such as laying of eggs, presence of cloaca and some
skeletal similarities. Thus they are connecting link between reptiles and
mammals.
*
Atavism:
It is the reappearance of certain ancestral characters
which had either disappeared or were reduced.
Some examples of atavism in human beings,
The power of moving pinna in some persons,
Greatly developed
canine teeth,
Exceptionally long dense hairs,
Short tail in some babies and
Presence of additional mammary glands in some
individuals.
Atavism is also observed in plants. In Citrus leaf the
lamina is separated from wing petiole by means of a joint or constriction.
Sometimes the winged part of the petiole is enlarged to produce two lateral
leaflets making the leaf trifoliolate.
It shows that Citrus leaf was once trifoliolate compound
but during evolution two leaflets have degenerated.
In many plants
(e.g., Rosa, Hibiscus, Oxalis, Poppy), some of the stamens and even carpels get
changed to petal-like structures indicating that stamens and carpels have
evolved from leaf-like structures.
*
Organ Systems:
The different systems of animal body are similar in many
groups of organisms, e.g., nervous system, blood vascular system, respiratory
system, excretory system, etc. Respiratory system of terrestrial vertebrates
has two lungs, a trachea, a larynx, nasal chambers and nostrils. Likewise, the
blood vascular system of all vertebrates contains a heart, arteries, veins and
lymph vessels.
Transport systems of plants have similar types of
conducting channels of xylem and phloem. The presence of similar organ systems
indicate a common ancestry. Despite broad similarity, the organ systems of
various groups have varied degree of specialization according to the habitat
and scale of evolution.
*Palaeontological
Evidences (Evidences from Fossil)
Palaeontology
is the study of fossils (latin word fossilis means anything dug out from earth).
Fossils are any kind of remnants models or impressions of organisms that lived
in past in Earth’s rocks. This branch of study was initiated by observations of
Leonardo da Vinci (Italian) regarded as father of palaeontology. However,
modern palaeontology was established by George Cuvier regarded as founder of
modern palaeontology.
The study of animal fossils is called palaeozoology
while study of plant fossils is called palaeobotany.
Fossils
provide the most direct evidence of evolution.
Fossilization
is the process of formation of fossils. During fossilization, entire Organism
or its parts often get buried in sand or mud. Most organisms after their burial
decay and disintegrate leaving no sign of their existence. The best place for
fossilization is ocean because salt water checks the decay of organisms. In
fact, the harder parts of the organisms after death settled to the bottom and
are covered by sediments. The process of settlement of more sediments and sand
goes on continuously in such a way that older fossils are preserved in deeper
and recent ones are preserved in the upper strata (layer). The land animals and
plants are also brought to the ocean with water stream and may also be
preserved here. The flow of molten lava and volcanic eruption have also caused
entire group of organisms to fossilize. The land plants and animals are
preserved by the collection of sand or soil on them after their death. In this
way, hard parts of the body are preserved and the soft parts disintegrate.
Types of
fossils
Depending
upon that what part of the organisms are preserved. There are several type of
fossils:
*Entire or
unaltered: The whole bodies of extinct organisms have been found frozen
in ice at poles or trapped in amber in forest of conifers. Example ancient
mammoths elephant like animal preserved in ice of Siberia.
*petrified: In this type of fossil soft parts of the
animals decompose but hard parts like bones, teeth, exoskeleton etc. remain as
fossils. This type of fossils are called as petrified fossils. Replacement of
organic parts by minerals deposit is called petrifaction. Example bones of
dinosaurs.
*Molds or
casts: it
is the replica of organisms left behind. They are formed when the materials
surrounding the buried organisms hardens and the organisms decay, leaving the
impression only which is an exact
replica of outline of Organism. Example: Gastropods of Portland.
*Compressions: It is
common type of plant fossil. In it, the internal structure of the plant is
completely decayed and lost leaving behind a thin carbon film giving an outline
of Organism. Example: plant fossils.
*Imprint: it is the fossil in the form of footprints,
tracks, tunnels, prints of skin, wings, etc. made in soft mud which became
fossilized as the mud got baked and preserved in sedimentary rock. Example:
footprints of dianosaurs.
Determination
of age of fossils
Age
of fossil is determined by analyzing the radioactive materials like carbon,
uranium, lead, potassium etc. present in the rock from which fossil is
recovered.
Age of fossil can be determined by any of the following process:
Carbon 14
dating method: (WF libby) Radiocarbon dating method is used to date
materials that once exchanged carbondioxide with atmosphere in other words
things that are living.
Carbondioxide
in atmosphere contains a constant amount of C-14 (a radioactive form). However,
once the organism dies, the amount of C-14 steadily decreases. By measuring the
amount of C-14 left in the organism, it is possible to determine the age of
fossil.
Each
radioactive element decays by a fixed amount in fixed time. Half- life is the
time required for half of the original sample of radioactive nuclei to decay.
Half –life of C-14 is 5730 years. This method works well for materials upto
around 50000 years old.
*Uranium - lead method:
It is
known that uranium is transferred into leads through several intermediate
strategies. U-238 decays into Pb-206
with half-life of 4.5 billion years. So, by estimating the amount of lead in a
rock, the approximate age of rock may be calculated.
*Potassium-
argon method: In this process the ratio of radioactive argon to radioactive
potassium of rock is calculated to find out its time of origin. Radioactive
potassium has half-life period of 1.3x 109 years.
*Missing link
The fossil organisms which show characters
of two different groups are called missing links.
Examples:
(a) Archaeopteryx (Archae —
primitive, old, pteryx = wing):
It was found in the rocks of the
Jurassic period. Archaeopteryx lithographica was discovered in 1861 by Andreas Wagner from the lithographic quarry at
Solenhofen, Bavaria, in Germany. This fossil is placed in the British Museum,
London. It displays the characters of both the reptiles and birds.
Reptilian Characters of
Archaeopteryx:
(a) The body axis is more or less
lizard-like,
(b) A long tail is present,
(c) The bones are not pneumatic,
(d) The jaws are provided with
similar teeth,
(e) The hand bears a typical
reptilian plan and each finger terminates in a claw,
(f) Presence of a weak sternum,
(g) Presence of free caudal vertebrae as found in lizards.
(a) Presence of feathers on the
body,
(b) The two jaws are modified
into a beak,
(c) The fore limbs are modified
into wings,
(d) The hind-limbs are built on
the typical avian plan,
(e) An intimate fusion of the
skull bones as seen in the birds.
From the above facts, it is clear
that the birds have been evolved from reptilian ancestors. Thus, Huxley called “birds are the glorified reptiles”.
(b) Ichthyostega:
It is a primitive fossil
amphibian and is a missing link between fishes and amphibians.
 |
| Ichthyostega |
(c) Seymouria:
It was a “missing link” between
amphibians and reptiles.
 |
| Seymouria |
(d) Lycaenops:
It was a mammal-like reptile. It
is considered a “missing link” between reptiles and mammals.
 |
| Lycaenops |
(e) Cynognathus (Dog Jaw):
It was a mammal-like reptile and
had characters of both reptiles and mammals. It was one of the ancient
reptilian ancestors of mammals.
 |
| Cynognathus |
(f) Basilosaurus:
This fossil whale had hind-limbs.
It links the aquatic mammals to their terrestrial ancestors.
 |
| Basilosaurus |
(g) Pteridosperms
(Cycadofilicales, Seed Ferns):
These are fossil plants which are
intermediate between ferns and seed plants.
There are more animal fossils as
compared to plants. It is due to presence of slow decaying harder structures in
their endoskeleton and exoskeleton.
*Embryological
Evidences (Evidences from Embryology):
These
evidences are based on the comparative study of the embryos of various animals.
Study of comparative embryology provide one of the most fascinating evidences
in favour of evolution.
Embryology is
study of process of development of a fertilized egg into adults.
(i)
Similarity in Early
Development:
All the
multicellular animals start their life
from the fertilized egg (zygote) which is single celled which is similar to
protozoa. The zygote undergoes
segmentation (cleavage) to produce a solid structure, the morula. The morula
develops into a single layered hollow blastula. The latter changes into either two
or three layered gastrula. The animals having two layered gastrula are said to
be diploblastic, e.g., coelenterates.
The animals
in which three layered gastrula is found are known as triploblastic, such as
frog, lizard, etc. Diploblastic gastrula consists of ectoderm and endoderm
while triploblastic gastrula consists of ectoderm, mesoderm and endoderm. These
two or three layers of gastrula are termed as primary germ layers, which give
rise to the entire animal. The developmental stages upto gastrula in all
metazoa are basically similar. Such a similar early development establishes a
close relationship among all multicellular animals and it suggests the origin
of all forms of metazoa from a single stock i.e. monophyletic origin.
(iii)
Resemblances among Invertebrate Larvae:
Annelids and
molluscs possess a similar type of larva called trochophore. Echinoderms and
hemichordates also have similar larvae. This larval resemblance points to a common
ancestry.
(vi)
Temporary Embryonic Structures:
Embryos often
possess structures which do not occur in the adults. For example: *Bird embryo
has tooth buds and gill clefts which are not found in the adult animal.
Presence of tooth buds has no relevance to the embryo as food is obtained from
yolk through special blood vessels. The adult which feeds on hard grains and
seed needs the teeth but is devoid of them.
The presence
of tooth buds in the embryos can be explained only on the assumption that:
(i) Birds
have developed from toothed ancestors;
(ii) Birds
have lost teeth during evolution;
(iii) The
bird embryo possesses some ancestral characters due to the persistence of some
genes that express their effect during developmental stages.
*Whale is an
aquatic mammal. It does not possess body hair. Its foetus or embryo possesses
hair which is shed before birth. Hairs are useless to the embryo because it is
well protected inside the mother’s body.
*Early
tadpople of frog possesses gills and tail, during metamorphosis these
structures disappear.
(vii)
Development of Vertebrate Organs:
Development
of many vertebrate organs (e.g., heart, brain, kidney) indicate the possible
path of evolution as well as the common ancestry of vertebrates. For example:
*During its
development the heart of a mammal or bird is initially two-chambered (as in
fishes), then three-chambered (as in amphibians and some reptiles) and
ultimately four-chambered. It clearly shows that birds and mammals have
originated from fishes through amphibians and reptiles.
*In all vertebrates,
the brain arises as an anterior enlargement of the neural tube. Soon it
develops two grooves and gets divided into three parts— fore brain, mid brain
and hind brain. Each of these parts develops further to attain the adult state.
*Vertebrates have
three types of kidneys— pronephric, mesonephric and metanephric. Pronephric
kidney occurs in hag fishes. Mesonephric kidney is found in other fishes and
amphibians while metanephric kidney is present in reptiles, birds and mammals.
In the mammalian or bird embryo, the kidney is initially pronephric then
mesonephric and ultimately metanephric.
(viii)
Evidences from Plant Embryos:
(a) In Pinus
the foliage leaves do not occur directly on the main stems but are borne in
clusters on the dwarf shoots. However, in the seedling state the foliage leaves
occur directly on the main stem indicating evolution of Pinus from ancestors
that possessed foliage leaves directly on main stems.
(b)
Australian species of Acacia possess phyllodes or foliaceous petioles instead
of normal bipinnate leaves as in other species of Acacia. Australian species
show all the transitional steps between bipinnate leaves and phyllodes during
the seedling stage.
(c) Many
bryophytes pass through a filamentous protonema stage before attaining adult
form. The filamentous protonema suggests algal ancestry for bryophytes.
(d)
Bryophytes and pteridophytes have ciliated male gametes or sperms. They require
an external source of water for swimming to the female sex organs. In
gymnosperms the sperms are transported by pollen tubes. Even then sperms of
cycas and Ginkgo are ciliated.
(ix)
Recapitulation Theory/Biogenetic Law:
In 1828, Von
Baer, the father of modern embryology, proposed Baer’s law which stated that
during embryonic development, the generalised features (such as brain, spinal
cord, axial skeleton, aortic arches, etc. are common to all vertebrates)
appeared earlier than the special features (like hair in mammals only, features
in birds only, limbs found in quadrupeds only) which distinguish the various
members of the group.
Later on this
law was modified as the biogenetic law by Ernst Haeckel in 1866. Haeckel’s
biogenetic law or recapitulation law states that “Ontogeny repeats phylogeny”. Ontogeny is the life history of an
organism while phylogeny is the evolutionary history of the race of that
organism. In other words an organism repeats its ancestral history during its
development.
Examples:
(a) In the
development of the frog a fish like tailed larva (tadpole) is formed, which
swims with the tail and respires by the gills. This indicates that the frog has
been evolved from a fish like ancestor.
(b) Tadpole
(larva) of Herdmania (urochordate) shows characters of chordates i.e., presence
of notochord, well developed dorsally placed central nervous system and tail.
However adult Herdmania does not have notochord and tail. Nervous system is
also very much reduced in adult Herdmania. Thus the larva shows its ancestral
characters.
(c) The
protonema, an early stage in the development of a moss and a fern gametocyte
resembles the filamentous green algae in structure, growth pattern and
physiology. This indicates an algal ancestry of the bryophytes and
pteridophytes.
(d) The
gymnosperms have normally become independent of water in fertilization. But the
primitive gymnosperms (e.g., Cycas and Ginkgo) have flagellated sperms and need
water for fertilization like the pteridophytes. This indicates that the
gymnosperms have descended from the pteridophyte-like ancestor.
*Evidences from Biochemistry
and Comparative Physiology:
The comparative study of
bio-molecules and physiological processes of living organisms provide a number
of evidences of common ancestry and evolution of different groups of organisms.
1. Protoplasm:
All living beings are made of
protoplasm, commonly called living matter. Its biochemical constitution is
similar in all the organisms. About 90% of the protoplasm is formed of four
elements — carbon, hydrogen, oxygen and nitrogen. Along with phosphorus and
sulphur; they constitute most of the organic compounds of living matter like
carbohydrates, proteins, lipids (fats) and nucleic acids. This provides support
for the common ancestry of all living organisms.
2. Nucleic Acids and Chromosomes:
The hereditary material is
present in the form of DNA. DNA is usually organised into chromatin fibres in
the nucleus and chromosomes in a dividing cell. It has the same chemical
composition in all the organisms. Genetic code, that expresses the effect of
DNA nucleotides, is universal. This provides support for the common ancestry.
3. Enzymes:
An organism has a number of enzyme
systems. A system possesses a similar set of enzymes in different organisms, so
much so that Kreb’s cycle has similar enzymes in both plants and animals. The
enzymes trypsin (protein digesting enzyme found in all animals from protozoa to
primates) and amylase (carbohydrate digesting enzyme found in many animals) are
the same throughout the animal kingdom. Vertebrates have a similar set of
digestive enzymes in their digestive tracts. Because of it, the digestive
enzymes of one animal can be safely administered to another animal including
human beings. This provides support for their the common ancestry.
4. Hormones:
They are bio-chemicals produced
by ductless or endocrine glands which in traces help in triggering reactions or
functions in other parts of the body. The hormones of vertebrates are both
chemically and functionally similar. In case of deficiency in human beings, the
hormones obtained from other vertebrates are taken as injections, e.g.,
insulin, thyroxin. This provides support for the common ancestry of all
vertebrates.
5. Metabolism:
Different metabolic reactions
like respiration, digestion, assimilation, muscle contraction, nerve conduction
(in animals) and photosynthesis (in plants) show a physiological harmony in
various living beings.
6. Photosynthetic Pigments:
All eukaryotic autotrophic plants
possess chlorophyll a. Chlorophyll b occurs in green algae and embryophytes.
The latter, therefore, must have originated from green algae. Other algae
possess chlorophyll c, d or e instead of b. They must have originated from a
common ancestor of algae.
7. Excretion:
Nitrogenous waste shows a
progressive detoxification in vertebrates. It is ammonia in fishes, urea in
amphibians, uric acid in reptiles and birds, and a combination of urea, uric
acid and other chemicals in mammals.
8. Blood and Lymph:
Blood and lymph are fluid
connective tissues which have the same composition and function in most of the
animals indicating a close relationship.
9. Blood Groups:
Human beings have four main blood groups— A, B, AB and О. A-B grouping is also found in apes but not in monkeys,
showing that man is more closely related to apes than to monkeys.
10. Oxy-haemoglobin Crystals:
Crystals formed from
oxy-haemoglobin of vertebrates show a relationship amongst the vertebrates.
Crystals of closely related species have the same pattern or configuration
while those of distantly related species have different configuration. For
example, crystals of birds have a basic resemblance and exhibit a basic
similarity from oxy-haemoglobin crystals of mammals, reptiles and amphibians.
11. Serum Tests (precipitin or
Blood Protein Tests):
Each race of individuals has
certain specific proteins which are not found in other races. Closely related
organisms show more similarity of these specific proteins than distantly
related forms. This can be checked through precipitin or serum tests.
When blood serum (antigen) of
human is mixed with blood of other animals like Rabbit, Pig, Rat etc. in their
body antibodies are produced and these antibodies react with human antigen and
causes precipitation. The relative degree of precipitation gives idea about
their closeness.
Serums of closely related animals
do not cause precipitation while those of distant relation cause highest
precipitation.
Similarly, blood serum tests of
diverse groups of vertebrates prove that birds are nearer to crocodiles than
other reptiles while human beings are related to apes, the latter to old world
monkeys, new world monkeys, etc.
*Biogeographical Evidences
(Evidences from Biogeography):
Biogeography
is the study of distribution of animals and plants on this earth. The evidences
of evolution based on biogeography (G. bios- life, ge- earth, grapho- to write)
are called biogeographical evidences. Pangaea (Gr. all earth); It is believed
that around the carboniferous period (about 345 million years ago) or slightly
earlier, all the present-day continents were in the form of a single big land
mass called pangaea. Later on, due to various geological changes, huge land
masses broke off and drifted apart from one another.
 |
| Continental drift |
As these land masses (now called continents) moved away, they got separated from each other by the seas. The seas acted as barriers and prevented free movements of organisms among the continents. As these continents had different environmental conditions, so plants and animals evolved there were of different varieties.
Biogeographical
evidences may be explained under the following headings.
1. Biogeographical Realms:
The
earth has been divided into six major biogeographic regions, called realms on
the basis of distribution of animals and plants. Dr. P.L. Sclater in the year
1858 proposed first time the division of the world into six realms or regions
according to the distribution of birds. In 1876 A.R. Wallace adopted it for all
the animals. These realms (regions) are: PalaearcticRealm,
Indian (Oriental) Realm, Australian Realm, Ethiopian Realm, Nearctic Realm and
Neotropical Realm.
(i) Palaearctic realm:
It
includes Europe, north of Himalaya, China, Sahara desert of Africa, Siberia and
a major part of Asia.
Important Animals: Anabas, Bufo, Rhacophorus, Alytes, Proteus,
Necturus, Varanus, Alligator, Hawks, Camel, Tiger, Seal, Panda.
(ii) Oriental realm:
It
includes India, Nepal, Malaysia, Philippines, Indonesia, SriLanka, Myanmar (Burma).
Important Animals: Carps, Cat fishes, Apods, Frogs, Draco, Python,
Cobra, King cobra, Crocodile, Gavialis. Peacock, Hornbills, Porcupines, Loris,
Gibbon, Rhinoceros, Elephants, Tiger, Lion.
(iii) Australian realm:
It
covers Australia, New Zealand, New Guinea.
Important Animals: Ceratodus (Australian Lungfish), Sphenodon,
Casuarius, Emu, Kiwi, Duck billed platypus, Spiny anteater, Opposum, Kangaroo,
Marsupial cat.
(iv) Ethopian realm:
It
includes Africa, Arabia and Madagascar.
Important
Animals:Protopterus (African Lungfish),
Rhacophorus, Crocodile, Chamaeleon, Python, Ostrich, Scaly- anteater,
Chimpanzee, Gorilla, Zebra, Elephants, Hippopotamus, Rhinoceros, Giraffe, Lion,
Tiger.
(v) Nearctic realm:
It
covers Canada, United States of America and Mexico.
Important Animals: Sucker fish, Tiger salamander, Amphiuma, He-
loderma (Poisonous Lizard), Alligator, Hawk, Opossum, Porcupine.
(vi) Neotropical realm:
It
includes the area of Central and South America and Island of West-Indies.
Important Animals:Lepidosiren (South American Lung fish),
Caecilians (Apoda), Hyla, Pipa, Rattle snake, Rhea, Opossum, Vampire bat,
Llama (Like Camel), Marsupial rat.
Biogeographical Realms |
Oriental realm is separated from Palaearctic realm by Himalayan Mountains.
Ethiopian realm and Australian realm are
separated by sea.
Oriental realm and Australian realm are
separated by Wallace’s line.
Palaearctic realm and Nearctic realm together
form Holoarctic region.
2. Discontinuous Distribution
of closely related species:
Sometimes
closely similar species exit at widely separated places without any
representative, in intervening territory. This is called discontinuous
distribution. Two specific examples of discontinuous distribution are given
below.
(a) Alligators:
They
occur only in south-eastern United States and eastern China. The North
American continent was connected with East Asia in early coenozoic. The
alligators were distributed over the entire region. But due to certain
barriers, the alligators of two regions were separated for long time and
developed some mutations. Therefore, these alligators are somewhat different
but they are related species of the same genus.
(b) Lung Fishes:
During
early stages of continental drift, South America, Africa, Antarctica and
Australia were interconnected. Later on they were separated. Antarctica was
shifted to a far away place. Now the lung fishes are only found in South
America, Africa and Australia.
If we look at a map of the world on a sheet of paper, cut out the outlines of South America and Africa and bring them together. We find that the right side of South America fits into the left side of Africa.
(c) Camels:
They
occur in Asia, while their nearest allies Limas are found in South America.
(d) Elephants:
They
are found in Africa and India and not in places with identical climate in
Brazil.
(e) Tapirs:
They
are found in tropical America and Malayan islands.
(f) Magnolias, Tulips and
Sassafras:
These
plants now grow naturally in the eastern USA and in China only. The reason is
the same as for the alligators.
3. Restricted
Distribution:
The
parts separated from the mainland have unique fauna and flora. For example,
Australia has:
(i) egg-laying and pouched mammals that occur only in Australia. This
restricted distribution may be explained in the following way. Australia
separated from the mainland of Asia during mesozoic era, before placental
mammals evolved. Placental mammals, being more adapted, eliminated the egg
laying and most of the pouched mammals in other parts of the world. The egg
laying and pouched mammals of Australia survived as placental mammals could not
reach their due to lack of land route,
(iii)
Deserts of America possess cacti while those of Africa have euphorbias,
(iv)
Double coconut is restricted to Seychles island.
4. Adaptive Radiation (=
Divergent Evolution):
Development
of different functional structures from a common ancestral form is called
adaptive radiation. The concept of adaptive radiation in evolution was
developed by H.F. Osborn in 1902. Homologous organs show adaptive radiation.
Examples:
(i) Darwin’s Finches of the
Galapagos Islands:
They
had common ancestors but now have different types of modified beaks according
to their food habits as shown in figure. Darwin differentiated thirteen species
of finches and grouped them into six main types — (a) Large ground finches, (b)
Cactus ground finches feeding on cacti, (c) Vegetarian tree finches, (d)
Insectivorous tree finches, (e) Warbler finches, (f) Tool- using or Woodpecker finches.
(ii) Australian Marsupials:
Darwin
explained that adaptive radiation gave rise to a variety of marsupials (pouched
mammals) in Australia in the same process of adaptive radiation as found in the
finches in the Galapagos Islands.
(iii) Locomotion in Mammals:
Adaptive
radiation based on locomotion in mammals is good example.
5. Convergent Evolution (=
Adaptive Convergence):
Development
of similar adaptive functional structures in unrelated groups of organisms is
called adaptive convergence or convergent evolution.
Examples:
(i)
Wings of insect, bird and bat show marked convergent evolution.
(ii)
Australian marsupials and placental mammals show convergent evolution, e.g.,
Placental wolf and Tasmanian wolf-marsupial.
 |
convergent evolution in Australian marsupials and placental mammals |
(iii)
Various aquatic vertebrates, not closely related show a marked convergent evolution.
(iv)
Anteaters such as spiny anteaters and scaly anteaters belong to different
orders of class mammalia, not closely related but have aquired similar
adaptations for diet of ants, termites and other insects.
Parallel Evolution:
When
convergent evolution is found in closely related species, it is called
“Parallel Evolution”. Example: development of running habit in deer (2-toed)
and horse (1-toed) with two vestigial splint bones. Tasmanian wolf is a
marsupial while wolf is a placental mammal. This also shows parallelism.