Evolution – Biology Notes Grade XI

Evolution – Biology Notes Grade XI

Evolution is the gradual continuous , irreversible process of change and development of simple and primitive things to complex and organized things. Charles darwin proposed tha term organic evolution which states that present complex organisms have evolved from simple and primitive form of life through modifications also called “Descent with modification”.

The earliest view for the evolution was based on mythology. According to Hindu Mythology, God Brahma created all living organism from his own body and all living things are made up of five basic elements air, water, fire, earth and sky.

Aristotle put forward his taxonomic views . He explained that higher organisms were evolved from lower organisms . He arranged the organisms in a graded series from lower to higher.

Evidences for organic evolution :

Morphological and Anatomical evidences

Taxonomy based upon similarities and disimilarities among the various groups of oraganisms was made. In all the vertebrates dorso-tubular nervechord, notochord, gillslits are present at least in embryonic stage. In fish heart is two chambered and only internal ear, in amphibians heart is three chambered and car has both middle and internal parts in reptile heart is incompletely four chambered , in mammals external ear pinnae also present . This shows that all the vertebrates are interrelated and they have evolved from some common ancestors.

Homologous organs

The organs having similar structure and develpment origin but appear differently and performance or function is also different are called homologous organs.
For example: Forelimbs of vertebrates like seal , bat , horse , man etc. appear different and perform different functions but they have similar structure and arrangement of bones, muscles, nerves, blood vessels etc.
This relationship between the structure having similar structure is called homology or divergent evolution.

Analogous organs

The organs having similar functions but different structures are called analogous organs.
 For example : Wings of birds and insects.
This relationship between the different groups due to their similar funtions are called analogy or convergent evolution.

Connecting link

The organisms having the character of two different groups. They proved that one group had evolved from another through gradual modifications are called connecting link.

For example : Peripatus is the connection link between annelida and arthopoda. The arthopoda charaters are compound eyes, antennae, semi-jointed legs. The annelida characters are wormlike body, segmental nephridial excretory organ.

Read this one too :   Lamarckism

Protopterus is the connecting link between fish and amphibians .
Ornithorychus and echidna are egg laying mammals. They are similar to mammals having mammary glands and are similar to reptiles because they give hard shelled eggs, abdominal testes , cloaca etc.

Vestigial organs

The organs which are reduced and non-functional but were functional in the ancestors and also functional in the related animals called vestigial organs.
Example : The vestigial organs in Man are as follows

  • Nictitating membrane
  • Verniform appendix
  • Coccyx
  • Ear muschles

Palaeotological evidences

  • It includes the study of fossils. Fossils are the remains of past organisms found in the sedimentary rocks. Fossils provides direct and most convincing evidences in support of organic evolution. Fossils includes hard structure like teeth, bone, goot print, shell etc. Fossils are found in different layers of rock. So, it is essential to estimate the age of the rocks in which fossils are found. To estimate the age of the fossils radioactive dating method i sused.
  • The most important finding from fossils record which help to explain evolutionary line are as follows :
  • Most primitive forms of life are recorded in the ldest reocks.
  • There was existance of diverse group of organisms which are not exactly similar to the organisms that exist today.
  • Different fossils are formed at different time periods.
  • There is succession of high and more complex form of fossils from older to recnet layer of rocks.
  • Large groups of organisms have become extinct but some species are persisting int he same or modified form.
  • Some organisms whcih remained scarce in one period of era become dominat in next period of era due to adoptive radiation.

Missing link

These are those extinct organisms which had the characters of two different groups. It shows the evolution of one group from another group.
For example : Archaeopteryx
Fossils of Archaeopteryx was discovered by Wagnar in Germany. It was about the size of crow and had characters of both reptiles and aves. The reptil charaters are teeth in jaw , claws in the fingers , a long tail etc. and aves characters are feathers, wings,etc.
These structure suggests that birds are evolved from reptiles like forms which become extinct. These fact explains the statement – “Birds are glorified reptiles “.

Read this one too :   GYMNOSPERM (Cycas & Pinus)


It is the evolutionary history of animals. Now, the scientist have collected the complete series of fossils of many animals . For example: Elepant, camel, horse etc. On the basis of these fossils we are able to construct the philogeny of many animals.

Phylogany of horse

The evolution of horse was started in North America about 60 millions years ago. The ancestor of modern horse was Eohippus. It was about the size of a fox with short limb , four toes in the fore limbs and three toes in the hind limbs . The probable of descent passes form Eohippus through Mesohippus , Miohippus, Merychippus, Pliohippus to the modern horse Eqqus. The changes took place during this evolution are as follows :

  • Enlargement in size.
  • Reduction in the number of side toes.
  • Enlargement of third digit , elongation in the limbs, fore head and neck also.
  • Development of premolar and molar teeths.

Embryological evidences

It is based on the camparative study of various animals. It shows that –

  • The early development of all triphoblastic and diploblastic animals are similar.
  • The embryo of all the vertebrates consists of gillslits, notchord, simple two chambered heart lateron these organs develped into different forms. This shows that all vertebrates have evolved from some common ancestors.

Recapitulation theory or Biogenetic law

It states that “Ontogeny repeats Philogany” which means that an individual during its own develpment passes through their evolutionary history.
For example : Fish like characters like fins , gills, tale with tale fins are present on the larva of a frog (i.e. Tadpole). This shows that amphibians have evolved from fish like ancestors.

Physiological and Biological evidences

  • Study of protoplasm in different organisms shows that it is made up of similar molecules like carbohydrates, nucleic acids, fats etc.
  • The similar enzymes of different animals are simlar in structure and mode of fraction.
  • The chromosomes of all the organisms are basically made up of nucleic acid and proteins.
  • Blood groups A, B , AB and O are found in the human beings , blood groups A and B also found in Apes. Thish show that man are very much simlar to Apes.

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