DARWIN’S THEORY OF EVOLUTION:

Organisms have unlimited capacity to reproduce
Organisms can reproduce offspring at a fast pace.
This is necessary for survival, because a higher number of offspring ensures that at least some of them could survive.
Each organism has to struggle for its day to day survival.
For example; a frog lays thousands of eggs at one go.
The spawn is released in water and it is left to fend for itself.
Most of the eggs are either washed away or are eaten by predators.
However, Some portions of eggs from the spawn develop into tadpoles.
Once again, many tadpoles are eaten by predators; leaving a few. which develop into adults.

Different individuals of a particular species have different traits.
Those with more suitable traits are selected by nature.
Each organism needs a particular trait for finding food and finding a mate.
Those with better traits are finally able to pass on their traits to the next generation.

Those organisms which are the fittest are able to survive, while others perish.
That is how many species become extinct and some Species continue to evolve over a period of time.

EVOLUTION AND CLASSIFICATION

The modern system of classification is based on evolutionary relationships.
Due to this, this is also known as phylogenetic classification.
The kingdom is the highest taxa, while the species is the lowest taxa.
Members of a species have a higher number of common characters, than members of a kingdom.
For example; all human beings belong to the species Homo sapiens.
Human beings can interbreed; irrespective of their race or skin colour.
All human beings come under the class mammalia; as do the monkeys, elephants and cows.
Apparently, each species of the class mammalian is quite different yet they have certain common characters; like hairs on the body and mammary glands in females.
Similarly, all animals are eukaryotes and the cell wall is absent in their cells.
The degree of similarity or dissimilarity shows that all animals have evolved from a common ancestor.

Some of the important sources which provide evidences for evolution are :

Organs which have common design but serve different functions in different animals are called homologous organs.
For example; the forelimbs of all tetrapods are composed of humerus, radio-ulna, tarsals and metatarsals.
Yet, the forelimbs of frogs are adapted to a jumping movement, those of birds are used for flying and those of humans are used for handling tools.
This shows that frogs, birds and humans have evolved from a common ancestor.

Organs which have different designs but serve a common function in different animals are called analogous organs.
Wings of birds and wings of bats are good examples of a pair of analogous organs.
Wings of birds are composed of all the bones of the forelimb and are covered with feathers.
Wings of bats are mainly composed of the digital bones and a thin membrane covering the structure.
Yet wings in both the organisms are used for flying.