Define evolution. How does it occur? Describe how fossils provide us evidences in support of evolution.

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Biological evolution is any change in the frequency of alleles in a population of organisms.   Evolution is a slow change in the heritable characteristics of biological populations over successive generations. Evolutionary processes give rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms, and molecules.

Evolution occurs due to the following 

i. Accumulation of variations over generations - The variations arise due to crossing over, recombination, fertilisation or  errors during DNA replication, mutation, recombination during sexual reproduction, etc . Geographical barrier  also plays a great role in the accumulation of these variations.

ii. Natural Selection - The organisms with the favourable mutations are selected by nature and have better chances of survival than those who do not have favourable variations.

iii. Genetic Drift refers to the fact that variations in gene frequencies within populations can occur by chance rather than by natural selection. Random genetic drift may be an important mechanism in evolutionary change in small population.

All this leads to changes in the heritable characters and finally results in evolution of the organism. 

Fossils − They represent plants and animals that lived millions of years ago and are now extinct. Different aged rock sediments contain fossils of different life-forms, which probably died during the formation of the particular sediment. 

It is observed that although fossils appeared different from the existing species they may show certain features similar to the existing species thus providing linkages between pre-existing and existing forms Provide information about the extinct species which were different from the existing species