In the mid 19th century, Charles Darwin (1809 – 1882), formulated the scientific theory of evolution by natural selection, published in his book “On the origin of species” in 1859.
Darwin’s idea were inspired by the observations that he had made during a sea voyage in a sail ship called H.M.S Beagle round the world, from 1831 to 1836.
Natural selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring. It is a key mechanism of evolution which involves the change in heritable traits of a population over time.
The concept of fitness is central to natural selection i.e., individuals that are more “fit” have better potentials for survival. Herbert Spencer (1820 – 1903) coined the phrase “survival of the fittest”, in his book “Principles of biology” in 1864, after reading Darwin’s book “On the origin of species”.
Alfred Russel Wallace (1823 – 1913) best known for independently conceiving the theory of evolution through natural selection by working in Malay Archepelago. The concept of natural selection was published by Darwin and Wallace in a “Joint presentation of papers” (1858).
Five basic concepts of Darwinism
The process that produces an alteration in DNA or chromosome’s structure or number is known as mutation.
The term ‘mutation’ was coined by Hugo Marie de Vries in 1901.
Any agent that cause mutation or increase the rate of mutation is known as mutagen.
Chromosomal mutation is a missing, extra, or irregular portion of chromosome. It is also known as chromosomal anomaly, chromosomal abnormality, chromosomal aberration or chromosomal disorder.
It can be classified into following three categories:
The wall of the human alimentary canal consists of four distinct layers: the mucosa, sub-mucosa, muscularis and serosa.
Fig: Transverse section of gut (diagrammatic representation)
Mucosa – The mucosa is the innermost layer of the gastrointestinal tract that is surrounding the lumen (open space within the tube). This layer comes in direct contact with digested food. This layer forms rugae in the stomach and villi in the small intestine. The mucosa is made up of epithelium – innermost layer, responsible for most digestive, absorptive and secretory processes. Mucosa also contains goblet cells which produces mucus that protects the epithelial surface.
Sub-mucosa – The sub-mucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymph vessels and nerves. In duodenum, glands are also present in it. Sub-mucosa supports the mucosa.