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Eyes: How They Work, Anatomy and Common Condition

 The eye is a complex sensory organ that allows us to see the world around us. The eye works by gathering light from the environment and converting it into electrical signals that can be interpreted by the brain. The basic structure of the eye includes the cornea, iris, pupil, lens, retina, and optic nerve. The cornea is the transparent outer layer of the eye that helps to focus incoming light. The iris is the colored part of the eye that regulates the amount of light that enters the eye through the pupil, which is the black circular opening in the center of the iris. The lens is a clear, flexible structure located behind the iris that helps to focus light onto the retina. The retina is a thin layer of tissue that lines the back of the eye and contains photoreceptor cells called rods and cones. These cells convert light into electrical signals that are sent to the brain via the optic nerve. The rods are responsible for the low light vision and detecting motion, while the cones are resp
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DARWINISM (Theory of Natural Selection)

Charles Robert Darwin was born on 12th Feb. 1809 in England. He was an English naturalist, geologist and biologist. Darwin travelled by H.M.S. Beagle ship, from 27 Dec. 1831 to 02 Oct. 1836 through South America, South Africa, Australia and Galapagos Islands. Later on he wrote a book "On the origin of species by means of Natural Selection" (1859). In this book he gave his theory of Natural selection.

Basic concepts of Darwinism :


Branching Descent and Natural selection are the two key concepts of Darwinian Theory of evolution and his theory is based on certain observations which are factual.

(1) Over production :


All organisms have the capability to produce enormous number of offspring or organisms (multiply in geometric ratio). Hence, theoretically population size will grow exponentially if everybody reproduced maximally but the fact is that population sizes in reality are limited.

(2) Struggle for existence :


Natural resources are limited and populations are stable in size (except for seasonal fluctuation) means that there had been competition for resources. Only some survived and grew at the cost of others that could not flourish. This is called struggle for existence. It is of three types :
  • Intra specific struggle : It is competition among the individuals of same species for same needs like food, shelter and breeding.
  • Inter specific struggle : It is the struggle among the individuals of different species for food and shelter. It is the most potent force for organic evolution.
  • Environmental struggle : This struggle is between the organisms and their environment. All organisms struggle with cold, heat, wind, rain, drought, flood etc.

(3) Variations and heredity :


Members of population vary in characteristics even though they look superficially similar i.e. population has built variation in characteristics. Those characteristics which enable some to survive better in natural conditions are called adaptive or useful variations while others are called as non-adaptive or harmful variations.

The novelty and brilliant insight of Darwin was, he asserted that variations, which are heritable and which make resources utilisation better for few will enable only those to reproduce and leave more progeny.

(4) Natural selection / Survival of the fittest : 


Individuals with more adaptive variations are "better fit" than the individuals with less adaptive variations. Hence, those who are better fit in an environment would be selected by nature and leave more progeny than others. Darwin called it Natural selection and implied it as a mechanism of evolution. Fitness is the end result of the ability to adapt and get selected by nature. The fitness, according to Darwin, refers ultimately and only to reproductive fitness. It is observed that all adult individuals  of a population don't have equal chances of matting; some males with better phenotype are preferred by females. This is called Sexual Selection.

(5) Origin of new species :


As a result of heritable variations and natural selection there would be a change in population characteristics and hence new forms appear to arise.

Criticism of Darwinism :

  1. The main Drawback of this theory is that Darwin didn't have the knowledge of genetics and he had no satisfactory explanation for the cause, origin and inheritance of variations.
  2. This theory only explained the survival of fittest but was unable to explain the arrival of fittest.
  3. Darwin was unable to explain why in a population only a few individuals develop useful variations and others have harmful variations.
  4. Criticism of Darwinism was based on sexual selection. Why only females have the right of selection for matting ?
  5. Darwin couldn't explain the existence of vestigial organs.
  6. Darwin was unable to differentiate the somatic and germinal variations.

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