Reproduction and Its Significance

Reproduction
Reproduction is the biological process by which existing organisms give rise to their offspring. Reproduction is carried out by different living organisms to perpetuate their species. Reproduction can be of two types – Sexual mode and Asexual mode.

• Reproduction prevents all kinds of organisms from becoming extinct.
• Reproduction is not necessary for carrying out life processes of an individual but helps in increasing the individuals in a population.
• Reproduction is essential in creating variations in species through genetic recombination. 
• Reproduction is necessary in the process of evolution by carrying favourable variations from one generation to another generation. 

Evolution
It is a long, drawn out process with gradual changes that take place over millions of years.  Evolution operates on different factors like variations, natural selection and adaptation. Evolution results in group of organisms living together in a well-defined environment.  Organisms living in this specific environment reproduce the offspring which are adapted to that specific environment. e.g. Over generations, giraffes developed long necks to eat leaves growing high up on trees.

Evolution occurs by stages
Evolution is a step by step process, it does not happen in few years. It takes generations by generations to exhibit variations over a large range. Organisms get adapted to their habitat by bring about certain changes in their structure. These changes are called as adaptations which bring about variations in the evolution. These variations are carried on to successive generations by natural selection.

• A classic example which supports the statement 'Evolution by stages' is the evolution of eye.
• In lower organisms like planaria, optic sensory structures are present  as patches which act as eye spots.
• In organisms like cockroaches, compound eyes are present which help them to see around their body.
• In organisms like octopus, eye is remarkable having  iris, circular lens, vitreous cavity, retina with photoreceptor cells. They lack a cornea. The lens in octopus does not change its shape but focusses through movement.
• In vertebrates, eye is a well-developed sensory structure with iris, ciliary muscles, pupil, cornea and a lens. The size of the pupil can be regulated. Shape of the lens can be adjusted in order to focus the image on the retina.

Evolution cannot be equated with progress

• Evolution is not exactly taking place from lower organisms to higher organisms but it can be related to the fact that higher and complex body forms evolved even when the low and simple forms are still existing. An individual also plays an important role in the evolutionary process. An individual contributes its specific genes into the gene pool of the population during transferring its genetic variations to its offspring. 
• These variations are carried on to successive generations by natural selection.
• If these genetic variations get accumulated over a period of time, alter the individuals of the subgroup and later give rise to new species.

Variations
These are the changes caused in the genetic material thereby bringing about changes in the offspring.

• Deoxyribonucleic acid is a genetic material present in chromosomes. 
• It carries information from parents to children.
• DNA replication is the process of creating similar copies of DNA during reproduction.
• Mutation leads to variation in DNA.
• Variations accumulated over a period of time are carried on to successive generations by natural selection. These genetic variations alter the individuals of the subgroup and later give rise to new species.
• Variations in species bring about diversity.
  

Variations caused by mutations can be beneficial or detrimental.

• Some variations might cause new born cells to die or survive. The surviving cells are similar to or different from each other.
• Variations can help species to evolve and survive in adverse conditions. e.g. Adaptation of giraffes, snakes and bacteria to the changing environment.
• Only advantageous variations occurring in an individual are able to exist in population. This is because variations which are detrimental cause the death of the individual. Hence, there is no chance for the detrimental variations to be carried to the next generation.

Varations are advantageous to species and not individuals

• Variations are advantageous to species as the changes which are acquired by variations are inherited by the offspring and certain alterations occur over many generations to make the changes acquired by variations perfect.
• Variations may prove to be advantageous or disadvantageous to the individual as the life span of the individual is so small. Variations acquired by the individual are not easily acceptable. Individual with variations is not easily accepted into the group.

Adverse changes in the environment can kill a species.