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 ...
Neo-Darwinism is a modified form of Darwinism along with recent researches of Weismann, De Vries, Stebbins, Dobzhansky, Sewall Wright, Mayr etc. According to this theory following factors are responsible for formation of new species :
- Rapid multiplication
- Limited food and space
- Struggle for existence
- Genetic variations
- Natural Selection
- Isolation : Isolation is a segregation of populations by some barriers which prevent interbreeding. The reproductive isolation between the populations due to certain barriers leads to the formation of new species.
NATURAL SELECTION
Natural selection is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny. A critical analysis makes us believe that variation results in changed frequency of genes and alleles in future generation. Coupled to enhance reproductive success, natural selection makes it look like different population and lead to new species formation.
Examples of Natural Selection :
(1) Industrial Melanism :
This phenomenon was studied by Bernard Kettlewell in England. In a collection of moths (Biston betularia) made in 1850s, i.e., before industrialization set in, it was observed that there were more white winged moths on trees than dark winged or melanised moths. However, in the collection carried out from the same area, but after industrialization, i.e., in 1920, there were more dark winged moths in the same area, i.e., the proportion was reversed.
The explanation put forth for this observation was that 'predators will spot a moth against a contrasting background'. Before industrialization set in, thick growth of almost white coloured lichen covered the trees in that background the white winged moth survived but the dark coloured moth were picked out by predators. During post industrialization period, the tree trunks became dark due to industrial smoke and soot. Under this condition the white winged moth did not survive due to predators while dark winged or melanised moth survived. Hence, moths that were able to camouflage themselves, i.e., hide in the background, survived. This understanding is supported by the fact that in areas where industrialization did not occur e.g., in rural areas, the count of melanic moths was low. This showed that in a mixed population, those that can better adapt, survive and increase in population size.
(2) Drug resistance :
The drugs which eliminate pathogens become ineffective in the course of time because those individuals of pathogenic species which can tolerate them survive and flourish to produce resistant population
MUTATION THEORY
This theory was proposed by Hugo de Vries based on his work on evening primrose (Oenothera lamarckiana). Large differences arising suddenly in a population are called mutations. Actually mutation are sudden changes of genetic material (DNA) and hence all are inheritable. In addition to recombination, mutation is another phenomenon that leads to variation in DNA. It is discontinuous source of variations and provides raw material for evolution. According to Hugo de Vries it is mutation which causes evolution and not the minor variations (Heritable) that Darwin talked about.
Mutations are random and directionless while Darwinian variations are small and directional. Evolution for Darwin was gradual while de Vries believed mutation caused speciation and hence called it saltation (single step large mutation).
Criticism :
(1) Natural mutations are not very common as Hugo de Vries thought.
(2) Mutations are normally recessive and harmful, while the characters taking part in evolution are usually dominant
Comments
Post a Comment