In this Darwin’s theory of evolution by natural selection post we have briefly explained about rapid multiplication, limited natural resources, struggle for existence, variation, natural selection, inheritance of useful variations, and, formation of new species.
Darwin’s Theory of Evolution by Natural Selection
Darwin’s theory of evolution by natural selection by Charles Darwin and Alfred Russel Wallace in their common publication. Both of them conducted scientific data collection from individual population survey.
Charles Darwin travelled for 5yrs expedition around the world on the ship H.M.S Beagle. During this journey, he made observation of several animals and plants. He keenly observed the similarities among organisms and draw evolutionary relationship.
In addition, economist Thomas Malthus’s report on workers recognized that competition between species leads to the struggle for existence. Considering Wallace’s view and Malthus observations of workers led Charles Darwin to propose the theory of natural selection in his book “Origin of Species”. The theory of natural selection is based on following points:
Every organism has enormous ability to reproduce to continuance of the species. All animals and plant tend to multiply in geometrical progression. For example, an organism will be double in Ist year, four times by 2nd year and 8 times in a third year and so on. Let’s see few example of organism to understand the potential of organism to multiply. These examples are as follows:
Paramecium: it has multiplication rate of 3 times in 48 hrs. if single paramecium will allow to grow and multiply in 5 years to give the mass equal to ten thousand times the size of earth.
Cod Fishes: Cod fish produces over 1 million eggs in a year. If all these eggs will rise to fishes, the whole Atlantic Ocean will be filled in next 5 yrs.
Oyster: An Oyster may lay 114,000,000 eggs at a single spawning. If all the oyster grow and survived upto adulthood for 5 generations, then number of oyster will be more than the number of electrons in the universe.
Elephant: Elephant has an average life span of 90 years and during the whole life span he can produce only six offspring’s. If all the offspring’s survived, single elephant pair would produce 19,000,000 elephants in 750 years.
Limited Natural Resources
Inspite the enormous capacity of an organism to reproduce, the number of individual species remains constant. It is due to increase in population in animal or plant requires more space and food. Ultimately the food to plant or animal is provided by the CO2 from air, water and mineral from soil. The amount of thes basic material is limited in universe. Hence, it does not allow the population of organism beyond the limit and an equilibrium is reached.
Struggle for existence
Due to shortage of food, water and space, there is severe competition among the off spring for existence. Every individual has few basic requirements, such as food, space, water, mate to reproduce and protection from enemies. In order to achieve basic needs, organisms compete with each other and it is known as struggle for existence. The struggle for an individual can be of 3 types:
Intraspecific Struggle: The competition of the individuals of the same species. For example, fight between two dogs for a piece of meat. War is another example of intraspecific struggle among different human.
Interspecific Struggle: The competition of the individuals of the different species. For example, tiger attacks on deer for food.
Environmental Struggle: Every individual struggle against the change in environment such as temperature, humidity, level of water, rain, climate etc.
Each and every individual varies in several aspects to other individual. Even the offspring’s produced by parents also differ in many aspects. The two individuals can be different from each other in their behaviour, color, size, strength etc.
Due to variation among different individual, they struggle towards their existence with different potentials. The variation in an individual may allow him to survive and complete its life cycle comfortable. Whereas, if the variations are unfavourable, the individual will struggle against every odds and as a result it may not be able to complete the life cycle. For example, fast running deer has better chance to escape from the tiger compared to the slow runner. Another factor is ability to adopt into the changed environment. Both Darwin and Wallace recognized the environment as the principal factor for natural selection. For example, plants with ability to hold more water and can be able to reduce loss of water will ultimately survive, despite the physical strength, height and other characters.
Inheritance of useful variations
The individual survived due to unique variation, mate and produces their off spring to complete their life-cycle. As a result, they transfer the useful variations to the next generations and allow the individual to multiply. Darwin believed that any variation which can help the individual to survive and help to favourable for struggle will be inherited. He considers the variation which may be acquired or inheritable.
Formation of new Species
As a result of struggle and natural selection, only the individual fits to the environmental conditions will survive and complete its life-cycle. As a result, the number of these individual will increase over the course of time compared to the less favourable organism. In addition, the variation favouring will be inherited to the next generation whereas unfavourable variation will be discarded. Due to continuous selection, a new organism will appear which will be different from their ancestral form.
Darwin’s theory of evolution by natural selection, Event in the evolution of giraffe from deer link ancestor
Let’s see the example of giraffe to understand the Darwin’s theory of natural selection. According to the theory of natural selection, in the beginning there were two different types of giraffe present on earth; (1) Deer like short height and (2) Long neck and forearms. Until the grass was available on land, both of these species were surviving and be able to complete their life cycle. With change in climate and reduction of grass, there might be a struggle for food. The giraffe with long neck and forearms can still be able to eat leaves on the tress but the deer like giraffe could not be able to reach there and died due to starvation. In due course, several round of the natural selection led the giraffe with long neck and forearms dominated the region and be present as new species.
Evidences to support Darwin’s Theory of Evolution
Artificial Selection: From ancient times, man is selecting good breed animals and plant for their use. In addition, they are performing cross breeding of these species to develop newer breed with desirable characters. The scientist supporting the Darwin’s theory explained the evolution through natural selection to give rise to newer species, just like following similar mechanism as artificial selection by man. They further added that natural selection is a slow process but much more complex compared to the artificial selection procedure.
Mimicry and Protective coloration: The mimicry and protective coloration is very common in several organisms as the product of natural selection. Most of this organism acquire the pattern of coloration by gradually changing color at each stage.
Correlation of nectarines and proboscis: The position of nectarines and proboscis in insect correlates well and matches well to facilitate pollination. This relation does not develop in single days but evolve gradually envisaged via the process of natural selection.
Evidences Against Darwin’s Theory of Evolution
Perpetuation of Vestigial Organs: Vestigial organs are selected despite the fact that they are not useful for animals but even then they are preserved generation over generation.
No explanation for variation: Darwin could not be able to explain the source and mechanism of generation of variation in organisms.
Distinction between continuous and discontinuous variations: According to theory, Darwin assumed that any variation essential for animal survival will be carried forward to next generation. We know that it is not true as per present knowledge of genetics.
Disapproval of Pangenesis theory of Darwin: Darwin put forward the theory of Pangenesis to explain the process of inheritance. It was disapproved by the experiments performed by August Weismann in 1892.