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Evolutionary biology is the study of the history of life forms on earth. The evolution of life on earth, different changes in flora and fauna around earth that co-exist along with human beings also forms parts of evolution.
Origin of Life
The origin of life is considered a unique event in the history of the universe. Huge cluster of galaxies comprises the universe. Galaxies contain stars and clouds of dust and smoke.
Big Bang Theory attempts to explain the origin of the universe. According to this theory, a huge explosion occurs that forms the different galaxies.
In the solar system of Milky Way galaxies, earth has been supposed to have formed about 4.5 billion years ago. There was no atmosphere in early earth. Water vapor, methane, carbon dioxide and ammonia released from molten mass covered the earth surface.
UV rays from the sun splits the water into hydrogen and oxygen. Life appeared 500 million years after the formation of earth.
There are different theories regarding the origin of life on earth-
Some scientists believe that life comes from other planets. Early Greek thinkers thought that units of life are called spores transferred from other planets.
According to other theories, life comes out of dead and decaying matters like straw and mud. This theory is called the theory of spontaneous origin.
Louis Pasteur experimentally proved that life arises only from pre-existing life.
Spontaneous theory of origin of life is dismissed after that.
Oparin and Haldane proposed that the first form of life could have come from pre-existing non-living organic molecules like RNA and protein etc. The formation of life is preceded by chemical evolution. At that time conditions on earth were- high temperature, volcanic eruption, reducing atmosphere containing CH4 and NHS3.
Miller experiment of Origin of Life- S.L. Miller in 1953, conducted an experiment to show the
origin of life on earth in the physical environment similar to the condition prevails at that time.
Miller created similar conditions of temperature and pressure in laboratory scale. He created electric discharge in a flask containing CH4, H2 and NH3 and water vapour at 8000C.
He observed formation of amino acids in the flask after 15 days of electric discharge. Similar experiments by other scientists found formation of sugars, nitrogen bases, pigments and fats.
Analysis of meteorite content also reveals similar compounds that reveal that similar process
are occurring elsewhere in the space. This experimental evidence about the origin of life is called chemical evolution of life.
Experimental representation of Miller’s experiment
The first non-cellular forms of life could have originated 3 billion years back. They could have been giant molecules like RNA, Protein, and Polysaccharide etc.
The cellular form of life was probably single cell and originates in water medium. The theory that first form of life arose slowly through evolutionary forces from non-living molecules is called biogenesis.
Evidence of Evolution: Evidence that evolution of life forms has taken place on earth have many proofs as mentioned below-
1. Paleontological evidence- different aged rock sediments contain fossils of different life forms that probably died during the formation of particular sediment. Fossils are remains of hard parts of life-forms found in rocks .The study showed that different from varied over time and certain life forms are restricted geological time span. Hence, new forms of life have arisen at different times in the history of earth.
2. Homologous organs- those organs that perform different functions but have similar origin and structures are called homologous organs. For example humans, cheetahs, bats and whales share similarities in pattern of bones of forelimbs although these forelimbs perform different functions in these animals. In these animals similar structures developed along different directions due to adaptation of different needs. This is called divergent evolution.
3. Analogous structures -they are not anatomically similar organs but perform similar functions. For example, the eyes of mammals and octopuses or flippers of penguins and dolphins.This is due to similar habitat that resulted in similar adaptive features in different groups of organisms. This evolution is called convergent evolution.
4. Biochemical evidence - similarities in proteins and genes performing a given function among diverse organisms give hints to common ancestry. These biochemical similarities point to the same shared ancestry as structural similarities among diverse organisms.
Evolution by natural selection- Industrial melanism
A case of natural selection was seen in England in 1850s, i.e., before industrialisation in a peppered moth (Biston betularia). This moth had two forms: grey colour and black colour (Carbonaria). In the early part of the nineteenth century , before industrialization only the grey coloured forms of moths were present; the dark forms were rare. The grey coloured moths were seen on the tree trunks covered with lichens and so they were able to escape from their enemies. Later on in 1920, due to the development of industries, post industrialization, the lichens were killed and the tree trunks looked dark due to the deposition of industrial soot. Birds, now were able to spot these moths and feed upon them.So the grey coloured moths were eaten by the birds and the dark coloured moths escaped from the birds. Then now the coal is replaced by the industries and oil and electricity is used.This has reduced the soot production and ultimately less deposition of soot on the tree trunks. These tree trunks have, now, again become grey in colour. Consequently, grey coloured moths have again increased in number. This example clearly brings out the action of natural selection.
Evolution by anthropogenic action - Resistance of mosquitoes to pesticides.
When DDT was introduced to control mosquitoes it was tremendously successful. Most of the
mosquitoes were sensitive to DDT and were therefore killed. In that population of mosquitoes, few mosquitoes become resistant to DDT and survive. They multiplied and now almost total population of mosquitoes become resistant to DDT.Same pattern has been observed in bacteria which are multidrug resistant due to excess use of drugs and medicines.
Adaptive Radiation- the process of evolution of different species in given geographical area
starting from a point and radiating to other areas of geography (habitat) is called adaptive radiation. Darwin’s finches represent one of the best examples of adaptive radiation.
Australian marsupials, each with different from other evolved from one ancestral stock, but all within Australian island continents.When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats), we can call this convergent evolution e.g Placental mammals and Australian marsupials.
Biological Evolution — the nature selected for fittest and fitness is based on characteristics which are inherited. Some organisms are better adapted to survive in an otherwise hostile environment. Fitness is the end result of the ability to adopt and get selected by nature.
Lamarck had said that evolution of life forms had occurred but driven by use and disuse of organs. He gave the example of giraffes evolving their neck by foraging leaves on tall trees and had to adapt by elongation of their necks.
Branching descent and natural selection are the two key concepts of Darwinian Theory of Evolution .Darwin theory of natural selection was based on certain observations like-
- Limited natural resources.
- Overpopulation
- Competition for resources
- Struggle for existence
- Survival of the fittest.
Mechanism of Evolution
Hugo deVries based on his work on evening primrose brought forth the idea of mutation. Mutation is the large difference arising suddenly in a population.
Mutations are random and directionless while Darwin variations are small and directional.
Hugo deVries believed that mutation causes speciation and hence called saltation (single step large mutation).
Hardy- Weinberg Principle- in a given population, frequency of occurrence of alleles or genes can be found out. These frequencies remain fixed and even remain the same through generation. This fact was represented by Hardy-Weinberg principles using algebraic equations.
This principle states that allele frequencies in a population are stable and are constant from generation to generation. The gene pool remains constant. This is called genetic equilibrium and the sum total of all the allelic frequencies is 1.
Binomial expansion of (p+q)2 = p2+2pq+q2=1 where p and gq represent the frequency of allele A and allele a in a population . The frequency of AA individuals in a population is simply p2 .This is simply stated in another ways, 1.e., the probability that an allele A with a frequency of p appearing on both the chromosomes of a diploid individual is simply the product of the probabilities, 1.e., p2 . Similarly of aa is q2 , of Aa 2pq. Hence, p2+2pq+q2=1.
When frequency is measured, the actual value varies that indicates the extent of evolutionary changes. Change of frequency in alleles (Hardy-Weinberg equilibrium) in a population resulted due to evolution.
The factors that affect Hardy-Weinberg equilibrium are-
- Gene migration or gene flow.
- Genetic drift
- Mutation
- Genetic recombination
- Natural selection.
During genetic drift sometimes change in alleles frequency is so different in a sample of population that they become a different species. The original drifted population becomes founder and that effect is called founder effect.
Brief Account of evolution
About 2000 million years ago the first cellular form of life appeared on earth.
Slowly single-celled organisms become multicellular forms and by the time 500 mya,invertebrates were formed and active.Jawless fish evolved around 350 mya.
Organisms started to invade from water to land. Fish with stout and strong fins could move on land and go back to water These animals called lobe fins evolved into the first amphibians.
Later, these amphibians evolved into reptiles. They lay shelled eggs. Then reptiles of different shapes and sizes dominated on earth, fish like reptiles e.g. Ichthyosaurs and the land reptiles e.g dinosaurs. The biggest of them was Tyrannosaurus rex.
Some of the reptiles evolved into birds and later some of them to mammals. Mammals were viviparous and more intelligent in sensing and avoiding danger .