Evolution Notes for Class 12 Chapter 6
Last Updated : 05 Apr, 2024
Evolutionary biology is the study of the evolutionary processes that produced the diversity of life on Earth. Earth came into existence sometime between 4 and 5 billion years ago. Life evolved on planet Earth about 3.5 billion years ago. Since then, approximately 15 million different species of organisms have evolved. But only about two million have been identified so far. In this article, we will discuss how the life of these, at first originated on earth and how such a vast variety of organisms evolved through variation and natural selection.
Origin of Life
The origin of life is considered a unique event in the history of the universe. Several theories have been put forth to explain the origin of life.
- Big Bang Theory: The universe is vast. Huge clusters of galaxies comprise the universe. The Big Bang theory attempts to explain to us the origin of the universe and the creation of life. Developed in 1927, it is considered the most credible scientific explanation of how the universe was created. It suggests that through a process of expansion and explosion, hydrogen gas was created which led to the formation of stars, and their death (supernova) led to the creation of life.
- Panspermia Theory: The panspermia hypothesis states that the seeds of life exist all over the universe and can be propagated through space from one location to another. For millennia, this idea has been a topic of philosophical debate. However, due to the lack of any validation, it remained merely speculative until a few decades ago.
- Theory of Spontaneous Generation: The theory of spontaneous generation believed that life originated spontaneously from non-living decaying and rotting matter like straw and mud. This theory was disproved by Louis Pasteur. Louis Pasteur demonstrated that life comes from pre-existing life. He showed that in pre-sterilized flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from ‘killed yeast’.
- Chemosynthetic Theory of Life: It was proposed by A.I. Oparin of Russia and Haldane of England. They proposed that the first form of life could have come from pre-existing non-living organic molecules, e.g. RNA, protein, etc., and that the formation of life was preceded by chemical evolution, i.e., the formation of diverse organic molecules from inorganic constituents. The conditions on earth were – high temperature, volcanic storms, and reducing atmosphere containing CH4, NH3, etc.
It was supported by S.L. Miller, an American scientist. In 1953, he created similar conditions on a laboratory scale. He created electric discharge in a closed flask containing Methane (CH4), Hydrogen (H2), Ammonia (NH3), and Water vapor (H2O) at 8000C. He observed the formation of amino acids. Miller's experiment showed how simple molecules could be assembled into the more complex molecules necessary for life by natural processes.
Also Read: Abiogenesis
The formation of complex organisms through ‘gradual change’ from simple ancestral types over the course of geological time is termed Evolution.
The theory of evolution is a shortened form of the term “theory of evolution by natural selection,” which was proposed by Charles Darwin and Alfred Russel Wallace in the nineteenth century. Darwin and a scientific contemporary of his, Alfred Russel Wallace, proposed that evolution occurs because of a phenomenon called natural selection. In the theory of natural selection, organisms produce more offspring than are able to survive in their environment.
Those that are better physically equipped to survive, grow to maturity, and reproduce. Those that are lacking in such fitness, on the other hand, either do not reach an age when they can reproduce or produce fewer offspring than their counterparts. Natural selection is sometimes summed up as “survival of the fittest” because the “fittest” organisms are the ones that reproduce most successfully, and are most likely to pass on their traits to the next generation. Natural selection was such a powerful idea in explaining the evolution of life that it became established as a scientific theory.
Evidence for Evolution
The evidence supporting organic evolution is derived from a number of fields of Biology. Those discussed here are:
- Palaeontological evidence: Different-aged rock sediments contain fossils of different life forms that probably died during the formation of the particular sediment. Some of them appear similar to modern organisms. They represent extinct organisms, e.g., Dinosaurs. A study of fossils in different sedimentary layers indicates the geological period in which they existed. Hence, new forms of life have arisen at different times in the history of Earth.
- Embryological evidence: It was proposed by Ernst Heckel based on the observation of certain features during the embryonic stage common to all vertebrates that are absent in adults. For example, the embryos of all vertebrates including humans develop a row of vestigial gill slits just behind the head but it is a functional organ only in fish and not found in any other adult vertebrates.
- Morphological evidence: Though organisms of different species and groups are quite different from each other, they still retain certain common features. Morphological evidence for evolution is derived from:
- Divergent evolution: Species from a common ancestral origin evolve similar anatomical parts (called homologous structures) but with dissimilar functions. For example, whales, bats, cheetahs, and humans (all mammals) share similarities in the pattern of bones of forelimbs.
- Convergent evolution: It creates analogous structures that have similar forms or functions but were not present in the last common ancestor of those groups. Both sharks and dolphins have similar body forms, yet are only distantly related: sharks are fish and dolphins are mammals.
- Molecular evidence: Similarities in proteins and genes performing a given function among diverse organisms give clues to common ancestry. These biochemical similarities point to the same shared ancestry as structural similarities among diverse organisms.
Also Read: Fossils - Tracing Evolutionary Relationships
Adaptive Radiation
Adaptive radiation is a rapid increase in the number of species with a common ancestor, characterized by great ecological and morphological diversity. This process occurs due to natural selection.
An example of adaptive radiation is Darwin finches, found on Galapagos Island. A large variety of finches is present in Galapagos Island that arose from a single species, which reached this land accidentally. As a result, many new species have evolved, diverged, and adapted to occupy new habitats. These finches have developed different eating habits and different types of beaks to suit their feeding habits. The insectivorous, blood-sucking, and other species of finches with varied dietary habits have evolved from a single seed-eating finch ancestor.
Biological Evolution
Branching descent and natural selection are the two key concepts of the Darwinian Theory of Evolution.
- Branching descent is the process of the development of a new species from a single common descendant. New species developed and become geographically adapted to a new environment. This results in reproductive isolation and finally to the development of a new species.
- Natural Selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others.
Darwin asserted that variations, which are heritable and which make resource utilization better for few will enable only those to reproduce and leave more progeny. Hence for a period of time, over many generations, survivors will leave more progeny and there would be a change in population characteristics and hence new forms appear to arise.
Mechanism of Evolution
Various theories about the mechanism of evolution have been proposed; some of them such as Lamarck’s theory of “Inheritance of acquired characters” and De Vries’ theory of ‘mutation’ are now of historical importance only. Darwin’s theory of Natural selection still holds ground but was modified with progress in genetics and developed into the Modern synthetic theory which is regarded as the most valid theory of evolution.
Darwin talked about variation but did not know about the sources of variation. With progress in genetics, the sources of variation were discovered and Darwin’s original theory of Natural Selection was modified. This new theory was termed Neo-Darwinism or Modern Synthetic Theory.
Hardy-Weinberg Principle
Hardy–Weinberg Principle mathematically explains the occurrence and consistency of gene frequency for a particular gene. The principle states that the allelic frequency remains constant through generations and the gene pool remains constant. This can be mathematically represented as:
(p+q)2 =1 or p2 +2pq+q2 =1
Factors affecting the Hardy-Weinberg principle:
- Mutation
- Genetic drift
- Natural selection
- Genetic recombination
- Gene flow
All these factors contribute to the change in gene frequency of a species in an area. If a few individuals from a species migrate to another place, the gene frequency changes again. It decreases from the place from where the individuals migrate and increase in the place they migrate to. If the frequency of the genes is high enough in the newly migrated land to start a new species, the migrated individuals become the founder species, and the effect is called the Founder effect. Thus, all these mechanisms contribute to the process of evolution.
Brief Account of Evolution
Evolution refers to the gradual change which occurs in an organism over a long duration of time. It is a slow-going process that results in the development of the organism. Life originated on Earth about 3.5 billion years ago. It is believed that there might have been the presence of simple elements on earth which may have given rise to simple organic and inorganic molecules. From these simple molecules, complex molecules like protein, DNA, etc. may have been formed.
Other evolutionary changes may have resulted in the formation of simple cells and the result of the continuous evolution on Earth is the several species of plants and animals that exist on Earth. Today, the diversity on earth varies from unicellular amoeba to a human beings and from unicellular Algae like Chlorella to a huge banyan tree.
Origin and Evolution of Man
Man belongs to the family Hominidae of the order Primates. Humanlike apes belong to the same order. With the passage of time, their ancestors evolved and became more and more different. The first-ever ancestors of humans are believed to have originated in Africa, eventually migrating to Europe, Asia, and the rest of the world. Man originated through several stages:
- Dryopethicus: It is the earliest known ancestor of man. They were found in some parts of Africa, Asia, and Europe. The evolution of man began with him. Dryopethicus was followed by Australopithecus.
- Australopithecus: These were 1.2 meters tall and could walk upright. They inhabited the African mainland. They had large jaws and human-like teeth.
- Homo habilis: They were five feet tall and could make use of tools. They are believed to have been able to speak.
- Homo erectus: They were more evolved beings. They were also upright and had a larger brain size. They had a prominent speech. They invented fire and were carnivorous.
- Neanderthals: Homo sapiens is the only extant species of hominin around today, but a few thousand years ago, there were a few other species that existed alongside anatomically modern humans – the Neanderthals, Denisovans and the Homo floresiensis. Today, scientists consider Neanderthals to be more of a subspecies of humans rather than a completely separate species.
- Homo sapiens: These are modern men. They developed the power of thinking, used tools, were omnivorous, and produced art. Their brain size was reduced to 1300 cc.
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