Revision: Origin and Evolution of Life Biology HSC Science (General) 12th Standard Board Exam Maharashtra State Board

The phenomenon of biogenesis states that living beings are neither produced spontaneously, nor created. Instead life comes from pre-existing life.

Organic evolution is a slow, gradual, continuous and irreversible change through which the present-day complex forms have descended from their simple pre-existing forms of the past.

The fittest individuals in nature are most likely to reproduce and pass on their good qualities to their offspring. It is called natural selection.

Differences between individuals of the same species that arise naturally and can be passed to offspring are called variations.

The origin of new species by gradual modification is called speciation.

The modern version of Darwin’s theory of natural selection, which incorporates genetics as the source of variations, is called Neo-Darwinism.

Insertion of a vector into the target bacterial cell is called transformation

Any random fluctuation (alteration) in allele frequency, occurring in the natural population by pure chance, is called genetic drift.

The structural, morphological change in chromosome due to rearrangement is called Chromosomal aberrations.

Gene flow is the movement of genes into or out of a population. Gene movement may be in the form of migration of organism, or gametes or segments of DNA Gene flow also alters gene frequency, causing evolutionary change.

The proportion of an allele in the gene pool, to the total number of alleles at a given locus, is called gene frequency.

Mutation breeding, sometimes referred to as “variation breeding”, is the process of exposing seeds to chemicals or radiation in order to generate mutants with desirable traits to be bred with other cultivars.

Organic evolution can be defined as slow, gradual, continuous and irreversible changes through which the present-day complex forms of the life developed (or evolved) from their simple pre-existing forms.

The total genetic information encoded in the sum total of genes in a Mendelian population is called gene pool.

Evolution is a slow and continuous process whereby complex forms of life have emerged from simpler forms over millions of years.

The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation. 

Homologous organs are those organs, which are structurally similar but perform different functions.

Large number of organisms get buried due to disasters like floods, earthquakes, volcanoes, etc. Remnants and impressions of such organisms remain preserved underground. These are called fossils.

The study of ancient life with the help of fossils is called palaeontology. Fossils are the dead remains of plants and animals that lived in the past in various geological layers.

The evidence of evolution indicated in the given diagram is vestigial organ.

Vestigial organs: Vestigial organs are useless organs that are degenerated or underdeveloped due to non-functionality through several years.

Vestigial organs are degenerate organs that are inadequately developed and non-functional. They could be useful in certain related and unrelated species, as well as in ancestors.

A species (both singular and plural) is a natural population of individuals or groups of individuals who are similar in all key morphological and reproductive characteristics, allowing them to interbreed freely and produce fertile offspring.

The process of formation of a new species from the pre-existing species is called speciation.

According to A.E. Emerson, speciation is the evolutionary process by which new species arise due to genetic divergence and isolation.

The process where organisms with favorable variations survive and reproduce more successfully than others is called natural selection.

The concept where only well-adapted individuals survive in the struggle for existence is called survival of the fittest.

• Hardy–Weinberg’s principle states that allele frequencies in a population remain constant from generation to generation in the absence of evolutionary forces.
• The total collection of all alleles in a population is called the gene pool.
• Genetic equilibrium means no change in allele frequencies over time.
• If p is the frequency of dominant allele and q is the frequency of recessive allele, then
  p + q = 1.
• Genotype frequencies are expressed as:
  p² (AA) + 2pq (Aa) + q² (aa) = 1.
• Any deviation from Hardy–Weinberg equilibrium indicates that evolution is occurring.
• The principle helps in detecting the role of natural selection and other evolutionary forces.

• Natural selection, proposed by Charles Darwin, explains evolution as a process where organisms with favourable variations survive and reproduce.
• Organisms produce more offspring than can survive, leading to a struggle for existence.
• Individuals with useful variations are better adapted to the environment and are selected by nature (“survival of the fittest”).
• These favourable traits are inherited by successive generations, resulting in gradual evolution and formation of new species.
• Darwin presented this theory in his book On the Origin of Species (1859), which became a landmark in evolutionary biology.

• Morphological similarities are observed in body parts of animals (e.g., mouth, eyes, nostrils, ears, hairs).
• In plants, similarities are seen in leaf shape, venation, petiole, etc.
• These structural similarities indicate a common ancestry and evolutionary relationship.

• The geological time-scale represents the entire history of the Earth and helps correlate geological and biological events in proper sequence.
• The Earth’s crust is made of layered rocks, and radioactive dating techniques estimate the age of the Earth to be about 4.5 billion years.
• Earth’s history is divided into five major eras: Archaeozoic, Proterozoic, Palaeozoic, Mesozoic, and Cenozoic.
• Each era is further divided into periods and epochs, marked by changes in climate, plant life, and animal life.
• The time-scale shows the gradual evolution of life, from simple unicellular organisms to complex plants, animals, and humans.
• Major events such as the origin of life, rise and extinction of dinosaurs, and evolution of mammals and humans are recorded in the geological time-scale.
• Scientists believe Earth is currently entering a sixth mass extinction (Anthropocene), mainly due to human activities causing rapid loss of biodiversity.

1. Meaning – Modern Synthetic Theory of Evolution is a combined explanation of evolution based on genetics, ecology, paleontology, anatomy and natural selection.
2. Major contributors – Important contributors include R. A. Fisher, J. B. S. Haldane, T. Dobzhansky, Julian Huxley, Ernst Mayr, G. G. Simpson and Stebbins.
3. Population concept – Evolution occurs in populations; a small interbreeding group within a species is called a Mendelian population.
4. Gene pool and gene frequency – The total genes present in a population constitute the gene pool, and the proportion of an allele in the gene pool is called gene frequency.
5. Factors causing evolution – Gene mutations, chromosomal changes, genetic recombination, migration and natural selection alter the gene pool.
6. Speciation – Reproductive isolation along with genetic variation and natural selection leads to the formation of new species.

Living organisms show basic characteristics such as metabolism, growth, reproduction, energy transformation and responsiveness.

The origin of life is still a mystery and many scientific fields have tried to explain how life began on Earth.

1. The theory of special creation states that all life was created by a supernatural power, but it has no scientific evidence.
2. The cosmozoic (panspermia) theory suggests that life came to Earth from outer space in the form of spores or microorganisms, but it does not explain the origin of life elsewhere.
3. The theory of spontaneous generation (abiogenesis) proposed that life arose from non-living matter, but this idea was disproved by Louis Pasteur.
4. The theory of biogenesis states that life arises only from pre-existing life, explaining continuity of life but not the origin of the first living organism.

• Meaning of evolution – Evolution means gradual and irreversible change in organisms, resulting in present-day complex forms from simpler ancestral forms.
• Organic evolution – Organic evolution is a slow, continuous process through which descendants become different from their ancestors over generations.
• Darwin’s view – According to Charles Darwin, evolution is “descent with modification”, where variations are inherited and lead to formation of new species.
• Weismann’s contribution – August Weismann disproved inheritance of acquired characters and proposed the theory of continuity of germplasm, stating that only germ cell variations are inherited.