Solutions-Class-12-Biology-Chapter-5-Origin and Evolution of Life -Maharashtra Board

(b) Oparin and Haldane

(b) Inorganic materials – organic materials – colloidal aggregates – Eobiont- cell.

(c) q²

(b) Flippers of dolphins and penguin

(c) Birds and reptiles

(d) Mutations are non-random and directional.

(c) Random mating

(b) tail is present

(a) Australopithecus

(b) 1400 cc

The sum total of genes of all individuals of interbreeding population or Mendelian population is called gene pool.

The proportion of an allele in the gene pool as compared with other alleles at the same locus is termed as gene frequency.

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.

All individuals of the same species form a group which is called a population.

Formation of new species from the pre-existing single group of organisms is called speciation.

The process of evolution which results in transformation of original species to many different varieties is called adaptive radiation.

If the variation occurs in population by chance alone and not by natural selection to bring change in frequencies of an allele, it is called genetic drift.

The Hardy-Weinberg equilibrium law states that at equilibrium point both the gene (allele) frequency and geneotypic frequency remain constant from generation to generation’. It occurs only in the diploid, sexually reproducing, large, free interbreeding population in which mating is random and no selection or other factors are present for changing the allele frequency.

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

Vestigial organs are imperfectly developed and non-functional organs which are in degenerate form, they may be functional in some related and other animals or in ancestor.

Homo sapiens sapiens is the scientific name of modern man.

Coacervates are colloidal aggregations of hydrophobic proteins and lipids (lipoid bubbles). Coacervates grew in size by taking up material from surrounding aqueous medium.

Jurassic period from Mesozoic era is known as age of Reptilia.

Australopithecus, the ancestor of human which is described as man with ape brain.

• Genetic drift is a random, directionless variation in allele frequency.
• It happens by chance in a tiny population.
• Because genetic drift may alter gene frequency, it becomes an evolutionary force.
• Sewall Wright created this notion, hence it is also known as the Sewall Wright effect.
• Some alleles of a population are lost or decreased by chance as a result of genetic drift, while others may be enhanced.
• Some time, a few individuals become isolated from the large population and they produce new population in new geographical area.
• Genetic drift is also called founders‘ effect because original drifted population becomes ‘founders’ in the new area.
• E.g. Non-adaptive character of huge horns in Antelope is fixed due to genetic drift.

Gene flow, genetic drift’ gene mutations chromosomal aberrations such as deletion duplication, inversion and translocation, genetic recombinations, natural selection, isolation are some of the factors which are responsible for changing the gene frequency.

• Here more number of individuals acquire peripheral character value at both ends of the distribution curve.
• Nature select extreme phenotypes and eliminate intermediate. Hence two peaks are formed in distribution of traits.
• This kind of selection is rare.
• It ensures the effect on the entire gene pool of a population, considering all mating types or systems.

• Fossils are studied under palaeontology.They are used in reconstruction of phylogeny
• Fossil study helps in studying various forms and structures of extinct animals.
• By understanding the structure of fossil, record of missing link between two groups of organisms can be deduced.
• By studying fossils various body forms and their evolution can be understood. They also help to understand the habit and habitat.
• Some fossils provide the evolutionary evidences such a connecting links.

Objections to Mutation Theory :

• Hugo de Vries observed the large and discontinuous variation. But these were chromosomal aberrations. Only gene mutations usually bring about minor changes.
• Rate by which mutations take place is very slow as compared to the requirement of evolution.
• Chromosomal aberrations are very unstable. The organisms with chromosomal aberration are usually sterile and thus chromosomal aberrations have little significance in evolution.

• The natural selection that disrupts the mean characters of the population, is called disruptive selection.
• Greater number of individuals acquire peripheral character value at both ends of the distribution curve. E.g. Finches with large size or small size, both will be selected.
• Extreme phenotypes are selected in evolutionary process and intermediate forms are eliminated.
• When distribution curve is plotted it shows two peaks for two extremes.
• Disruptive selection is rare because, nature always tries to balance the characters.
• It ensures the effect on the entire gene pool of a population, considering all mating types or systems.

Example of disruptive selection :

• African seed cracker finches are types of seed-feeder birds which have different sizes of beak. The seeds available to them were of small and large sized.
• Large beak sized birds feeds on large seeds While small beak sized birds feed on small seeds. Such large and small birds thus thrive well. However, intermediate beak sized birds are unable to feed on either type of seeds so they starve and their population was decreased gradually. Natural selection eliminated them and thus the population of finches appear disrupted.

• Wings of butterfly are made up of chitin. They neither have bones, nor muscles in the wings.
• The bat’s wings are actually patagium. They have muscles and bones just as those seen in all vertebrate limb series.
• Therefore, these two examples cannot be homologous. However, both the animals use the wings for flight.
• This is an indication that their function is similar but structure is different, hence they are analogous organs.

• Adaptive radiation is an evolutionary process that results in the change of original species into numerous new variants.
• Darwin's Finches are a well-known example of adaptive radiation. When Charles Darwin visited the Galapagos Islands, he discovered finches, which are a type of little bird.
• According to Darwin's findings, the American main land finch species was the original one that must have moved to the Galapagos islands.
• Since environmental conditions here were different, they adapted in various ways to the differing environmental conditions of these islands.
• Original bird had a beak suited for eating seeds, but the changed feeding pattern has changed the shape of beaks too. Some birds also show altered beaks for insectivorous mode. Thus, this demonstrated adaptive radiation.
• Adaptive radiation in Australian Marsupials is also Well studied. In Australia, there are many marsupial mammals who evolved from common ancestor.
• Adaptive radiation leads to divergent evolution.

• Industrial melanism is the best example of natural selection which was studied by Kettlewell. In U.K. there are two varieties of peppered moths, Biston betularia and Biston carbonaria.
• Before industrialisation, in Great Britain, Biston betularia were more in number than Biston carbonaria. B. Betularia is greyish white while B.carb0naria is melanic form.
• These nocturnal moths rest on tree trunk during day. White-winged moth can camouflage well with the lichen covered whitish barks of trees. They thus escaped the attention of the predatory birds. But at the same time melanic forms were visible due to white barks of the trees. Their number was thus reduced as they were preyed upon by birds.
• Later there was an industrial revolution, which ultimately resulted in air pollution causing dark soot to settle on the barks of the trees. Lichens too were destroyed and the melanic forms were now at advantage. Melanie forms could camouflage with black tree trunks and their number increased. White-winged moth become clearly seen in changed colours of the trees and thus they were easily caught by predatory birds. This caused decrease in their number.
• Natural selection thus acted in changed environmental conditions and helped in the establishment of a phenotypic traits. The changed traits were more adaptive aunt hence were selected. Natural selection encourages those genes or traits that assure highest degree of adaptive efficiency between population and its environment.

• Urey and Miller performed an experiment to prove Oparin’s theory of chemical evolution.

• They selected a spark discharge apparatus that consisted of closed system of glass having tungsten electrodes, flask for water boiling, a side tube connected to a vacuum pump, a cooling jacket and U-shaped trap.
• The entire apparatus was first evacuated and made sterile and pre-biotic atmosphere was created in it.
• The flask was filled with some water and mixture of methane, ammonia and hydrogen in the ratio of 1 : 2 : 2 were slowly passed through the stopcock, without allowing air.

• Heat was supplied to the flask at very low temperature causing water to boil. The flask simulated the ocean present on primitive earth. Process of evaporation and precipitation was simulated by using heating mantle and condenser respectively.
• Water vapours along with other gases were circulated continuously through continuous electric sparks. These sparks were given to the mixture for several days causing the gases to interact. This too simulated lightning.
• Mixture of CH₄, NH₃ and H₂ gases passed through a condenser and was condensed to liquid.
• The liquefied mixture was collected in the U-shaped trap, present at the bottom of the apparatus. It was found that variety of simple organic compounds (urea, amino acids, lactic acid and sugars) were formed in the apparatus.

This experiment provides the evidence in support to the fact that simple molecules present in the earth's early atmosphere combined to form the organic building blocks of life.

• Isolation refers to the division of a species' population into smaller units. Some barrier prevents organisms belonging to these subunits from interbreeding. These are known as isolating mechanisms.
• They obstruct genetic interchange and gene flow.
• Because of isolating mechanisms in nature, organisms diverge gradually, leading to speciation.

The isolating mechanisms are of two types namely, geographical isolation and reproductive isolation.

Geographical Isolation :

• Geographic isolation refers to a barrier in the form of physical distance or a geographical barrier.
• Geographic obstacles such as rivers, oceans, mountains, glaciers, and so on split the original population into two or more divisions.
• Because organisms cannot pass the barriers on their own, interbreeding between separated groups is avoided.
• The divided populations have unique gene pools and do not interbreed because they encounter diverse environmental conditions and gain new features through mutations.
• Each subgroup then develops independently, resulting in the emergence of new species.
• E.g. Darwin’s Finches, African elephant, Loxodonta and Indian elephant, Elephas.

Reproductive Isolation :

• Two populations may be occupying the same area, they may not be separated by geographical barrier, but then also they are reproductively isolated.
• Such reproductive isolation occurs due to change in genetic material, gene pool and structure of genital organs.  Such differences prevent interbreeding between population. Such isolation later leads to speciation.

Different types of reproductive isolations :

Reproductive isolation is of two types, viz, pre-zygotic and post-zygotic isolating mechanisms.

Pre-zygotic : Pre-zygotic or pre-mating isolating mechanisms do not allow individuals to mate with each other at all.

By various mechanisms the two groups remain isolated. These mechanisms are of following types :

• Habitat isolation
• Seasonal isolation
• Ethological isolation
• Mechanical isolation

Post-zygotic : In post-zygotic or post-mating isolating mechanisms, the two individuals can mate but the result of mating is not favourable. Thus the populations remain isolated without the actual genetic exchange.

Post-mating isolating mechanisms are divided into the following categories :

• Gamete mortality :In gamete mortality, there is death of gametes. Sperm transfer may take place but the egg is not fertilized due to gamete mortality.
• Zygote mortality : In zygote mortality, the zygote is formed but it fails to thrive. Though the egg is fertilized the zygote does not survive.
• Hybrid sterility : In this isolation, there is the formation of hybrid as the gametes or zygotes do not die but the hybrid formed is sterile. Sterile hybrid cannot contribute genetically to further generations.

Genetic variations : The change in gene and gene frequencies is known as genetic variation.

Genetic variations are caused by following factors :

Mutations :

• A mutation is a sudden permanent heritable alteration.
• Mutations can arise in the gene, the chromosomal structure, or in chromosome number.
• Point mutation or gene mutation refers to mutations that occur inside a single gene. This leads to the change in the phenotype of the organism, causing variations.

Genetic recombination :

• During gamete development in sexually reproducing organisms, genetic material is exchanged between non-sister chromatids of homologous chromosomes. This is known as crossing over.
• It generates new genetic combinations, resulting in variety.
• Fertilisation between opposing mating gametes results in numerous recombinations, which result in phenotypic differences. As a result, the frequencies of alleles change.

Gene flow :

• Gene flow is movement of genes into or out of a population.
• Gene movement may be in the form of migration of organism, or gametes (dispersal of pollens) or segments of DNA (transformation).
• Gene flow also alters gene frequency causing evolutionary changes.

Genetic drift :

• Any random fluctuation (alteration) in allele frequency, occurring in the natural population by pure chance, is called genetic drift.
• For example, when the size of a population is severely reduced due to natural disasters like earthquakes, floods, fires, etc. elimination of particular alleles from a population becomes possible.
• Smaller populations have greater chances for genetic drift. It results in the change in the gene frequency.
• Genetic drift is also an important factor for evolutionary change.

Chromosomal aberrations :

• The structural, morphological change in chromosome due to rearrangement of genes is called chromosomal aberrations.
• Due to changes in the gene arrangement or gene sequence variations are caused.