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

Origin And Evolution Of Life Class 12 Exercise Question Answers Solutions Maharashtra Board

Class 12 Biology Chapter 5 Exercise Solutions Maharashtra Board

Biology Class 12 Chapter 5 Exercise Solutions

1. Multiple Choice Questions

Question 1.Who proposed that the first form of life could have come from pre-existing non-living organic molecules?
(a) Alfred Wallace
(b) Oparin and Haldane
(c) Charles Darwin
(d) Louis Pasteur
Answer: (b) Oparin and Haldane
In simple words: Oparin and Haldane independently proposed that early Earth conditions, with non-living organic molecules, could have led to the spontaneous formation of the first life forms.

🎯 Exam Tip: Remember Oparin and Haldane's hypothesis is central to understanding the chemical evolution theory for the origin of life.

Question 2.The sequence of origin of life may be
(a) Organic materials - inorganic materials - Eobiont - colloidal aggregates - cell.
(b) Inorganic materials - organic materials - colloidal aggregates - Eobiont - cell.
(c) Organic materials - inorganic materials - colloidal aggregates - cell.
(d) Inorganic materials - organic materials - Eobiont - colloidal aggregates - cell.
Answer: (b) Inorganic materials - organic materials- colloidal aggregates - Eobiont - cell.
In simple words: Life's origin is theorized to start from basic inorganic elements, forming complex organic molecules, then aggregates like coacervates (colloidal aggregates), primitive life forms (Eobiont), and finally true cells.

🎯 Exam Tip: This sequence outlines the key stages of chemical evolution leading to the first primitive cells, a fundamental concept in the origin of life studies.

Question 3.In Hardy-Weinberg equation, the frequency of homozygous recessive individual is represented by-
(a) \( p^2 \)
(b) pq
(c) \( q^2 \)
(d) 2pq
Answer: (c) \( q^2 \)
In simple words: In the Hardy-Weinberg principle, \( q \) represents the frequency of the recessive allele, so \( q^2 \) denotes the frequency of individuals possessing two copies of this recessive allele (homozygous recessive).

🎯 Exam Tip: Understanding the terms \( p^2 \), \( 2pq \), and \( q^2 \) and their representation of genotype frequencies is crucial for Hardy-Weinberg calculations.

Question 4.Select the analogous organs.
(a) Forelimbs of whale and bat
(b) Flippers of dolphins and penguin
(c) Thorn and tendrils of bougainvillea and Cucurbita
(d) Vertebrate hearts or brains
Answer: (b) Flippers of dolphins and penguin
In simple words: Analogous organs are structures that have similar functions but different evolutionary origins and underlying structures, like the flippers of dolphins (mammal) and penguins (bird) for swimming.

🎯 Exam Tip: Distinguish carefully between homologous (similar structure, different function, common ancestor) and analogous (different structure, similar function, convergent evolution) organs.

Question 5.Archaeopteryx is known as missing link because it is a fossil and share characters of both
(a) Fishes and amphibians
(b) Annelida and Arthropoda
(c) Reptiles and birds
(d) Chordates and non-chordates
Answer: (c) Reptiles and birds
In simple words: Archaeopteryx is considered a transitional fossil because its anatomical features exhibit characteristics of both reptiles (like teeth and a long bony tail) and birds (like feathers and wings), indicating an evolutionary link between these two groups.

🎯 Exam Tip: Missing links like Archaeopteryx provide strong fossil evidence for evolutionary transitions between major taxonomic groups.

Question 6.Identify the wrong statement regarding evolution.
(a) Darwin's variations are small and directional.
(b) Mutations are random and non- directional.
(c) Adaptive radiations leads to divergent evolution.
(d) Mutations are non-random and directional.
Answer: (d) Mutations are non-random and directional
In simple words: The incorrect statement is that mutations are non-random and directional; in reality, mutations are random changes in genetic material and do not occur in a specific direction towards adaptation.

🎯 Exam Tip: A key principle of modern evolutionary theory is that mutations are random, providing the raw material for natural selection to act upon, rather than being directed towards a specific outcome.

Question 7.Gene frequency in a population remain constant due to ....................
(a) Mutation
(b) Migration
(c) Random mating
(d) Non-random mating
Answer: (c) Random mating
In simple words: Random mating ensures that all individuals have an equal chance of reproducing and passing on their genes, thus maintaining the existing gene frequencies in a population under ideal conditions of no other evolutionary forces.

🎯 Exam Tip: Random mating is one of the five conditions required for a population to be in Hardy-Weinberg equilibrium, where gene frequencies remain constant from generation to generation.

Question 8.Which of the following characteristic is not - shown by the ape?
(a) Prognathous face
(b) Tail is present
(c) Chin is absent
(d) Forelimbs are longer than hind limbs
Answer: (b) Tail is present
In simple words: Apes, unlike monkeys, are characterized by the absence of a tail, which is a distinguishing feature in their evolutionary lineage.

🎯 Exam Tip: Knowing the key distinguishing features of apes (like absence of a tail) is important for understanding primate evolution and human ancestry.

Question 9..................... can be considered as connecting link between ape and man.
(a) Australopithecus
(b) Homo habilis
(c) Homo erectus
(d) Neanderthal man
Answer: (a) Australopithecus
In simple words: Australopithecus is widely considered a crucial connecting link in human evolution, displaying characteristics of both apes (smaller brain size) and early humans (bipedal locomotion).

🎯 Exam Tip: Australopithecus plays a pivotal role in the study of hominin evolution, bridging the gap between earlier ape-like ancestors and later Homo species.

Question 10.The Cranial capacity of Neanderthal man was ....................
(a) 600 cc
(b) 940 cc
(c) 1400 cc
(d) 1600 cc
Answer: (c) 1400 cc
In simple words: Neanderthal man had a significantly large cranial capacity, averaging around 1400 cc, which is comparable to or even slightly larger than that of modern humans.

🎯 Exam Tip: Cranial capacity is a key indicator used to track brain size evolution in hominins; remember that Neanderthals had large brains, challenging some earlier assumptions about their intelligence.

2. Very Short Answer Questions

Question 1.Define
(i) Gene pool
Answer:The sum total of genes of all individuals of interbreeding population or Mendelian population is called gene pool.
(ii) Gene frequency
Answer:The proportion of an allele in the gene pool as compared with other alleles at the same locus is termed as gene frequency.
(iii) Organic evolution
Answer: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.
(iv) Population
Answer:All individuals of the same species form a group which is called a population.
(v) Speciation
Answer:Formation of new species from the pre-existing single group of organisms is called speciation.
In simple words: (i) A gene pool is the complete set of all genes and alleles in a population. (ii) Gene frequency is the relative occurrence of a specific allele in that gene pool. (iii) Organic evolution refers to the gradual, continuous, and irreversible changes in life forms over time from simple ancestors. (iv) A population is a group of individuals of the same species living in a specific area. (v) Speciation is the evolutionary process leading to the formation of new distinct species from existing ones.

🎯 Exam Tip: Clearly defining these fundamental terms is crucial for a strong understanding of genetics and evolutionary biology concepts.

Question 2.What is adaptive radiation?
Answer:The process of evolution which results in transformation of original species to many different varieties is called adaptive radiation.
In simple words: Adaptive radiation is an evolutionary process where an ancestral species diversifies rapidly into many new forms, each adapting to a different ecological niche.

🎯 Exam Tip: Adaptive radiation is a key mechanism for increasing biodiversity, often occurring after major environmental changes or colonization of new habitats.

Question 3.If the variation occurs in population by chance alone and not by natural selection and bring change in frequencies of an allele, what is it called?
Answer: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.
In simple words: When allele frequencies in a population change purely due to random events, rather than selective pressures, this phenomenon is known as genetic drift.

🎯 Exam Tip: Genetic drift has a more pronounced effect on small populations, where random fluctuations can lead to significant changes in gene frequencies, including the loss or fixation of alleles.

Question 4.State the Hardy-Weinberg equilibrium law.
Answer:The Hardy-Weinberg equilibrium law states that at equilibrium point both the allelic frequency and genotypic frequency remain constant from generation to generation, in the diploid, sexually reproducing, large, free interbreeding population in which mating is random and there is absence of any other factors that change the allele frequency.
In simple words: The Hardy-Weinberg law states that in a large, randomly mating population with no mutation, migration, or natural selection, both allele and genotype frequencies will remain constant across generations.

🎯 Exam Tip: The Hardy-Weinberg principle serves as a null hypothesis in population genetics, allowing scientists to detect when evolutionary forces are at play in a real population.

Question 5.What is homologous organs?
Answer:Homologous organs are those organs, which are structurally similar but perform different functions.
In simple words: Homologous organs are body parts in different species that share a similar fundamental structure due to a common ancestry, even if they have evolved to perform different functions.

🎯 Exam Tip: Homologous structures provide strong evidence for divergent evolution, illustrating how species can evolve different adaptations from a common structural blueprint.

Question 6.What is vestigial organ?
Answer: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.
In simple words: Vestigial organs are reduced and often non-functional structures in an organism that were fully functional in its ancestors or related species.

🎯 Exam Tip: Vestigial structures are important evidence of evolution, demonstrating anatomical remnants of an organism's evolutionary past.

Question 7.What is the scientific name of modern man?
Answer:Homo sapiens sapiens is the scientific name of modern man.
In simple words: The binomial nomenclature for modern humans is Homo sapiens sapiens, designating our specific subspecies within the Homo genus.

🎯 Exam Tip: Remembering the correct scientific classification, including the subspecies designation, is important in evolutionary biology and anthropology.

Question 8.What is coacervate?
Answer:Coacervates are colloidal aggregations of hydrophobic proteins and lipids which grew in size by taking up material from surrounding aqueous medium.
In simple words: Coacervates are spherical aggregates of macromolecules, like proteins and lipids, that form spontaneously in aqueous solutions, considered a primitive step towards cell-like structures.

🎯 Exam Tip: Coacervates are important in the Oparin-Haldane hypothesis as they represent a model for how early protocells might have formed, encapsulating molecules and creating an internal environment.

Question 9.Which period is known as 'age of Reptilia'?
Answer:Jurassic period from Mesozoic era is known as age of Reptilia.
In simple words: The Jurassic period, part of the Mesozoic Era, is famously called the 'age of Reptilia' due to the widespread dominance and diversification of dinosaurs and other reptiles during this time.

🎯 Exam Tip: Associating geological periods with the dominant life forms helps to organize and recall key events in Earth's evolutionary history.

Question 10.Name the ancestor of human which is described as man with ape brain.
Answer:Australopithecus, the ancestor of human which is described as man with ape brain.
In simple words: Australopithecus is the hominin ancestor referred to as "man with ape brain" because it possessed a relatively small brain size similar to apes, yet exhibited human-like bipedal walking.

🎯 Exam Tip: This description highlights the mosaic nature of Australopithecus, showing a mix of primitive and derived traits crucial for understanding early human evolution.

Short Answer Questions

Question 1.Genetic drift.
Answer:1. Genetic drift is random, directionless fluctuation that takes place in allele frequency.
2. It occurs by pure chance, in small sized population.
3. Genetic drift becomes an evolutional factor as it can change the gene frequency.
4. Sewall Wright has given this concept and hence it is also known as Sewall Wright effect.
5. Due to genetic drift, some alleles of a population are lost or reduced by chance and some others may be increased.
6. Some time, a few individuals become isolated from the large population and they produce new population in new geographical area.
7. 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.
In simple words: Genetic drift is the random change in allele frequencies in a population due to chance events, particularly significant in small populations, leading to the loss or increase of certain alleles. This process, also known as the Sewall Wright effect or founder effect, can lead to the fixation of non-adaptive traits.

🎯 Exam Tip: Emphasize the randomness and the greater impact of genetic drift on small populations, differentiating it from natural selection which is directional.

Question 2.Enlist the different factors that are responsible for changing gene frequency.
Answer: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.
In simple words: Gene frequency changes are driven by several evolutionary forces including gene flow (migration), random genetic drift, new gene mutations, structural chromosomal aberrations (like deletions or translocations), genetic recombination, natural selection, and isolation of populations.

🎯 Exam Tip: Listing these factors accurately demonstrates a comprehensive understanding of the mechanisms that drive evolution and deviate populations from Hardy-Weinberg equilibrium.

Question 3.Draw a graph to show that natural selection leads to disruptive change.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र 'विघटनकारी चयन' (Disruptive selection) को दर्शाता है, जो प्राकृतिक चयन का एक प्रकार है। ग्राफ में 'व्यक्तियों की संख्या' (Number of Individuals) Y-अक्ष पर और 'समय' (Time) X-अक्ष पर है। यह मूल जनसंख्या के वितरण को दिखाता है, जिसके बाद चयन के कारण दो अलग-अलग पीक वाली जनसंख्या बनती है, जो मध्यवर्ती लक्षणों के खिलाफ चयन को इंगित करती है।
In simple words: Disruptive selection is a type of natural selection where extreme phenotypes are favored over intermediate ones, leading to a population with two distinct phenotypic groups. This process can eventually lead to the formation of new species.

🎯 Exam Tip: For diagrams, focus on clearly labeling axes and understanding how the shape of the curve changes to represent the selective pressure, in this case, favoring two extremes.

Question 4.Significance of fossils
Answer:1. Fossils are studied under palaeontology. They are used in reconstruction of phylogeny.
2. Fossil study helps in studying various forms and structures of extinct animals.
3. By understanding the structure of fossil, record of missing link between two groups of organisms can be deduced.
4. By studying fossils various body forms and their evolution can be understood. They also help to understand the habit and habitat.
5. Some fossils provide the evolutionary evidences such a connecting links.
In simple words: Fossils are vital records of past life, enabling palaeontologists to reconstruct evolutionary lineages (phylogeny), study extinct organisms, identify transitional forms ("missing links"), and understand ancient environments and biological changes over geological time.

🎯 Exam Tip: Highlight the role of fossils as direct evidence of evolution, emphasizing their use in understanding past biodiversity, environmental conditions, and evolutionary relationships.

Question 5.Write the objections to Mutation theory of Hugo de Vries.
Answer:Objections to Mutation Theory:
1. Hugo de Vries observed the large and discontinuous variation. But these were chromosomal aberrations. Only gene mutations usually bring about minor changes.
2. Rate by which mutations take place is very slow as compared to the requirement of evolution.
3. Chromosomal aberrations are very unstable.
4. The organisms with chromosomal aberration are usually sterile and thus chromosomal aberrations have little significance in evolution.
In simple words: Criticisms of de Vries' Mutation Theory include that his observed large variations were often chromosomal aberrations rather than typical gene mutations, that the rate of mutation is usually too slow to drive rapid evolution, and that chromosomal aberrations often lead to sterility, limiting their evolutionary impact.

🎯 Exam Tip: While de Vries correctly identified mutations as sources of variation, understanding the limitations and specific types of mutations he observed is key to appreciating the development of modern evolutionary synthesis.

Question 6.What is disruptive selection? Give example.
Answer:Disruptive selection:
1. The natural selection that disrupts the mean characters of the population, is called disruptive selection.
2. 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.
3. Extreme phenotypes are selected in evolutionary process and intermediate forms are eliminated.
4. When distribution curve is plotted it shows two peaks for two extremes.
5. Disruptive selection is rare because, nature always tries to balance the characters.
6. 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.
In simple words: Disruptive selection is a mode of natural selection that favors individuals at both ends of the phenotypic spectrum over intermediate phenotypes, leading to a bimodal distribution. An example is the African seed cracker finches, where birds with either very large or very small beaks thrive due to the availability of only large or small seeds, while intermediate-beaked birds struggle to find food and are selected against.

🎯 Exam Tip: Clearly explain how disruptive selection operates by favoring extremes and how this can eventually lead to speciation if sustained, using the finch example effectively.

4. Match The Columns

Question 1.

🎯 Exam Tip: Familiarity with the key scientists and their theories is essential for understanding the historical development of evolutionary thought.

5. Long Answer Questions

Question 1.Would you consider wings of butterfly and bat as homologous or analogous and why?
Answer: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.
In simple words: The wings of a butterfly and a bat are analogous organs. While both structures serve the same function of flight, they have distinct structural origins: butterfly wings are chitinous membranes without bones, whereas bat wings are modified vertebrate forelimbs with bones and muscles, indicating different evolutionary paths despite similar utility.

🎯 Exam Tip: When comparing structures, always analyze both function and underlying anatomical organization to correctly classify them as homologous or analogous, thereby inferring evolutionary relationships.

Question 2.What is adaptive radiation? Explain with suitable example.
Answer:1. Adaptive radiation is the process of evolution which results in transformation of original species to many different varieties.
2. The well-known example of adaptive radiation is Darwin's Finches. When Charles Darwin went on his voyage to Galapagos islands, he noticed finches which is a variety of small birds.
3. According to Darwin's observations, the American main land species of finches was the original one which must have migrated to the different islands of Galapagos.
4. Since environmental conditions here were different, they adapted in various ways to the differing environmental conditions of these islands.
5. 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.
6. Adaptive radiation in Australian Marsupials is also well studied. In Australia, there are many marsupial mammals who evolved from common ancestor.
7. Adaptive radiation leads to divergent evolution.
In simple words: Adaptive radiation is an evolutionary process where a single ancestral species rapidly diversifies into many new species, each adapted to fill different ecological niches. A classic example is Darwin's Finches on the Galapagos Islands, where a single South American finch ancestor colonized the islands and evolved into distinct species with varied beak shapes, each suited to different food sources, illustrating divergent evolution.

🎯 Exam Tip: Use Darwin's finches as the primary example, as it clearly illustrates the core principles of adaptive radiation: diversification from a common ancestor, adaptation to different environments/niches, and leading to divergent evolution.

Question 3.By talking industrial melanism as one example, explain the concept of natural selection. OR Explain natural selection in action by quoting the example of industrial melanism.
Answer:1. 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.
2. Before industrialisation, in Great Britain, Biston betularia were more in number than Biston carbonaria. B. Betularia is greyish white while B.carbonaria is melanic form.
3. 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.
4. 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. Melanic 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.
5. Natural selection thus acted in changed environmental conditions and helped in the establishment of a phenotypic traits. The changed traits were more adaptive and hence were selected. Natural selection encourages those genes or traits that assure highest degree of adaptive efficiency between population and its environment.
In simple words: Industrial melanism in peppered moths (Biston betularia) exemplifies natural selection. Before industrialization, light-colored moths were camouflaged against lichen-covered trees, while dark moths were visible and preyed upon. Post-industrialization, soot darkened tree trunks and killed lichens; now, dark moths were camouflaged, and light moths became visible, leading to an increase in dark moth population and a decrease in light moths, as predators preferentially consumed the less camouflaged forms.

🎯 Exam Tip: This example perfectly illustrates how environmental changes can alter selective pressures, demonstrating natural selection's role in shifting allele frequencies and driving evolutionary change within a population.

Question 4.Describe the Urey and Miller's experiment.
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र यूरे और मिलर के प्रयोग के उपकरण को दर्शाता है, जो प्रारंभिक पृथ्वी की परिस्थितियों को अनुकरण करके कार्बनिक अणुओं के निर्माण को प्रदर्शित करता है। इसमें एक बंद प्रणाली है जहाँ पानी को उबाला जाता है, वाष्प को मीथेन (\( \text{CH}_4 \)), अमोनिया (\( \text{NH}_3 \)) और हाइड्रोजन (\( \text{H}_2 \)) जैसी गैसों के साथ मिलाया जाता है, और इलेक्ट्रोड द्वारा स्पार्क्स (बिजली) उत्पन्न किए जाते हैं। फिर मिश्रण को संघनित (condense) करके जैविक यौगिकों वाले तरल को एक U-आकार के जाल में एकत्र किया जाता है।
1. Urey and Miller performed an experiment to prove Oparin's theory of chemical evolution.
2. 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.
3. The entire apparatus was first evacuated and made sterile and pre-biotic atmosphere was created in it.
4. 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.
5. 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.
6. 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.
7. Mixture of \( \text{CH}_4 \), \( \text{NH}_3 \) and \( \text{H}_2 \) gases passed through a condenser and was condensed to liquid.
8. 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.
In simple words: Urey and Miller's experiment simulated early Earth conditions by circulating water, methane, ammonia, and hydrogen gases through a closed system, applying electric sparks to mimic lightning, and then cooling the mixture. This process successfully generated various organic compounds like amino acids, providing experimental evidence for the chemical origin of life from inorganic precursors.

🎯 Exam Tip: Remember the components of the apparatus, the gases used (and their ratio), the energy sources (heat and electric sparks), and the significant outcome of amino acid formation, as this experiment is foundational to understanding abiogenesis.

Question 5.What is Isolation? Describe the different types of reproductive isolations.
Answer:1. Isolation means separation of the population of a particular species into smaller units. The organisms belonging to these subunits are prevented from interbreeding due to some barrier. These barriers are called isolating mechanisms.
2. They prevent the genetic exchange and gene flow.
3. Due to isolating mechanisms in nature the divergence among organisms takes place gradually leading to speciation. The isolating mechanisms are of two types namely, geographical isolation and reproductive isolation.
I. Geographical Isolation - The barrier in the form of physical distance or geographical barrier is called geographical isolation. The original population gets divided into two or more groups by geographical barriers such as river, ocean, mountain, glacier, etc. Organisms cannot cross the barriers on their own and hence interbreeding is prevented between isolated groups. The separated groups experience different environmental factors and they acquire new traits by mutations. The separated populations develop distinct gene pool and they do not interbreed. Each subgroup then evolves differently which results into formation of new species. E.g. Darwin's Finches, African elephant, Loxodonta and Indian elephant, Elephas.
II. 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.
III. Different types of reproductive isolations - Reproductive isolation is of two types, viz. pre-zygotic and post-zygotic isolating mechanisms.
1. Pre-zygotic or pre-mating isolating mechanisms do not allow individuals to mate with each other at all.
2. By various mechanisms the two groups remain isolated.
3. In post-zygotic or post-mating isolating mechanisms, the two individuals can mate but the result of mating is not favourable.
4. Thus the populations remain isolated without the actual genetic exchange.
In simple words: Isolation is the separation of populations by barriers that prevent interbreeding, leading to distinct evolutionary paths and ultimately speciation by hindering gene flow. It comprises geographical isolation, where physical barriers like mountains separate populations, and reproductive isolation, which prevents interbreeding even if populations are sympatric, through pre-zygotic (mating prevention) or post-zygotic (unfavorable offspring) mechanisms.

🎯 Exam Tip: Clearly differentiate between geographical and reproductive isolation, and further between pre-zygotic and post-zygotic mechanisms, providing examples for each type of barrier to interbreeding.

Question 6.What is Genetic variations? Explain the different factors responsible for genetic variations.
Answer:Genetic variations - The change in gene and gene frequencies is known as genetic variation. Genetic variations are caused by following factors:
(i) Mutations - Sudden permanent heritable change is called mutation. Mutation can occur in the gene, in the chromosome structure and in chromosome number. Mutation that occurs within the single gene is called point mutation or gene mutation. This leads to the change in the phenotype of the organism, causing variations.
(ii) Genetic recombination - In sexually reproducing organisms, during gamete formation, exchange of genetic material occurs between non-sister chromatids of homologous chromosomes. This is called crossing over. It produces new genetic combinations which result in variation. Fertilization between opposite mating gametes leads to various recombinations resulting into the phenotypic variations. These result in change in the frequencies of alleles.
(iii) 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.
(iv) 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.
(v) 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.
In simple words: Genetic variation refers to the differences in genes and gene frequencies within a population, which are primarily caused by mutations (sudden heritable changes in DNA), genetic recombination during sexual reproduction (like crossing over), gene flow (movement of genes between populations), genetic drift (random changes in allele frequencies), and chromosomal aberrations (structural changes in chromosomes).

🎯 Exam Tip: For each factor, briefly explain how it introduces or alters genetic diversity, emphasizing their collective role as the raw material for evolution.

Complete The Chart

Question 1. Complete the chart:

🎯 Exam Tip: Memorizing the dominant life forms for each geological era is crucial for questions on evolutionary timelines. Pay attention to the correct spelling of each era and animal group.