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Class 12 Biology Chapter 7 Evolution
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IV. Long Questions for 5 Marks : (Explanatory)
Q.1. How do the photosynthetic organisms evolved? Discuss.
Ans : At the beginning atmosphere was devoid of oxygen the first life- forms were probably anaerobes. They probably depended on organic raw materials in the form of some molecules found on the chemical broth making the sea water. These were most probably chemoheterotrophs or chemoautotrophs. All were anaerobes. They probably ruled the sea between 4280 to 3500 million years ago. They utilised chemical compound to derive might belonged to this group.
Consequence of variation in response to changing need led to evolution of photosynthetic organisms. The early photosynthetic organisms were anaerobic in nature. The photosynthetic anaerobes were primitive bacteria. These bacteria used to frap solar energy but did not produce oxygen because they did not make use of water. The first aerobic photosynthetic organisms probably were some cyanobacteria-like organisms evolved 3300 to 3500 million years ago. They were the earliest oxygen producing photo- autotrophs with the origin of chlorophyll molecules the diversification in organisms had started. The earth began to be green.
With the increasing accumulation of free oxygen in the environment the reducing environment gradually transformed into an oxidising one leading to further changes in life forms and consequent further evolution. As a result of this, the cell organelles like nucleus, mitochondria etc. developed. The eukaryotes emerged in ocean nearly 1,600 million years ago. Further, the aerobic organisms began to oxidise food to liberate energy. Autotrophic organisms being the only type of organisms capable of synthesising food, became the source of food for heterotrophs. The photosynthesis and respiration began to balance each other from the very early age.
Q.2. What is organic evolution? Give the evidences of organic evolution.
Ans : There are different forms of animals and plants in the nature. The development of new complex living organism both plants and animals on the surface of earth as a result of slow but continuous changes. This mechanism is called the organic evolution. The term evolution mean to hardly change from one form to another (Latin e = out + volvere = to roll).
(a) Evidences from Comparative Anatomy and Morphology: Comparative anatomy of animals provides the most extensive evidences of organic evolution. Though the structure and functions of different organs of animals are varied, the basic pattern of organisation are aim the same in a group or a class of animals or plants.
A few examples anatomical similarities are as follows :
(i) Homology : Homologous organs are structurally and ontogenetically similar due to common ancestry but may have varied functions. For example, there appears little similarity between flippers of whale, wings of bats, forelimbs of horse and the hands of human being. All the structures are dissimilar in look and the functions are also quite different. But all these structure possess almost same number of bones, muscles, nerves and blood vessels and their mode of development are also almost similar. All these structures have common origin and are built on the same basic pattern, but due to their adaptation for different functions they are superficially dissimilar. The existence of homologous organs suggests that all these vertebrate animals have inherited the basic structure from 4 common ancestral stock.
- Forelegs of Amphibia
- Flippers of Sea
- Wings of Birds
- Wings of Bat
- Forelegs of Horse
- Hands of Human
(ii) Analogy : They have structural differences but have functional (1) similarities means they are coming from the same environment.
Wings of Bat and wings of Butterfly are the common example of analogous organs.
(iii) Vestigial Organs : The most convincing morphological evidence of evolution is provide by the vestigial organs. Sometimes an organ which is well developed in one group may be present in rudimentary and functionless condition in another group. Such reduced and functionally useless organs are called vestigial organs. Vestigial organs were once functional and fully developed in the ancestral form but have lost their significance and are reduced structurally in the present form.
The presence of all these vestigial organs in different animals can only be explained by the fact that the ancestors of these animals had used these organs. Because of the change of environment or mode of life these organs became nonfunctional and so became reduced in size.
The common vestigial organs of Man are :
(a) Vermiform appendix.
(b) Coccyx.
(c) Nictitating membrane.
(d) Wisdom teeth.
(e) Body hair.
(f) Ear muscle.
(g) Abdominal muscle.
(h) Nipples of Male (Man)
(i) Clitoris of female.
(b) Evidences from Comparative Embryology : The study of the development of an organism from egg to the adult is known as embryology. Amongst the evidences of organic evolution, the embryological evidences are found to be the most conclusive evidences of evolution. It has been observed that the embryos of different groups of animals have many similarities in their development than the adults. For example, the embryonic development of all groups of vertebrates have striking resemblance during their early stage of development. All multicellular animals start their development is a single zygote. The zygote divides mitotically to form the blastula. The cleavage, blastula and gastrula are almost similar in all the groups of vertebrates.
Embryos of all groups of vertebrates possess gill pouches in their early development but; functional gills are found only in fishes and in tadpoles of amphibians. The avian and mammalian embryos possess simple tubular heart and same pattern of aortic arches that are found in fishes. The vertebrate embryos possess notochord which is present only in protochordates.
Why all these structures found only in lower chordates, develop in the embryos of reptiles, birds and mammals? It is because these animals have evolved from a common ancestor.
(c) Evidences from Palaeontological Records or Geological Evidences : Palaeontology means the study of fossils of ancient plants and animals. The direct evidence of organic evolution is provided by fossils. Fossils provide the missing links in the evolution of the present day flora and fauna.
Fossil remains are found deep under the soil. They are therefore have to be excavated carefully. Different types of plant and animal fossils have been discovered in different geological strata. Whenever any kind of fossil is discovered in any strata it becomes necessary to ascertain the geological time at which the organism was in existence. It is also necessary to ascertain its evolutionary position by linking it up wich fossils of related forms found in its lower and upper strata in the same place or in other places. It is also necessary to know whether such fossils have been found in other places and in other geological strata. The period in which the animals or plants were existed can be determined by study of fossil records.
Q.3. What is fossil? How fossils were formed?
Ans : Fossils are the remains of the past. The organic body or to parts under the sedimentary rock undergo change in which the organic molecule were replaced by inorganic molecular and therefore become solid and non decomposable. They are to form impression upon sedimentary rocks. They show evidence of evolution.
Igneous’ rocks formed the original earth crust upon which the layers of soil that sustained life gradually formed. Due to physical and chemical action the igneous rock got warm out. Torrential rain of those days brought down the soil, organic matter and the worn out rocks to sea and other low areas. The debris settled down layer upon layer. The lower layers gradually hardened due to pressure from above layers. These lower layers gradually formed the sedimentary rocks and became fossils. Along with the debris various kinds of organisms also got hurried under the sedimentary rocks. The process of fossilization depends upon environmental condition, physical, chemical and biological action.
Those parts of the plant or animal bodies which were less immune to microbial decomposition became fossils. Those plant or animal parts which were quickly buried and oxygen was not available for decomposition, rapid infiltration of minerals took place which replaced the organic fractions converting the plant or animal parts into fossils. Fossils are highly compressed because of heavy pressure of sedimentary rocks that formed above. Such fossils are without any trace of organic fraction in them and became completely petrified. Other forms of fossils such as cast, impression etc. are also found. Coal, petroleum, limestone, graphite etc. are also some kinds of fossils.
Q.4. Discuss the theory of Natural selection of Darwinism.
Ans : Darwinism : Darwin suggested that the animals always adopt themselves to their environment and to ensure the continuity of their race, they reproduce their own kinds. Those who fail to adopt themselves to their environment perish in due course and the fittest survive ruling over the land. Thus there always prevails some sort of selection in nature and this is known as Natural Selection. He published his monumental work on the “Origin of Species by Natural Selection’ in the book Origin Species.
Basis of Darwinism :
(a) Over production : It is true that, organisms tend to overproduce. If all the offsprings of any species remain alive and reproduce then these will overcrowded the earth within a few years. But it is evident that in nature such enormous increase in number is never occurred. The number of offspring is maintained more or less constant under natural conditions.
(b) Struggle for existence : Because of overproduction of offsprings the struggle for existence occurs among the organisms as food and space remain almost constant.
The struggle for existence may be :
(i) Interspecific : It is the competition among the organisms of different species living together.
(ii) Intraspecific : It is the competition among the individuals of a species.
(iii) Environmental struggle : This is the struggle against the environmental conditions such as drought, flood, etc.
(c) Survival of the fittest : As a result of everlasting competition among organisms, the stronger win and survive, while the weaker are rooted out. Since the unfitted and weak are rooted out, the number of organisms on the earth remains almost constant.
(d) Variation and heredity : The everlasting competition among the (p) organisms has compelled them to change according to conditions so that they can utilize the natural resources and can survive successfully. Therefore, it is difficult to find out two individuals alike. A change of survival value is transmitted to the next generation so that the progeny is still more suited.
(e) Natural selection : After observing the above mentioned fact. Darwin summarised his conclusion under the heading ‘Natural Selection’. He said that due to inheritance, the successive generations tend to become better adapted to their environment. These adaptations are preserved and accumulated and establish new species.As natural selection continues the latter descendants after few generations become markedly distinct from their ancestors. Due to different environmental conditions, from a single ancestral species two or more species may arise.
T.H. Huxley, Herbert Spencer, D.S Jordan and Asa Gray also proposed the same theory.
Q.5. Give the Darwin’s concept of common ancestry and adaptive radiation.
Ans : Darwin’s theory of evolution is based on the fact that all the different types of species of plants and animals found today are the descendants of common ancestors. Gradual variation among them occured due to changes in organism-environment relationship in both time and space. The organism- environment relationship is different for different populations at different places and time. If a homogeneous population living in a particular environment migrates to a new habitat the organism- environment relationship may change. Depending upon the organism-environmental relationship the nature may select out those combination of genes which are better adapted under the given circumstances. In long course of time a few members of the population may show distinct adaptation which is marked by variation of characters.
The environment selects out the better gene combination rom the existing gene pool of the population. The newly adapted individual if become reproductively isolated from its parent population, it establishes itself as a new species and breeds among the newly adapted individual. Such a population which is different, even marginally, from its parent population may migrate to new habitat. If the migrated species overcome competition and adapt to a new geographic area and evolve through further specification, the evolutionary line radiating from the initial population to new geographic area is said to have undergone adaptive radiation.
The adaptive radiation from initial stock first gave rise to races or varieties which had attended the level of species through further adaptation and reproductive isolation. The following two examples will illustrate the concept.
Darwin’s finches are examples of adaptive radiation from a common ancestor. Galapagos islands are situated on the equator some 900 km. west of Ecuador. These islands were originated form volcanic eruption which were later occupied by plants and animals. A few plants and birds originally entered into the islands and later being separated from the parent stock evolved through selection by nature. Darwin had noticed that there were 13 different species of finches Fish were similar to the species in the mainland. The original species migrated to the new lands which provided greatly relaxed selection pressure because of the absence of predators and competitors.
The relative isolation of the islànds permitted the establishment of unique hind races or subspecies. From these had arisen the 13 different species. These species had given rise to more subspecies. But the fact that in the mainland there was only one species. So the 13 species had a common ancestor on the mainland. The marine species of giant iguana wizard similarly arose from terrestrial species to avoid competition on land. The finches also to avoid competition developed different food habit and therefore they occupied different ecological places. Their beaks had evolved to suit different kinds of food.
The homology of forelimbs of mammals illustrates common ancestry. The forelimbs of bat, hale, monkey, pig, horse, mole, ant eater conform to the basic pentadactyl pattern. They were probably originated from common ancestor and evolved in different lines depending upon the need in different ecological niches.
Q.6. Discuss the Hardy Weirnberg’s principle. When the equilibriums get disturbed? Explain briefly.
Ans : Hardy weinberg principle (1908) defined genetic structure of a non- evolving population. It explains that under certain conditions of stability population size is very large, random mating occurs and no allele has selective value then allelic frequencies remain constant from generation to generation in sexually reproducing animals or organisms. If other factors remain constant the frequency of particular genes and alleles will remain constant in a population through generations. Changing gene frequencies would indicate that evolution is taking place. Therefore evolution occurs when the Hardy-weinberg equilibrium or genetic equilibrium is upset. In other words evolution is a departure from Hardy-Weinberg equilibrium.
The dominant alleles do not, infact, replace recessive ones because their proportions do not change, The genotypes are said to be in Hardy-Weinberg equilibrium.
The Hardy-Weinberg principle in algebraic terms is written as an equation. Suppose a population of 100 cats contains 84 black and 16 white cats. In this case, the respective frequencies would be 0.84 (or 84%) and 0.16 (or 16%). The white homozygous (BB) or Heterozygous (Bb) and the recessive character (bb).
Let the letter p designate the frequency of one allele and the latter of the frequency of the alternative allele, Because there are only two alleles, p and q must always equal to 1.
The equation can be expressed as :
(p+q)² = p² + 2pq + q²
(Individuals of BB) (Individuals of Bb) (Individuals of bb)
Sperm Ovum
P = 0.6 (B) P = 0.6 (B)
q = 0.4 (b) q = 0.4 (b)
p² = BB = 0.36 (white)
pq = Bb = 0.24 (white)
+ pq = Bb = 0.24 (white)
+ q² = bb = 0.16 (black)
| Phenotype | White cat | White cat | Black Cat |
| Genotype | BB | Bb | bb |
| Frequency | P² + | Pq + pq | + q² |
| of genotype | 0.36 | 0.48 | 0.16 |
| in population | – | – | – |
| Frequency of | 0.36 + 0.24 | 0.24+ | 0.16 |
| gametes | = 0.6B white | – | = 0.4(b) black |
Q.7. Explain with the help of suitable examples that ‘Anthropogenic actions have caused evolution of species’
Ans : In evolution different group of animals are formed from the common ancestor having different ecological, ethological and reproductive character. The living form have homology and then they develop the analogy. This is also termed as macro evolution which divides a group to many new groups. The newly formed groups has the relationship with ancestral one. For example, the Darwin’s finches, which were found the galapagos island had the similarity although the lived in different places having different characters, establishes the anthropogenic action which causes evolution to the species.
Q.8. What is artificial selection? Is artificial selection can affects the process of natural selection? If it is explain how.
Ans : The process of selecting genetically improved and domesticated plants and animals by man, is called artificial selection. It is also called as 8. man made selection.
Artificial selection is made by man which may affect the natural selection process.
(i) Artificial selection is done by man with desired characters only which may or may not be selected by the nature.
(ii) The selected breeds in Artificial selection are interbreed while the organisms naturally selected may not interbreed.
(iii) Artificial is made by man and can repeated several times.
For example, to obtain the comes with high milk yield, animal breeders takes only those calves which are produced by high yielding varieties of the cows. These calves are interbreed to produce the next generation of cows when the process of artificial selection is repeated for a number of generation, a good breed of high milk yielding cames is produced.
Q.9. ‘Mammals and birds have evolved from reptiles, In what way does a comparative study of their embryology establish the validity of this statement?
Ans : If we study the embryological evidences in the reptiles, birds and mammals, are come to conclusion that birds and mammals are evolved from the reptiles.
(a) The early developmental phases of embryo in mammals and birds have the similarities with reptiles where fertilization always internal.
(b) Reptilian, Birds and Mammalian embryo follows the same cleavage, morula, Blastula and gastrula and finally producing three layered structure.
(c) They have similar notochord, tail, eye and ear rudiment.
(d) In the embryonic stages, birds and mammals all closely related to The reptiles.
(e) The development heart of reptilia which is three chambered leads to avian heart (four chambered) and mammalian (four chambered).
(f) Birds are called the glorified reptiles because of the presence of reptilian characters.
(g) Archaeopteryx establishes the relationship between the reptiles and birds while the duck bill platypus establishes the relationship between the reptiles and mammals.
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