Heredity and Evolution Class 10 Science Notes Maharashtra State Board
Heredity and Hereditary Changes
You know that heredity is the transfer of biological characteristics from one generation to another via genes. Johann Gregor Mendel is a pioneer of modern genetics. Around the period of 1886, it took a long time for him to understand the conclusions of his research about heredity. In 1901, the reasons behind the sudden changes were understood due to the mutational theory of Hugo de Vries. Meanwhile, in 1902, Walter and Sutton observed the paired chromosomes in the cells of grasshoppers; until then it was not known to anyone. Research started in the direction of finding the nature of genetic material when it was proved that genes are carried via chromosomes. In 1944, a trio of scientists Ostwald Avery, Mclyn McCarthy, and Colin MacLeod proved that except for viruses, all living organisms have DNA as genetic material.
In 1961, the French geneticists Francois Jacob and Jack Monad proposed a model for the process of protein synthesis with the help of DNA in bacterial cells. It helped to uncover the genetic codes hidden in DNA. Thereby, recombinant DNA technology emerged which has vast scope in the field of genetic engineering. The science of heredity is useful for the diagnosis, treatment, and prevention of hereditary disorders, the production of hybrid varieties of animals and plants, and industrial processes in which microbes are used.
Transcription, Translation and Translocation
With the help of RNA, the genes present in the form of DNA participate in the functioning of cells and thereby control the structure and functioning of the body. Information about protein synthesis is stored in the DNA and synthesis of appropriate proteins as per requirement is necessary for the body. These proteins are synthesized by DNA through the RNA. This is called ‘Central Dogma’. mRNA is produced as per the sequence of nucleotides in DNA. Only one of the two strands of DNA is used in this process. The sequence of nucleotides in mRNA being produced is always complementary to the DNA strand used for synthesis. Besides, there is uracil in RNA instead of thymine of DNA. This process of RNA synthesis is called ‘transcription’.
The mRNA formed in the nucleus comes in the cytoplasm. It brings in the coded message from DNA. The message contains the codes for amino acids. The code for each amino acid consists of three nucleotides. It is called a ‘triplet codon’.
Dr Har Govind Khorana, a scientist of Indian origin has made an important contribution to the discovery of triplet codons for 20 amino acids. For this work, he was awarded the Nobel Prize in 1968, along with two other scientists. Each mRNA is made up of thousands of triplet codons. As per the message on mRNA, amino acids are supplied by the tRNA. For this purpose, tRNA has an ‘anticodon’ having a complementary sequence to the codon on mRNA. This is called as ‘translation’.
The amino acids brought in by tRNA are bonded together by peptide bonds with the help of rRNA. During this process, the ribosome keeps moving from one end of mRNA to another by the distance of one triplet codon. This is called ‘translocation’. Such many chains come together to form complex proteins. These proteins control various functions in the body of living organisms and their appearance too.
Living organisms can produce new individuals like themselves due to genes only and some of those genes are transmitted to the next generation without any changes. Due to this, some of the characteristics of parents are transmitted to their offspring. However, sometimes sudden changes occur in those genes. Sometimes, any nucleotide of the gene changes its position which causes a minor change which is nothing but the ‘mutation’. Some mutations may be minor but some may be considerable. Ex. The mutation may cause genetic disorders like sickle cell anemia. This is an everlasting process and it is one of the proofs of Darwin’s theory of natural selection.
Evolution
Evolution is the gradual change occurring in living organisms over a long duration. This is a very slow-going process through which the development of organisms is achieved. All the stages in changes that occurred in various components ranging from stars and planets in space to the biosphere present on the Earth should be included in the study of evolution. The formation of new species due to changes in specific characteristics of several generations of living organisms as a response to natural selection is called evolution. 3.5 billion years ago, life had been non-existent on the Earth. In the beginning, there may have been only simple elements in the ocean on the Earth and simple types of organic and inorganic compounds may have been formed from those. Complex compounds like proteins and nucleic acids may have formed over a long period from those simple compounds. The first primitive type of cells may have been formed from a mixture of different types of organic and inorganic compounds.
A number of those cells may have increased at the cost of surrounding chemicals. There may have been some differences among those cells and according to the principle of natural selection, some may have shown good growth and some may have perished and could not adjust to the surroundings. At present, crores of species of plants and animals with huge diversity regarding shape and complexity are present on the Earth. Animal diversity ranges from the unicellular Amoeba and Paramoecium to man and giant whales. The plant diversity consists of various species ranging from unicellular Chlorella to the huge banyan tree. Life exists on Earth everywhere from the equator to both the poles. Organisms are present in all the places like air, water, land, rock, etc. Humans have shown curiosity about the origin of life and the reasons for such a great diversity in life present on Earth since the ancient period. Different theories about the origin and evolution of life have been proposed till today of which the theory of ‘Gradual development of living organisms is accepted.
Theory of Evolution:
According to this theory, the first living material (protoplasm) has been formed in the ocean. In due course of time, the unicellular organism was formed. Gradually, changes occurred in the unicellular organisms from which larger and more complex organisms were formed. All those changes were slow and gradual. The duration of all these changes is at must 300 crore years. Changes and development in living organisms had been all round and multi-dimensional and this led to the evolution of different types of organisms. Hence, this overall process is called organizational evolution. The progressive development of plants and animals from the ancestors having different structural and functional organization is called evolution.
Evidences of Evolution
Collective thinking on all the above-mentioned theories implies that evolution is the everlasting process of change. However, it needs proof to prove it. Following are various proofs available in support of the theories mentioned above.
1. Morphological Evidence
Various similarities like the structure of the mouth, the position of eyes, the structure of nostrils and ear pinnae, and thickly distributed hairs on the body are seen in animals whereas similarities in characters like leaf shape, leaf venation, leaf petiole, etc. occur in the case of plants. This indicates that there are some similarities in those groups and hence it proves that their origin must be the same and must have common ancestors.
2. Anatomical Evidence
If you carefully observe the pictures, there doesn’t seem any superficial similarity between the human hand, the cat’s foreleg, the flipper of the whale, and the patagium of the bat. Similarly, the use of each of those structures is different in respective animals. However, there is a similarity in the structure of bones and bony joints in the organs of each of those animals. This similarity indicates that those animals may have a common ancestor.
3. Vestigial Organs
Degenerated or underdeveloped useless organs of organisms are called vestigial organs. In living organisms, sudden development of new tissues or organs for living in a changing environment is not possible. Instead, existing organs undergo gradual changes. Mostly, a specific structure in the body is useful in certain situations. However, the same structure in different situations may become useless or even harmful. Such structure begins to degenerate under such a situation as per the principle of natural selection. It takes thousands of years for a structure to disappear. Such organs are seen in different phases of disappearance in different animals. Such organs, though non-functional in certain organisms, may be functional in other organisms i.e. it is not vestigial in other organisms.
Appendix, which is useless to humans, is a useful and fully functional organ in ruminants. Similarly, muscles of the ear pinna, which are useless to humans, are useful in monkeys for movement of the ear pinna. Various vestigial organs like the tailbone (coccyx), wisdom teeth, and body hairs are present in the body of a human being.
4. Paleontological Evidence
A question may arise in your mind which organisms existed millions of years ago? How can we tell this? Now this secret has been hidden in the Earth. Many 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 fossils is an important aspect of the study of evolution.
Carbon consumption of animals and plants stops after death and since then, only the decaying process of C-14 has occurred continuously. In the case of dead bodies of plants and animals, instead of remaining constant, the ratio between C-14 and C-12 changes continuously as C-12 is non-radioactive. The time passed since the death of a plant or animal can be calculated by measuring the radioactivity of C-14 and the ratio of C-14 to C-12 present in their body. This is the ‘carbon dating’ method. It is used in paleontology and anthropology to determine the age of human fossils and manuscripts. Once the age of a fossil has been determined by such a technique, it becomes easy to deduce information about other erstwhile organisms. It seems that vertebrates have been slowly originated from invertebrates.
The carbon dating method is based on the radioactive decay of naturally occurring C-14 and it was developed by Willard Libby. He was awarded with Nobel Prize (1960) for this invention. The age of the materials determined by this method is published in the journal Radio Carbon’.
5. Connecting Links
Some plants and animals show some morphological characteristics by which they can be related to two different groups; hence they are called ‘connecting links’. Ex. In Peripatus, characters like segmented bodies, thin cuticles, and parapodia-like organs are present. Similarly, these animals show tracheal respiration and open circulatory systems similar to arthropods. This indicates that Peripatus is the connecting link between Annelida and arthropods.
Similarly, duck-billed platypus lays eggs like reptiles but shows a relationship with mammals too due to the presence of mammary glands and hairs. Lungfish perform respiration with lungs irrespective of being fish. These examples indicate that mammals evolved from reptiles and amphibians from fishes.
6. Embryological Evidence
A comparative study of embryonic developmental stages of various vertebrates given in the picture shows that all embryos show extreme similarities during initial stages and those similarities decrease gradually. Similarities in the initial stages indicate the common origin of all these animals.
Darwin’s Theory of Natural Selection
Charles Darwin collected innumerable specimens of plants and animals and depending upon the observations of those specimens; he published the theory of natural selection which preaches the survival of the fittest. For this purpose, Darwin published a book titled ‘Origin of Species’. While explaining the concept, Darwin says that all organisms reproduce prolifically. All the organisms compete with each other in a life-threatening manner. In this competition, only those organisms sustain which show the modifications essential for winning the competition.
However, besides this, natural selection also plays an important role because nature selects only those organisms that are fit to live and the rest perish. Sustaining and selected organisms can perform reproduction and thereby give rise to new species with their specific characteristics. Darwin’s theory of natural selection was widely accepted for long duration. However, some objections were raised against the theory. Some of the main objections are-
- Natural selection is not the only factor responsible for evolution.
- Darwin did not mention any explanation about useful and useless modifications.
- There is no explanation for slow changes and abrupt changes.
Irrespective of all these objections, Darwin’s work on evolution has been a milestone.
Charles Robert Darwin (1809-1882)
This English biologist proposed the theory of evolution. He showed that all the species of living organisms have gradually evolved throughout thousands of years from a common ancestor. He proposed that the principle of natural selection is responsible for this evolution.
Lamarckism
Jean-Baptiste Lamarck proposed that morphological changes occurring in living organisms are responsible for evolution and the reason behind those morphological changes is the activities or laziness of that organism. He called this concept as principle of ‘use or disuse of organs’.
Further, he said that the neck of the giraffe has become too long due to browsing on leaves of tall plants by extending their neck for several generations; similarly, the shoulders of the ironsmith have become very strong due to frequent hammering movements. Wings of birds like ostrich and emu have become weak due to no use. Legs of birds like swans and ducks have become useful for swimming due to living in water and snakes have lost their legs by modifications in their body for burrowing habit. All these examples are types of ‘acquired characters’ and are transferred from one to another generation. This is called as theory of inheritance of acquired characters or Lamarckism.
The development of organs due to specific activities or their degeneration due to no use at all was widely accepted but the transfer of those characteristics from generation to generation was rejected. It has been verified many times that modifications brought in us are not transferred to the next generation and thereby Lamarck’s theory was disproved. The living organism can transfer the characteristics which it has acquired, to the next generation. This is called ancestry of acquired characters.
Lamarck proposed that the activities of the organisms are responsible for their evolution. This French naturalist proposed that each animal or plant undergo some changes in its life span and those changes are transferred to the next generation and such changes occur in subsequent generations too.
Speciation
The formation of new species of plants and animals is the effect of evolution. Species are the group of organisms that can produce fertile individuals through natural reproduction. Each species grows in specific geographical conditions. Their food, habitat, reproductive ability, and period are different. However, genetic variation is responsible for the formation of new species from earlier ones. Besides, geographical and reproductive changes are also responsible. Similarly, geographical or reproductive isolation also leads to speciation.
Human Evolution
The biodiversity that is known today has been said to be formed from very simple unicellular organisms due to evolution. In this evolution, the origin of human evolution can be shown in the picture given below. The last dinosaurs disappeared approximately seven crore years ago. At that time, monkey-like animals are said to have evolved from some ancestors who were more or less similar to the modern lemurs. The tail of these monkey-like animals of Africa is said to have disappeared about 4 crore years ago. They developed due to enlargement in the brain their hands were also improved and thus ape-like animals were evolved. Meanwhile, these ape-like animals reached South and North-East Asia and finally evolved into gibbons and orangutans. The remaining ape-like animals stayed in Africa and from them, gorillas and chimpanzees evolved about 2.5 crore years ago. The evolution of some of the 2 crore-year-old species of apes seems to occur differently. They had to use their hands more for eating food and other work.
Those apes started to live on land as the forests started to decline due to the dry environment. Their lumbar bones developed in such a way that they started to stand in erect posture in grasslands and thereby their hands became available for use, anytime. These first human-like animals with erect postures which were using their hands evolved about 2 crore years ago. The first record of a human-like animal is with us in the form of a ‘Ramapithecus’ ape from East Africa. Afterwards, this ape grew in size and became more intelligent, and thus the ape of South Africa evolved about 40 lakh years ago. The morphology of these human-like animals started to appear like a member of the genus Homo, about 20 lakh years ago, and thus skilled humans developed. About 15 lakh years ago, human walking with an erect posture evolved. It may have existed in China and Indonesia of the Asian continent.
The evolution of upright man continued in the direction of developing its brain for the period of about 1 lakh years and meanwhile, it discovered the fire. The brain of 50 thousand-year-old man had been sufficiently evolved to the extent that it could be considered a member of the class- wise-man (Homo sapiens). Neanderthal man can be considered as the first example of a wise man. The Cro-Magnon man evolved about 50 thousand years ago and afterward, this evolution had been faster than the earlier.
About 10 thousand years ago, wise men started to practice agriculture. It started to rear the cattle herds and established the cities. Cultural development took place. The art of writing was invented about 5000 years ago and thus history has been started. Modern sciences emerged about 400 years ago and industrial society was established about 200 years ago now we have reached this stage, and still, we are searching for the details of the roots of human ancestry.
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