Throughout history, all human cultures have offered and followed their own explanations for the origin of the world and of human beings and other creatures. However, Charles Darwin is the founder of the scientifically approved and modern theory of evolution.
In 1859, he published On the Origin of Species by Means of Natural Selection, a treatise that established the theory of evolution and the role of natural selection in determining its course.
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Later on, Darwin extended his theory of natural selection to human evolution through books like The Descent of Man and Selection in Relation to Sex (1871).
Evolution in its contemporary meaning in biology typically refers to the changes in the proportions of biological types in a population over time.
Before Darwin's theorisations, people used to attribute the origin of Earth’s living things, with their exceptional capacity to adapt, to the design of an omniscient God. Darwin accepted the facts of adaptation.
However, he showed that the multiplicity of plants and animals, with their unique adaptations, could be explained by a process of natural selection, sans the handiwork of any designer agent.
Darwin's achievement had profound intellectual and cultural implications that outlasted his multi-pronged evidence which convinced contemporaries of the fact of evolution.
Molecular Biology and Evolution
Molecular biology was the most important line of investigation to be applied to evolutionary studies, after the 1950s. In fact, the laboratory techniques of DNA cloning and sequencing have provided a new and powerful means of investigating evolution at the molecular level.
Major research efforts, such as the Human Genome Project, have helped to further improve the technology for obtaining long DNA sequences rapidly and inexpensively. By the first few years of the 21st century, the full DNA sequence had been obtained for more than 20 higher organisms.
Revision Notes for Class 10 CBSE Evolution
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Students can refer to these notes for quick revision before the exam to give an edge to their preparation level and increase their final score.
These notes are specific to CBSE Class 10 Science Chapter 9 (Heredity and Evolution). The notes have been designed according to the revised CBSE syllabus. All the topics and concepts are discussed with conciseness and clarity.
The study of heredity and variation is collectively known as genetics. Heredity is defined as the transmission of characteristics from parents to offspring.
The differences in characters of parents and offspring are known as variation. There are 2 types of variation:
- Somatic variation occurs in the somatic cells of the body. These variations are not inherited or transmitted to the next generation. So, they are also known as acquired traits.
- Gametic variation occurs in the germ cells of the body. These variations are inherited by the next generation. So, they are known as inherited traits.
Importance of Variation
- It is the basis of heredity.
- It is the basis of evolution also.
- It increases the chances of the survival of the organism according to the changing environment.
Causes of Variation
The most common causes of variations are mutation, recombination, and random mating. Recombination or crossing over is one of the important reasons for variation. It is an exchange of chromosome segments at the time of gamete formation.
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Mendel: The Father of Genetics
Gregor Johann Mendel was an Austrian Monk who conducted research on pea plants to understand the concept of heredity. This research laid the groundwork for modern genetics. Mendel devised 3 basic laws of inheritance.
Laws of Mendel
- The Law of Dominance says that a gene has two contrasting alleles and one always expresses itself in the organism. It is called the dominant gene and it expresses in any possible combination.
- The Law of Segregation says that traits get segregated completely during the formation of gametes without any mixing of alleles.
- The Law of Independent Assortment says that the traits can segregate independently of different characters during gamete formation.
The traits that express themselves in an organism in every possible combination and can be seen are called dominant traits. For example, in Mendel’s experiment, the tall trait in pea plants tends to express itself more than the short trait. So, the tall trait of the plant is said to be dominant over the short trait.
A trait that is not expressed in the presence of a dominant allele is known as recessive. So, recessive character/trait is present in an organism but cannot be seen if a dominant allele exists.
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When only 1 character is considered while crossing 2 organisms, such a cross is known as a monohybrid cross. The ratio of characters, arising out of this cross, at F2 generation is called the monohybrid ratio.
When 2 characters are considered while crossing 2 organisms, such a cross is known as a dihybrid cross. The ratio of characters, arising out of this cross, at F2 generation is called the dihybrid ratio.
In the context of Biology, inheritance refers to the transfer of traits from one generation to another.
The process of determining the sex of an individual, based on the composition of the genetic material is called sex determination. The sex of an embryo is determined by different factors in different animals.
For example, in humans, sex determination happens on the basis of the presence or absence of the Y chromosome. XX is female and XY is male. An ovum always contains an X chromosome. An ovum, upon fusion with Y containing sperm, gives rise to a male child and upon fusion with X containing sperm gives rise to a girl child.
Traits are characteristic features of an organism, manifested in a physical form that is visible or in a physiological aspect of the organism.
The traits that are acquired by an organism over the period of its lifetime are called acquired characters. These characters may or may not get transferred to the next generation.
The traits that are inherited from the parents are called inherited characters. These traits always get transferred to the next generation. However, depending on the dominance or recessiveness it may or may not be expressed. Examples include height, skin colour, and eye colour.
The differences in the DNA sequences among every organism leading to the diverse gene pool are called genetic variations. These differences lead to different/varied physical characters or biochemical pathways.
The phenomenon by which a favourable trait in a population of a species is selected is called natural selection. Changing natural conditions exert equal pressure on all existing species.
The species/organisms which are better adapted to the changing conditions survive and reproduce. In other words, they are selected by nature. The species/organisms which cannot adapt perish, that is, they are rejected by nature.
It is the process of formation of a new species from existing ones due to several evolutionary forces like genetic drift, isolation of populations, natural selection, etc.
Speciation leads to diversity in the ecosystem and diversity lead to evolution.
Natural selection plays an important role in deciding the traits that survive in a population. However, random fluctuations in gene variants are seen on many occasions. This phenomenon is known as genetic drift.
Genetic drift is a change in the frequency of an existing allele in a small population. Genetic drift may cause a gene variant to disappear from the population, thereby reducing genetic variation.
Gene flow is the transfer of genes from one population to the next.
A population is a community or a group of animals, plants, or any living organism that can reproduce with each other and have fertile, viable offsprings.
Charles Darwin: The Father of Evolution
Charles Darwin was an English naturalist and biologist. A 5-year expedition in a ship called HMS Beagle to Galapagos Island helped him develop his theory of evolution.
Evolution and Fossils
Evolution is a tangible change in the heritable characteristics of a population over several generations. These changes can give rise to a new species or the species might change themselves to become better adapted to the surrounding environment.
Origin of Species
Following a successful expedition on the HMS Beagle, Charles Darwin wrote his observations down in the form of a book, The Origin of Species. The book contains a detailed theory of evolution, mostly based on natural selection.
Origin of Life: Haldane’s Theory
JBS Haldane, a British scientist, theorized that life originated from organic and lifeless matter. His theory was developed upon and proved by Urey and Miller’s experiment. It came to be known as the theory of abiogenesis.
Evolutionary Evidence: Fossils
Among the abundance of evidence to support the theory of evolution, fossils happen to be the largest body. Fossils are the preserved remains of ancient animals or plants that died millions of years ago.
Fossils help us understand the anatomy and even physiology of organisms and how evolution worked, leading to the formation of the organisms that we see today.
Formation of Fossils
Fossils are formed by the following steps:
- Organisms die and get buried in mud and silt.
- The soft tissues of the body of the organisms wither away quickly, leaving behind the hard bones or shells.
- Over time, sediments form over it and harden into rocks.
- As the bones decay, minerals seep in to replace the contents, cell by cell. This process is called petrification.
- Once the bones decay completely, they leave behind the cast of the animal. This cast is our fossil.
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Evolutionary relationships of animals can be deduced by studying the homologous organs and analogous organs.
- Homologous organs are those which have a similar structure but different function.
- For example, the wings of birds and the forelimbs of mammals have a similar structure but are modified to suit different functions.
- The tendril of a pea plant and the spine of a barberry plant are modified leaves, but perform different functions.
- Analogous organs are those which have a similar function but a different structure and origin too.
- For example, the wings of bats, birds, and insects are all used for flying but are structurally very different.
- The leaves of opuntia and peepal perform photosynthesis, but the leaves of Opuntia are modified stems, whereas peepal leaves are normal leaves.
Evolution by Stages
Evolution is a slow process that undergoes several stages. Almost every animal and plant that we see today have undergone these evolutionary stages. Complexities do not evolve overnight but are the results of evolution that occur bit by bit. The gradual evolutionary process is called evolution by stages.
A single species can, sometimes, evolve into several different species due to artificial selection. For example, the cabbage family. A single ancestor in the cabbage family gave rise to several different species due to the selection of different traits.
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The evolutionary relationship among different biological species is called phylogeny. Phylogeny gives rise to an evolutionary tree.
In molecular phylogeny these relationships are studied at the hereditary molecular level, mainly using DNA sequences. The study involves the analysis of DNA composition and gene comparison between different species.
Humans have been known to belong to the primate family. The humans of the contemporary world are genetically connected to chimps and other primates. The origins of Homo sapiens can be traced back to Africa, from where they migrated to all other parts of the world.
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While the complete picture is yet to emerge on the evolutionary process of humans from primates, a larger picture of human evolution has indeed formed. Some of our ancestors include Dryopithecus, Ramapithecus, Australopithecus, Homo erectus, Homo sapiens Neanderthalensis, Cro-magnon man, and finally us, the Homo sapiens.
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