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Evolution

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Evolution is a theory in Biology that describes the change in the characteristics of a species of plants, animals, and other living things on Earth generation by generation and this depends upon the process of natural selection. At the time of reproduction, the genes are passed from parents to offsprings. The characteristics are expressions of these genes. Different characteristics exist in the given population of species either due to genetic variation or due to mutation. Over a period of time, these characteristics tend to become more common or rare in a given population of species.

HISTORY

The scientific theory of Evolution through the method of natural selection was introduced by ‘Charles Darwin’ and ‘Alfred Russel Wallace’. This came into existence in the 19th century. The introduced theory is commonly known as Darwin’s Theory of Evolution as well. Darwin even wrote a book on the topic of Evolution that was named ‘On the Origin of Species’. According to Darwin’s observation in the theory of natural selection, the offsprings are often produced more in number but the survival rate is low. 


There are three things observed with regard to this theory:

  1. In all the species, the characteristics vary with regard to their physiology, morphology, and behavior.

  2. The rate of survival and reproduction depends upon the characteristic that has passed on.

  3. Characteristics can pass on from generation to generation.


So we can say that with each passing generation the offspring tend to survive by getting the particular trait from the parents and transform in their features and characteristics with respect to certain environmental conditions that enable them to adapt to a particular habitat. This is how evolution takes place generation by generation.


Whatever the form of creatures that are present today on Earth, have the Last Universal Common Ancestor (LUCA) that lived nearly around 3.5 to 4 billion years ago. This can be assessed by studying fossil records that include Biogenic Graphite to fossilized organisms underground or some evolution data encrypted in each passing generation that can give us an approximate characteristic detail of any living being in the history. 

The study of the existing pattern of biodiversity has been classified under 3 categories:

  • Speciation( this is the formation of new species with time )

  • Anagenesis( the changes that occur within a particular species with respect to each passing generation )

  • Extinction( loss of any particular species with time like Dinosaurs ) 


The Greek philosophers like Empedocles and Anaximander of the presocratic times also believed that there is a possibility a particular type of organism can descend from another variety. These thoughts were carried forward even to Roman times. During medieval times people followed the concepts of Aristotle who was considered as the father of western logic, however, much to the contradiction of the classical data the people believed that whatever was present in nature has something to do with divinity in-universe. Hence, this understanding was brought under Christian learning. However, the examples that were set by Aristotle that how new types of living things could come into existence was still considered.


Then came the Pre-Darwinian times when the modern approach of understanding rejected the ideologies put forward by Aristotle. They only accepted the explanation of natural phenomena with regard to physical laws. The term ‘Species’ was given by John Ray to some fixed natural types. The biological classification was then introduced by Carl Linnaeus in the year 1735. Later, Erasmus Darwin put presented the idea that all warm-blooded animals evolved from a single microorganism.

The first theory of Evolution came into existence in the 19th century( 1744-1829 ) when Jean Baptiste Lamarck proposed the theory of ‘Transmutation of species’.

ROLE OF GENES IN EVOLUTION

The process of Evolution takes place at the Genomic level. Genes are the basic unit of inheritance. Genes are composed of DNA. Any changes in the sequence of DNA will affect the composition and also affect the way Genes are expressed. Our ancestral history and the evolutionary data are encrypted in the DNA. When two different species have similar DNA and Genes, then we can predict that those species might have shared similar Ancestors. For example, humans are said to be evolved from apes. With time, the DNA undergoes changes. While some changes are beneficial for a living being and help them to adapt to the environment and habitat better, other changes might tend to become harmful especially if any defect is involved as this defect can be carried further into the passing generations. The rest of the Genes remain as it is throughout the Evolution.


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HEREDITY

Heredity depends upon two main factors which are Genotype and Phenotype.

There are certain traits that are controlled by the Genes. This is called as Genotype. For example, some facial features and eye color of the person are inherited from one of the parents. To be precise, if one of the parents has the blue color of the eye and it is inherited to the offspring then it is called the ‘blue eye trait’.


The set of characteristics of an individual with regard to the interaction between their genotype and the environment are called Phenotype. It is due to this factor, many features of the individual’s phenotype are not inherited. Phenotype is observable by nature. There is a strong combination of Genes with the environment that presents itself on the appearance and behavior of the organism. Various examples can be accounted for that, ranging from the size of the bird’s beak, stripes on the animals back, and hair color, etc. 

GENE FLOW

Gene flow is also called as migration. This is nothing but carrying the genetic material by an individual from one population to another. This migration takes place by several factors like pollens flying from one place to another, mating between humans of different races and ethnicity will yield to offsprings carrying traits of both parents. For example, an individual of India shifts to the USA, and mates with an individual of the American race will yield to children who have characteristics of both parents. Gene transfer between two species will also lead to the formation of Hybrid organisms. 


Gene flow between different species take place in two ways;

  1. Vertical gene transfer: The gene flow between two populations of the same species is done through migration and carried ahead through reproduction, thus Vertical gene transfer from parents to offsprings take place.

  2. Horizontal gene transfer: This is the gene transfer between two different species. The genes can transfer from lower organisms to higher organisms or the gene transfer takes place from Endosymbiont to the host. 

The genetic variation of the population depends upon the Gene flow. Gene flow can be restricted within specific species by putting barriers between the population separating them thus maintaining the unique identity of the particular population.


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NATURAL SELECTION

The theory of Natural Selection proposed by Darwin has four major components:

  • Inheritance: few traits are indefinitely passed on from parent to offsprings whereas few traits have the influence of the environment and Habitat.

  • Variation: Even within a population each individual exhibits different variations like color, body size, features, etc though they belong to the same species.

  • High population growth rate: Few species have more offsprings each year than the number of resources present for them. This leads to a struggle for resources between the population.

  • Differential survival and reproduction: Those organisms that have the trait which can survive the struggle for resources will lead to reproduction and thus will contribute more offsprings to the future generation.

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FAQs on Evolution

1. What is the scientific definition of evolution in Biology?

Evolution in Biology refers to the gradual change in the characteristics of living organisms over generations, through processes like natural selection, mutation, gene flow, and genetic drift. These changes result in the diversity of species observed today.

2. How does natural selection drive the process of evolution?

Natural selection acts on the variation within a population, allowing individuals with traits that are better adapted to their environment to survive and reproduce. Over time, these advantageous traits become more common in the population, driving evolution.

3. Which types of evolution are described in the CBSE Class 12 syllabus?

The main types of evolution are:

  • Convergent evolution: Different species evolve similar traits independently.
  • Co-evolution: Two species evolve in response to each other.
  • Adaptive radiation: Small groups rapidly evolve into new species to exploit new habitats.

4. What evidence supports the theory of evolution in the CBSE curriculum?

Evidence for evolution includes:

  • Fossil records showing changes in species over time
  • Comparative anatomy highlighting homologous and analogous structures
  • Molecular evidence such as DNA similarities between species
  • Biogeography explaining distribution of species

5. Why is the study of heredity important in the context of evolution?

Heredity determines how genetic traits are passed from parents to offspring. Understanding heredity helps explain how genetic variations arise and are inherited, which is essential for the process of evolution and adaptation within populations.

6. Describe the differences between genotype and phenotype with examples.

The genotype is the genetic makeup (DNA sequence) of an organism, while the phenotype is the observable characteristics resulting from the interaction between genotype and environment. For example, a plant's genotype may carry genes for tall height, but nutrient availability (environment) affects whether it actually grows tall (phenotype).

7. What role do mutations play in evolution as per CBSE guidelines?

Mutations are changes in the DNA sequence. They introduce new genetic variations in a population. Some mutations can be beneficial, aiding adaptation and survival, while others can be neutral or harmful. Beneficial mutations may spread in a population through natural selection, leading to evolution.

8. Explain gene flow and its significance in evolution.

Gene flow is the movement of genetic material between populations, for example through migration or reproduction. It increases genetic diversity and can introduce new traits to a population, influencing evolutionary change and sometimes preventing speciation by mixing gene pools.

9. How has the modern theory of evolution improved our understanding compared to Darwin's original theory?

The modern theory, or modern synthesis, combines Darwin's ideas of natural selection with the principles of genetics. It explains evolution as changes in allele frequencies in populations, incorporating mutation, gene flow, and genetic drift alongside natural selection.

10. What are the main misconceptions about evolution addressed in NCERT Class 12?

Common misconceptions include:

  • Evolution is a linear or goal-directed process (it is not; evolution is branching and non-directional).
  • Individuals evolve during their lifetime (in reality, populations evolve over generations).
  • All traits are adaptive (some may be neutral or vestigial).

11. How does speciation occur, and why is it vital to evolution?

Speciation occurs when populations of the same species become isolated and evolve independently due to genetic drift, natural selection, or barriers to gene flow. Over time, these changes can result in the formation of new species, increasing biological diversity.

12. In what ways do human activities influence evolution in other species?

Human activities, such as selective breeding, habitat destruction, pollution, and introduction of invasive species, can alter the natural selection pressures on other organisms. This can lead to rapid evolutionary changes, such as antibiotic resistance in bacteria or pesticide resistance in pests.

13. Why is the concept of a last universal common ancestor (LUCA) important in evolutionary biology?

The LUCA represents the shared origin from which all present-day life descended. Studying genetic and molecular evidence helps trace evolutionary relationships among diverse species, showing the unity of life.

14. What is the difference between vertical and horizontal gene transfer?

Vertical gene transfer refers to genetic material passing from parent to offspring. Horizontal gene transfer is the movement of genes between unrelated species, often observed in microorganisms. Both processes contribute to genetic variation and evolution.

15. How can understanding evolution help in addressing health challenges such as genetic disorders or developing new medicines?

Understanding evolution assists in tracing the origins of genetic disorders, predicting how pathogens may evolve resistance, and informing the design of new drugs or treatments. Evolutionary principles guide strategies in combating diseases and improving public health.