CLASS X BIOLOGY - Heredity and Evolution

Notes

Genetics

Genetics is the branch of biology that deals with study of heredity and variations. The term was given by Bateson in 1906.

Mendel

Gregor Johann Mendel is known as Father of genetics as he discovered the mechanism of transmission of characters from one generation to the next. He chose Garden Pea (Pisum sativum) as his experimental material.

Monohybrid cross

It is a cross between two organisms of the same species which studies the inheritance of two factors of traits of a single character. e.g. tall and dwarf Pea plant.

Dihybrid Cross

It is a cross between two organisms of same species which is made to study inheritance of two pairs of factors or traits of two characters.  

Variation:

Variations are differences present in morphological, physiological and other traits found among individuals belonging to the same family, race and species.

Significance of variation:

1. It is the source of raw material for evolution.
2. Animals are able to adapt themselves to the changing environment.
3. Organisms are better suited to face the struggle for existence.
4. Variations give the organisms an individuality of their own.
5. Without variation, there would be no science of heredity as all individuals of a race, would be identical in all aspects

Speciation

Mutation

Organisms have evolved in number of ways to protect their DNA from changes. In spite of these mechanisms, however, changes in the DNA occasionally do occur. Any change in the DNA sequence is called a mutation. Mutations can be caused by errors in replication, transcription, cell division, or by external agents. E.g. nuclear radiation can lead to mutation

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Evolution
Evolution may be defined as a gradual development of more complex species from pre-existing simpler forms.
It is an extremely slow process and has occurred over millions of years,as revealed by fossil evidences.
Evolution has thus resulted in the diversity of organisms, influenced by environmental selection

Darwin

Charles Robert Darwin (1809-1882), a naturalist, worked on the theory of evolution. He gave Theory of Natural Selection which is also known as Darwinism. The theory states that organic evolution occurs through natural selection and accumulation of inheritable variations which provide structural and functional superiority to some individuals over others in their survival and reproduction.

Natural Selection

Natural selection is the process by which species adapt to their environment. Natural selection leads to evolutionary change when individuals with certain characteristics have a greater survival or reproductive rate than other individuals in a population and pass on these inheritable genetic characteristics to their offspring.

Genetic Drift

Genetic drift refers to the change in a type of genes in a population due to a random occurrence. In other words, a random circumstance causes a certain genetic trait to become more common or rarer over time.  

Traits

A trait is a distinct variant of a phenotypic character of an organism that may be inherited, environmentally determined or somewhere in between.

 For example, eye color is a character or abstraction of an attribute, while blue, brown and hazels are traits.

Acquired traits

An acquired trait is defined as a characteristic or trait that produces a phenotype that is a result of an environmental influence. Acquired traits are not coded for in the DNA of an individual and therefore cannot be passed down to offspring during reproduction. In order for a characteristic or trait to be passed down to the next generation, it must be part of the individual's genotype

Inherited traits

A trait or character that is genetically inherited or passed down from generation to generation

Origin of Life on Earth (Chemical Evolution of Life)

 Speciation

Speciation is arising of a new species from a sub-population of a species which is geographically or reproductively isolated over a long period of time from the other population of the same species.

Evolution and Classification

Tracing Evolutionary Relationships

Homologous Organs

Analogous Organs

Vestigial Organs

Fossils

A fossil is evidence of an organism that lived long ago. These are remnants or impressions of the extinct organisms which existed on earth millions of years ago.

Fossilisation

Fossils can be of following types:

• Trace Fossils: These are indirect evidence of the dead organism. E.g. footprint, trail etc.
• Casts: In rocks, the spaces are filled with dead organism making ts its replica or cast.
• Molds: organism buried in sediment and decayed leaving an impression.
• Petrified Fossils: minerals replacing the hard part of the organism
• Frozen Fossils: Organism trapped in ice or in tree sap which hardened later.

Fossils help us understand the process of evolution in following ways:

1. These are the clues to the past, thus trace the path of evolution.
2. Help in building evolutionary relationships among the present organisms. E.g. fossil evidence like Archaeopteryx and some characteristics of present-day birds like this hoatzin suggest
   that dinosaurs might have been the ancestors of today’s birds.
3. Fossils help us in learning diversity of life and animal behaviour in past. This helped in understanding ancient environment and climate and categorizing geological time scale.