Samacheer Class 10 Science - Origin and Evolution of Life

(i) When there is a change in the environment, the animals respond to the change.
(ii) They develop adaptive structures. The characters developed by the animals during their life time, in response to the environmental changes are called acquired characters.
(iii) Lamarck’s use and disuse theory states that if an organ is used constantly, the organ develops well and gets strengthened. When an organ is not used for a long time, it gradually degenerates.
Degenerated wing of kiwi is an example for organ of disuse
Degeneration of wings in kiwi is due to disuse of wings over generations since they have learns to walk on land for all their needs. This is an acquired character in response to their habitat. According to Lamarck such characters are passed to the off springs by inheritance.

(i) Ethnobotany is the study of a region’s plants and their practical uses through the traditional knowledge of the local culture of people^
(ii) The term Ethnobotany was coined by J. W. Harshberger in 1895 to include the study of plants used by the primitive and aboriginal people.
Importance of Ethnobotany :
(i) It provides traditional uses of plant.
(ii) It gives information about certain unknown and known useful plants.
(iii) The ethnomedicinal data will serve as a useful source of information for the chemists, pharmacologists and practitioners of herbal medicine.
(iv) Tribal communities utilize ethnomedicinal plant parts like bark, stem, roots, leaves, flower bud, flowers, fruits, seeds, oils, resins, dyes, gum for the treatment of diseases like diarrhoea, fever, headache, diabetes, jaundice, snakebites, leprosy, etc.

Evolution is the gradual change occurring in living organisms over a period of time. Formation of new species due to changes in specific characters over several generations as response to natural selection, is called evolution. Principles of Darwinism - Theory of Natural selection proposed by Charles Darwin :
Overproduction :
(i) Living beings have the ability to reproduce more individuals and form their own progeny. (Multiply in Geometrical manner.)
(ii) This will increase reproductive potential leading to over production.
Struggle for existence: Due to over production, a geometric ratio of increase in population occurs. This creates an intense competition among the organisms for food and space leading to struggle.
The struggle for existence are of three types:
(i) Intraspecific struggle: Competition among the individuals of same species.
(ii) Interspecific struggle: Competition between the organisms of different species living together.
(iii) Environmental struggle: Natural conditions like extreme heat or cold, drought and floods can affect the existence of organisms.
Variations :
(i) The occurrence of variation is a characteristic feature of all plants and animals. Small variations are important for evolution.
(ii) According to Darwin favourable variations are useful to the organism and unfavourable variations are harmful or useless to the organism.
Survival of the fittest or Natural selection:
(i) During the struggle for existence, the organisms which can overcome the challenging situation, survive and adapt to the surrounding environment. Organisms which are unable to face the challenges, are unfit to survive and disappear.
(ii) The process of selection of organisms with favourable variation is called as natural selection.
Origin of species
(i) According to Darwin, new species originates by the gradual accumulation of favourable variations for a number of generations.
Hence natural selection is a driving force for evolution since it leads to formation of new species.

Homologous organs: Organs that have the same origin from a common ancestor and similar basic structure or embryonic development, though they may have different functions. Example: human hand, forelimb of cat, flipper of whale, bat wing.
Analogous organs: Organs that perform similar functions but have different evolutionary origins and different structural organization. Example: wings of insect and wings of bird (insect wing vs bird wing).

Common modes of fossilization in plants (concise):
- Permineralization (petrifaction): Minerals (silica, calcium carbonate) infiltrate and harden in cell spaces, preserving wood and bone structure.
- Carbonization: Organic volatile elements are lost under pressure and heat, leaving a carbon film (common for leaves).
- Impression/mold and cast: Organism leaves an impression (mold) in sediment; minerals may fill it to form a cast.
- Compression: Plant parts are compressed in sediments leaving flattened fossils or impressions.
- Replacement: Original material is dissolved and replaced by minerals molecule by molecule.
- Preservation: Entire organisms/parts preserved in amber, peat, or ice (rapid exclusion of decay agents).

(i) No. Wings of a dragon fly are different from the wings of crow. In the case of a crow, wings represent modified forelimbs.
(ii) Wings of a dragon fly and crow have the same function. They serve to fly but their origin and developmental pattern are different. Such organs which look similar and perform similar function but have different origin and developmental pattern are called analogous organs.

(i) Fossils are evidence for evolution.
(ii) Fossils are the most important proof for evolution of life on earth over millions of years. They show that life on earth was once different from life found on earth today.
(iii) By Studying fossils, help scientists to gather information about plants and animals which lived in the past ages.
(iv) A plant fossil is any preserved part of a plant that has died long back.
(v) Fossils may be a prehistoric impression that may be hundred to millions of years old.
(vi) Majority of the plant fossils are disarticulated parts of plants, it is rare to find plants to be preserved as whole.
Importance of fossils :
(i) They throw light on phylogeny and evolution of plants.
(ii) Fossil plants give a historical approach to plant kingdom.
(iii) Thus even if fossils are not available as a whole, imprints or separated parts will also be able to give us lot of information about evaluation.