EVOLUTION

 

WHAT ARE EVIDENCES FOR EVOLUTION?

Paleontological evidence:

• Fossils are remained of hard parts of life-forms found in rocks.
• Different-aged rock sediments contain fossils of different life-forms who probably died during the formation of the particular sediment.
• They represent the extinct organisms (e.g. Dinosaurs).
• A study of fossils in different sedimentary layers indicates the geological period in which they existed.
• The study showed that life-forms varied over time and certain life forms are restricted to certain geological time-span.
• Hence new lives have arisen at different times in the history of earth.
• All this called Paleontological evidence.

Comparative anatomy and morphological evidence:

• Comparative anatomy and morphology shows similarities and differences among organisms of today and those that existed years ago.

Divergent evolution:

• Whale, bats, cheetah and human share similarities in the pattern of bones of forelimbs.
• These forelimbs perform different functions in these animals, they have similar anatomical structure – all of them have humerus, radius, ulna, carpals, metacarpals and phalanges in their forelimbs.
• Hence in these animals, the same structure developed along different directions due to adaptation to different needs.
• This is divergent evolution and these structures are homologous.
• Homology indicates common ancestry.
• Other examples of homologous organ are vertebrate hearts and brains.
• Thorn of Bougainvilleaand tendrils of Cucurbitarepresent homology.

Convergent evolution:

• Wings of butterfly and of birds look alike.
• They are anatomically similar structure though they perform similar function.
• Hence analogous structures are a result of convergent evolution.
• Eye of octopus and eye of mammals.
• Flippers of Penguins and Dolphins.
• Sweat potato (root modification) and potato (stem modification).

Biochemical evidences:

• Similarities in proteins and genes performing a given function among diverse organisms give clues to common ancestry.

 Embryological support for evolution:

• Proposed by Ernst Heckel based upon observation of certain features during embryonic stage common to all vertebrates that are absent in adult.
• The embryos of all vertebrates including human develop a row of vestigial gill slits just behind the head but it is a functional organ only in fish and not found in any other adult vertebrates.
• This is disproved on careful study performed by Karl Ernst von Baer. He noted that embryos never pass through the adult stages of other animals.

Evolution by natural selection:

• Based on observation of moth population in England made in 1850.
• Before industrialization set in, it was observed that there were more white-winged moths on trees than dark-winged or melanised moths.
• After industrialization i.e. 1920 there were more dark-winged moths in the same area i.e. the proportion was reversed.

Evolution by anthropogenic action:

• Excess use of herbicides, pesticides etc., has only resulted in selection of resistant varieties in a much lesser time scale.
• This is also true for microbes against which we employ antibiotics or drugs against eukaryotic organisms/cell.
• Hence resistance organisms/cells are appearing in a time scale of months or years and not in centuries.
• These are the examples of evolution by anthropogenic action.
• Evolution is a stochastic process based on chance events in nature and chance mutation in the organisms.

WHAT IS ADAPTIVE RADIATION?

Darwin’s Finches:

• In Galapagos Islands Darwin observed small black birds later called Darwin’s Finches.
• He realized that there were many varieties of finches in the same island.
• All the varieties, he came across, evolved on the island itself.
• Form the original seed-eating features, many other forms with altered beaks arose, enabling them to become insectivorous and vegetarian finches
• This process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation.

Australian marsupial:

• A number of marsupials each different from the other evolved from an ancestral stock. But all within the Australian island continent.
• When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats), one can call this convergent evolution.
• Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be ‘similar’ to a corresponding marsupial (e.g. placental wolf and Tasmanian wolf-marsupial).

BIOLOGICAL EVOLUTION:

• The essence of Darwinian Theory about evolution is natural selection.
• The rate of appearance of new forms is linked to the life cycle or the life span.
• There must be a genetic basis for getting selected and to evolve.
• Some organisms are better adapted to survive in an otherwise hostile environment.
• Adaptive ability is inherited.
• It has genetic basis.
• Fitness is the end result of the ability to adapt and get selected by nature.
• Branching descent and natural selection are the two key concepts of Darwinian Theory of Evolution.

Lamark theory of evolution: (theory of inheritance of acquired characters)

• French Naturalist Lamark had said that evolution of life forms had occurred but driven by use and disuse of organs.
• He gave the example of Giraffes who in an attempt to forage leaves on tall trees had to adapt by elongation of their necks.
• They passed on this acquired character of elongated neck to succeeding generations.
• Giraffes, slowly over the years, came to acquire long necks.

 

CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
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