Evolution
Evolution: -The sequence of gradual changes which takes place in the primitive organisms over millions of years and new species are produced. Since the evolution is of living organisms, so it also called Organic Evolution.
Sources of Variations:
1. Errors in DNA copying
2. Combination of two set of traits during sexual reproduction.
An Illustration of Evolution
12 red beetles live in a green leafy bush and grows by sexual reproduction and generate variations.
Situation – 1
-Crows eats these beetles.
-Due to variation during reproduction, one green beetles evolved, which later produced green progeny.
-Crows cannot see green colored beetles over green leaves and hence can not eat them so population of green beetles has increased.
-Green colored beetles were given a survival advantages.
Situation - 2
-This time due to variations during reproduction, one blue beetle evolved, which later increased its population.
-Crows can see both red and blue beetles and therefore, eaten both.
-One day an elephant stamps on the bushes and kill most of the beetles. By chance, surviving all beetles were blue.
-Thus, the blue beetle population expands slowly.
-There is no survival advantages on this variation but it provide variation without adaptation.
Situation – 3
-Once the bushes suffer from a disease and amount of leaf available for beetles have reduced.
-So, the beetles were poorly nourished and their weight has decreased.
-After few years the plant overcame from the diseases and provided enough food for the beetles.
-So beetle came back to its original size and weight.
-This change did not inherit over the generation.
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Since genes control traits, we can say that the frequency of certain genes in a population changed over generations. This is the essence of the idea of evolution.
In the first situation the variation became common because it gave a survival advantage. In other words, it was naturally selected.
In the second situation, the color change gave no survival advantage. Instead, it was simply a matter of accidental survival of beetles So, accidents in small populations can change the frequency of some genes in a population, This is the notion of genetic drift, which provides variation without any adaptations.
In the third situationno genetic change occurred due to crisis of food So, somatic changes or acquired traits cannot be a reason of evolution.
Natural Selection (Survival of Fittest): According to Charles Darwin “In the struggle of existence, the organisms having the fittest variations can only survive and can propagate their characteristics to the next generation. And the organisms having unfavorable variations will perish.”
Genetic Drift: - Change in the gene frequency of any small population, which does not provide any survival advantage.
Acquired Traits: -The traits which are acquired by any organisms during its life time are acquired traits. These traits are non-inherited as these are changes in the non-reproductive tissues of the organisms.
1. Therefore, the experiences of an individual during its lifetime cannot be passed on to its progeny, and cannot direct evolution.
2.. A scientist named Weismann conducted an experiment on mice by cutting their tail for 21 generation. But in every generation, he found mouse with a normal length of tail.
Acquired and Inherited Traits
Acquired Traits / Inherited Traits1.These are the traits which are developed in an individual due to special environmental conditions
2.They cannot be transferred to the progeny
3.They cannot direct evolution eg. Low weight of starving beetles. / 1.These are the traits which are passed from one generation to the next.
2.They get transferred to the progeny.
3.They are helpful in evolution. eg. Colour of eyes and hair
Micro evolution: It is the evolution which is on a small scale. eg. change in body color of beetles.
Geneflow:It is exchange of genetic material by interbreeding between populations of same species or individuals
Speciation: - It is the process in which one or more species arise from existing species. New species which arise due to speciation do not interbred with parents. Generally, Geographical isolation leads to speciation.
Relationship of Evolution and Classification
1.Classification of organisms resembles their evolutionary history.
2.The more characteristic two species have in common the more closely they are related.
3.The more closely they are related, the more recently they have a common ancestor.
Evidences of evolution: -
There are a number of common features in different organisms which provide evidence to show evolutionary relationship. The main evidences of evolution are from the study of: -
Homologous organs, Analogous organs and Fossils
Homologous Organs These are the organs of different organisms which are similar in structure but different in functions.
E.g.: - The fore limbs of amphibians, reptiles, birds, and mammals have similar structures but different functions.
•Frog (amphibian) uses its fore limb to raise the front of the body. Lizard (reptile) uses its fore limb for walking and running. Birds fore limbs are modified as wings for flying. Mammals use the fore limbs for grasping, walking, running, swimming, flying etc. This shows evolutionary relationship that all vertebrates have evolved from a common ancestor.
Homology in Plants: -Leaves of pitcher plant modified to pitcher
Leaves of cactus are modified to spines
Leaves of peas are modified to tendrils.
Analogous Organs These are the organs of different organisms which are different in structures but similar in functions.
Eg :- The wings of butterfly, bird and bat have different structures but similar functions. Wings of bats are skin folds stretched between elongated fingers but wings of birds are covered by feathers all along the arm.
This shows that different organism can develop similar body structure in the similar environment.
Analogy in Plant -
•Sweet potato (Modified Root) and Potato (modified Stem)
•Tendril of pea(leaf) and tendril of Cucurbitacae(stem).
• Tendril of pea(leaf) Tendril of Pumpkin (stem)
FossilsThey are the preserved traces or remains of living organisms of geological past.
-When organisms die, their dead bodies decompose and get lost.
-But some part of the body dies not decompose.
-Ex:- dead insect will not decompose in hot mud, it will harden and retain the impression of the insect body parts.
•i)Fossil of wooly mammoth
•ii) Archaeopteryx (fossil bird)
iii) Dead insect caught in hot mud.
•Various Fossils: Depending upon their differences in appearance and preservations, they are
•1. Fossil tree trunk
•2. Fossil Invertebrate (Ammonite)
•3. Fossil Invertebrate (Trilobite)
•4. Fossil Fish (Knightia)
•5. Fossil Dinasour Skull (Rajasaurus)
•Complex organs evolved because they provided survival advantages at the past.
•Complex organs like eyes are created slowly over generation. Different organism’s have different kind of eyes which shows different evolutionary origins.
•There are some organs which used to perform some function but later adapted to do some other function like feathers were evolved for providing warmth and later adapted for flight.
Artificial Selection:
•It is a mode of selection in which humans actively choose which traits should be passed onto next generation.
•Farmers chose cattle with beneficial traits to breed.
•For crops farmer’s choose the best variation and allows them to breed in the next generation.
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Artificial Selection in Wild Cabbage
•It is a mode of selection in which humans actively choose which traits should be passed onto next generation.
•Farmers chose cattle with beneficial traits to breed.
•For crops farmer’s choose the best variation and allows them to breed in the next generation.
•Phylogeny: It is the evolutionary history of an organism.
•Molecular Phylogeny: It is the branch of Phylogeny that analyses DNA sequences, to gain information on organism’s evolutionary relationship.
•Ex- If we will examine DNA of a bird and cat, there will be significant difference, indicating that the two-species separated a long time ago.
•But a cat and a rat will show more similarities.
Evolution Versus Progress
•Evolution cannot be said to “progress” from lower forms to higher forms.
•So, it is not always one species is eliminated to give rise to a new one.
•And new species may not be better than previous species
•So, evolution is a process of generating diversity and shaping the diversity through environmental selection.
Human Evolution
•There is a great diversity among human beings in their form and features around the world.
•Human beings evolved in Africa.
•Some of them stayed there and others migrated to different parts of the world.
• Then due to genetic variations and the environmental changes in different geographical regions they developed changes in their forms and features.