Monday, April 30, 2012

5. Evolution

Evolution is an orderly change from one form to another.
Evolutionary Biology: Study of history of life forms.
Big Bang Theory states that universe originated about 20 billion years ago, by a thermonuclear explosion (big bang) of a dense entity.
The earth was formed about 4.5 billion years ago.
There was no atmosphere on early earth. Water vapour, CH4, CO2 & NH3 released from molten mass covered the surface.
H2O           →  H2 + O2
NH3 + O2   →   water
CH4 + O2   →    CO2
Then the ozone layer was formed. As it cooled, the water vapour fell as rain to form oceans.
Theories of origin of life
1.    Theory of spontaneous generation (Abiogenesis): It states that, life came out of decaying and rotting matter like straw, mud etc.
Louis Pasteur demonstrated that life comes only from pre-existing life and disproved this theory.
He showed that in pre-sterilized flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose.
2.    Biogenesis: Proposed by Francisco Redi, Spallanzani & Louis Pasteur. It states that, life originates from pre-existing life.
3.    Cosmic theory (Theory of Panspermia): It states that, the units of life (spores) were transferred to different planets including earth.
4.    Theory of special creation: It states that, living & non-living was created by some supernatural power (God).
5.    Theory of chemical evolution: Proposed by Oparin & Haldane. It states that, the first form of life was originated from non-living inorganic & organic molecules such as CH4, NH3, H2O, sugars, proteins, nucleic acids etc. i.e. “Abiogenesis first, but biogenesis ever since”.
Urey-Miller experiment
Harold Urey & Stanley Miller conducted an experiment to prove theory of chemical evolution. They created a condition similar to that of primitive earth (i.e. high temperature, volcanic storms, reducing atmosphere containing CH4, NH3, H2O, H2 etc).
They made electric discharge in a closed flask containing CH4, NH3, H2 and water vapour at 800o C. As a result, some amino acids are formed.
In similar experiments, others observed formation of sugars, nitrogen bases, pigment and fats.
First non-cellular form of life originated 3 billion years ago.
They were RNA, proteins, Polysaccharides etc.
1. Paleontological evidences
Paleontology: The study of fossils.
Fossils: These are remnants of life forms found in rocks (earth crust). Fossils are written documents of evolution.
Significance of fossils:
a.    To study phylogeny (evolutionary history or race history). E.g. Horse evolution.
b.   To study the connecting link between two groups of organisms. E.g. Archaeopteryx.
c.    To study about extinct animals. E.g. Dinosaurs
d.   To study about geological period by analyzing fossils in different sedimentary rock layers. The study showed that life forms varied over time and certain life forms are restricted to certain geological time spans.
2. Morphological and Anatomical evidences
Comparative anatomy and morphology shows that different forms of animals have some common structural features. This can be explained as follows:
a.    Homologous organs and Homology
-    Homologous organs are the organs having fundamental similarity in structure and origin but different functions. This phenomenon is called Homology.
-    E.g. Human hand, Whale’s flippers, Bat’s wing, and Cheetah’s foot. All these perform different functions, but are constructed on the same plan.
-    Homology can be seen in skeleton (e.g. humerus, radius, ulna, carpals, metacarpals & phalanges), heart, blood vessels, excretory system, brain etc.
-    Homology in plants: E.g. Thorns of Bougainvillea and tendrils of Cucurbita.
-    The origin of homologous organs is due to Divergent evolution (It is the process by which related species become less similar in order to survive and adapt in different environmental condition).
-    Homology indicates common ancestry.
b.   Analogous organs and Analogy
Analogous organs are the organs having similar function but different structure and origin. This phenomenon is called Analogy. E.g.
§ Wings of insects (formed of a thin flap of chitin) and wings of birds (modified forelimbs).
§ Eyes of Octopus (retina from skin) and mammals (retina from embryonic brain).
§ Flipper of Penguins and Dolphins.
§ Sweet potato (modified root) & Potato (modified stem).
§ Trachea of insects (from ectoderm) and lungs of vertebrates (from endoderm).
Origin of analogous organs is due to Convergent evolution (It is the process by which unrelated species become more similar in order to survive and adapt in similar environmental condition).
3. Adaptive radiation (Biogeographical evidences)
Adaptive radiation (evolution by adaptation) is the evolution of closely related species in a given geographical area starting from a point. E.g. 
o  Darwin’s finches (seen in Galapagos Islands).
o  Australian marsupials.
o  Placental mammals in Australia.
Placental mammals: Mole, Anteater, Mouse, Lemur, Flying squirrel, Bobcat, Wolf.
When more than one adaptive radiation is appeared in an isolated geographical area, this leads to convergent evolution.
E.g. Australian Marsupials and Placental mammals.
Placental mammals
Australian Marsupials
Marsupial mole
Ant eater
Numbat (Ant eater)
Marsupial mouse
Spotted cuscus
Flying squirrel
Flying phalanger
Tasmanian tiger cat
Tasmanian wolf
4. Biochemical evidences
-       Similarities in proteins and genes
-       Similarities in other biomolecules and metabolism.
5. Evidences for evolution by natural selection
Natural selection is the process by which the organisms that are best suited for their environment survive and reproduce. Some evidences are given below.
Industrial melanism (In England)
§ Before industrialization (1850s): There were more white winged moths (Biston betularia) on trees than dark winged or melanised moths (Biston carbonaria).
Reason: There was white coloured lichen covered the trees. In that background the white winged moths survived but the dark coloured moths were picked out by predators.
§ After industrialization (1920): More dark winged moths and less white winged moths.
Reason: The tree trunks became dark due to industrial smoke and soot. No growth of lichens. Under this condition the white winged moth did not survive because the predators identified them easily. Dark winged moth survived because of suitable dark background.
o Excess use of herbicides, pesticides, antibiotics or drugs etc resulted in selection of resistant varieties (Natural selection by anthropogenic action).
1.   Lamarckism (Theory of Inheritance of
Acquired characters)
Proposed by Lamarck. It states that evolution of life forms occurred by use and disuse of organs. E.g.
o Evolution by use of organs: Long neck of giraffe is due to continuous elongation to forage leaves on tall trees. This acquired character was inherited to succeeding generations.
o Evolution by disuse: Disappearance of limbs in snakes.
This theory was eliminated out because it is proved that the characters are inherited only through genes.
2.   Darwinism (Theory of Natural selection)
Proposed by Charles Darwin. It is based on 2 key concepts
·     Branching descent
·      Natural selection
Branching descent: It explains that all organisms are modified descendants of previous life forms.
Natural selection: Consider a bacterial colony (say A) growing on a given medium. If the medium composition is changed, only a part of the population (say B) can survive under new condition. This variant population outgrows the others and appears as new species, i.e. B is better than A under new condition. Nature selects for fitness.
Work of Thomas Malthus on populations influenced Darwin.
Natural selection is based on the following facts:
·      Heritable minor variations
·      Overproduction by organisms  
·      Limited natural resources
·      Struggle for existence
·      Survival of the fittest
Population size grows exponentially if everybody reproduces maximally (E.g. bacterial population). In fact, population size is limited due to competition for resources (Struggle for existence). Only some survives (Survival of the fittest).
Darwin said that the organisms with heritable variations make resource utilization better. They reproduce and leave more progeny. It leads to a change in population characteristics and new forms appear.
Darwin ignored about origin of variation and mechanism of speciation.
Hugo de Vries proposed Mutation Theory of evolution. He conducted some experiments on Oenothera lamarckiana (evening primrose) and believed that evolution takes place through mutation and not by minor variation.
Differences between Darwinian variation & mutation
Darwinian variation
Minor variation
Large variation
Slow and directional
Random, sudden & directionless
Gradual evolution
Speciation by saltation (single step, large mutation)
Hardy- Weinberg Principle
-    It says that allele frequencies in a population are stable and constant from generation to generation.
-    The gene pool (total genes and their alleles in a population) remains a constant. This is called genetic equilibrium (Hardy-Weinberg equilibrium).
-    Sum total of all the allelic frequencies = 1
-    E.g. In a diploid, p and q are the frequencies of alleles A & a respectively.
The frequency of AA = p2 (i.e. the probability of an allele A with frequency p is the product of the probabilities, i.e. p2)
The frequency of aa = q2
The frequency of Aa = 2pq
Hence p2 + 2pq + q2 = 1   [binomial expansion of (p+q)2]
Change of frequency of alleles in a population causes disturbance in genetic equilibrium. This is due to evolution.
Factors affecting Hardy-Weinberg equilibrium
a.    Gene migration: Gene flow from one population to another. Here gene frequencies change in both populations. There would be a gene flow if migration happens multiple times.
b.   Genetic drift: The accidental gene flow causing change in frequency. Sometimes, the change in frequency is so different in the new sample of population that they become a different species. The original drifted population becomes founders and the effect is called founder effect.
c.    Mutation: Mutations result in formation of new phenotypes. Over few generations, this leads to speciation.
d.   Genetic recombination: It is the reshuffling of gene combinations during crossing over resulting in genetic variation.
e.    Natural selection:  3 types.
        i.  Stabilizing selection: Here, more individuals acquire    mean character value and variation is reduced.
         ii.   Directional selection: Here, individuals of one extreme are more favoured.
       iii.      Disruptive selection:
} Proterozoic era: 2000 million years ago (mya):
   First cellular forms of life.
   Some of the cells had the ability to release O2 as the light reaction in photosynthesis.
   Single celled organisms → Multicellular organisms
} Paleozoic era:
   500 mya: Invertebrates.
   400-600 mya: First land organisms (plants).
   400 mya: Arthropods invaded the land.
   350 mya: Jawless fish. Fish with stout and strong fins could move on land and go back to water.
   320 mya: Sea weeds and few plants.
   Amphibians to reptiles. They lay thick-shelled eggs which do not dry up in sun unlike those of amphibians.
   In the next 200 million years reptiles dominated on earth. Giant ferns (Pteridophytes) were present but they all fell to form coal deposits slowly.
} Mesozoic era:
   200 mya: Some of the land reptiles went back into water to evolve into fish-like reptiles (E.g. Ichthyosaurs).
   The land reptiles were dinosaurs. They include
} Tyrannosaurus rex: Largest dinosaur (20 feet in height, huge fearsome dagger-like teeth).
}  Pteranodon
}  Brachiosaurus
} Coenozoic era:
   65 mya: Dinosaurs suddenly disappeared
   First mammals (shrew-like). Their fossils are small sized.
   In South America, there were mammals resembling horse, hippopotamus, bear, rabbit etc. Due to continental drift, when South America joined North America, these animals were overridden by North American fauna.
   Due to continental drift, Australian marsupials survived because of lack of competition from any other mammals.
·   Dryopithecus & Ramapithecus (15 mya):
Hairy. Walked like gorillas & chimpanzee.
Dryopithecus: ape-like.
Ramapithecus: man-like.
Fossils of man-like bones found in Ethiopia & Tanzania.
·   Man-like primates (3-4 mya): Height up to 4 feet.
·   Australo-pithecus (2 mya):
In East African grass lands.
Hunted with stone weapons. Ate fruits.
Homo habilis:
First human-like being (hominid).
Brain capacity: 650-800 cc. Did not eat meat.
·   Homo erectus (1.5 mya):
Large brain (900 cc). Ate meat.
·   Neanderthal man (40,000- 1 lakh yrs ago):
Brain 1400 cc.
Lived in East & Central Asia.
Used hides to protect their body.
Buried their dead.
·   Homo sapiens (Modern man): 10,000 to 75,000 yrs ago.  
   Pre-historic cave art developed about 18,000 years ago. Agriculture & settlements: 10,000 years ago.

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