Evolution Class 12 and NEET

 Evolution 

6.1 Origin of Life 

6.2 Evolution of Life Forms - A Theory 

6.3 What are the Evidences for Evolution ?

6.4 What is Adaptive Radiation ?

6.5 Biological Evolution 

6.6 Mechanism of Evolution 

6.7 Hardy - Weinberg Priciple

6.8 A Brief Account of Evolution 

6.9 Origin and Evolution of Man

Introduction( NCERT )

NCERT Line : Evolutionary Biology is the study of history of life forms on earth. What exactly is evolution? To understand the changes in flora and fauna that have occurred over millions of years on earth, we must have an understanding of the context of origin of life, i.e., evolution of earth, of stars and indeed of the universe itself.

Explanation : Evolutionary Biology ka matlab dharti par life ki history ko padhna hai. Evolution asal mein kya hai? Lakhon saalon mein plants (flora) aur animals (fauna) mein jo badlav aaye hain, unhe samajhne ke liye humein life ki shuruat, dharti, taaron aur pure universe ke evolution ko samajhna hoga.

NCERT Line : What follows is the longest of all the construed and conjectured stories. This is the story of origin of life and evolution of life forms or biodiversity on planet earth in the context of evolution of earth and against the background of evolution of universe itself.

Explanation : ab aage jo kahani bataae jaaegi vah sabse lambi or logo dyara banaae gai or anumaan ( Ideas + guesses ) par aadharit kahani hai. yah kahani jeevan ki suruaat or earth par alag - alag jeevo ( biodiversity) ke vikaas ki hai,jo earth ke vikas or pure brahmand ke vikas ke sandarbh me samjhao jaati hai.

What is Evolution ?

Evolution is the process by which organisms change over generations.

Example : pehle apes ( monkey jaise ) the, dheere - dheere changes hue, aaj humans ban gaye. Isko hi Evolution kahte hai.

Evolution = Time ke saath change + survival + new species ka banana.

or, Evolution = Variation + Natural Selection + Survival

6.1 Origin of Life ( NCERT )

NCERT Line : When we look at stars on a clear night sky we are, in a way, looking back in time. Stellar distances are measured in light years. What we see today is an object whose emitted light started its journey millions of year back and from trillions of kilometres away and reaching our eyes now.However, when we see objects in our immediate surroundings we see them instantly and hence in the present time. Therefore, when we see stars we apparently are peeping into the past.

Explanation : Jab hum clear night sky me stars ko dekhte hain, tab hum actually past dekh rahe hote hain. Aisa isliye kyunki stars bahut zyada door hote hain aur unki distance light years me measure ki jaati hai.Aaj hum jo taara dekh rahe hain, uski light ne apni journey millions of saal pehle shuru ki thi aur trillions of kilometres door se ab hamari aankhon tak pahunchi hai.
Lekin jo cheezein hamare paas hoti hain (jaise insaan, ghar, tree), unki light ko aane me bahut hi kam time lagta hai, isliye wo hume instant dikhai deti hain. Is wajah se hume lagta hai ki hum unhe present time me dekh rahe hain.

Stars ko dekhna = universe ka past samajhna

Universe ka past = elements ka origin samajhna

Elements ka origin = life ka origin samajhne ka base

👉 Stars ko dekhkar hum indirectly samajhte hain ki life ke liye zaroori cheezein kahan se aayi.

NCERT Line : The origin of life is considered a unique event in the history of universe. The universe is vast. Relatively speaking the earth itself is almost only a speck. The universe is very old – almost 20 billion years old.Huge clusters of galaxies comprise the universe. Galaxies contain stars and clouds of gas and dust. Considering the size of universe, earth is indeed a speck. The Big Bang theory attempts to explain to us the origin of universe.

Explanation : Life ki shuruat universe ki history mein ek anokhi ghatna hai. Universe bahut bada hai aur uske muqable Earth sirf ek chhota sa chhota particles (speck) hai. Universe bahut purana hai—lagbhag 20 billion saal purana.Universe badi-badi galaxies ke clusters ( bahut saare galaxies ka ek group) se bana hai. Galaxies mein taare aur dhool-mitti/gas ke baadal hote hain. Universe ke size ke hisab se earth sahi me bahut hi chhoti hai. Big Bang theory humein universe ki shuruat samjhane ki koshish karti hai.

Big Bang Theory 

The Big Bang theory attempts to explain to us the origin of universe.

[ Big Bang theory hume universe ki shuruaat ko samajhaati hai. Lagbhag 13.8 billion saal pehle universe ek bahut chhoti, bahut garam aur dense state mein tha, Achanak woh state phailne (expand hone) lagi

 Aaj bhi universe expand ho raha hai ]

Formation of Earth 

It talks of a singular huge explosion unimaginable in physical terms. The universe expanded and hence, the temperature came down. Hydrogen and Helium formed sometime later.The gases condensed under gravitation and formed the galaxies of the present day universe. In the solar system of the milky way galaxy, earth was supposed to have been formed about 4.5 billion years back.

Explanation : Ek bahut bada aur samajh se pare explosion hua tha, jise hum normal tareeke se imagine nahi kar sakte — isi ko Big Bang Theory kehte hain.Explosion ke baad universe dheere-dheere phailne laga (expand hua).

Jaise-jaise phaila, waise-waise uska temperature (garmi) kam hota gaya.Thoda time baad sabse simple gases bani — Hydrogen aur Helium.

 Ye hi universe ke sabse pehle elements the.Ye gases gravity (gurutvakarshan) ki wajah se ek jagah ikattha hone lagi.

 Dheere-dheere ye milkar galaxies (taaron ke bade samuh) ban gayi.Milky way galaxy ke solar system me earth bani, earth lagbhag 4.5 billion year pahle bani thi.

Formation of Atmosphere, Water and Ocean

There was no atmosphere on early earth. Water vapour, methane, carbondioxide and ammonia released from molten mass covered the surface. The UV rays from the sun brokeup water molecule into Hydrogen and Oxygen and the lighter H2 escaped. Oxygen combined with ammonia and methane to form water, CO2 and others. The ozone layer was formed. As it cooled, the water vapor fell as rain, to fill all the depressions and form oceans.

Explanation : pahle earth par koi atmosphere nahi tha.earth us samay bahut garam ( molten ) tha, jisse gases water vapour, methane, carbon dioxide,ammonia nikli jo milkar atmosphere banati hai. Sun ki UV rays ne water ko hydrogen or oxygen me tor diya, isme hydrogen gas halke hone ke karan space me nikal gai ( escape),or oxygen ne methane or ammonia ke saath react Kiya or water and carbon dioxide banne lage. oxygen se ozone layer bani jo harmful rays se protection deti hai. uske baad earth dheere - dheere cool hui or water vapour se clouds se rain bana or rainwater jama hokar ocean banaya. 

Note : Life appeared 500 million years after the formation of Earth or almost 4 billion years back.( Lagbhag 4 billion years pahle )

 Theories of Life 

Did life come from outerspace ? Some scientists believe that it came from outside. Early Greek thinkers thought units of life called spores were transferred to different planets including earth.' Panspermia ' is still a favourite idea for some astronomers. For a long time it was also believed that life came out of decaying and rotting matter like straw, mud etc. This was the theory of spontaneous generation.

Explanation : Kiya life outer space ( antriksha ) se aai hai ? Kuchh scientists maante hai ki life baahar se aai hai. Purane Greek thinkers ka maanna tha ki life ki units, jise spores kahte hai, Earth sahit dusre planet par transfer hui thi. ' Panspermia( space se aana )  ' aaj bhi kuchh astromets ka ek favourite idea hai. Kaafi time Tak ye bhi maana jaata tha ki life sadi - gali cheezo( jaise bhusa, kitchen ) se paida hui thi, isea theory of spontaneous generation kaha gaya.

Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life.[ Louis Pasteur ne experiments se proof kiya ki life sirf pehle se maujood life (pre-existing life) se hi aati hai.]

He showed that in pre-sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from ‘killed yeast’. [Unhone dikhaya ki sterilized flasks mein 'killed yeast' se life nahi bani, lekin open flask mein 'killed yeast' se naye organisms paida ho gaye.]

Spontaneous generation theory was dismissed once and for all.[ Iske baad Spontaneous generation theory ko hamesha ke liye khatam (dismiss) kar diya gaya.]

However, this did not answer how the first life form came on earth. [Magar, isse ye pata nahi chala ki Earth par sabse pehli life kaise aayi.]

Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (e.g. RNA, protein, etc.) [Russia ke Oparin aur England ke Haldane ne kaha ki pehli life non-living organic molecules (jaise RNA, protein) se aayi hogi.]

and that formation of life was preceded by chemical evolution, i.e., formation of diverse organic molecules from inorganic constituents. [Aur life banne se pehle 'chemical evolution' hua tha, matlab inorganic cheezon se organic molecules bane.]

The conditions on earth were – high temperature, volcanic storms, reducing atmosphere containing CH_4, NH_3, etc.[ Us waqt Earth ke conditions aise the—high temperature, volcanic storms, aur ek reducing atmosphere jisme CH_4 aur NH_3 thi.] 

In 1953, S.L. Miller, an American scientist created similar conditions in a laboratory scale. [1953 mein, S.L. Miller ne lab ke andar bilkul waisi hi conditions create ki.]

He created electric discharge in a closed flask containing CH_4, H_2, NH_3 and water vapour at 180°C. [Unhone ek band flask mein CH_4, H_2, NH_3 aur water vapour ke saath electric discharge (bijli jaisa jhatka) diya.]

He observed formation of amino acids. [Unhone dekha ki wahan amino acids ban gaye hain.]

In similar experiments others observed, formation of sugars, nitrogen bases, pigment and fats. [Dusre experiments mein logon ne sugars, nitrogen bases, pigments aur fats bante huye dekhe.]

Analysis of meteorite( space rocks) content also revealed similar compounds indicating that similar processes are occurring elsewhere in space. [Meteorites( like a rock or metal )  ki jaanch se bhi aise hi compounds mile, jisse pata chala ki space mein bhi aise process ho rahe hain.]

With this limited evidence, the first part of the conjectured story, i.e., chemical evolution was more or less accepted.[ In sabooton ke saath, 'chemical evolution' wali baat ko kaafi had tak maan liya gaya.]

We have no idea about how the first self replicating metabolic capsule of life arose. [Humein abhi ye nahi pata ki life ka pehla 'self-replicating' capsule (jo apni copy bana sake) kaise bana.]

Miller's Experiment Diagram (Figure 6.1)

This diagram explains how S.L. Miller simulated "Early Earth" in a lab.

Boiling Water :  Ye 'Ocean' ko represent karta hai. Pani ko boil karke steam (vapour) banaya gaya.

Gases (CH_4, NH_3, H_2O, H_2) : Ye early earth ka atmosphere tha (Methane, Ammonia, Water, Hydrogen).

Spark Discharge :  Ye 'Lightning' (bijli) ko represent karta hai, jo reaction ke liye energy deti hai.

Condenser : Ye steam ko thanda karke wapas liquid (baarish) banata hai.

Water containing organic compounds Experiment ke baad is pani mein Amino Acids mile, jo life ke building blocks hain.

The first non-cellular forms of life could have originated 3 billion years back. [Life ki pehli non-cellular forms shayad 3 billion saal pehle shuru hui thi.]

They would have been giant molecules (RNA, Protein, Polysaccharides [Wo bade molecules rahe honge jaise RNA , Protein, Polysaccharides,etc.]. These capsules reproduced their molecules perhaps. [ In ' capsules ' ne saayad apne molecules ko reproduce ( copy) karna suru kiya.

The first cellular form of life did not possibly originate till about 2000 million years ago. [Life ki pehli proper 'cell' wali form shayad 2000 million (yani 2 billion) saal pehle tak paida nahi hui thi.]

These were probably single-cells. [Shuruat mein ye sirf single-celled organisms (ek cell wale jeev) rahe honge.]

All life forms were in water environment only. [Us waqt sabhi tarah ki life sirf paani (ocean) ke andar hi maujood thi.]

This version of a biogenesis, i.e., the first form of life arose slowly through evolutionary forces from non-living molecules is accepted by majority. '[Biogenesis' ka ye version—ki pehli life non-living molecules se dheere-dheere evolutionary forces ki wajah se bani—zyadaatar log maante hain.]

However, once formed, how the first cellular forms of life could have evolved into the complex biodiversity of today is the fascinating story that will be discussed below. [Magar, ek baar cell banne ke baad, wo aaj ki itni badi aur complex biodiversity (itne saare jaanwar aur plants) mein kaise badli, ye ek bahut interesting kahani hai jo aage batayi jayegi.]

Summary : Ye page batata hai ki life kaise shuru hui. Pehle log 'Panspermia' (space se aana) aur 'Spontaneous Generation' (gandagi se banna) par yakin karte the. Pasteur ne spontaneous generation ko galat sabit kiya. Phir Oparin-Haldane ne 'Chemical Evolution' ki theory di, jise Miller ne lab mein amino acids banakar support kiya.

6.2 Evolution of life forms - A Theory 

Conventional religious literature ( prampragat Dharmik likhi hui kitabe or dharm granth ) tells us about the theory of special creation. According to this theory :

i) Species were created as such: Sabhi jeev (animals/plants) jaise aaj dikhte hain, waise hi banaye gaye.

ii) Diversity was always the same: Biodiversity hamesha se aisi hi thi aur aisi hi rahegi.

iii) Earth is 4000 years old: Dharti sirf 4000 saal purani hai.

The Scientific Challenge (Charles Darwin):

H.M.S. Beagle( sail ship ) : Charles Darwin ne is jahaz mein baith kar puri duniya ka safar (sea voyage) kiya.

Conclusion: Unhone kaha ki aaj ke living organisms sirf aapas mein hi nahi, balki un ff se bhi milte-julte hain jo millions of years pehle rehte the.

Extinctions: Bahut saari life forms ab khatam (extinct) ho chuki hain, aur nayi forms samay ke saath evolve hui hain.

There has been gradual evolution of life forms. [Life forms mein badlav (evolution) hamesha dheere-dheere (gradual) hua hai.]

Any population has built in variation in characteristics. [Kisi bhi population (jaise insaan ya janwar) mein pehle se hi thode-thode farak (variations) hote hain. ( jaise sab insaan ka height or colour ek jaisa nahi hota)]

Those characteristics which enable some to survive better in natural conditions (climate, food, physical factors, etc.) would outbreed others that are less-endowed to survive under such natural conditions."

Hinglish: Wo khaas gun jo kisi jeev ko uske mahol (mausam, khana) mein zinda rehne mein madad karte hain, wo unhe dusron ke muqable zyada bache paida karne ka mauka dete hain. Jo "fit" nahi hain, wo peeche chhoot jaate hain.

 "Another word used is fitness of the individual or population."

Hinglish: Iske liye ek term use hota hai—Fitness.

 "The fitness, according to Darwin, refers ultimately and only to reproductive fitness."

Hinglish: Darwin ke hisaab se fitness ka matlab body banana nahi, balki 'Reproductive Fitness' hai—yani kaun kitne bache paida karke apni nasal aage badha sakta hai.

"Hence, those who are better fit in an environment, leave more progeny than others."

Hinglish: Isliye, jo apne mahol mein zyada "fit" hote hain, unke bache (progeny) dusron ke muqable zyada hote hain.

 "These, therefore, will survive more and hence are selected by nature."

Hinglish: Ye fit log zyada survive karte hain aur Nature (prakriti) inhi ko chunti hai aage badhne ke liye.

 "He called it natural selection and implied it as a mechanism of evolution."

Hinglish: Is process ko Darwin nke 'Natural Selection' kaha aur bataya ki evolution isi tarah kaam karta hai.

 "Let us also remember that Alfred Wallace, a naturalist who worked in Malay Archipelago ( island group jagah hai ) had also come to similar conclusions around the same time."

Hinglish: Yaad rakhein ki usi waqt Alfred Wallace naam ke ek aur scientist ne bhi bilkul yahi baat kahi thi.

 "In due course of time, apparently new types of organisms are recognisable."

Hinglish: Samay beetne ke saath, inhi badlavon ki wajah se naye tarah ke jeev (organisms) ban jaate hain.

"All the existing life forms share similarities and share common ancestors."

Hinglish: Aaj ke sabhi jaanwar aur plants aapas mein milte hain aur un sabke purvaj (ancestors) ek hi the.

 "However, these ancestors were present at different periods in the history of earth [epochs( day), periods(month), and eras( year)]."

Hinglish: Lekin ye purvaj dharti ki history mein alag-alag samay (epochs/periods) par maujood the.

"The geological history of earth closely correlates with the biological history of earth."

Hinglish: Dharti ki zameen ki history aur life ki history ek-dusre se puri tarah judi hui hain.

 "A common permissible conclusion is that earth is very old, not thousand of years as was thought earlier but billions of years old."

Hinglish: In sabse ye sabit hota hai ki dharti sirf hazaron saal nahi, balki arabon (billions) saal purani hai.

Charles Darwin (Brief for NEET)
British naturalist (1809–1882)

Proposed the theory of evolution in his book On the Origin of Species

Explained how species evolve over time through natural selection

 Natural Selection 

“Survival of the fittest” (term popularized by Herbert Spencer)

 Organisms with favorable traits survive and reproduce more, passing those traits to the next generation.

 Main Principles :

1. Overproduction

Organisms produce more offspring than can survive

Example: A frog lays thousands of eggs, but few survive

2. Variation

Individuals in a population show differences

Variations can be:

Genetic (important for evolution)

Environmental

3. Struggle for Existence

Three types:

Intraspecific (within same species)

Interspecific (between different species)

Environmental (against nature: climate, disasters)

4. Survival of the Fittest

Only organisms best adapted to the environment survive

“Fitness” = ability to reproduce, not just strength

5. Natural Selection

Nature “selects” beneficial traits

Unfavorable traits are eliminated

6. Inheritance of Useful Variations

Useful traits are passed to offspring

Over generations → evolution occurs

Note :

• Giraffe’s long neck: those with longer necks survived better and reproduced more
• Natural selection acts on phenotypes, but evolution occurs at genetic level
• It is a slow, continuous process
• Leads to adaptation and sometimes speciation

6.3 What are the Evidences for Evolution 

NCERT Line : “Evidence that evolution of life forms has indeed taken place on earth has come from many quarters.”

  Hinglish Explanation:

Is baat ke bahut saare proofs mile hain ki Earth par living organisms time ke saath change hue hain, yani evolution hua hai.

NCERT Line : “Fossils are remains of hard parts of life-forms found in rocks.”

Hinglish Explanation : Fossils = purane jeev-jantu ya plants ke bache hue hard parts (bones, teeth, shells etc.) jo rocks me milte hain.

NCERT Line : Rocks form sediments and a cross section of earth ' s crust indicates the arrangement of sediments one over the other during the long history of earth. Different aged rock sediments contain fossils of different life - forms who probably died during the formation of the particular sediment.

Hinglish Explanation : Rocks dheere-dheere tootkar chhote particles ban jaate hain.

In particles ko sediments kehte hain.

Ye sediments time ke saath ek ke upar ek layer banate rehte hain.

Sabse purani layer niche hoti hai.

Nayi layer uske upar banti jaati hai.

Earth ko agar side se kaatkar dekhen (cross section), to hume ye sari layers dikhengi.

Isse Earth ki purani history samajh aati hai. Alag-alag age ki rock layers me alag-alag organisms ke fossils milte hain.

Jab koi plant ya animal mar gaya hoga, tab uske remains sediments ke andar dab gaye honge.

Baad me woh fossils ban gaye.

Purani layers → purane organisms ke fossils

Nayi layers → naye organisms ke fossils

Isse scientists ko pata chalta hai ki time ke saath life forms change hue hain (evolution hua hai).

Example:

Bahut niche wali layers me dinosaur fossils mil sakte hain.

Upar wali recent layers me modern animals ke fossils milte hain.

NCERT Line : Some of them appear similar to modern organisms ( figure 6.2) . They represent 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 - spans. Hence, new forms of life have arisen at different times in the history of earth.All this is called palaeontological evidence.

Hinglish Explanation : Kuch fossils aaj ke modern organisms jaise dikhte hain. Iska matlab hai ki purane organisms aur aaj ke organisms ke beech relation ho sakta hai. Example ke liye, kuch dinosaurs ki body structure birds aur crocodiles se milti hai. Kuch fossils aise organisms ke hote hain jo ab Earth par nahi milte. Aise organisms ko extinct organisms kehte hain, jaise dinosaurs.

Scientists jab different rock layers (sedimentary layers) me mile fossils ka study karte hain, to unhe pata chalta hai ki kaunsa organism kis geological period me jeeta tha. Jo layers niche hoti hain woh zyada purani hoti hain aur jo upar hoti hain woh comparatively nayi hoti hain.

Fossils ki study se ye bhi pata chala ki life forms time ke saath change hote gaye. Kuch organisms sirf ek particular geological time period me hi exist karte the aur baad me extinct ho gaye. Isse conclude kiya gaya ki Earth ki history me different times par naye-naye life forms develop hue.

Evolution ke proof ke roop me fossils ki study ko Palaeontological evidence kaha jata hai.