Population is a word which has been deduced as very important because of its relevance to ecology and evolution. Nobody knows what it exactly is,but they define it to be a group of same kind of people.For technicality,'kind' means species and people means an indivduals who can be any living thing which can reproduce.
It is good to imagine 'population' as a community of individuals of hens(choose your favourite animal) living in a place which is surrounded from all sides by mountain so they can't get out or for that matter,nobody can come in. The bussinesss of food and sex is left for reader to choose and imagine in the above context.
If you don't want to read all of this and want to learn Hardy-Weinberg law,please understand that it is not a very big thing ,it just says a very simple thing which is as follows. If you have a pair of dice of red and blue color and someone else has both red color and some might have blue colored ones,but everyone has a pair of dice.Now you can count the total number of dice and figure out say 70% of dice are red and 30% are blue. But still the people holding red pair of dice say 48% and blue colored dice pair are 37% and the people with mixed color dice are 15%. Now if all of you throw your dice in a tub and mix it and pick two dice up,the proportion of people holding blue pair dice would 9% ,red pair dice 49% and mixed color dice 42%.And if you repeat the game,same thing will happen.THe reason is as simple as this,if you mix things and randomly pick again,frequency of getting something is equal to probability of drawing a pair of things from an urn or something without replacment. Nowgo read the text.
Now you ask, i see somewhat different kind of hens here.There are 1000 hens(what a feast it would make), some are white colored,some are red colored and some pinkish. How could this happen.These all have the same kind of meat and egg,why be different. Why couldn't everyone be same.
Well,if you think of a context when suddenly a fox or an eagle(who eats hens) came into the mountains and hens didn't know where to go. They will be killed,of course but who will be killed first.
Fox is color-blind,it can only see difference in white and black(and their linear combination).So would it observe the white hens or red hens first. My bet is on white hens. Say it did,and it will go and grab and eat 'em up.
Now he's full and everyday he starts eating these hens at a rate of say 1 hen per say.
Now hens are no foolish,they can have sex. They will go, find a cock and mate. Now white hens mating with white cocks can give rise to two different kind of offspring ( i am not talking about its gender). The offspring could be pink or white.
To make matters more concrete,lets do some number crunching with notations.or in other words,make a model.
DNA is unit of heredity.What you pass to your son is not only your wealth(to sons it seems to be the only thing),definitely not your blood(like in movie they keep on mentioning the blood) but your DNA and also what you are is the combination of DNA of both your mother and father(ask your dad,mom might be shy).
Since DNA is the only thing which is passed from parents to offspring, a DNA has to have information ,in principle,to construct an entire human being (you).
That means DNA determines your hair color, your lungs bronchioles density,the eye color and maybe starch content(if you are a pea).
Now the color of hens is determined by their DNA.
Different colored hens have different DNA. And egg production(and hence offspring) requires sex,that means the offspring must be getting two of those DNA's. So we can think that everyone alive must have two DNA's.But characteristics or technically phenotype is determined by combination of both DNA's so you cannot expect same character as parents to show up in offspring or their mixture for that matter and some traits may not be visible but be there(those are called carriers) and show up in some other generation(although Caucasian parents cannot possibly have black kid).
So lets symbolise.
Red hens have AA DNA or gene,if you will(AA because there are two copies of A).
White hens have BB gene.
Pink ones have AB or BA. (dont worry its commutative,not like the quaternions).
So we try to find out what would be gene frequency/proportion dynamics when their was no fox and couple of other assumptions.
What is proportion of A genes in the population
total no of A genes = 2 copies per hen of red color + 1 copy per hen of pink ones
total no of B genes = 2 copies per hen of white color+ 1 copy per hen of pink ones
total no of all genes = total no of hens *2 (since every hen has 2 of them)
Let us say p (its not convenient or something but it gives a feeling of what i have learnt) is the frequency of A genes in the population and q is the frequency of B genes.
Naturally p= total no of A genes/ total no of all genes
q= total of B genes / total no of all genes
And p+q= (total no A genes + total no of B genes)/(total no of hens)
=1
Say red hens (AA) are P in proportion.
White hens (BB) are Q in proportion.
Pink ones(AB) are R in proportion.p = (2 P + R) /2(P+Q+R) = P + R/2
q= (2 Q + R)/2(P+Q+R) = Q+ R/2
Notice P+Q+R=1
Say out of 1000 hens there are 238 white hens, 478 red ones ,rest you figure out.
P = 238/1000
Q= 478 /1000
Its not so vague as it looks,just put some numbers.
So p =(2*478 + (you figured))/2000 = 0.62
q= (2*238 + c'mon)/2000 = 0.38
Now if just get to figure out how this frequency changes after every mating(from now on sex is for humans,mating is for animals) we would be able to say how many of each color are present in each generation,all you have to do is to multiply the gene frequency by the total no of people in the population.
like in above case you wanted to know how many of each color are there
for red hens.
no of red hens = frequency of red hens * total no of red hens
= 0.62 * 0.62 *1000
= 384.4
WTF. we had 478 red hens and calculation shows only 384 (give or take) hens.
Clearly something is wrong (or wrongly assumed).
The equation
no of red hens = frequency of red hens * total no of red hens
is assuming that if we have A gene and B gene in population and we randomly pick two genes (hence making an individual) ,then the probability of picking two A's with(actually it does not matter much here,if it did you could take 10^5 hens) replacement is p*p. And hence there must be p*p*total hens amount of red hens.
But we are drawing our sample from a thoroughly mixed box of genes,which might be a wrong assumption for this generation.
Now since the mating would be random and genes will get mixed,we will show later that in the next generation(and henceforth) the above equation will start working. This is the famous(i dont know why) Hardy Weinberg Law.
So one assumption we make now is that the mating is random i.e unlike human society ,no male in hens have mating with a female just becoz she is white (or red).
So to know the gene frequency in next generation of kids (when the current hens are dead,they have die before kids start mating,take it as an assumption or kill the old hens) we need to know what is the mating frequency.that is how often do people mate.Obviously,if some kind of hens are more in number,they will mate more often(not the same individual,the same kind).
Frequency of mating is directly proportional to the number of hens for the mating group
lets make a table of all possible matings. of course,Aa * aa is same as aa *Aa, remember mating is multiplicatively commutative(unfortunately it does not form a vector space because of addition operation not being defined).
All possible kind of matings are
AA * AA
AA *AB
AA *BB
AB *AB
AB *BB
BB* BB
Mating frequency is just the product of the number of each type of individual
mating type mating frequency
AA * AA P*P
AA * AB P*R
AA * BB P*Q
AB * AB R*R
AB * BB R*Q
BB * BB Q*Q
Now we want to know what kind of offspring each of the different type of mating will give rise to.
For this, since i don't have the patience to write, i copy.
Mating type |
Mating frequency
|
How many will be AA |
How many
AB |
How many
BB |
AA * AA |
P*P |
all |
0 |
0 |
AA * AB |
P*R |
half |
half |
0 |
AA * BB |
P*Q |
0 |
all |
0 |
AB * AB |
R*R |
quarter |
half |
quarter |
AB * BB |
R*Q |
0 |
half |
Half |
BB * BB |
Q*Q |
0 |
0 |
all |
Wherever you see half or quarter,it means that out of all the matings in the respective mating type,how many of each type would be produced,to see the logic behind the values,visualise from a given mating type (say AA*AB) choose A sperm from (AA) and A egg from AB,you will get AA as final product or if you choose A from AA and B from AB,you will get AB as final product,the probability of AA producing A sperm is 1 while the probability of AB producing A or B is half each. And mating implies multiplication of probabilities.
Making the table more quantitative
Mating type |
Mating frequency
|
How many will be AA |
How many
AB |
How many
BB |
AA * AA |
P*P |
1 |
0 |
0 |
AA * AB |
2 P*R |
½
|
½
|
0 |
AA * BB |
2 P*Q |
0 |
1 |
0 |
AB * AB |
R*R |
¼
|
½
|
¼
|
AB * BB |
2 R*Q |
0 |
½
|
½
|
BB * BB |
Q*Q |
0 |
0 |
1 |
|
New P |
P*P +2P*R/2+ R*R/4
|
(P+R/2)^2 |
p^2 |
|
New R |
2 P*Q + 2R*Q/2+2P*R/2 |
2(P+R/2)(Q+R/2) |
2pq |
|
New Q |
Q*Q +2 R*Q/2 + R*R/4 |
(Q+R/2)^2 |
q^2 |
Now look at the last three rows of this column
we have predicted the frequency of observing red hens ,white hens and (jeez) the pink ones too.
Number crunching starts.
P (the red ones) will be observed (0.62*0.62*100)=38.4% of times
R (the pink ones) will be observed (0.62*(1-0.62)*100)=47.1% of times
Q(the white ones) will be observed (0.38*0.38*100) = 14.4% of times
by the fox.
So the no of hens in a population of 1000 is
384 red ones, 471 pink ones,144 white ones.
Observe that this number is same as what we predicted sometimes back from random mating of people and stuff like that.
The result is nothing more tough. In general, whatever be the color frequency of indivduals in the population, their gene frequecies are say p and q for 2 colors, after one round of mating,in which,all the information about the number of red hens and white ones is lost,what you will see is the combination of red,white and pink with a frequency of p^2, q^2 and 2pq.
Notice the change we see saw in our population.
The gene frequnecy never changed .
p = 0.62 and q=0.38
only its distribution among individuals changed.
Initially we have 62 % red and 38% of white gene in the whole population but 478 red and 238 white and 284 pink ones.
After all the genes got combined and were again randomly distributed among individuals,there are
384 red ones,144 white ones, 471 pink ones.
Now this number cannot change until and unless some males start liking some females(of a particular color) more than other(non-random mating) or the different color provide different survival advantage to the hens which is true for our fox who eats only white (natural selection acting).
On a different note, this tells us that fundamental process of mixing can prevent a population from losing out some gene. A gene being in low frequency does not mean that eventually by mating after several generations it will be gone.No.Rather in simplistic cases, it wont.
On a different(er) note, G.H Hardy was a mathematician who figured this law out in a conversation with Punett (why does a matrix get to be called punett square) regarding the population still having deleterious genetic diseases which are still not gone (which is by virtue of carriers).