Ever wondered why there are only two sexes? What is so evolutionary about the number 2? Since your sex is determined by the two chromosomes X,Y, I guess my question boils to why are there in general only 2 chromosomes determining the gender? Why not 3 for example?
So in a hypothetical world in which 3 chromosomes determine the gender, there would be 9 genders. Ofcourse taking this wild idea further, it would be interesting to ponder on the sexual habits of the people in this hypothetical world... :)
(Shyam has a nice proof in the comments)
I also found an article on bbc about the same topic: Why are there only two sexes?
There is an article in slate on the same topic.
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16 comments:
Perha[s, it is for the same reason why a binary search is more efficient than a ternary search. Perhaps, two sexes can achieve more diversity in the gene pool than either one sex (which achieves no diversity) or more than two sexes. It has to be proved mathematically though !
Because two is company and three is crowd.why is there no YY type?
Interesting question. As far as natural selection goes, creation of sexes is not a frivolity. The should be just enough sex-determining chromosomes to propogate a species. If there are more, it is just a waste, isn't it? You are really diluting the species.
So, why is one not a good number? Then, as Shyam says, you do not get diversity in the gene pool. Having the genes of two parents gives you a better chance of survival that having the genes of just one.
Btw, 3 sex-determining chromosomes do not necesarily imply 9 genders. Once again, nature is too functional to be able to afford that sort of luxury. Mammals have two sex chromosomes but only 2 genders - not 4. (The male chromosome has *no* other function than to guarantee "maleness".)
Remember, reproduction is a 'binary operation'.
If it was a unary operation, the probability of reproduction is high but there is no diversity in gene
pool. Ternary operation would reduce the probability.
You would have had to find two life mates instead of just one. Therefore, binary operation is optimal.
Also, child bearing females (atleast for mammals)
have to evolve separately from males. So, female sexes
(F1, F2 etc)
would have to exist which are distinct for male sexes
(M1, M2 etc)
Best population mix is n/2 males and n/2 females which
produces n/2 * n/2 combinations which would be greater
than any other break up (say n/3 * 2n/3 ). This optimal
ratio explains why there are two chromosomes XX and XY which gets 50 % chance of producing a male or female.
Now within males or females, number of different sexes
(M1, M2) or (F1, F2 etc)
would still only produce n/2 * n/2 combinations at best. Also M1 will eventually dominate M2 if M1 is better. Also after many generations of intermarriage,
a homogeneous M would appear and so would a homogeneous
F.
Hence only two sexes.
My proof anyway.
Shyam,
Not that I am saying that the final result is wrong :), but there seems to be some assumptions here that are untrue in the real world. The first paragraph is fine, it is the later mathematical part I have objection with. Let us see them one by one --
a)Females and males share pretty much all of the chromosomes. The only thing that is different is one part of one out of 23 chromosomes i.e. Y. So, the contention of females evolving separatly from males is false for the most part.
b) best population mix is n/2*n/2 only assuming that each male/female has one partner. That is not true in nature. Nature makes sure that there are more females then males, because males are dispensible, while females are not. 1 male produces enough sperm for 100s of females. That is why female sex is always at an advantage in terms of disease, longetivity etc.
Kautilya,
1. I am not an expert on biology and so I cannot comment on the assumption if females would require
special child bearing mechanisms to be evolved.
I do not need females to evolve separately but that
only females can bear child. This provides me with
two broad categories of sexes (males and females) and
a child to be produced by one male and one female.
2. Best mix of n/2 males and n/2 females should be accurate. Each new child can be produced in n/2 * n/2 ways (which is better than n/3 * 2n/3 ways or n/4* 3n/4 ways). I did not make an assumption on monogamy. Even if polygamy is practised, each new child can be produced by just one male and one female in n/2 * n/2 ways. It is the number of ways a child can be produced that produces higher probability of diversity in gene pool. Therefore n/2 males and n/2 females is the best mix.
Anyway the way the chromosome for men is structured, probability of boy child and girl child is equal in natural reproduction. This should be accurate.
Kautilya,
1. I am not an expert on biology and so I cannot comment on the assumption if females would require
special child bearing mechanisms to be evolved.
I do not need females to evolve separately but that
only females can bear child. This provides me with
two broad categories of sexes (males and females) and
a child to be produced by one male and one female.
2. Best mix of n/2 males and n/2 females should be accurate. Each new child can be produced in n/2 * n/2 ways (which is better than n/3 * 2n/3 ways or n/4* 3n/4 ways). I did not make an assumption on monogamy. Even if polygamy is practised, each new child can be produced by just one male and one female in n/2 * n/2 ways. It is the number of ways a child can be produced that produces higher probability of diversity in gene pool. Therefore n/2 males and n/2 females is the best mix.
Anyway the way the chromosome for men is structured, probability of boy child and girl child is equal in natural reproduction. This should be accurate.
Shyam,
I understand now what you meant. But, in nature the probablility is not 50% and that is what I wanted to point out. Nature has a tendency to produce more felamles then males.(See previous post)
Theoretically, it has been shown that ratio of men to women would tend to 50%. This result was from Ron Fisher and listed here http://www.newscientist.com/lastword/article.jsp?id=lw989.
I think what Jasmeet is pointing out is that in the current society this might not be true (with female infanticide and all). But I think even in states like UP where this is so true, there are articles now pointing out that females have now become so much in demand that the guys are paying money to women to marry them and over time I think the situation will rectify itself.
Interesting article Vinod.
Ok, now for some shameless self-promotion, in order to increase hitcount on my blog,;) here is the link to my blog--
http://kautilya.chhabra-inc.com
You are welcome to leave comments.
mmm hmm! and why does the world - the planet earth have to have different climates/weather? why not just one average all year round or four seasons everywhere? and dont say because our world is round and the sun is bla bla bla. just a question, like why is there only two sexes in general
Sorry dude, I dont have a link to the article :). But I think it is an interesting point as to how humans with medicine, genetic engineering, natal screening etc, would affect evolution in the long run?
Well Brother-in-law...
Today's question:
"Why are there only two sexes in general?"
the same question 200 years from now:
"Why are there only women on earth?"
Textbook Answer:
There used to be another race of human beings called "men". They were scheming and violent and caused unrelenting damage to humanity with their "biggest monkey in the jungle" mentality. Their inability to evolve and master simple skills such as getting along with others and putting the toliet seat down made them pretty much obsolete. After the biochemical revolution of the early 21st century (which enabled women to reproduce without the help of men) men simply fell out of fashion and slowly became extinct. A movement for an endangered species act started a rescue program in the 2030s, but the short lived program folded in 2036 when Madame President Chelsea Clinton and Congress voted to pull funding in favor of world peace and ecological efforts.
Sister in law:
My work has been cut out for me as far as response goes. All I have to do is refer you to the beautiful song in My Fair lady: http://www.stlyrics.com/lyrics/myfairlady/ahymntohim.htm
:)
I think a relevant issue is how many chromosomes determine the gender of an offspring (let's call this 'n') as well as how many types of chromosomes there are (let's call this 'm'). Humans (and perhaps many other lifeforms) have a chromosome pair (n = 2)determining the sex of the offspring, as well as two types of chromosomes (m = 2). We can imagine having different values of 'n' and 'm'.
The question is how many different genders are possible for different values of n & m. The answer is the number of terms in the multinomial expansion of: (a1 + s2 +....+am)^n = C(n + m - 1, m - 1).
n = m = 1 (hermaphrodite)
# = C(1, 0) = 1
n = 1, m = 2 (theoretically impossible)
# = C(2, 1) = 2
n = 2, m = 1 (possible, but redundant chromosome)
# = C(2, 0) = 1
n = m = 2 (humans)
# = C(3, 1) = 3
n = 2, m = 3
# = C(4, 2) = 6
n = 3, m = 2
# = C(4, 1) = 4
n = 3, m = 3
# = C(5, 2) = 10
Some observations: Humans could possibly have one more gender (corresponding to YY) but they don't most likely because as Shyam pointed out, there would not be an optimal mating scheme.
Let's assume that the multiple genders are divided into some genders that are capable of giving birth and some that are not (if all of them could, then we default to the hermaphodite case and no diversity in the gene pool). Hence there are 'different' types of females F1, F2, F3....and different types of males M1, M2 .. etc.
If we assume that all males can mate with all types of females (ie. M1:F1, M1:F2, M2:F1...so on and so forth), then as long as n and m yield an even number of genders, then we have an optimal mating scheme. So the question is why don't we have (n, m) = (2, 3), (3, 2) and (3, 3)? These would even lead to more diversity in a sense.
I think we are still in the process of evoulution. Probably all species had more genders but over the time the weaker chromosomes got eliminated and we ended up in only two genders. Down the line we might have one gender and able to reproduce by only one gender.
http://news.bbc.co.uk/2/hi/science/nature/6196225.stm
As a funny note in case of Humans males have to reporduce since they carry the combination of XY else we mgith be extinct..so be ready future generations!!!
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