Can current evolutionary theory explain identical twins?
I can understand that fraternal twins result more-or-less from an accident of the woman's body. Still, interesting to see that natural selection does seem to place a typical limit on the "litter size" for humans. But the fact of multiple eggs seems coincidental enough to me not to require an explanation.
What I'm not so sure of is whether identical twins are so obviously coincidental. As I understand it, they're a mistake of the doubling process after the egg is already fertilized. A totally different kind of mistake from fraternal twins. For an egg to make a random mistake at the cell-division level seems like it would be catastrophic, so the fact that it works so well now seems to suggest that natural selection has had a hand in making it safe. But if that's the case, then doesn't that imply that identical twinning has some survival value? Some survival value that is not, or at least not necessarily conferred on fraternal twins?
Am I totally missing some obvious point, or wading to far into the deep end?
Please, religious people, feel free not to answer. I've heard your claims before and find them unhelpful to my personal endeavor. Feel free to pursue your own way without me. Thank you.
And those of you who think that evolutionary theory has nothing to do with it, please don't post an answer. I recommend that you read Dawkins or Coyne, highly respected biologists.
People! I want to claw my eyes out! Evolutionary theory is FOUNDATIONAL to cell biology. How can you say that evolutionary theory has nothing to do with this? Read some Gould, people! Oh, never mind!
- secretsauceLv 71 decade agoFavorite Answer
There may be a remote connection between twinning, in general, and primate evolution, especially human evolution.
If you look through all of the animal kingdom, you see a pattern. Multiple births are very common among many mammal species, most bird species (penguins are an interesting exception ... most penguins lay two eggs, while emperor and king penguins lay a single egg), most reptile species, all amphibious species, etc.
And if you look at the lifestyle of each animal you can see that the more investment by the parents in the survival of the young, the fewer offpring produced at a time, and the longer the incubation or gestation.
In other words, survival of the greatest number of offspring is always the driving factor. (Of course you know this as natural selection 101.) This can be accomplished either by producing a lot of offspring, or by maximizing the chances of survival of each offspring.
There are a lot of factors involved in the survival of each offspring ... and you have to look at the investment of time and energy by one or both parents, both while the offspring is incubating or gestating, and during early childhood.
In mammals, you also have to recognize that when *size* is part of the survival strategy of the species, (e.g. horses, elephants, giraffes) then large animals tend to have large babies, and therefore multiple births is very dangerous for the mother. Where multiple births happen, they have to be smaller and more premature as a result, which reduces their chance of survival after birth, unless there is increased invesment by the parents.
So what you have in humans is a species where single-births is the norm ... and the mechanisms that produce multiple offspring has been mostly eliminated by natural selection.
*Mostly* eliminated ... but not completely.
In fact, triplets, quadruplets, and other multiple births illustrate that the human reproductive system still has a lot of this capability ... but the size of the resulting baby is smaller as the numbers go up, and the chances of survival lower. It is only with the advances of modern medicine that such multiple births have much of a chance of survival at all.
As I understand it, about one-third of twins are identical (monozygotic) twins. There doesn't seem to be any survival advantage of identical vs. fraternal twins ... so there is no reason to see this as some sort of natural selection advantage. And while early mutations can indeed be catastrophic during the first few days after fertilization (a) not all are ... and in fact, all evolution is driven by mutations that occur either during replication of gametes, or in the early stages of cell division; and (b) monozygotic twinning is not a "mutation" in the genetic sense. It is clearly a process that occurs quite often and quite safely in other species ... so it is not the division of the zygote that is the problem, but whether the placenta and other reproductive components are OK to handle it.
In other words, twinning (both identical and fraternal) and other multiple births should be seen not in terms of what advantages it produces ... but rather as a vestigial process that has not been completely eliminated in our species. (People who think I'm saying that twins are "less evolved" are not reading carefully.)Source(s): P.S. Stephen Jay Gould is my absolute favorite science writer ... but the above is not based on anything specific I recall him writing about twins.
- Bob BLv 71 decade ago
Evolutionary theory isn't related to this process- this is a cell biology question.
Identical twins are caused due to a temporary fault in mitosis- the fertilised ovum divides into two as normal, but then they actually come apart. After that, they replicate as normal. They don't always replicate as normal after that, and in such cases the pregnancy simply fails. As you might have guess, it doesn't happen all too often, which is why identical twins are so rare. It's just a particular fault that the body sometimes has- remember our bodies aren't perfect, and some glitches happen more than others.
Natural selection doesn't really apply here- natural selection only really applies to organisms that have already been born. The only way you could say it helps is that having two children means there is a greater chance that at least one will survive. This is why animals like fish tend to lay hundreds and hundreds of eggs- at least a few are likely to get through.
The only real point your missing is that not everything has to have a logical evolutionary reason for it to occur. No organism is perfect, and all kinds of problems abound. Evolution only really tries to get rid of problems if they are having a serious effect on the species ability to survive as a whole. The fact that some tiny percentage of the population will have twins doesn't really pose any danger to our survival, so evolution won't weed it out.
- gardengallivantLv 71 decade ago
Evolution selects between allelic variations for a phenotypically expressed trait but there is no evidence that monozygotic twins are generated by a genetic trait.
One line of evidence against monozygotic (MZ) twins being a genetic variation is that their incidence is constant world wide with ~4/1000 births.
Dizygotic (DZ) twinning rate varies. Some populations have a much higher incidence of DZ twins than others. Asians 3/1000 but Europeans 7-10/1000. Some reports say 10 and as many as 40/1000 in some African states.
Families with high rates of DZ twinning can be tracked in genealogies but MZ twins cannot.
MZ twinning cannot be considered a hereditary trait on this basis.
MZ twinning happens with variable results in the early division stages. It does not appear to be regulated. Early twinning, in the first 2 days after fertilization, results in two placentas that can be separate or fused but this is only ~30% of all MZ births.
Most, ~70% MZ twinning is a failure of the blastocyst cells to migrate properly from the zona pellucida.. This is as likely to be due to environmental factors that the blastocyst fragments. One placenta forms but each twin has an amniotic sac.
If the splitting is later 9-12day after fertilization there can be conjoined twins because they share the amniotic sac. This is no more than 1% of MZ births.
Triplets can also be MZ. Two consecutive zygote splits where one twin has a lethal mutation or error so is lost.Source(s): http://emedicine.medscape.com/article/977234-overv... Aneuploidy in monozygotic twins may be due to unequal cell division creating discordant twins. http://8e.devbio.com/article.php?id=111
- CharlieeLv 41 decade ago
identical twins are to do with the idea of cloning.
they are natural clones.
when an egg implants in the uterus and begins to multiply, the cells are undifferentiated or unspecialized. identical twins occur when a cell separates from the ball of cells, and then also implants onto the uterus wall. two separate babies are growing. they are identical because the original cell that was separated from the ball of cells and the cells are the same, and so the DNA is the same. this process is not genetic but pure coincidence.
this is pretty much it.
fraternal twins can be genetic and are a result when the woman's ovary produces two eggs in one month, and then they are both fertilized. the ovary is said to go into spasm, which isn't necessarily bad, because you end up with twice the cuteness. it can be bad when your ovary spasms and you end up with 6 fertilized eggs....
i hope this helped
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- KTDykesLv 71 decade ago
Why would a theory accounting for the origin of species seek to explain the occurrence of identical twins? It's be rather like looking at a map of Poland in order to try and locate Glasgow.
<<And those of you who think that evolutionary theory has nothing to do with it, please don't post an answer. I recommend that you read Dawkins or Coyne, highly respected biologists.>>
But they don't claim evolutionary theory somehow explains identical twins. Nor did Darwin, another author you might like to consult on what evolutionary theory happens to be about.
- Anonymous1 decade ago
> Can current evolutionary theory explain identical twins?
Probably an accident, just one of those things.
> limit on the "litter size" for humans
Important from an evolutionary standpoint. We have arboreal monkeys in our ancestry. There's only room enough for one baby to cling to momma when she's running through the trees, and thus it's more efficient of energy for her to give birth to only one at a time.
> doesn't that imply that identical twinning has some survival value
Nope. Most people get wisdom teeth, and there's no survival value in that. Quite the contrary; in plenty of people the wisdom teeth become a source of infection.
- 1 decade ago
It sort of does, Identical twins are the splitting of one fertilized egg, or "zygote". In fraternal twins a women can have another egg lined up for ovulation during her fertilization period, that can become fertilized, even after her current egg is fertilized as well. It's not uncommon for a women who is pregnant to have a period after she is pregnant and this is why. IDENTICAL TWINS somewhat support the evolution theory.Source(s): Bio
- Bastion 「A」Lv 71 decade ago
I think you've already answered your own question.
Monozygotic twins are the result of a non-fatal error during the early stages of division. There are obviously errors that are fatal, or produce naturally fatal (ie: fatal without modern medicine) such as conjoined twins that demonstrate that it isn't always a perfect process.
Assuming that it can't be a 'coincidence' given the evidence that it does, apparently, 'just' occur, would be foolish.
- OmenLv 51 decade ago
Twins are explained in biology, it doesn't have much to so with the theory of Evolution, there is nothing spectacular about twins, dogs have identical pups in droves, Lion, cats etc and so on the same. They are not a mistake.
- Anonymous5 years ago
Typically, these kind of conditions are related to mis-transfered sex chromosomes and leave the individuals sterile as a result. It's basically a deformaty so it doesn't really factor into evolution. I can see that you're arguing that evolution is the foundation of biology but that doesn't mean it encompasses all elements of biology as well. The fact that we've been outside of natural selection for a long time now kind of puts us beyond the scope of evolution anyway.