Surely at least once in life each of us thought about what it was like to have a brother or sister twin, but twins are not always of the same sex. In fact, the phenomenon of twins long – standing evolutionary puzzles. Identical twins develop from a single fertilized egg, which randomly splits in two, but twins of different sex appear when two eggs are released and fertilized. But the reason remained a mystery for many years. Now scientists finally know why the birth of twins is evolutionary reasonable.
Gemini — children born from one mother, fetal development which took place in the course of a pregnancy. Twins are born as the result of some sorts after a short time of each other.
What science knows about the twins?
Since the release of the famous book by Michael Bulmer 1970 on the biology of twins in Homo Sapiens, biologists wondered about that, if favored by natural selection double ovulation or, like identical twins, the occurrence of twins of different sexes – the result of chance. Let me remind you that the double ovulation means that the ovary has two eggs in one cycle.
As the authors of a new study in The Conversation, at first glance, this seems unlikely. Splitting of the embryo, resulting in the formation of identical twins is not hereditary, but the incidence of identical twins is independent of other aspects of human biology. It seems random in every sense of the word, however, researchers believe that twins are genetically determined.
The work, published in the journal Nature Ecology & Evolution, researchers used computer modeling to try to explain why natural selection favors production of two eggs, despite the low survival rates of twins and the risk of multiple births for mothers. In human populations without access to medical care, the birth of twins, apparently, is of little use. Twins more likely to die in childhood than single children, and mothers of twins the risk of dying in childbirth is much higher.
Like other apes, women, apparently, is designed to give birth to one child at a time. But if the birth of twins, for a light costing this price, why evolution still is not eliminated? Paradoxically, in populations with high fertility in mothers of twins often more offspring to the end of life than other mothers. This suggests that the birth of twins may have an evolutionary advantage, at least for mothers.
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Why are twins so little?
In the study, researchers built a simulation and a mathematical model based on data on survival of mother, child and fetus of the real populations. This allowed them to do something impossible: during the simulation they were able to control the process of ovulation one or two eggs during a cycle.
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The researchers also modeled a different strategy where he switched women with ovulation one egg at ovulation two at different ages. Then the authors compared the number of surviving children of women with various forms of ovulation. The results of computer simulation showed that women who went from single to double ovulation under the age of 25 years, had more children than those who have always produced one egg or two eggs are released. How to write the authors of the article, the data indicates that with age, natural selection favors the unconscious switch from single to double ovulation. But why is this happening?
The reason why such a transition is best, is the survival of the fetus is the probability that a fertilized egg will give birth to a child quickly decreases with woman’s age. Thus, the transition to the production of eggs increases the likelihood that at least one of them will give birth. More articles about the mysteries of nature and the world in which we live, read up on our channel in Yandex.Zen. There are items that are not on the website!