Surely at least once in a lifetime, each of usI thought about what it would be like to have a twin brother or sister, but the twins are not always the same gender. In fact, the phenomenon of twins is one of the long-standing evolutionary mysteries. Identical twins develop from one fertilized egg, which accidentally splits into two, but twins of different sexes appear when two eggs are released and fertilized. But the cause of what was happening remained a mystery for many years. Now scientists have finally learned why the birth of twins is evolutionarily justified.
Twins - children born from one mother, intrauterinethe development of which took place during one pregnancy. Twins are born as a result of one birth after a short time one after another.
What does science know about twins?
Since the release of the famous book by Michael Ballmer1970 on the biology of twins at Homo Sapiens, biologists wondered whether natural selection favored double ovulation or, like identical twins, the appearance of twins of different sexes was the result of chance. Let me remind you that double ovulation means that two ovules leave the ovary in one cycle.
As the authors of a new study in TheConversation, at first glance this seems unlikely. The splitting of the embryo, resulting in the formation of identical twins, is not inherited, and the frequency of occurrence of identical twins does not depend on other aspects of human biology. This seems random in every sense of the word, however, researchers believe that multiple pregnancy is genetically determined.
In a paper published in the journal Nature Ecology& Evolution, scientists used computer modeling to try to explain why natural selection favors the release of two eggs, despite the low survival rate of twins and the risk of twin births for mothers. In human populations without access to medical care, the birth of twins seems to be of little use. Twins more often die in childhood than single children, and twin mothers have a much higher risk of dying during childbirth.
Like other humanoid monkeys, women,apparently arranged to give birth to one child at a time. But if the birth of twins is so costly, then why has evolution still not eliminated this? Paradoxically, in populations with high fertility, twin mothers often have more offspring by the end of life than other mothers. This suggests that giving birth to twins can have an evolutionary advantage, at least for mothers.
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Why are there so few twins?
In the course of the study, scientists built simulationand mathematical models based on maternal, child, and fetal survival data from real populations. This allowed them to do something completely impossible: during the simulation, they could control the process of ovulation with one or two eggs during the cycles.
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Researchers also modeled variousstrategies where women switched from ovulating one egg to ovulating two at different ages. Then, the authors compared the number of surviving children in women with various forms of ovulation. Computer simulation results showed that women who switched from single to double ovulation under the age of 25 years had more children than those who always released one egg or always released two eggs. As the authors of the article write in the work, the obtained data indicate that with age, natural selection favors an unconscious switch from single to double ovulation. But why is this happening?
The reason this transition is beneficiallies in the survival of the fetus - the likelihood that a fertilized egg will lead to the birth of a child quickly decreases with the age of the woman. Thus, the transition to the release of two eggs increases the likelihood that at least one of them will lead to birth. Read more articles about the mysteries of nature and the world in which we live on our channel in Yandex.Zen. There are articles that are not on the site!