In humans, twins usually occur in only about 1-3% of all newborns. Although twins are at a much higher risk of prenatal and postpartum health problems for mothers and children than singletons, twins can be found in all populations. Given these risks, natural selection seems to have prevented twins from becoming more common in evolution. But why didn't the evolution of natural selection completely prevent the emergence of twins?
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A popular hypothesis is that the survival risk of twins is partially hidden in natural selection because twins are accompanied by a higher fertility rate. The conventional idea is that women who are more fertile than the average person are also more likely to excrete more than one egg when ovulating - making twins a sign of high fertility. Many studies have analyzed population data and obtained results consistent with this view.
Now, however, the new study shows that previous analysis is flawed. Alexandre Courtiol, a researcher from Leibniz zoo and Wildlife Research Institute in Germany, explained: "previous studies are problematic because they can't tell us whether mothers with twins give birth more frequently because they are particularly able to give birth or because more frequent childbirth increases the chance that one of these births is twins."
The new results show that people who have twins are not particularly fertile. Previous science confused causality. Ian Rickard of the University of Durham, UK, the lead author of the study, added: "if a mother gives birth more frequently, one of these births is more likely to be twins - just as you are more likely to win a prize if you buy more lottery tickets, or have a car accident if you drive often." When considering the "lottery effect", the researchers found that mothers who are more likely to have twins actually have fewer children - a result that contradicts previous studies.
To re-examine the relationship between twins and fertility, the international team of 14 scientists combined large birth outcome data sets from several regions of pre industrial Europe (now Finland, Sweden, Norway, Germany and Switzerland). "All this data comes from ancient parish records, which have been carefully digitized and transcribed," said virpi lummaa, co-author of the study and from the University of Turku, Finland Fran ç OIS Rousset, co-author of the Institute of evolutionary sciences in Montpellier, France, added: "in order to avoid the statistical traps that plagued previous studies, we must also deploy efficient and carefully calibrated statistical procedures."
Figuring out what forms the relationship between twins and fertility is not only an academic interest, but also a public health issue. In fact, biomedical research seeking to improve women's fertility has compared mothers with and without twins. However, Erik postma, co-author from the University of Exeter in the UK, pointed out that "this research design ignores many factors affecting women's fertility frequency, which will mask any real physiological differences between mothers with and without twins." In short, comparing a group of mothers with twins with a group of mothers without twins may mask the effects of twins and fertility genes, or create an illusion in the absence of these effects.
"We still don't know much about twins, but our research shows that twins are not eliminated by natural selection for two reasons: first, twins are the result of double ovulation, which compensates for reproductive aging, and all mothers except the youngest mother can benefit from it; second, when the risk of early death of twins is not too high, twins are related to a larger family size, although women with twins Fewer births. This is because the birth of twins brings two offspring, not one, "Courtiol concluded.