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Many Mallard in the East are of domestic origin or have bred with domestic ducks . Bears may have become much bigger than today and then smaller , or more than one type of bear , of different sizes , may have existed together , as we see in N America today . Quite a few studies have concluded that the blood types of the mother and the fetus can affect the chance of a spontaneous miscarriage , several have inferred selection against the A allele , and at least one study suggested that natural selection would favor a male population with high frequencies of the O phenotype ( because the fetus would have an ABO phenotype similar to the mother 's ) . Five-year plans have become more workable three-year plans . The Graduate School has identified some programs that have undergone significant attrition and no longer exhibit the vitality they should .