Below is a text with blanks. Click on each blank, a list of choices will appear. Select the appropriate answer choice for each blank.
下面的文本有一些空格。点击每个空格会出现一组选项，请为每个空格选出最合适的答案。
In The Origin of Species, Darwin provided abundant evidence that life on Earth has evolved over time, and he proposed natural selection as the primary mechanism for that change. He observed that individuals in their inherited traits and that selection acts on such differences, leading to change. Although Darwin realized that variation in heritable traits is a prerequisite for , he did not know precisely how organisms pass heritable traits to their offspring. Just a few years after Darwin published The Origin of Species, Gregor Mendel wrote a groundbreaking paper on inheritance in pea plants. that paper, Mendel proposed a model of inheritance in which organisms transmit discrete heritable units (now called genes) to their offspring. Darwin did not know about genes, Mendel’s paper set the stage for understanding the genetic differences on which evolution is based.
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