In his later years, Darwin encountered a boundary problem that evolution could not solve, a problem proposed by engineer Jenkin, that new small variations would be completely drowned in the normal mating of individuals, and small variations produced by natural selection would be ignored in a large number of individual mating. That is to say, the advantages of parents may not be reflected in the offspring, which is contrary to the theory of evolution. Darwin could not give this answer.
The person who answered this question was Gregor Mendel (1822-1884), Mendel began his plant research from 1956, he chose peas among many plants, scientific results sometimes require a certain amount of luck, pea is a self-pollinating plant, short life cycle is easy to observe, easy to pollinate, the flower color is very convenient for statistics and control of traits. After 8 years of hard work, Mendel successfully proposed his first and second laws of genetics. The first law is that the two genetic factors can remain relatively independent in the case of combination, and the paired genetic factors to form offspring are separated and recombined, that is, the combination of AA and Ab factors will be formed, AA, Ab, Aa, ab four combinations and present a certain proportion. The second law, the law of free combination, is that the genetic factors of an organism are not selective, free, and random after the formation of offspring. Morgan turned the focus of his research to genetics after Mendelian genetics received attention, and through the study of Fruit flies, he verified that Mendel's first and second laws applied to animals, and the genetic factors of the offspring of Fruit flies were fully consistent. However, there are also some non-conformities, the use of white-eyed male fruit flies and red-eyed female fruit flies to produce the first generation of offspring do not exist white-eyed fruit flies, red-eyed offspring mating and then appear white-eyed male fruit flies and all are red, that is to say, the factors that control color are chained fixed on chromosomes, which is the third law of genetics found by Morgan.
This means that the laws of evolution are not only reflected in the macroscopic level of life, but also apply to the genes, the basic units of heredity. Genes are the smallest independent units of inheritance, do not mix, can not divide, the whole of the gene composition of a whole for genotype, is the collection of all genes, in the process of sexual reproduction, genotype will divide and recombine, each person's entire gene half from the father, the other half from the mother. The smallest unit of life is made up of genes, which support the basic structure and performance of life and store various information about life. Modern comprehensive evolution combines Darwinian evolution and genetic inheritance to form the following main contents:
1. It is believed that natural selection determines the direction of evolution and makes organisms develop in the direction of adapting to the environment. Advocating two-step adaptation, variation needs to be tested by choice to form adaptation.
2. Population is the basic unit of biological evolution, and the evolutionary mechanism belongs to the genetics of life as a whole. According to Dubzansky, populations are genetically mixed individual taxa, and the essence of evolution lies in the change of gene frequency and genotype frequency within the population, and the resulting change in biological type.
3. Mutation, selection, isolation of the mechanism of speciation and biological evolution, mutation provides the initial material for the inheritance of genes, and is the source of biological variation.
By the 1970s, on the basis of synthetic evolution, synthetic theories at the molecular level emerged. At the molecular level, natural selection is the elimination of "good" alleles and the elimination of "bad" alleles, while there are multiple selection patterns at the molecular level in nature [1].
[1] Shen Yinzhu and Huang Zhanjing, Evolutionary Biology, pp. 12-13.