CBSE Class 10 Answered

Heredity lies at the heart of evolution. The variations in each generation are the raw material for natural selection, while the continuity from one generation to the next allows the changes wrought by natural selection to have long-term effects.

Gregor Mendel's 1866 paper on plant hybridization formed the basis for the modern study of genetics, which was used in the 1940s in support of Darwin's theory of evolution.

Much of Mendel's research concerned hybridism and its role in evolution. He transplanted unusual wild varieties of plants to his garden, and when they failed to converge with the known domestic forms he concluded that environmental influence, as in Lamarckian evolution, could not account for the modification of species .

Mendel's idea that some species might begin as hybrids was introduced by Carolus Linnaeus in the eighteenth century.

Bishop (1996) points to Mendel's "population approach" as an influence of a more modern evolutionary view, but Mendel's emphasis on populations was very different from Darwin's. Darwin saw a large population, interacting with every aspect of the environment, as the necessary focus of evolutionary study. Mendel used large numbers not because he thought it was necessary to observe the very mechanism but for the practical purpose of arriving at numerical laws which could provide insight into the mechanism. He believed that the mechanisms relevant to evolution lay in the organisms themselves, but they could not be directly observed so it was necessary to study the average behaviour of a large group and treat the data statistically.

Mendel aimed to provide some of the detail that his predecessors had failed to investigate. To this end, he began his discussion of the evolutionary significance of hybrids by making a clear distinction between "variable" and "constant" hybrids. The offspring of variable hybrids would display Reversion, while constant hybrids would breed true. Mendel's object was to determine which circumstances could give rise to which types, and whether constant hybrids could be produced in a reproducible experiment.

Mendel never reached a point where he could make a definite conclusion about the role of hybrids in the origin of species.

Darwin knew that the lack of an explanation for heredity left a big gap in his theory of natural selection.
Darwin, like many of his contemporaries, speculated that characteristics of the parents were blended -- like mixing paint -- as they passed to the offspring.

Mendel read Darwin with deep interest, but he disagreed with the blending notion, hypothesizing instead that traits, such as eye color or height or flower hues, were carried by tiny particles that were inherited whole in the next generation. According to him, an adaptive mutation could spread slowly through a species and never be blended out. Darwin's theory of natural selection, building on small mutations, could work.