Lecture 3

The two most influential biologists in history, Darwin and Mendel, were contemporaries and yet the initial acceptance of their ideas suffered very different fates. Darwin was concerned with the evolution of complex traits (and hence concepts from population and quantitative genetics), while Mendel was concerned with the transmission of traits that had a simple genetic basis (often a single gene). Modern genetics and evolutionary theory was dependent on a successful fusion of their two key ideas (Mendel’s that genes are discrete particles, Darwin’s of evolution by natural selection). Against this background, its interesting to consider the initial fates of both of their original papers. In 1859, Darwin published his Origin of Species. It was an instant classic, with the initial printing selling out within a day of its publication. His work had an immediate impact that restructured biology. However, Darwin’s theory of evolution by natural selection, as he originally presented it, was not without problems. In particular, Darwin had great difficulty dealing with the issue of inheritance. He fell back on the standard model of his day, blending inheritance. Essentially, both parents contribute fluids to the offspring, and these fluids contain the genetic material, which is blended to generate the new offspring. Mathematically, if z denotes the phenotypic value of an individual, with subscripts for father (f ), mother (m) and offspring (o), then blending inheritance implies zo = (zm + zf )/2 (3.1a)