Today we know that certain chemicals and radiation alter the structure of DNA, thus changing (or mutating)genes. Genes are the ultimate instructions for the characteristics of an organism. Knowing about genetics, sexual reproduction and that genes can mutate would have helped Darwin to explain which of the following aspect of his theory? survival of the fittest inheritance of acquired characteristics sources of variation natural selection

Knowing about genetics, sexual reproduction, and mutations would have helped Darwin explain the **sources of variation**.<br /><br />Darwin's theory of evolution by natural selection rests on the existence of variation within a population. He observed that individuals within a species are not identical and that some variations are heritable, meaning they can be passed from parents to offspring. However, Darwin didn't know *how* these variations arose. He observed the *fact* of variation but lacked a mechanism to explain it.<br /><br />Genetics, sexual reproduction, and mutations provide that missing mechanism.<br /><br />* **Genetics:** Shows how traits are inherited through genes.<br />* **Sexual Reproduction:** Through recombination (the shuffling of genes during the formation of sperm and egg), sexual reproduction creates new combinations of genes in offspring, leading to variation.<br />* **Mutations:** Provide a source of entirely new genetic variations. Changes in DNA sequence can lead to new traits, some of which may be beneficial, harmful, or neutral in a given environment.<br /><br />While Darwin's theory included the concept of "survival of the fittest" and "natural selection" (which are essentially two sides of the same coin), he struggled to explain the *origin* of the variations upon which natural selection acted. The discovery of genetics, mutations, and a deeper understanding of sexual reproduction provided that crucial piece of the puzzle. The inheritance of acquired characteristics, while a popular idea in Darwin's time (proposed by Lamarck), is *not* supported by modern genetics and was not part of Darwin's theory.<br />