# Secondary Sexual Characters Variable explained — The Origin of Species > Chapter companion for The Origin of Species by Charles Darwin. ## What happens Darwin argues that secondary sexual characters are highly variable, more so than other traits, because they are shaped by sexual selection which is less rigid than natural selection. He notes that differences between sexes often mirror differences between species of the same genus, supporting common descent. Variability in these parts is exploited by both natural and sexual selection to adapt species and sexes to their roles. ## Themes in this chapter ### Variation and Inheritance High variability of secondary sexual characters and their inheritance through sexual selection. ### Common Descent and the Tree of Life Species of the same genus and sexes of the same species share a common progenitor, explaining parallel differences. ## Character check-ins ### Charles Darwin Author presenting the argument on variability of secondary sexual characters and their relation to species differences. ### Sir John Lubbock Cited for observations on crustaceans illustrating the law that sexual and specific differences appear in the same organs. ## Key lines > I think it will be admitted by naturalists, without my entering on details, that secondary sexual characters are highly variable. Darwin asserts that naturalists generally agree that secondary sexual traits vary greatly. _Opening claim establishing the chapter's focus._ > The cause of the original variability of these characters is not manifest; but we can see why they should not have been rendered as constant and uniform as others, for they are accumulated by sexual selection, which is less rigid in its action than ordinary selection, as it does not entail death, but only gives fewer offspring to the less favoured males. Though the origin of variability in these traits is unclear, their lack of uniformity is due to sexual selection being less strict than natural selection, since it reduces offspring rather than causing death. _Explanation of why secondary sexual characters remain variable._ > It is a remarkable fact, that the secondary differences between the two sexes of the same species are generally displayed in the very same parts of the organisation in which the species of the same genus differ from each other. Notably, the same body parts that differ between sexes also tend to differ between species within a genus. _Key observation linking sexual and specific differences._ > I look at all the species of the same genus as having as certainly descended from the same progenitor, as have the two sexes of any one species. Darwin views all species in a genus as descended from a common ancestor, just as the two sexes of a species are. _Core statement of common descent applied to both species and sexes._ ## Links - HTML: https://www.betterreads.online/discover/the_origin_of_species_se/chapters/secondary-sexual-characters-variable - Book: https://www.betterreads.online/discover/the_origin_of_species_se - All chapters: https://www.betterreads.online/discover/the_origin_of_species_se/chapters