Evolution is powered by variation: the differences in DNA sequences. One hugely important form of difference is copy number variation, where genes are duplicated or deleted from one generation to the next. In this Ri event, Aoife McLysaght from Univeristy of Dublin explains how copy number variations gave us colour vision, a sense of smell and haemoglobin in our blood, before exploring the role they play in diseases such as cancer, autism and schizophrenia. The event 'Too Much of a Good Thing' was presented at the Ri on Friday 28 March and forms part of the Ri's all-women line up for Friday Evening Discourses in 2014 as part of a year-long celebration of women in science. The Ri is on Twitter: http://twitter.com/ri_science and Facebook: http://www.facebook.com/royalinstitution and Tumblr: http://ri-science.tumblr.com/ Our editorial policy: http://www.rigb.org/home/editorial-policy Subscribe for the latest science videos: http://bit.ly/RiNewsletter