Why is a loss of genes considered an example of evolution?

It depends on which gene specifically is being lost. That gene could code for a negative trait and it would make sense to get rid of it. Like the article says(para 4), "we also know that most of the genes were not important for survival because they were lost." There's another reason for evolution to get rid of certain genes. Also(para 5), "If this happens, it won't be the end of males. Instead, a new pair of non-sex chromosomes likely will start on the path to becoming sex chromosomes". The Y chromosomes may die out, but they believe a new type of sex chromosome may form. Perhaps this new sex chromosome would be superior to it's predecessor.