Remember when the first mp3 players came out?   They could hold a thousand songs, which seemed like an impossibly huge number at the time.    But of course, there are millions of songs out there.   And according to new research published this week, over the last fifty years, popular music has changed -- evolved, actually -- in ways that even Darwin himself would find satisfying.   Using techniques normally reserved for evolutionary biologists, a group of British researchers -- including experts in both music and biology -- analyzed the songs on the Billboard Hot 100 charts from 1960 to 2010.   Their aim was to use the principles of evolutionary biology to quantify the development of music over time in the most concrete, scientific way possible.    Their theory is that, like life, culture evolves. So in the same way that you can analyze changes in organisms over time, you should be able to study changes in culture.    It’s just that, instead of studying living things -- like animals -- you’re studying popular music.   And instead of certain species, you’re focusing on different genres of music -- like rock, techno, disco, and jazz.   In that way, the different elements of each song can be studied like the traits of an individual organism -- traits that might mark an evolutionary link between songs over time.   The researchers started this evolutionary study by analyzing more than 17,000 songs, categorizing them based on certain defining characteristics.    For example, songs with major chords were classified as classic country, classic rock, or love songs, while those with more dissonant chords counted as jazz or blues.   Treating this data set like a fossil record, the team then looked for signs of sudden bursts of diversity, or unusual spikes in particular qualities in songs -- like their chord structure.   Three evolutionary spikes in the data stood out, showing up in the musical record around 1964, 1983, and 1991 -- times that, the researchers say, were associated with major evolutionary changes in popular music.   In 1964, for example, rock and soul were taking over, while doo-wop was on the decline.    Then, in 1983, new wave and hard rock were emerging.   But the biggest spike by far was in 1991, which marked the rise of rap -- mostly at the expense of rock’s former dominance.     Basically, the scientists say, these genres were evolving through a form of artificial selection, with both artists and listeners favoring some traits of music over others. The more that people selected for those traits, the more prevalent they became across all popular music over time.   And the dominant player here -- the most “evolutionarily successful,” as biologists might say -- has been rap.   According to the researchers, rap has had the greatest effect on what the rest of popular music has come to sound like, as some of its signature traits -- like energetic speech and a lack of a chord structure -- began to show up more and more in the Hot 100 charts.   And rap’s success has also spurred diversity, with more and different kinds of artists creating new variations within the genre.   The team says it plans to use their new method to study other evolutionary concepts, like competition, in popular music.   But for now, you can just imagine that -- every time you put your headphones on -- it’s like a little battle for the survival of the fittest going on between your ears.   In other news, new research has uncovered genetic links between two distinctive sleep disorders: sleepwalking and sleep terrors.   Sleepwalking is a condition in which subjects perform complex tasks -- like sitting up, walking around, or driving -- when they’re asleep. They aren’t aware of their surroundings, and they usually don’t remember what they did when they wake up.   Sleep terrors, meanwhile, are much more than your average nightmare. Instead of walking, these subjects demonstrate fear in their sleep, often by screaming, crying or flailing around, and while it usually affects children, the symptoms can persist into adulthood.   In a new study, Canadian sleep-scientists interviewed parents about their sleeping habits, and those of their children -- ultimately studying nearly 2000 children over 12 years.   The researchers found that the children of parents with a history of sleepwalking were much more likely to sleepwalk themselves.   Specifically, they found, children of parents who never sleepwalked had a 22.5 percent chance of nighttime wanderings.   But if one parent had a history of this behavior, then they were three times more likely to sleepwalk.    And if both parents were sleepwalkers? Their child was seven times as likely to sleepwalk.   It also turned out that children with sleep terrors were more likely to have parents who sleepwalked.    And among those children, the condition had a much greater chance of sticking around past age five.    Both disorders are thought to come from a similar mechanism in the nervous system, when someone emerges from deep, slow-wave sleep, just enough to show symptoms.    So it could make sense that both sleepwalking and sleep terrors would be influenced by the same set of genes.   It’s just another of the delightful qualities you can pass on to your offspring!    Thanks for joining me for this SciShow News, brought to you by our patrons on Patreon. If you want to help, just go to Patreon.com/SciShow. And don’t forget to go to YouTube.com/SciShow and subscribe!