David Kaplan explains why a simple definition of 'species' is hard to come by in our fifth In Theory video. QUANTA MAGAZINE Website: https://www.quantamagazine.org/ Facebook: https://www.facebook.com/QuantaNews Twitter: https://twitter.com/QuantaMagazine You can also sign up for our weekly newsletter: http://eepurl.com/6FnWj. David Kaplan is a theoretical particle physicist at Johns Hopkins University and a producer of the award-winning documentary Particle Fever. Filming for this video by Petr Stepanek. Editing and motion graphics by Ryan Griffin. Music by Podington Bear. Quanta Magazine is an editorially independent publication launched by the Simons Foundation.