I wrote a comment about it on HN some time ago that explains the problems of the definition of a species from my layman&#x27;s point of view:<p><a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10478762" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10478762</a><p>In particular, take this example:<p>&gt; Consider the following: imagine that you assembled a gathering of all your ancestors in the male line for say, the past two million years. Let&#x27;s say, for illustrative purposes, that the gap between each generation is twenty years. So you&#x27;d have 5 people to represent a century - you, your father, your grandfather, your great-grandfather, and your great-great grandfather. That makes 50 people per millennium, 500 people per ten thousand years, 5,000 people per hundred thousand years, and 50,000 people per million years. So two million years of ancestry gets you 100,000 people, the size of a small city, or about enough people to fit in the Rose Bowl stadium.<p>&gt; So really, not that many people in the grand scheme of things. Now line all those people up in order going all the way back to the first guy, who we&#x27;ll just call Adam. The difference between Adam N and Adam N+1 is very small. There&#x27;s no point at which you can say &quot;Adam N is this species and Adam N+1 is that other species.&quot; But there&#x27;s a hell of difference between you and Adam.<p>We can have a useful handwavy definition of &quot;species&quot;, but there&#x27;s never going to be a way to define the word in such a way that you can always filter every organism that ever lived into one clear bucket or another.<p><i>Lots</i> of species can interbreed. Are lions and tigers the same species? OK, maybe we can modify that and say &quot;with fertile offspring.&quot; But sometimes the offspring <i>are</i> fertile among couplings we would say are definitely interspecies. For example, some mules are actually fertile!<p>On the other hand, there&#x27;s species that are very genetically similar that can&#x27;t interbreed. And there&#x27;s species that are very genetically similar that simply <i>don&#x27;t</i> interbreed or maybe can&#x27;t without human intervention. (Silly example: teacup breed of dog and huge breed of dog.)<p>For basically every definition of species you can think of, nature will throw an example at you that makes it look kind of silly. I suppose you <i>could</i> just deal with the silliness, put your foot down, and say &quot;lions and tigers are the same species, deal with it.&quot; But what if something like the humanzee is proven possible? (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Humanzee#Feasibility" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Humanzee#Feasibility</a>)