Speciation is the formation of a new species. A species is defined as a group of organisms that can interbreed successfully with each other. In allopatric speciation, two populations can get physically separated and become two species. In sympatric speciation, on the other hand, a new species starts in the same location as the parental species due to polyploidy or hybridization.
One of the fun things about teaching biology is you start really understanding how the human mind works, and how that really doesn't work very well when studying the natural world. We love categories but nature kind of ignores our loves and desires for categories. One of the categories that human beings love to put animals and other creatures into is the idea of what is a species. We've even created this thing called speciation which is how new species get formed.
Well, what is a species? A scientist might define it as a group of organisms that can interbreed successfully with each other. Now there are some obvious examples of what is a species. I'm a human, you're a human. Anybody who's watching this in about four or five years is a human. We can all successfully interbreed with each other. We're not but we could. But, when you look at something like a chihuahua and a Great Dane, they look very different but they're members of the same species. If you give one of them a step ladder they can successfully interbreed. But then there's other things which you look at them and think, hmm, wolf and dog they look very similar. Usually they don't successfully interbreed and there are some kinds of flowers where I look at them and I think that flower and that flower are identical. But to a botanist "Oh no. They're separate species" and they'll, if you try to crossbreed them, failure. So, you have to accept all of what I'm talking about with a little bit of a grain of salt and just understand that this is our best descriptions of something that in the real world can get kind of messy.
So, again speciation is the formation of a new species. Now one way that we see it in the evolutionary timeline is we see what's sometimes called phylletic speciation. That's where you have some kind of organism and then if you look back through the fossil records you can see slowly over time, that one organism, its descendants always look a little bit different until finally you get two clearly related members of the same group but you look at their structures and you think about it and you realise that if you had.