Randomly occurring mutations can lead for adaptations that are selected for over time. Eventually, enough genetic changes can accumulate until a completely new species is formed. Speciation describes the evolution of a new species. The opposite of speciation is extinction, which is when a species is lost. Although there are some debate over how to properly define a species, generally speaking, a species is when two similar organisms can successfully reproduce and produce a fertile offspring. For example, if two dogs can reproduce and produce puppies, they can go and have puppies of their own, then they are part of the same species. A mule is not a separate species. Mules are produced by mating a donkey with a horse, and the resulting mule is sterile, meaning that it cannot reproduce. Since mules cannot reproduce, they cannot have viable offspring, and it is not a separate species, rather it is a donkey-horse hybrid. So where do we get new species and why do animals lose the ability or motivation to interbreed. In allopatric speciation, a new species forms after some physical barrier like a mountain range or a river, separates one species into two different areas. Over time, mutations accumulate and two different species form. With sympatric speciation, new species form in the same general place, likely due to some micro-environment. For example, fish that live in different parts of the same lake can diverge into two different species. One of my favorite examples of speciation occurred with mosquitoes living in London. During the blitz in World War II, to stay from bombing, Londoners were forced into the London Underground, the famous subway system. Mosquitoes are parasites who feed on humans, and they followed the humans underground. Eventually, the mosquitoes living underground diverse from those who lived above the ground, the new sub-species of mosquitoes had different reproductive behaviors that are more reflective of life underground and no longer interbred with the above ground mosquitoes, forming a new subspecies found only in the London Underground. Now that we've talked about where new species come from, let's switch gears and talk about the opposite process, extinction. Extinction is the permanent loss of a species. Species can be extinct, meaning they don't exist anymore, or extinct in the wild, meaning that they only live in captivity, places like zoos or aquariums. Although we usually hear about extinction as a negative thing, extinction is a normal biological process. There is a background extinction rate reflecting the loss of about 0.1 to one extinction per year per million species. Sometimes this level goes way up, and these are what are called mass extinction events. Over Earth's history there were five mass extinctions. One of the most important mass extinctions was 65 million years ago when the dinosaurs went extinct, which gave room for mammals to begin to flourish. Some biologists have hypothesized that we're now in the midst of a sixth mass extinction. Rather than the background extinction rate of 0.1 to one extinctions per year per million species, it is now estimated the extinction rate is between 20 and 200 extinctions per million species each year. This is mainly due to habitat loss, pollution, ecological disruptions like invasive species or overharvesting. We'll come back to ecology and these ecological disruptions and why they're so important later in this course. We'll learn more about the sixth extinction in the next video and return to the impact of the sixth extinction on biodiversity later in this module.
