[Carina writes...] There are over 22,000 species of trees in the American tropical forests. In just 15 patches of forest, each about the size of two football fields, scientists catalogued over 1,000 tree species in Yasuní National Park in Ecuador, more than the 620 tree species native to all of the US and Canada.
These are the numbers that blow my mind and get me excited about trying to understand the origins of biodiversity, especially in the tropics. I have good company— I think about 30% of ecologists and evolutionary biologists start talks with a slide full of colorful, beautiful organisms, and say that the ultimate goal of their research is to understand where all this diversity comes from.
Imagine it’s the year 2100, and we are sending some colonies of people to Jupiter, and others to Mars. We can’t leave home without our best friends and helpers, dogs! NASA decides that Labrador Retrievers will be the most useful space pups, and away they go, with several mom and dad dogs so there is a renewable supply. Because of the different atmospheres and gravities, dogs on Mars do better when they are small and compact, while really big dogs do better on Jupiter. It doesn’t take long, maybe a couple hundred years, before colonists choosing the healthiest dogs have bred Mars labs to be the size of Chihuahuas and Jupiter labs to be the size of Great Danes.
That’s essentially how speciation works. We went from only Earth Labs to mini Mars dogs plus giant Jupiter dogs, and they’ve become different species because they can’t live in each other’s habitats and breed with each other.** The Earth Labs haven’t gone away either, but they will probably still encounter some of the same problems we discussed for a Mars-Jupiter pairing. And you might imagine that even Earth Labs aren’t going to look the same in a few hundred years— we’ll have bred them to be EVEN MORE ADORABLE OMG!!!
can no longer interbreed. Doggie sizes are scaled to each other but not to their home planets.
There are different categories of isolating “barriers.” Some are applicable before mating can even happen, like if populations can’t survive in each others’ habitats, or if mating won’t work. For example, plants might flower at different times, or birds might not recognize another species’ song. Other barriers are applicable after mating, like genetic incompatibilities causing babies to be inherently messed up, or poor competitors compared to babies that aren’t hybrids.
Importantly, not all of these barriers have to be present to prevent successful breeding. There could be just one that is really strong, such as different flowering seasons that prevent two plant species from ever hybridizing in nature. On the other hand, complete reproductive isolation could be from a bunch of weaker barriers adding up, so the odds of successful hybrid babies are virtually zero by the time you account for all the problems, like with our Mars and Jupiter dogs.
In all cases (or nearly all, depending who you talk to***), speciation on Earth happens when populations become geographically isolated for some reason, like our dogs on different planets. For example, when the Isthmus of Panama emerged from the sea, it cut off the Atlantic and Pacific tropical fish from each other.
Speciation is just a byproduct of these geographically separated populations adapting to different conditions. Like all evolutionary processes, except human-directed domestic breeding, it just happens, without the organisms trying to make it happen or planning ahead. In other words, there is no natural selection for speciation.**** Even in our dog example, breeders were not trying to make it difficult for Mars dogs to go to Jupiter and make hybrid pups, it just happened that way as a result of different breeding preferences based on the environment.
Speciation can seem a bit mysterious because it’s a process that takes a long time to happen. And there are still some unanswered questions! Is geographic isolation totally necessary for speciation? Why does speciation seem to happen faster at lower latitudes? What is a species and how does speciation work in bacteria and other clonal organisms? But in a nutshell, speciation is the accumulation of complete reproductive isolation in populations that are geographically separated for a while. Hopefully it seems a little less mysterious now, and if you want waaaay more detail, check out Talk Origins and the book Speciation by Coyne and Orr.
**In the real world, even if Great Danes and Chihuahuas can’t mate, dogs are still one species because there are intermediate sized dogs that can mate with anyone, and the puppies will do fine anywhere as long as they’re adorable enough to make human friends. The key to our example is that the two breeds have adapted to very different environments while in complete isolation.
***Some scientists think that “sympatric speciation,” where speciation happens without geographic isolation, is possible. Some disagree. We’re going with the “sympatric speciation doesn’t happen” scenario here.
****That is, USUALLY there is no natural selection for speciation. However, there could be rare situations where populations become almost completely reproductively isolated while separated. Then if they come back into contact and hybrids are really unhealthy, organisms that prefer mates in their own group could have healthier babies with same preferences. Thus there is selection to “reinforce” reproductive barriers so mating happens more within groups than between groups.