Transcription
[Music] The average person on the street can probably tell you that cats and dogs are different species. And chances are, they can come up with some reasons why. Cats and dogs look different. They behave differently. They don't make kitten-puppy hybrids. The same goes for squirrels and raccoons. We know intuitively that they're different species.
But what about these squirrels or these trees? Well, it turns out the squirrels are different species, and the trees are the same species. But how do we know that? And what is a species, anyway?
Well, it depends on who you ask. If you ask a biologist, they might tell you that species are groups of interbreeding natural populations that are reproductively isolated from other such groups. Some would add the idea of potential to reproduce. For example, if two groups are separated by a barrier, but they could reproduce if they got together, then they're still the same species.
An ecologist might give you a different definition. A species is a set of organisms adapted to a particular set of resources in the environment. Some might add that species have different physical characteristics or morphologies. Between these two concepts, we get something that's pretty broadly accepted across most of science. Sounds good, right?
But in nature, things can get pretty fuzzy. For one, how meaningful is it that two groups have the potential to interbreed if they never meet in nature? Elk live in North America, and Red Deer live in Eurasia. In captivity, they can produce fertile offspring, but they could not possibly meet in nature. Stickleback fish live in isolated groups all across the Northern Hemisphere. Some groups look different, and they don't meet in nature, but they can reproduce in an aquarium tank.
Second, some groups that everyone agrees are different species can interbreed. Take the polar bear and the grizzly bear. They look different. They live in different habitats. They eat different foods. DNA evidence suggests these two groups have been reproducing separately for hundreds of thousands of years. Clearly a different species, right? But they can and do produce hybrid offspring in nature. By the way, this is not unusual. Coyotes and grey wolves are different species, but they have interbred. Coyotes on the east coast of North America have a significant amount of DNA not only from wolves but also from dogs. There are many more examples of hybridization between species.
If two groups look alike, we have to work really hard to even know if they have interbred or not. Beyond all of these challenges, this species concept only fits with some types of living things, mainly plants and animals. Lots of living things, including bacteria, reproduce without a partner. So this whole interbreeding metric doesn't work for them. It doesn't work for defining fossil species either. They can't reproduce at all. Species of bacteria and fossils are usually defined by how different they are, measures that can be pretty arbitrary. Bacteria species are defined by differences in their DNA sequences. Fossil species are defined by differences in their anatomy, their age, and where they were found. Scientists in other fields use entirely different definitions.
All of this is to say that the best way to define a species depends on what you're studying and what you're trying to do. There is no universal definition. To take a page out of the book of the philosophers, it's actually helpful to stop looking at species as things and instead look at speciation as a process that generates transient things. Species aren't fixed entities; they change.
Evidence from fossils tells us that species that lived in the past were different than the ones that are alive today. Add to this evidence from DNA, anatomy, and embryos, and we know that living things change over time. The world we live in today is just a snapshot in a long evolutionary timeline. Looking from this perspective, we can see that there is not one moment in time where an old species becomes a new, different species, and there's not a moment in time where one species becomes two. In our snapshot of today, some groups are clearly different species. Many others are in the process of speciation, and some groups are even in the process of coming back together, just as they have in the past. These processes will continue into the future. Some of the boundaries are fuzzy, and that's okay.
All of this may leave you wondering, why do we even use the word species at all? Well, there are a lot of reasons why this species concept is important. One is conservation. If we hope to protect a group from habitat destruction or potential extinction, we need to be able to define the group we're trying to protect. Another is that this concept of species helps us and the scientific community to categorize and communicate about groups of living things. When scientists study speciation as a process that is driven by natural selection, they gain insights into some of the biggest issues faced by humankind, like antibiotic resistance, world hunger, and cancer. Instead of getting too caught up in what a species is, you may be better served to focus on the context in which the term is being used. Different contexts call for different definitions.