Imagine you are a biologist in a national park. To help monitor the park’s biodiversity, you need to count all the different species of plants. As you start collecting samples, you quickly realize this is no easy task. Where does one species end and the next one begin?
Here’s one popular definition: a species is a group of living things that can reproduce with one another to create fertile offspring. This definition is called the biological species concept. For example, horses and donkeys are considered separate species. Of course, a female horse and a male donkey can successfully reproduce. But their offspring—a mule—will be infertile.
Scientists also have other ways of identifying separate species. These techniques are based on things like differences in DNA sequences, evolutionary history, and physical appearance. But why use so many different tools? Why not just stick with the biological species concept?
Did you know? Many bacteria and fungi are known to scientists only from their DNA sequences. So far, no one has been able to find the right set of conditions to grow them in a lab.
Setting the mood
Well, it turns out it can be pretty difficult to test whether two animals can reproduce, especially in a laboratory. Stainless-steel lab benches, bright fluorescent lights, and other unfamiliar sights and sounds don’t make for a very romantic environment! To get in the right mood, most living things—even microbes, like bacteria and fungi—need just the right combination of light, nutrients, and temperature. For example, mycologists (scientists who study fungi) sometimes get their fungi to reproduce by including V8 Juice in their nutrient media.
Even when all of the other conditions are right, communication between potential mates can be complicated. Imagine being in a room with a person you’ve never met from the other side of the world. That room might be full of cozy couches and romantic lighting, but if you don’t speak the same language, wooing this person would be pretty tricky!
Did you know? DNA barcoding is a technique that uses a short, standardized DNA segment to identify species. In some cases, it has identified entirely new species. This technique was developed at the University of Guelph.
Darwin’s finches have a similar problem. They’re a group of about 15 closely-related species of birds that live on different islands. Over time, each species has evolved in a range of different ways, including body and beak size, diet, and mating songs. So if male finches from one island meet females from another, their mating songs just don’t hit the right notes. And if communication doesn’t occur, neither does mating.
So, according to the biological species concept, should these finches should be considered different species? Since they can’t normally reproduce, the answer would seem to be yes. However, there have been cases where male finches have been raised by parents of a different species. These males have learned the mating songs of their adopted species and successfully mated with females from that species! So you could also argue that they should be considered members of the same species after all.
It can be especially hard for scientists to apply the biological species concept in the wild. It can take years of careful, detailed observation to determine the parents and fertility of each new offspring, and to decide whether or not closely-related organisms are separate species. Imagine how much more difficult this would be in the case of plants and animals that are endangered or live in remote areas!
Other organisms don’t even reproduce sexually, making it virtually impossible to apply the biological species concept. Asexual reproduction is the production of offspring by a single living thing—no partner required! For example, parthenogenesis is a process where a single individual produces offspring that are genetically identical to it (except for random errors during DNA replication).
Parthenogenesis is common in bacteria and fungi, but it can also occur in some plants and animals. For example, a group of whiptail lizards is believed to consist only of females. And they only produce female offspring! Other living things, such as some worms and slugs, are hermaphroditic. That means they have both male and female sexual organs. Some hermaphroditic species still require a partner to reproduce, but some can self-fertilize.
Did you know? Organisms that don’t reproduce sexually can be at risk for a lack of genetic diversity. Whiptail lizards get around this by having twice as many chromosomes as other closely-related lizards that reproduce sexually.
For all of these reasons, biologists have developed a range of different species concepts. For example, the phenetic species concept is based on tiny differences in the shape, size, and appearance of different body parts. It is very useful to entomologists (scientists who study insects). Meanwhile, the genotypic species concept is based on differences in DNA sequences. It is useful when studying tiny microorganisms whose mating habits and appearance can be difficult or impossible to observe.
Telling biologists they can only use the biological species concept would be a little bit like telling all carpenters that the only tool they can use is a hammer! It is still an extremely valuable tool. But other species concept are also needed to help scientists discover, understand, and protect the Earth’s biodiversity.
Basic information on species:
Information on different forms of reproduction: