You probably think you know what a species is. Look at the zebra picture below. How many species do you see?
Look at the butterflies below. How many species do you see?
Look at the cockroaches below. How many species do you see?
Are you correct? I will tell you the correct answer. But first will I challenge your notion of what a species is and in the process test your faith in biologists.
Perceptions of species
In my last post I discussed how the idea of species allows us to measure biodiversity. I also said that species are independently derivable units because living things truly exist as species, and they are not just a man-made category. The idea of species is surely a wonderful and elegant thing. However, species in practice are a terrible mess.
The dictionary definition of species is “the major subdivision of a genus or subgenus, regarded as the basic category of biological classification…” . This is a terrible scientific definition. 1. You could divide a genus (one of Linnaeus’ units) however you want, it doesn’t have to be scientific in any way. 2. Even if it is regarded as the basic unit of classification, that says nothing about what patterns that category should follow. The scientific definition of “species” needs to be clearer than that.
Unfortunately science doesn’t do much better. Here is quite a concise source that describes only 26 of the many proposed definitions for a species.
There is no consensus on what definition we should use for species!
Everyone intuitively thinks they know what a species is, but it’s extremely difficult to fit any single definition to the many real-world possibilities. Let me give you some examples of what I mean.1) It seems important that individuals of the same species be able to mate with each other. But a lion and a tiger can mate with each other and most of use agree they shouldn’t be the same species. Also, a lion in 1800 and a lion in 2015 cannot mate with each other (unless…) but we probably think they should be the same species. 2) It seems important that individuals of the same species look similar. But how similar? Human males and human females can mate with each other but we look different. With some other animals these differences are more obvious or extremely exaggerated. [slideshow_deploy id=’664′]
3) It seems important that individuals of the same species be closely related to each other. But what happens when closely related things look more different from each other than their distant relatives? Does this actually happen? Keep reading.
These three problems confound our ability to have a single species concept that works for all of life. We are also in need of a species concept that is practically useful, not just intellectually satisfying, which makes the problem even worse.
Let’s go back to those pictures we looked at in the beginning and see what we think.
The “correct” answers
Ok, the zebras.
There are 8 species of zebras. At first this might be surprising but only if you didn’t look close enough. The stripes between the species are quite different. Although this is a little baffling at first, these are good species most people can agree on.
Now the butterflies.
There are actually 4 species here. Look closer. Do you see the differences? No? That’s because there really aren’t any.
Then why do we call these different species? Well these species are actually different enough where we know they probably don’t mate with each other. The differences, however, are not in the way they look. The differences are in their genes1. There is enough genetic difference between them where we can say that they probably don’t mate with each other. Sooner or later they will probably start to look different, but they haven’t evolved enough where their “speciesness” is visible to the naked eye. Not everyone will agree on these species (although I believe they deserve to be called species).
Finally, the cockroaches. As most of your experiences with cockroaches, this one will make you angry. How many species are there?
Actually, there’s only two2. The reason why there’s only two is that cockroaches, like a lot of other animals are “polymorphic”. That means that single species have multiple forms. For the species on the right, the adults look very different from the juveniles. The juveniles themselves look quite a bit different from each other as well. For the species on the left we have the same issue where the juvenile looks different from the adults, but also the male and female adults look different from each other.
Take a look at the example of these damselfly wings. All of the wings in each box have varying degree of differences, but they are all the same species4. Scientists aren’t sure why they look so different but they are pretty sure they interbreed with each other.
These are only a few reasons why “species” is one of the most difficult ideas in biology. Unfortunately almost everything else in biology relies on the assumptions that we are dealing with species! My previous post discusses a bit about why.
Do species truly exist?
The question it comes down to in the end is, “Do species truly exist?”. If they do then we would want to know if every living animal on the planet belong to a species. These are very different questions. I refuse to answer either one for fear of being blacklisted from my next conference. Make up your own mind using this last example that will truly “break the biology-internet”.
This figure shows a series of salamanders. Using genetics, we know that population A is very closely related to population B and they mate with each other. The same goes for population B & C, they can mate with each other and they are closely related. The same goes for C & D, D & E, E & F, and F & G. Population A & G (and then B & G) are the most distantly related of all and they cannot mate in places where they both occur. So each neighboring pair (AB, BC, CD, DE, EF, FG) can all mate except for A & G (or B & G) 5.
Where are the species here? This biological paradox is a special case we call a “ring species”. Do we agree on how to define ring species? I’ll let you answer that for yourself.
1. John M. Burns, Daniel H. Janzen, Mehrdad Hajibabaei, Winnie Hallwachs, and Paul D. N. Hebert. 2007. DNA barcodes and cryptic species of skipper butterflies in the genus Perichares in Area de Conservación Guanacaste, Costa Rica. PNAS. 105, 6350–6355, doi: 10.1073/pnas.0712181105
2. Evangelista DA, Bourne G, Ware JL (2014) Species richness estimates of Blattodea s.s. (Insecta: Dictyoptera) from northern Guyana vary depending upon methods of species delimitation. Systematic Entomology 39: 150–158. doi: 10.1111/syen.12043
3. Evangelista DA, Chan K, Kaplan KL, Wilson MM, Ware JL. 2015. The Blattodea S.S. (Insecta, Dictyoptera) of the Guiana Shield. ZooKeys 475:37-87
4. Sanchez Herrera M, Kuhn WR, Lorenzo-Carballa MO, Harding KM, Ankrom N, Sherratt TN, et al. 2015. Mixed signals? Morphological and molecular evidence suggest a color polymorphism in some neotropical polythore damselflies. PLoS One 10:e0125074.
5. Craig Moritz, CJ Schneider and DB. Wake. 1992. Evolutionary Relationships Within the Ensatina Eschscholtzii Complex Confirm the Ring Species Interpretation.Syst Biol 41 (3): 273-291. doi: 10.1093/sysbio/41.3.273
Dominic Evangelista is a PhD candidate studying the biodiversity and systematics of the cockroaches of the Guiana Shield. Follow him on twitter @Roach_Brain or ask him a question about cockroaches!