A large part of science is taking a really simple obvious thing, looking at it a bit closer, and realising it’s actually not very simple at all. Seriously, every answer to a question seems to lead to like 5 new questions! Take ‘species’ for example. Everyone knows what a species is- it’s different… things, like a cat is a different species from a dog. Obvious! So then- what about a dog and a wolf? Are they the same species? Or a horse and a donkey?
Not so simple after all! This week we’ll be taking a look at species, what they are, what can make them separate, and what can make them join together. This will include all kinds of wonderful words like pizzlies, ligers and hinnies!
What is a species?
Let’s start with what is a species. In the 1700s naturalists knew that there was loads of different plants and animals, and some were more similar than others. A Swedish botanist/zoologist called Carl Linneaus decided our understanding of the natural world was a bit messy so went about trying organising it, by grouping together organisms (animals and plants) that were similar. They were sorted at many different levels e.g ‘Kingdom’ (Animal, plant, fungi, protist and monera- single celled organisms), phylum e.g chordata who all have a backbone, then class, order, family, genus and species. I won’t give examples of all of those, but basically the creatures in each sub-group get more and more similar. So, species are the smallest level of similarity in terms of grouping things biologically (well, there is sub-species but let’s just ignore that for now).
What makes it into this classification then? One of the best definitions of a species is ‘the widest group of organisms that can mate and produce fertile offspring’. So a dog and a dog can make puppies which can then go on to have more puppies, but a cat and a dog can’t make a little baby hybrid, so are not the same species.
Some exceptions to the rule
You might have noticed I wrote ‘one of the definitions’ above. You see, whilst it’s a good definition, it doesn’t actually cover all bases. We have agreed that dogs are a species, but a Chihuahua and a Great Dane can’t mate and produce fertile offspring because, well, there are size issues… but a Chihuahua and a Jack Russel could mate, and a Jack Russel and a Beagle, and so on till you get to Great Danes so… are they a species then or not?
To help with these ambiguous situations, there are a few different ways of defining a species, including similarity in behaviour, DNA, morphology (physical structure) and ecological ‘niche’ (lifestyle). With these added definitions, dogs are all the same species because of their many similarities. On the other hand dogs and wolves, who can make fertile offspring together and share DNA ancestrally, are not the same species as they have very different behaviours and niches.
How species separate
There are a few different ways a species, with the same ancestry, might become separated like this. One is niche. With dogs one sub-group was taken into domestication by people, and then selectively bred to be different, and this is why we also have many different kinds of cows, horses and cats too.
Another is isolation. Sometimes species start to split off because they live in communities that are cut off from each other. Say you have two populations of lizard that live on either side of the mountain. Over time one population happens to get taller and taller, and the other smaller and smaller because of local conditions encouraging these traits. If they are brought back together they can’t mate anymore, and as there is no mixing of these populations there isn’t any medium sized lizards to bridge the gap, they would become separate species. This has historically happened with freshwater fish species who live in lakes. They can’t head over to another lake to see their cousins, so over time become more and more different. A great example of this is African cyclid fish, who have become very diverse in a short space of time due to their isolation, and so are studied by many scientists for just this reason.
Can they rejoin?
Now for the crazy interesting part. Species can split apart, but what happens when they come together. We already know that only the same species can produce fertile offspring together, but there are different species which can come together to produce infertile offspring (so they just have the one generation and never get the chance to be grandparents). Perhaps the most famous is when a female horse and a male donkey breed, and you get a mule. They are a hybrid of the two, and are infertile due to the different number of chromosomes coming from each parent. When you get a male horse and a female donkey, the baby is called a hinny. There are very rare cases when a mule does get a foal, but this is like a one in a million stroke of genetic luck. In fact it is so rare that in ancient Greece the phrase “when a mule foals” is sort of like our “when pigs fly” phrase today (or the Nigerian “when chickens grow teeth” which is a funny one for the imagination).
Of course there are other examples too. Next up we have the love story between tigers and lions. Very similar, but still different species. However, they can also have infertile offspring together called ligers (male lion, female tiger) or tigons (other way around). The most recent hybrid discovered, first seen in 2006, is the pizzly (grolar) bear. This is when a grizzly bear and a polar bear have cubs, and is thought to be happening more often as polar bears are spreading south into grizzly territories as the Arctic is getting more difficult to survive in.
So everything is mixing and splitting?
From all this you can see ‘species’ is more of a concept than a super stable set thing. Species can split apart over time, and sort of blob together too, though usually they don’t get much further than one generation. However, having species is a really useful way for scientists to classify the natural world and build family trees which show how they are historically related. Sometimes these classifications need tweaking when methods like analysing DNA get better, so it’s all still an ongoing process of discovery!
For more info:
Different levels of species classification broken down: http://ikonet.com/en/visualdictionary/static/us/species_classification
A little bit about Carl Linnaeus: https://www.britannica.com/biography/Carolus-Linnaeus/Classification-by-natural-characters
This is not any high-brow article, it’s literally just pictures of random animal hybrids: http://www.viralnova.com/real-animal-hybrids/
Title photo: Quanta Magazine