What is a species, anyway?

This is Part 1 of a four part miniseries on the process of speciation; how we get new species, how we can see this in action, and the end results of the process. This week, we’ll start with a seemingly obvious question: what is a species?

The definition of a ‘species’

‘Species’ are a human definition of the diversity of life. When we talk about the diversity of life, and the myriad of creatures and plants on Earth, we often talk about species diversity. This might seem glaringly obvious, but there’s one key issue: what is a species, anyway? While we might like to think of them as discrete and obvious groups (a dog is definitely not the same species as a cat, for example), the concept of a singular “species” is actually the result of human categorisation.

In reality, the diversity of life is spread across a huge spectrum of differentiation: from things which are closely related but still different to us (like chimps), to more different again (other mammals), to hardly relatable at all (bacteria and plants). So, what is the cut-off for calling something a species, and not a different genus, family, or kingdom? Or alternatively, at what point do we call a specific sub-group of a species as a sub-species, or another species entirely?

This might seem like a simple question: we look at two things, and they look different, so they must be different species, right? Well, of course, nature is never simple, and the line between “different” and “not different” is very blurry. Here’s an example: consider that you knew nothing about the history, behaviour or genetics of dogs. If you simply looked at all the different breeds of dogs on Earth, you might suggest that there are hundreds of species of domestic dogs. That seems a little excessive though, right? In fact, the domestic dog, Eurasian wolf, and the Australian dingo are all the same species (but different subspecies, along with about 38 others…but that’s another issue altogether).

Dogs
Morphology can be misleading for identifying species. In this example, we have A) a dog, B) also a dog, C) still a dog, D) yet another dog, and E) not a dog. For the record, A-D are all Canis lupus of some variety; and are domestic dogs (Canis lupus familiaris), C is a dingo (Canis lupus dingo) and is a grey wolf (Canis lupus lupus). E, however, is the Ethiopian wolf, Canis simensis.

How do we describe species?

This method of describing species based on how they look (their morphology) is the very traditional approach to taxonomy. And for a long time, it seemed to work…until we get to more complex scenarios like the domestic dog. Or scenarios where two species look fairly similar, but in reality have evolved entirely differently for a very, very long time. Or groups which look close to more than one other species. So how do we describe them instead?

Cats and foxes
A), a fox. B), a cat. C), a foxy cat? A catty fox? A cat-fox hybrid? Something unrelated to cat or a fox?

 

Believe it or not, there are dozens of ways of deciding what is a species and what isn’t. In Speciation (2004), Coyne & Orr count at least 25 different reported Species Concepts that had been suggested within science, based on different requirements such as evolutionary history, genetic identity, or ecological traits. These different concepts can often contradict one another about where to draw the line between species…so what do we use?

The Biological Species Concept (BSC)

The most commonly used species concept is called the Biological Species Concept (BSC), which denotes that “species are groups of interbreeding natural populations that are reproductively isolated from other such groups” (Mayr, 1942). In short, a population is considered a different species to another population if an individual from one cannot reliably breed to form fertile, viable offspring with an individual from the other. We often refer to this as “reproductive isolation.” It’s important to note that reproductive isolation doesn’t mean they can’t breed at all: just that the hybrid offspring will not live a healthy life and produce its own healthy offspring.

For example, a horse and zebra can breed to produce a zorse, however zorse are fundamentally infertile (due to the different number of chromosomes between a horse and a zebra) and thus a horse is a different species to a zebra. However, a German Shepherd and a chihuahua can breed and make a hybrid mutt, so they are the same species.

zorse
A zorse, which shows its hybrid nature through zebra stripes and horse colouring. These two are still separate species since zorses are infertile, and thus are not a singular stable entity.

You might naturally ask why reproductive isolation is apparently so important for deciding species. Most directly, this means that groups don’t share gene pools at all (since genetic information is introduced and maintained over time through breeding events), which causes them to be genetically independent of one another. Thus, changes in the genetic make-up of one species shouldn’t (theoretically) transfer into the gene pool of another species through hybrids. This is an important concept as the gene pool of a species is the basis upon which natural selection and evolution act: thus, reproductively isolated species may evolve in very different manners over time.

RI example
An example of how reproductive isolation maintains genetic and evolutionary independence of species. In A), our cat groups are robust species, reproductively isolated from one another (as shown by the black box). When each species undergoes natural selection and their genetic variation changes (colour changes on the cats and DNA), these changes are kept within each lineage. This contrasts to B), where genetic changes can be transferred between species. Without reproductive isolation, evolution in the orange lineage and the blue lineage can combine within hybrids, sharing the evolutionary pathways of both ancestral species.

Pitfalls of the BSC

Just because the BSC is the most used concept doesn’t make it infallible, however. Many species on Earth don’t easily demonstrate reproductive isolation from one another, nor does the concept even make sense for asexually reproducing species. If an individual reproduced solely asexually (like many bacteria, or even some lizards), then by the BSC definition every individual is an entirely different species…which seems a little excessive. Even in sexually reproducing organisms, it can be hard to establish reproductive isolation, possibly because the species never come into contact physically.

This raises the debate of whether two species could, let alone will, hybridise in nature, which can be difficult to determine. And if two species do produce hybrid offspring, assessing their fertility or viability can be difficult to detect without many generations of breeding and measurements of fitness (hybrids may not be sustainable in nature if they are not well adapted to their environment and thus the two species are maintained as separate identities).

Hybrid birds
An example of unfit hybrids causing effective reproductive isolation. In this example, we have two different bird species adapted to very different habitats; a smaller, long-tailed bird (left) adapted to moving through dense forest, and a large, longer-legged bird (right) adapted to traversing arid deserts. When (or if) these two species hybridised, the resultant offspring would be middle of the road, possessing too few traits to be adaptive in either the forest or the desert and no fitting intermediate environment available. Measuring exactly how unfit this hybrid would be is a difficult task in establishing species boundaries.

 

Integrative taxonomy

To try and account for the issues with the BSC, taxonomists try to push for the usage of “integrative taxonomy”. This means that species should be defined by multiple different agreeing concepts, such as reproductive isolation, genetic differentiation, behavioural differences, and/or ecological traits. The more traits that can separate the two, the greater support there is for the species to be separated: if they disagree, then more information is needed to determine exactly whether or not that should be called different species. Debates about taxonomy are ongoing and are likely going to be relevant for years to come, but form critical components of understanding biodiversity, patterns of evolution, and creating effective conservation legislation to protect endangered or threatened species (for whichever groups we decide are species).

 

Why we should always pander to diversity

Diversity in the natural world

‘Diversity’ is a term that gets used a lot these days, albeit usually in reference to social changes and structures. However, diversity is not merely a human construct and reflects an extremely important aspect of the natural world at a variety of levels. From the smallest genes to the biggest ecosystems, diversity is a trait that confers a massive range of benefits to individuals, populations, species and even the entire globe. Let’s dissect this diversity down at different scales and see how beneficial it can be.

Hierarchy of diversity
The generalised hierarchy at life, with diversity being an important component of each tier. At the smallest tier, genes underpin all life. The collection of genetic diversity is often summarised into a population (as a single cohesive genetic unit). Several populations can be pooled together into a single (usually) cohesive speciesDifferent species are then components of a larger community (which in turn are components of a broader ecosystem).

Genetic diversity

At the smallest scale in the hierarchy of genetic differentiation, we have the genes themselves. It is a well-established concept that having a diversity of genetic variants (alleles) within a population or species is critical to their future adaptation, evolution and persistance. This is because different alleles will have different benefits (or costs) depending on the environmental pressure that influences them; natural selection might favour one allele over another at one time, but a different one as the pressure changes. Having a higher number of alleles within the population or species means that there is a greater chance at least a few individuals will possess an adaptive gene with the changing environment (which we know can be quite rapid and very, very strong). The diversity serves as a ‘buffer’ against extinction; evolution by natural selection functions best when there are many options to choose from.

Without this diversity, species run the risk of having no adaptive genes at the ready to deal with a selective pressure. Either a new adaptive gene must mutate (or come about in other ways, such as through gene flow from another population or species) or the population/species will suffer and potentially go extinct. As strong selection causes the species to dwindle, it enters what is referred to as the ‘extinction vortex’. Without genetic diversity, they can’t adapt: thus, more individuals die off, causing more genetic diversity to be lost from the population. This pattern is a vicious cycle which can inevitably destroy species (without serious intervention).

Extinction vortex
A very dramatic representation of the extinction vortex.

For this reason, captive breeding programs aim to maintain as much of the genetic diversity of the original population as possible. This reduces the probability of entering a downward extinction spiral from inbreeding depression and helps to maintain populations into the future (both the captive one and the wild population when we reintroduce individuals into the wild).

“Population”  diversity

Because genetic diversity is critically important for species survival, we must also try to preserve the diversity of the entire gene pool of a species. This means conserving highly genetically differentiated populations within a species as a priority, as they may be the only ones that possess the necessary adaptive genes to save the rest of the species. This adaptive genetic variation can then be introduced into other populations in genetic rescue programs and serve as a means to semi-naturally allow the species to evolve. Evolutionarily-significant units (ESUs) are one measure of the invaluable nature of genetically unique populations.

Although many more traditional conservationists strongly believe that ESUs should be managed entirely independently of one another (to preserve their evolutionary ‘pedigree’ and prevent the risk of outbreeding depression), it has been suggested that the benefit of genetic rescue in many cases significantly outweighs this risk of outbreeding depression. For some species, this really is an act of rescue: they are at the edge of extinction, and if we do nothing we condemn them to die out.

Introducing genetic material across populations (or even species!) can generate new functional genes that allow the recipient species to adapt to selective pressures. This might sound very strange, and could be extremely rare, but examples of adaptive genetic material in one species originating from another species through hybridisation do exist in nature. For example, the black coat of wolves is a highly adaptive trait in some populations and is encoded for by the Melanocortin 1 receptor (Mc1r) gene. However, the specific mutation in Mc1r gene that generates the black coat colour actually first originated in domestic dogs; when wild wolves and domestic dogs interbred, this mutation was transferred into the wolf gene pool. Natural selection strongly favoured this new variant, and it very rapidly underwent strong positive selection. Thus, the adaptiveness of black wolves is thanks to a domestic dog mutation!

Species diversity

At a higher level of the hierarchy, the diversity of species within a particular community or ecosystem has been shown to be important for the health and stability of said community. Every species, however small or seemingly unimpressive, plays a role in the greater ecosystem balance, through interactions with other species (e.g. as predator, as prey, as competitor) and the abiotic environment. While some species are known to have very strong impacts on the immediate ecosystem (often dubbed ‘keystone species’, such as apex predators), all species have some influence on the world around them (we’re especially good at it).

Species interactions flowchart

The overall health and stability of an ecosystem, as well as the benefits it can provide to all living things (including humans) is largely determined by the diversity of species. For example, ‘habitat engineers’ are types of species that, by altering the physical environment around them (such as to build a home), directly provide new habitat for other species. They are a fundamental underpinning of many incredibly vibrant ecosystems; think of what a reef system would look like if there were no corals in it. There’d be no anemones growing colourfully; no fish to live in them; no sharks to feed on these non-existent fish. This is just one example of a complex ecosystem that truly relies on its inhabiting species to function.

Ecosystem jenga
Much like Jenga, taking out one block (a species) could cause the entire stack (the ecosystem) to collapse in on itself. Even if it stands up, however, the system will still be weaker without the full diversity to support it.

Protecting our diversity

Diversity is not just a social construct and is an important phenomenon in nature, at a variety of different levels. Preserving the full diversity of life, from genetic diversity within populations and species to full species diversity within ecosystems, is critical to maintaining healthy and robust natural systems. The more diversity we have at each level of this hierarchy, the greater robustness and security we will have in the future.