What is a species, and how many species are there?

What is a species?

The most common definition of a species is that a species is a group of organisms that can successfully breed to produce fertile offspring. This is known as the biological species concept.

This species definition divides up animals, plants and other forms of life into groups based on their reproduction. For example, deer species such as the sika and roe are considered separate as they can’t have fawns together.

While some closely related species, such as the horse and the donkey, can sometimes have offspring together, these are often infertile. As a result, animal hybrids such as the mule aren’t considered to be species.

However, as our scientist Dr Jeff Streicher explains, this biological meaning of a species isn’t always that helpful.

“The biological species concept is the definition most often used for species, but it doesn’t always hold,” he says. “Tigers and lions, for example, are different enough to be regarded as separate species, but their hybrids, known as ligers and tigons, can sometimes be fertile.”

“It’s possible that if their distributions overlapped enough, the features that make lions and tigers unique could eventually be eroded. This means that, by the strict definition of the biological species concept, these animals wouldn’t be separate species even though most people recognise them as such.”

Some groups of animals, such as birds, produce hybrids much more easily than others. More than 10% produce fertile hybrids, with different penguin species among those which can breed to have healthy chicks.

Dr Gemma Bramley, an expert in plant species from Kew Gardens, adds that this is even more common in plants.

“It can be harder to define a plant species than an animal species based on the biological species concept,” Gemma explains. “Different plant species can much more easily hybridise and produce fertile offspring, so it’s more difficult to know where one species starts and another begins.”

The biological species concept has fallen out of favour with many scientists because of the difficulties in testing it. Instead, they’ve turned to new ways of defining species instead.

Other species definitions

While the biological species concept might be the most commonly known, it’s not the only way of defining a species. Instead, scientists have come up with many other concepts that recognise that species share some quality that makes them unique.

Palaeontologists, for example, don’t study living organisms, so it can be very difficult to say anything about reproduction from fossils alone. Instead, they often use the morphological species concept, where extinct species are identified using similarities in their size and shape.

However, this definition has its own issues when it comes to species complexes – groups of closely related species that are so similar they’re very hard to distinguish on appearance alone. In these cases DNA is often used to help identify the differences between them.

Some scientists go one step further and use a genetic species concept to define a new species. However, this means it can only be identified by genetic testing, which makes the concept less useful for scientists out in the field and impossible for those working with fossils.

“While DNA is an important line of evidence to help us understand and, in some cases, redefine a species, it’s preferable to combine it with similarities and differences in their features,” Gemma explains. “As a plant taxonomist, I create identification keys and descriptions based on characteristics. Defining a species solely on genetic differences alone would be impractical in most circumstances.”

Ultimately, there are as many ways to identify a species as there are scientists. Each case is different, with researchers using one or a combination of these concepts to convince others they have found a new species, and not just variation within a pre-existing group.

Dr Rupert Collins, our Senior Curator of Fishes, says that it’s best to treat a species as “the result of an ongoing process, rather than a definable quality”.

“When populations become reproductively isolated from one another, be it for ecological, geographical or behavioural reasons, they’re on the path to becoming separate species,” he explains. “Over time differences will accumulate and manifest in different ways.”

“Generally, the longer two distinct populations are apart, the easier it is to find enough evidence to say that they’re separate species – but where we draw the line is up to us.”

While most scientists agree that defining what a species is can be difficult, it’s vital to draw the line somewhere. Laws on food safety, biosecurity and conservation all rely on identifying species, meaning this concept still has an important role to play.

What is speciation?

The definition of speciation is that it is the process by which new species form. When different populations of a single species are separated and can no longer interbreed, it can eventually lead to a new species evolving, or speciating.

The most common form of speciation is geographic speciation, also known as vicariant or allopatric speciation. This happens when populations of a single species become separated by features of the landscape such as canyons, mountains or seas.

Over time, as the habitats on either side of the divide change, the two isolated populations can evolve either by natural selection to become better suited to their environment or by a random process known as genetic drift. Eventually, this can lead to them becoming different species.

Some of the best-known examples of geographic speciation come from the Galápagos Islands, which formed as a result of volcanic activity over the past few million years. Charles Darwin documented that animals such as finches, mockingbirds and giant tortoises weren’t the same as their relatives on different islands or on the South American mainland.

The bird species had developed different beak shapes and the tortoise species had evolved different body sizes, shell shapes and limb lengths to allow them to cope with the conditions on their particular island. These observations contributed to Darwin’s thinking around evolution and natural selection, eventually leading to him publishing his book On the Origin of Species.

One of our evolutionary experts, Dr Natalie Cooper, adds that geographic speciation can happen at a range of scales. It doesn’t only happen across large divides.

“While allopatric speciation is mostly caused by large features, such as a river, it depends on the organism in question,” she says. “A small insect species, for instance, might speciate on two sides of a small stream if it’s unable to cross.”

“For flying animals, such as birds, however, the distances involved have to be much bigger, as flight allows them to overcome many obstacles.”

Sympatric speciation

The speciation process isn’t always the result of geography. Sympatric speciation is a term that covers all the different ways, or isolating mechanisms, that cause populations living in the same environment to lose the ability to breed with each other.

Mechanical isolation, for example, is when two populations can’t physically reproduce because they’re now too different in shape or size. One of our insect experts, Dr Gavin Broad, says it’s particularly common in invertebrates.

“In insects, there’s significant selective pressure on the genitalia of a species to be different from its relatives,” he explains. “The difference in structure makes it harder, if not impossible, for members of different species to reproduce successfully.”

“It’s particularly important in flies and moths, where genital structure is sometimes the only way to identify a particular species from among relatives that look similar.”

Speciation can also be a matter of time. Temporal isolation occurs when different populations start to breed at different times. Different species of Acropora corals, for instance, have evolved because they consistently spawn at different times, meaning they don’t have a chance of reproducing.

Changes in behaviour can also lead to speciation. Songs to attract potential mates, and other courtship rituals, are often regulated by genes, especially in insects. If these genes are altered, one population may no longer recognise the mating behaviours of another, leading to behavioural isolation and a new species evolving.

“Courtship behaviour can distinguish some species, and it comes about as a result of sexual selection,” Gavin says. “There are wasps that produce courtship songs by vibrating their wings at a particular frequency, for example, while there are flies that have specific courtship dances.”

“As these behaviours are genetically controlled, flies that lose some steps after a mutation may not be recognised by others, promoting speciation.”

On scales ranging anywhere from hundreds to millions of years depending on the organism, one or more of these processes eventually results in a distinct population becoming a new species.

How many species are there?

There’s no exact figure for how many species live on Earth. As of 2024, more than 2.1 million species have been scientifically described and named, but this is likely to be nowhere near the true number living on the planet.

To start with, large parts of Earth haven’t been explored. Areas that are hard to get to, such as tropical forests, deserts or the depths of the ocean, are probably home to many species that just haven’t been discovered yet.

Then there’s the problem of microscopic organisms. Species of bacteria, for instance, are hard to define, and the speed at which they mutate means that new ones are forming all the time. As a result, it’s notoriously difficult to estimate how many there are, with scientists suggesting numbers from the low millions into the trillions.

Even if microbes are ignored, there’s still a wide range of estimates for the number of species on Earth. One of the most common ways of estimating is to focus on well-studied organisms, such as birds and butterflies, and compare them to less studied groups.

“By comparing the known diversity of well-studied groups to less well-known organisms, we can estimate how many unknown species there might be,” Gavin explains. “We can then extrapolate these relationships across the world to give us an estimate of the overall number of species.”

“Alternatively, we can make these estimates using the rates at which new species are being described. If the naming of new species is slowing down, it suggests that we’re reaching a point where all these species have been described. We can then compare between different groups and taxonomic ranks to give us a total species estimate.”

At the moment, most papers estimate that there are around 5–10 million species on Earth, excluding microbes. If microscopic life is included, then the best estimates put the total number of species in the billions.

While narrowing down these estimates is a never-ending challenge, as new species evolve and others become extinct, it’s one that scientists continue to tackle as they try to understand all of Earth’s biodiversity.