The ever-changing tree of life: How we classify living things

From Aristotle to DNA, biological classification has evolved — and so have the wolves

Image by Interior Design adapted and licenced through Shutterstock.

How do we decide what counts as a species? It seems simple enough — lions and tigers look different, so they must be separate species. But nature isn’t always so clear-cut. Some species are more genetically similar than expected and blur the lines of classification. This is the case for the red and grey wolf, two animals classified as distinct species yet so genetically similar that their separation is up for debate.

To understand how we got here, we need to go back to when humans first tried to make sense of life’s diversity.

From Aristotle to Linnaeus: The birth of classification

The ancient Greek philosopher Aristotle was arguably one of the first to systematically classify living organisms. He divided animals into those with blood and those without blood. While crude, his work laid the foundation for later classification systems in biology.

Fast forward to the 18th century, and Swedish botanist Carl Linnaeus revolutionised biological classification. He introduced binomial nomenclature, the system we still use today, where every species is given a two-part Latin name (e.g., Homo sapiens for modern humans). Linnaeus grouped organisms by physical traits, but his system was hierarchical, recognising that some species were more closely related than others.

This approach worked well — until Darwin came along.

Darwin, DNA, and the rise of phylogenetics

In 1859, Charles Darwin published On the Origin of Species, changing how we think about life forever. He argued species aren’t fixed but rather evolve over time through natural selection. This meant classification should reflect evolutionary relationships, not just superficial similarities.

For over a century, scientists struggled to reconstruct the true evolutionary tree of life. Then came DNA sequencing in the late 20th century, allowing researchers to compare genetic material. This led to phylogenetics, the modern approach to classification based on shared ancestry.

But genetics didn’t always make things simpler. In some cases, it made classification messier, especially when species weren’t as distinct as we once thought.

The wolf problem: When genetics and classification collide

Take the red wolf (Canis rufus) and the grey wolf (Canis lupus). They are classified as separate species, yet their genetic differences are surprisingly minor. In fact, there is more genetic variation within each species than between them. Some studies suggest the red wolf is not a distinct species at all, but a hybrid of grey wolves and coyotes (Canis latrans).

This raises uncomfortable questions. If species are meant to be genetically distinct, how do we justify calling the red wolf its own species? And if we reclassify it as a hybrid, do we stop protecting it under conservation laws? The U.S. Endangered Species Act lists the red wolf as a species, making its classification not just a scientific issue, but a legal and ethical one.

The red wolf isn’t alone in causing classification headaches.

Other species that challenge the rules

The polar bear and the brown bear (Ursus maritimus and Ursus arctos) are another example. Despite their obvious physical differences, they regularly interbreed, producing fertile hybrids known as grolar bears. Are they truly separate species, or just two variations of the same one?

Then there’s the Eastern and Western meadowlark, two nearly identical songbirds that share territory in North America. They look the same, but their songs are different, and they rarely interbreed. Should we define species by appearance or behaviour?

These cases show that no single rule can define what a species is. Instead, scientists use multiple approaches, each with its strengths and weaknesses.

How we classify life today

Modern classification relies on three main concepts:

1. The biological species concept defines species based on reproductive isolation (they don’t breed with others). This works well for animals, but not for asexual organisms like bacteria.
2. The morphological species concept groups species by physical traits. This is useful for fossils, but can be misleading when organisms that are quite different genetically look similar (e.g., identical-looking meadowlarks).
3. The phylogenetic species concept uses genetics to trace evolutionary history. This is more precise, but can result in endless splitting of species.

No single concept is perfect, which is why classification remains a work in progress and in practice is determined by scientific committees and taxonomic groups that assess evidence and make consensus decisions. For example, organisations like the International Commission on Zoological Nomenclature and the International Botanical Congress oversee classification standards, but debates often arise.

Some taxonomists advocate for splitting species based on small genetic differences, while others argue for broader, more inclusive definitions. These different approaches mean species classification can be inconsistent and is often subject to change — what one committee recognises as a species, another might reclassify as a subspecies or hybrid.

The future of classification

The way we classify life will continue to evolve. New species will be discovered, old ones will be redefined, and some like the red wolf may challenge our very idea of what it means to be a species.

The tree of life is still growing. And with every new discovery, we realise just how tangled its branches really are.

Jonathan Meddings is an author and advocate from Melbourne, Australia.