麻豆传媒

HOMO SAPIENS likes to categorise. Putting things in boxes helps us understand the complexities of the world around us 鈥� until it doesn鈥檛.

Take the apparently simple organising principle of a species. You might have learned at school that a species is a group of individuals that can breed to produce fertile offspring. But this is just one of at least 34 competing definitions by researchers working in different fields. Ecologists tend to categorise based on lifestyle. Palaeontologists focus on form. Geneticists sequence genomes and then create family trees based on shared, genetically encoded characteristics.

The problem is that evolution 鈥� the origin of species 鈥� is intrinsically about gradual, random change (see 鈥�Think you understand how evolution works? You鈥檙e probably wrong鈥�). Charles Darwin recognised that organisms live in populations that can diverge and evolve in different directions, especially if they face different environmental challenges. At some point, they become so distinct that we classify them as separate species. It just isn鈥檛 easy to pin down exactly when.

We could just accept that 鈥渟pecies鈥� is a fluid, imperfect concept that helps us understand and conserve the diversity of the natural world. Unfortunately, it doesn鈥檛 always. This year, for example, genetic analysis of the world鈥檚 largest amphibian, the critically endangered Chinese giant salamander, revealed it was actually three species, not one, each requiring different conservation interventions.

Traditional species concepts are further undermined by high levels of hybridisation in nature. One prominent example is the critically endangered red wolf found in the south-east US, which is now thought to be a cross between a coyote and a grey wolf.

Our species isn鈥檛 immune to these confusions. In recent years, we have discovered that our ancestors interbred with other hominins, including Neanderthals, Denisovans and a mysterious 鈥渟pecies X鈥�. Does this make us hybrids, or does it mean those hominins weren鈥檛 separate species, but simply different versions of us?

There probably isn鈥檛 a clear answer to such questions. In the end, the whole idea of fixed species appeals to ideas of immutability in nature that now seem rather outdated. Biology is messy, and doesn鈥檛 bend to our desire for clean classifications.

Cutting-edge science throws up all sorts of controversial, nebulous and mind-bending concepts. Here鈥檚 your guide to how to think about some of the fiddliest of them:

• In the quantum world, uncertainty reigns 鈥� or is it all in the mind?
• Think you understand how evolution works? You鈥檙e probably wrong
• Why information could be our route to the universe鈥檚 deepest secrets
• Who do you think you are? Why your sense of self is an illusion
• Big bang retold: The weird twists in the story of the universe鈥檚 birth
• Firms and governments use the internet to spy on us. Should we care?
• D鈥檕h! Why human beings aren鈥檛 as intelligent as we think
• Extinction is a fact of life. Could we stop it 鈥� or even reverse it?
• No more goody two shoes: Why true altruism can鈥檛 exist
• Alien life could be weirder than our Earthling brains can ever imagine
• Why it鈥檚 time to call time on the 鈥榥ature vs nurture鈥� debate
• Dark energy: Understanding the mystery force that rules the universe