‘Colour helps Darwinian fitness in nature — humans are impacting this hidden world too’

Hopi Hoekstra

is an evolutionary biologist teaching at

Harvard University

. Speaking to Srijana Mitra Das at

Times Evoke

, she discusses why colour evolved among birds and animals — and how human beings are now changing nature’s palette:

What is the core of your research?
The question that unites all the research we conduct in our lab is, how do organisms adapt to their environment? We address this by using an integrative approach, which means our work encompasses fields that range from evolution to ecology and genetics to neuroscience.

Is there an evolutionary backdrop to the range of colours we see in animals and birds?
Colour contributes to differences in organisms’ ability to survive and attract in order to reproduce — these are the two fundamental components of Darwinian fitness.

Further, studying colouration at a molecular and genetic level enables us to make connections between genes and understand how genetic development works. One of the most striking facts about the development of colour patterns in nature is that this actually evolved very quickly — presumably, that is because colour has a profound impact on evolutionary fitness. We have studied systems where colour differences evolved over just a few thousand years. We have also done fieldwork showing the evolution of colour occurring over just a handful of generations — for example, some of our early work studied whole body changes in colour and we found that while a mouse is usually brown, some species living on the beaches of

Florida

have developed a white colour to blend in better with the sand. Colour really matters for fitness and so, there are very strong and rapid evolutionary responses around it.




What are the functions of colour which help survival in the natural world?
Colour has many functions. One of the simplest examples is camouflage — this can protect animals from visually hunting predators. Interestingly, this can also be important for a predator species to be less visible to its prey. Another function, especially in diurnal organisms like birds or those which are active during the day, is to attract mates. This is why birds, especially males, can be incredibly showy in their colours and have behaviours highlighting that showiness.

Colour is also used to advertise the danger a venomous organism poses to others. Toxic organisms can be brightly coloured to advertise that toxicity, such as with the poison dart frog. In the plant world, colour evolved to attract pollinators.

DO NOT DISTURB: Animals like the poison dart frog found in central and South America are vividly coloured to highlight their toxicity. Picture courtesy: iStock


Are there colours hidden from human eyes?

Yes. We humans see the world only through our particular visual system — but different organisms view the world through their visual system. There is a whole hidden world of colours and patterns across nature which we cannot see but which is seen by other organisms. Animals can often see in a wider or expanded visible range or spectrum than humans — butterflies and birds, for instance, can see in ultraviolet (UV) which we humans cannot. There is also a very fine co-evolution between the visual systems of organisms and the things that are important for their fitness in a particular environment.

I always tell my students to be very careful when thinking of the evolution of colour because we see the world only through our visual spectrum — and this differs from other species on

Earth

.



How should we humans think about this amazing world of colours?
This is one of the most distinct traits which makes us appreciate the diversity we see in the natural world. Due to the limitations of our visual system, we humans only get to see a part of this diversity — this is a reminder to us that there is even more hidden biodiversity that we are unaware of.

WHEN WE HUMANS DRAW: The soot-carrying emissions of the

Industrial Revolution

even turned the peppered moth's wings dark. Picture courtesy: iStock



Have humans impacted colours in nature?
A classic example is the change in the wing colour of peppered moths — as the Industrial Revolution took off and more soot was produced from the mid-19th century onwards, trees in England grew darker. This was mirrored in the frequency of dark-coloured peppered moths, an iconic example of very rapid colour evolution due to human impacts. As humans change the environment now, that is affecting all the organisms sharing this space. We’re impacting both colour evolution as well as the visual environment which species can see.

Animals that change colour seasonally, like snowshoe hares that are white in winter and brown in summer, find winters getting shorter and snow reducing. This has affected the landscape where they live — so, we’re seeing some populations that no longer change colour. This is happening in waterbodies too — stickleback fish are often brightly coloured which is important for their mate choice and reproduction. But as water bodies get more and more polluted, it’s changing the visual environment inside lakes and streams. And since the original colours and patterns of these fish are no longer as clearly visible, these are changing too.

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