Studying Darwin’s finches has been the life’s work of the renowned British evolutionary biologists Rosemary and Peter Grant. For several months every year for 40 years, the husband-and-wife team visited the Galápagos Islands in the eastern Pacific to meticulously track the fate of thousands of finches on two small islands there. The Grants demonstrated that evolution by natural selection can be observed in the wild in real time: they were the first to see and measure it in action in nature. One Step Sideways, Three Steps Forward is Rosemary Grant’s new memoir. It reflects on her far-from-straight path to becoming a biologist, living with a family on the Galápagos – for 10 years the scientists’ two daughters accompanied them – and the joy of sharing ideas with her partner. Rosemary, 87, is senior research biologist, emeritus at Princeton University in New Jersey. The book comes ahead of a new edition of 40 Years of Evolution – the Grants’ classic account of their study of the finches – due out later this year. The Galápagos are famous for their unusual and unique species including giant tortoises, marine iguanas and flightless cormorants. But what’s the Charles Darwin connection? Darwin spent five weeks there in 1835 on his journey around the world, and the islands were key to solidifying the concept of evolution through natural selection [put forward in On the Origin of Species in 1859]. While there, he collected a variety of specimens of different-sized songbirds. Later, back in England, he was advised that they were all species of finch. He reasoned that their diversity in terms of body size and beak shape evolved over time as they adapted for feeding on different food sources [which vary by island]. Today we recognise 17 distinct species of Galápagos finches and there is also one on Cocos Island. We now know they all evolved from a single ancestral species in the last one to two million years – probably a small flock of finches that came over from mainland South America. Where did the idea to study finches on the Galápagos come from? We knew from earlier work that the adaptive radiation of Darwin’s finches was very young, and it was just this kind of situation that Darwin had suggested would be the place to look to for evidence of species formation. Also, many of the islands have never been inhabited by humans (what we saw would be completely natural) and the archipelago is prone to drought (which causes food shortages and large numbers of bird deaths and therefore potentially natural selection). We began our fieldwork in 1973 on the very small island of Daphne Major, a steep-sided volcanic crater. We went with two of Peter’s postdocs and our children aged six and eight. We added Genovesa, a larger and flatter island further to the north, in 1978. We ran studies on the two islands in parallel for a decade before concentrating on just Daphne because, being smaller, it was possible to follow the fate of every individual finch. We would catch the birds, band them and take body and beak measurements along with a small blood sample for later DNA analysis. We would also see what they fed on, monitor the quantities of food available and record their songs (which differ between species and are important when it comes time to choose a mate). What was island living like? How did you keep your kids occupied? We had to take all our research equipment, camping supplies, food and water. Critical was the water and often we would have a boat deliver new containers at intervals. They were five gallons each and, on Daphne, we had to haul them up to our kitchen – which was a cave in the cliff we shaded with an awning to keep cool. [Our children] Nicola and Thalia loved it and always wanted to come. They would help a bit and were good at finding nests for us. They did their own research, too, and it actually led to publications – Nicola on mockingbirds and Thalia on doves. We also took books and they had a violin. Any scary incidents? Luckily, we had no major accidents or illnesses but Peter was nearly attacked by a shark off Daphne. We didn’t use fresh water for washing – it was too valuable. Instead, we would dive into the sea, cover ourselves with shampoo, then dive in again to rinse. Both the children and I had already washed and Peter came down. He had shampoo on when I saw this shark come up from the depths and go towards his shoulder before suddenly turning away. Later back home we read that sodium lauryl sulphate, found in many shampoos, had been found to have shark repellent effects. We looked at our bottle’s ingredients and sure enough, there it was. After that we washed in buckets! Was Darwin right about how evolution would occur? While Darwin had provided the clue about where to see evolution, he thought any observations of change would be minute: big changes would take millions of years. We found change happened much quicker. The evolutionary process that Darwin outlined is character displacement (or divergence) caused by competition with another species. Over a period of nearly 30 years we were able to see the average beak and body size of the population of one of Daphne’s main finch species, the medium ground finch (Geospiza fortis), become smaller after its larger members were out-competed for food by another invading finch species during a drought. But we also saw other methods of speciation that Darwin hadn’t anticipated involving the rare hybridisation (or interbreeding) of species – including the formation of an entirely new lineage. This remarkable and unexpected process began with the arrival of a single male large cactus finch (Geospiza conirostris) on Daphne from an island in the far south of the archipelago. Though whether the lineage ultimately succeeds or succumbs remains to be seen. What’s the message for species conservation that has come from your work? That we have got to keep populations capable of further natural change, which means you can’t just save one species. A full complement of related species provides the opportunity for genetic input and variation. On both Daphne and Genovesa, if it weren’t for the multiple finch species interbreeding occasionally, we would have likely seen extinctions with the droughts. Any advice for those juggling parenting with professional life? Don’t rush. Not everything has to go in a straight line. Try to make time to enjoy being with your children and introduce them to a diversity of experiences. We were struggling financially and my PhD seemed elusive, but one of the best times in our life was being on an uninhabited island alone as a family. Did you ever resent how easily your husband achieved his career goals given how much harder it was for you? Never. Peter was unusual among men of his time in being very supportive of what I wanted to do (which was to get back to research and do my PhD). The Galápagos work was ours: we did it together, we published together and he would always bring me into the conversation. Most men would have said they couldn’t possibly work with their wives! One Step Sideways, Three Steps Forward by Rosemary Grant is published by Princeton University Press (£25). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply