The conclusion comes from a long-term study of the Rum red deer by researchers at the UK Universities of Edinburgh, St Andrews and Cambridge, and at the Australian National University, which now has field records and genetic data spanning 45 years. Female red deer (hinds) give birth to a single calf each year, and hinds that give birth earlier have more offspring over their entire lifetime. The research shows that this is in part because the genes that make hinds give birth earlier each year also increase the reproductive success of the animals that bear them, and as a result, genes for breeding earlier become more common over time.

As the climate changes worldwide, advances in the timing of reproduction have been documented in many wild animals and plants. However, this article is among the first to demonstrate the role of Darwinian evolution (that is, genetic change by natural selection) in advancing the timing of reproduction. Genetic evolution in wild animal populations is often thought to be slow and irrelevant on the time scale of human lives, but this result shows that genetic change can be sufficiently fast to be observed over a few decades.

The article also shows that genetic change is not everything. The rapid change in birth dates in Rum's deer is due to the combined effects of direct changes in physiology or behaviour in response to climate change, changes in the population demographics, and evolution. However not all of the advance in dates can be explained by these multiple mechanisms and the long-term outcome of these processes for the deer population is unclear.