Diffuse phenotypic structure in native chickens across Java and Wallacea is consistent with human-mediated connectivity and decentralised selection

The domestic chicken is one of the most widespread animals on Earth, yet its story is not just one of biology but of human history. For thousands of years, people have raised these birds for food and ceremony, moving them across landscapes and selecting them for specific looks or behaviors. This long partnership means that a chicken's appearance is often a record of where it has been and how people have cared for it. When populations of animals are separated by mountains or oceans, they usually drift apart, developing distinct features that make them look different from their neighbors. Scientists call this process differentiation, and it is a standard way to understand how species adapt to their local environments. However, when humans move animals around or when farmers share breeding stock, those boundaries can blur, keeping populations looking more alike than geography alone would predict. Understanding whether a bird's look is shaped more by the land it lives on or by the people who raise it helps us see how human activity writes itself into the natural world.

In a recent study focused on Indonesia, researchers set out to map the physical diversity of native chickens across the islands of Java and Wallacea. They traveled to 259 households spread across eight provinces, gathering birds from a wide range of local farming systems. At each home, they recorded details about how the farmers raised their flocks and what traits they preferred when choosing which birds to keep. The team then measured the size and shape of the chickens and analyzed the colors of their feathers using a precise system that breaks down hue, saturation, and brightness. By comparing these physical traits against the location of the farms and the specific practices of the farmers, they aimed to see if the birds formed distinct groups based on where they lived or if they remained a mixed, continuous population.

The results revealed a landscape of variation that defied simple boundaries. While there were measurable differences in size and shape between provinces and even between the islands, the differences were not sharp. Instead of finding clear clusters where one group of chickens looked entirely different from another, the researchers found a vast amount of overlap. A chicken from one village could look very much like a bird from a neighboring island, and the range of colors and sizes within a single location was often as large as the differences between locations. This pattern suggests that the phenotypic structure of these chickens is diffuse, meaning the traits are spread out and blended rather than separated into neat, isolated boxes. The largest share of the variation found in the study came from differences between individual birds within the same household or village, rather than between different regions.

This widespread mixing points directly to the role of human management. The study indicates that the decentralized nature of smallholder farming, where individual farmers make their own choices and likely exchange birds with neighbors, prevents the populations from becoming genetically or physically isolated. Even though the islands are separated by water and the farming environments vary, the constant movement of birds and the shared preferences of farmers keep the populations connected. The researchers argue that geographic barriers and different local environments alone are not strong enough to create distinct groups when people continue to move animals between them. The chickens remain a fluid population, shaped more by the flow of human exchange than by the static lines on a map.

The study concludes that while the chickens show signs of local adaptation, the overall picture is one of continuity. The authors note that while the physical traits are linked to human movement and selection, the underlying genetic story remains to be fully told. It is possible that the birds are sharing genes in real time, or that they are simply holding onto ancient variations that have not yet been sorted out by isolation. What is clear, however, is that the visible diversity of these chickens is a testament to the ongoing, decentralized connections between the farmers and their flocks across the Indonesian archipelago. The birds do not belong to rigid categories defined by geography; they belong to a network of human care that keeps them looking remarkably similar across a vast and varied landscape.