Changing invaders

How introduced rats have changed during their colonization of Pacific islands.

Humans have been transporting vertebrates to islands worldwide since prehistory, often with dramatic impact on native fauna. These introduced species appear to change very rapidly. The direction, mechanism and drivers of these evolutionary changes parallel those of macro-evolutionary changes but on a spatially delimited scale, the island. This allows for the study of the separate factors involved in multifactorial evolution.

A.A.E. (Alexandra) van der Geer

Postdoctoral researcher
Endless forms

I want to test predictions on how introduced species adapt evolutionarily to local environments within a known time frame. I will use the Polynesian rat as a proxy species. This species provides a “natural experiment” in evolution, because it is represented by multiple introduced populations with a vast geographic and well-documented temporal range, occurring on a multitude of islands, each with different characteristics. I will correlate its evolutionary changes with various features, including time, space, the presence of native competitive species, geographic features and climate. To describe its evolution, I will quantify size and shape changes of the skull and lower jaw with geometric morphometrics, a method that captures the shape of organisms.

My preliminary data from various islands across the entire range of the species show a remarkable size and shape variation. I will further test if the observed changes have a genetic background or are due to drift. I hypothesize that the Polynesian rat is rapidly adapting to local conditions in a predictable way. My main aim is to analyse how this species evolves. Additionally, data on how an invasive species interacts with environmental conditions and the native fauna is urgently needed, especially with the increasing pace of introductions today due to globalized transport.


  • Frietson Galis (Naturalis)
  • Martin Rücklin (Naturalis)
  • Philip Mitteroecker (University of Vienna, Austria)
  • Ross MacPhee (AMNH, New York, U.S.)
  • James Russell (University of Auckland, New Zealand)
  • Lawrence Heaney (Field Museum, Chicago, U.S.)