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Organism: Darwin’s small ground finch, Geospiza fuliginosa.

Field sites: The arid lowlands (0-100 m above sea level) and the humid highlands (500-600 m above sea level) of Santa Cruz Island, Galapagos Archipelago.

Background: Positive assortative mating tends to maintain adaptations and individual niche specialization. However, adaptations and niche specializations are not always favoured across generations. In such cases, a loss of assortative mating could increase offspring adaptive potential and thereby offspring fitness. Range expansion into a novel habitat, with novel selection pressures, presents a scenario where assortative mating may be lost via relaxed selection on mate choice.

Hypothesis: A loss of assortative mating should be favoured in the highland colonist population of G. fuliginosa.

Methods: We measured the beaks of 23 nesting pairs from the highland and lowland populations in January and March 2001 and 2002. We used correlation analysis to examine assortative pairing for beak length. We determined the distribution of beak lengths for females and males within each ecological zone to rule out limited mates as a mechanism for the loss of assortative pairing.

Results: As predicted, we found positive assortative pairing for beak length in the lowland source population but not the highland colonist population. In addition, we found no evidence for limited mates.