Rebekah Christensen

Postgraduate Student

Contact Details

Academic Qualifications

• Bachelor of Science (Honours)

Research

Sexual Selection and Divergence in Darwin's tree finches

Darwin’s finches are one of the best-studied examples of adaptive radiation to date. The group consists of 15 species of finch, 1 found on Cocos Island (Costa Rica) and the remainder found throughout the Galapagos Islands (Ecuador). Following on from my Honours research on song in Darwin’s small tree finch (Camarhynchus parvulus) I am now investigating the role of sexual selection in promoting divergence in Darwin’s tree finches (Camarhynchus spp.).

Sexual selection, the differential mating success of phenotypes, may contribute to divergence when mate preferences drive the evolution of novel characteristics (as observed in runaway selection) or when divergent mate preferences among a population lead to reproductive isolation between groups. In Darwin’s finches, song is a sexually selected signal functioning in species recognition and mate choice. Viable hybrids are observed between species of Darwin’s finches, thus prezygotic isolating mechanisms such as song are particularly important for maintaining reproductive isolation in this group. During my Honours study I found a correlation between song and bill morphology in Darwin’s small tree finch such that song is a reliable indicator of bill size and shape. Thus, females are able to assess the bill size of prospective mates by their song characteristics and select their mate accordingly. A unique feature of this system is that the character upon which sexual selection is acting (the bill) is also an important character upon which natural selection acts. Bill morphology is also highly heritable in Darwin’s finches, thus mating preferences based on bill size (and facilitated by male morphologically-references song cues) could contribute to divergence.

My study incorporates the small, medium, and large tree finches and my work is carried out across two islands of the Galapagos, Santa Cruz Island and Floreana Island. My research includes three main components:

1. investigation of foraging behaviour to better understand the relationship between environmental conditions and bill morphology

2. investigation of song characteristics and their relationship to bill morphology across the three species

3. investigation of female mate preferences in relation to male song and bill morphology.

Publications

• Christensen, R., Kleindorfer, S. (2007) Assortative pairing and divergent evolution in Darwin’s Small Tree Finch, Camarhynchus parvulus. Journal of Ornithology 148(4):
  463-470.
• Christensen, R., Kleindorfer, S., Robertson, J. (2006) “Song is a reliable signal of bill morphology in Darwin’s small tree finch, Camarhynchus parvulus, and vocal performance predicts male pairing success.” Journal of Avian Biology 37(6): 617-624.

Conference Presentations

Grants & Funding

Supervisors

Dr Jeremy Robertson

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