Unravelling the dynamics of hybridisation and its implications for ecology and conservation of Darwin’s tree finches

Author: Katharina Johanne Peters

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Peters, Katharina Johanne, 2016 Unravelling the dynamics of hybridisation and its implications for ecology and conservation of Darwin’s tree finches, Flinders University, School of Biological Sciences

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Abstract

The significance of hybridisation for biodiversity has been the subject of a long-standing

debate. Hybridisation has been characterised as being detrimental for biodiversity and

speciation as it can blur the borders between distinct species. Contrastingly, hybridisation

has also been described as a creative evolutionary process generating increased genetic

variation and facilitating adaptation. Only few study systems enable us to observe

hybridisation in real time, which has limited our knowledge of its consequences for the

ecology and conservation management of contemporary species. This study investigates

current hybridisation between two species of Darwin’s finches (small tree finch

Camarhynchus parvulus and medium tree finch C. pauper) on Floreana Island, Galápagos,

Ecuador, and tests key variables related to foraging ecology, song, gene flow, and

parasitism in hybrids and their two parental species.

The current ecological positions of hybrids in relation to parental species are important

to identify possible selection pressures that could favour different phenotypes across

vertical or horizontal clines. I examined foraging behaviour in relation to vertical habitat

use in Darwin’s tree finches and hybrid birds as the proportion of hybrids increased

across the decade. Both parental species changed foraging height or behaviour with

increasing hybrid density, while hybrid foraging behaviour was consistent across years.

These findings suggest that parental species and hybrids may be experiencing different

selection pressures, and the increasing hybrid abundance could be influencing the

foraging behaviour of their parental species. Given the importance of rapid assessment for

regular biodiversity monitoring, I investigated if hybrid birds could be acoustically

identified, by comparing their song with song of the two parental species. While C. pauper

had a distinct song, hybrid birds and C. parvulus song was indistinguishable and their

respective populations could therefore not be surveyed individually. Acoustical surveys across the decade 2004–2013 showed 52 % decline of the critically endangered C. pauper,

highlighting the need for targeted conservation actions. Next, I examined the role of

female choice as a driver of the hybridisation using a powerful combination of pairing

observations and genetic analysis with nine microsatellite markers. I found that C. pauper

females did not discriminate heterospecifics and frequently paired with C. parvulus males,

while C. parvulus females were never observed to pair with C. pauper males. Hybrid

females paired predominantly with hybrid and C. parvulus males, resulting in

asymmetrical introgression with gene flow skewed towards C. parvulus. These findings

support the formation of a hybrid swarm comprising C. parvulus and hybrids of various

generations while C. pauper retains most of its genetic purity.

Reproductive success is a key measure of biological fitness. I analysed nesting success in

Camarhynchus and Geospiza fuliginosa and identified parasite intensity due to larvae of the

introduced fly Philornis downsi, whose parasitic larvae have been identified as the primary

cause of nestling mortality. Hybrid birds had lowest in-nest P. downsi numbers, providing

the first evidence of hybrid fitness in this system.

This thesis uses a combination of behavioural, genetic and monitoring methods to assess

the survival of hybrids in a rapidly evolving vertebrate system. Under conditions of

extreme natural selection from the recently introduced fly P. downsi, hybrid fitness was

higher than that of the parental species as measured by fewer parasites per nests. I have

identified the role of sexual selection in forming the hybrids via female choice of

heterospecific males, and the role of natural selection in maintaining the hybrid offspring,

It is my hope that the findings of this thesis will encourage conservation efforts of the

Darwin’s finch species complex including the hybrid birds.

Keywords: Darwin's Finches, Camarhynchus, Hybridisation, Galápagos, conservation, point-count survey, microsatellites, hybrid fitness, foraging ecology

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2016
School: School of Biological Sciences
Supervisor: Sonia Kleindorfer