The Trinidadian guppy, Poecilia reticulata, is a tropical freshwater fish with a long history as a model species for the study of evolution and adaptation to changing environments. The guppy is widespread in Trinidad, and many rivers on the island are host to multiple populations subject to varying levels of predation. Population structure in the guppy is influenced by several factors, including colonization history, presence or absence of barrier waterfalls within rivers, and both documented and accidental human-mediated introduction events. This study used genetic data from both microsatellite markers and mtDNA to investigate guppy population structure in 25 rivers and lakes across Trinidad and Tobago, with particular focus on the north shore Marianne and Paria Rivers. Most sites were located in the Northern Range Mountains of northern Trinidad, where rivers are divided into three major aquatic areas – the Caroni drainage, the Oropouche drainage, and the north shore. Results show a deep genetic divide between populations in the west-flowing Caroni drainage and those in the east-flowing Oropouche drainage, likely due to the colonization of these two drainages from two separate branches of the Orinoco, a large river located on the South American mainland. On Trinidad’s north shore, guppies collected in rivers on the western side of the island appeared to be genetically related to Caroni drainage guppies, while those in rivers on the eastern side of the north shore were predominantly related to Oropouche drainage guppies but showed evidence of admixture from the Caroni. Detailed study of Marianne and Paria River guppy populations showed downstream-biased gene flow in both rivers, with waterfalls in the Marianne limiting the movement of guppies in that river. Evidence of migration between the Marianne and Paria River watersheds was also found at two separate locations.

Cooperative behaviour among unrelated individuals is an evolutionary paradox. Research suggests that an individual’s propensity to cooperate and its response to experiencing cooperation or defection from its social environment consistently varies among individuals and as a function of external factors. The biological and psychological underpinnings of such behavioural variation remain unknown; they can, however, provide more insight into the evolution and maintenance of cooperation among non-kin. This thesis explores the proximate effects of experiences of cooperation or defection from the social environment, as well as possible proximate drivers of cooperative behaviour, using the Trinidadian guppy (Poecilia reticulata) as a study system. Firstly, the behavioural rules underpinning an individual’s decision to cooperate or not with unfamiliar individuals in the presence of specific or non-specific information were explored. When fish had information about their social partner’s cooperativeness, they behaved in a manner consistent with direct reciprocity, copying their partner’s last move. When paired with an ostensibly novel partner, a different, or at least additional, behavioural rule seemed to be employed. In order to help understand the drivers of individual variation in cooperative behaviour, phenotypic selection on cooperativeness was carried out over three filial generations, resulting in fish of high cooperativeness (HC) and low cooperativeness (LC). The divergence of individual cooperativeness observed between the two phenotypic selection lines suggests that cooperative behaviour in the context of predator inspection is at least in part heritable. Cooperative behaviour of F3 fish was found not to correlate with boldness or exploratory behaviour; HC and LC fish did, however, differ in some aspects of sociability and agonistic behaviour. Possible proximate neuromodulatory mechanisms underlying these differences in cooperativeness were also explored, focusing on brain expression patterns for the isotocin receptor (itr) gene in F3 females. HC females were found to have higher mid-section itr expression levels than LC females. Finally, I explored the effects of experiencing cooperation or defection on monoaminergic neurotransmission, which is thought to instantiate the effects of such experiences on the individual’s internal state. My findings suggest that experiencing cooperation or defection from the social environment affects internal state; this phenomenon may be crucial for the appropriate adjustment of the behavioural response to such experiences, and for the emergence of behavioural rules such as generalised reciprocity. Taken together these results suggest that neuromodulatory mechanisms are pivotal for the perception of stimuli from the social environment in the tested cooperative context and that variation in cooperative behaviour may be underpinned by individual differences in the structural properties of such systems. They also provide insight into how behavioural input may affect the behavioural response to such experiences, and ultimately how such mechanisms may lead to the evolution and maintenance of cooperation.