The Population Consequences of Sex and Conflict

Natural selection generally operates at the level of the individual, or more specifically at the level of the gene. As a result, individual selection does not always favour traits which benefit the population or species as a whole. The spread of an individual gene may even act to the detriment of the organism in which it finds. Thus selection at the level of the individual can affect processes at the level of the organism, group or even at the level of the species. As most behaviours ultimately affect births, deaths and the distribution of individuals, it seems inevitable that behavioural decisions will have an impact on population dynamics and population densities. Behavioural decisions can often involve costs through allocation of energy into behavioural strategies, such as the investment into armaments involved in fighting over resources or increased mortality due to injury or increased predation risk. Similarly, behaviour may act o to benefit the population, in terms of higher survival and increased fecundity. Examples include increased investment through parental care, choosing a mate based on the nuptial gifts they may supply and choosing territories in the face of competition. Investigating the impact of behaviour on population ecology may seem like a trivial task, but it is likely to have important consequences at different levels. For example, antagonistic behaviour may occasionally become so extreme that it increases the risk of extinction, and such extinction risk may have important implications for conservation. As a corollary, any such behaviour may also act as a macroevolutionary force, weeding out populations with traits which, whilst beneficial to the individuals in the short term, ultimately result in population extinction. In this thesis, I examine how behaviours, specifically conflict and competition over a resource and aspects of behaviour involved in sexual selection, can affect population densities, and what the implications are for the evolution and ecology of the populations in question. It is found that both behaviours related to individual conflict and mating strategies can have an effect at the level of the population, but that various factors, such as a feedback between selection and population densities or macroevolution caused by species extinctions, may act to limit the intensity of conflicts that we observe in nature.The actions of selfish individuals can have a negative impact for everybody in a group. This is best highlighted in the tragedy of the commons , a parable which deals with the destruction of shared resources by humans. In this parable, individual cattle herders graze their animals on a common land. As no single individual owns the land, it is to each herders advantage to place an extra animal on the common. This is because the extra animal will not cost as much, as the herder does not own the land, any damage caused by one extra animal will be shared by all the herders, not only the one who puts an extra animal to graze. Eventually, all herders will realise that it is better to graze more animals, and the common will be destroyed. The parable holds an important truth: when common resources are unmanaged and shared by individuals they are vulnerable to over-exploitation at the collective expense of all individuals managing the resource. This is indicated by the current state of our fisheries, where each fisherman does better in the short term by fishing more from the sea, until eventually the stocks will collapse. The natural world suffers a similar problem to the tragedy of the commons seen in human societies. Evolution by natural selection favours genes which relentlessly increase their reproduction within a population. As a result, it is inevitable that many genes which code for selfish behaviours will evolve. This means that natural selection does not always result in progress from the point of view of the greater good of all individuals, but rather can result in a very bad collective situation where each individual suffers as a result of the intense competition. It is well known in animals that there is no moral restraint, and that it is the individuals that survive and reproduce the most relentlessly that will survive. When genes arise in a population which selfishly reproduce at the expense of their less selfish counterparts, a situation can arise where so many individuals suffer from the selfish behaviour that the whole population will be driven extinct. Some bacteria are social, meaning that they rely on other members of their species to help them to reproduce. Such help can come in the form of helping to form a stalk to aid a reproducing fruiting body. Individuals in the stalk have little to gain, as they don t reproduce, so all individuals would prefer to be in the fruiting body. Under some conditions, it is possible that cheaters may arise which compete so selfishly that they never end up helping to build the stalk, but rather always end up in the fruiting body. This could, in some situations, result in the population being driven extinct. When extinction is a direct result of evolution at the individual level, it is known as evolutionary suicide. It is not only selfish behaviour, involving competition with other members of the population, which can cause evolutionary suicide. Behaviour related to finding a mate can also influence the number of individuals in the population. For example, too many males can lead to a situation where intense competition for females will actually cause harm to the females themselves. This has been shown in the common lizard, where females suffered greater injuries when there were more males in the population. The injuries proved to be so great, that it was predicted that the population could be driven extinct within 40 generations. While too many males can be dangerous for females, when there are too few males, then females may not be able to find a mate. This can sometimes be so bad that it results in the population becoming extinct. Males of the Saiga antelope are hunted for their horns in Central Asia. In recent years, this has led to a reduction in the number of males available to females, and is thought to be a big reason that the species is on the verge of extinction. When looking at how the number of individuals in a population changes over time, ecologists generally ignore male behaviours. However, incorporating such behaviours, for example in the case where males are aggressive or where they are limiting, can actually change the predictions of such models. If evolution and individual behaviour can affect the demography and the number of individuals in a population, then it is certain to affect the risk of the population going extinct. If individuals are selective for which are too competitive or too selfish can drive a population extinct, then they will not be seen at the species level in the long term. While such individuals do better in the short term, eventually they drive the population in which they are living extinct, meaning that in the long-term it is not always favourable to be too competitive. Similarly, if the number of individuals in a population decreases as a result of competition, then the conditions will also change: there are less individuals to compete with, and therefore the benefit an individual gets from conflict will be reduced. Thus there is a feedback between conflict and the number of individuals in a population which can help to not only reduce the level of conflict, but also prevent the population from going extinct. In all, this thesis shows that the behaviour of individuals in natural populations should be taken seriously when looking at the number of individuals in a population, and that behaviours involved in sex and conflict can have profound influence at the level of the population. However, extinctions and other factors may come in to play to reduce the level of conflict we observe in nature. Hence, conflicts in the natural world may not as be as tragic as scientists may have originally expected