The cichlid story illustrates many of the broader features of evolutionary ecology, the science that involves both ecological and evolutionary knowledge. Evolutionary biology is the field concerned with understanding how biological lineages change through time (anagenesis), split (cladogenesis), and ultimately go extinct. Ecology is concerned with the interaction of organisms with their environment. The organisms can be considered at various levels of a hierarchy, comprising the individual, the population (groups of individuals of the same species), and the community (groups of interacting populations from different species). Communities in turn comprise the biotic component of ecosystems, which also include their interactions with the abiotic world. Ecology asks how individuals behave in different environments,what determines population size, and the properties of communities and ecosystems, such as their diversity. Knowing all this, why do ecology and evolution interact and how do they do so?
A basic answer, and one that does not require much in-depth study, is that both fields are concerned with understanding similar characteristics. For example, both evolutionary biologists and ecologists would consider species richness as one of the key variables they want to understand. Both too would want to understand why species richness varies across environments, such as different lakes in the case of cichlids, and across clades, such as haplochromines versus other cichlids or cichlids versus other fish.
Another answer, that requires some knowledge of the subject, is that evolutionary and ecological processes are affected by each other (Figure 1.6). They do this in many ways: one way is through adaptation. Darwin's and Wallace's greatest discovery was an understanding of the way in which this occurs: evolution through natural selection. Organisms vary in form (phenotype). These forms are heritable because of variation in their underlying genetics (genotype). The phenotypes interact with their environment, and some are more successful than others for a variety of reasons: they may survive or reproduce better. This differential success is called natural selection. Thus, the individuals that contribute to the gene pool of the next generation are a subset of those that were born and will pass on that subset of characteristics to the next generation through their genotype. In this way the population changes through time. A second type of selection process is normally distinguished from natural selection: sexual selection. Sexual selection causes evolution of traits affecting mating success in males and females. Both natural selection and sexual selection come about from phenotypes interacting with their environment, and for this reason selection is generally viewed as an ecological process. Natural selection is responsible for the evolution of traits, such as cichlid jaw shape, which governs their ecological niche. Sexual selection is responsible for traits, such as the bright male coloration of cichlids, that influence their mating success.
So ecology, through the medium of selection, causes anagenesis, evolution within lineages. Ecology can also influence cladogenesis, the other big evolutionary process.We saw this in the role that water clarity plays in speeding up or slowing down rates of cichlid speciation and extinction.
Evolution can also affect ecology, and this interaction occurs at several levels of the ecological hierarchy (Figure 1.6). If you know about the environment, you can sometimes accurately predict, or at least in retrospect understand, the phenotypes that are favoured. Evolutionary biologists need to do this routinely, and it will be a repeated theme throughout this book. Ecology at the level of the individual is largely concerned with trying to predict how individual traits should be related to the environment through selection pressure. Behavioural ecology is the field that asks what behaviours would suit particular environments, such as the mate preferences seen in haplochromine cichlids. It is one of the richest parts, but only one of
the parts of evolutionary ecology. Hence evolution by natural selection affects ecology at the level of the individual.
The traits that evolve within species are often relevant to population and community processes. For example, each species has a characteristic reproductive rate, size, and length of life. These are important characteristics in determining how many individuals of a species can exist in any one place, and how variable the populations are. Species also vary in their ecological specialization; for example, how many other species they eat or which eat them. Haplochromine cichlids, for example, have very specialized jaws. These interactions evolve through natural selection, but they also structure communities. Knowing about one should help us to understand the other.
The second major evolutionary process, cladogenesis is also important for an understanding of ecology. To produce species-rich communities, such as in East African lakes, species have to be formed and not go extinct. Both evolution within lineages and the origin and death of lineages are processes that might have contributed. Thus evolution influences every level of the field of ecology and maybe key to understanding some of the basic ecological properties of our planet.
In the following chapters, we will explore the ways in which the two fields of ecology and evolution interact, see what we have learnt about the world as a result, and along the way build up a picture of how exactly these interactions occur. However, the book will describe something else about evolutionary ecology that cannot be fully appreciated without an overall view of the field. It is that the topics which the field addresses are mutually supportive, such that understanding of one aids understanding of others. For example,we can understand the rates of speciation in cichlids from a knowledge of speciation and extinction mechanisms, and we can understand those from a knowledge of sexual selection and sex determination. Ultimately then, workers in one area will benefit from an awareness of other areas. This is what makes a synthesis worthwhile. Knowing how these interactions between topics occur reveals interesting features about how our living universe is shaped, and provides another aspect to the bigger picture that the field depicts. The next chapter looks at how organisms became complex from very simple beginnings.
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