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The meanings of fitness in evolutionary biology

Bouckaert, Hugo (1995) The meanings of fitness in evolutionary biology. PhD thesis, Murdoch University.

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This thesis identifies and explores the meanings of different concepts of fitness currently used in evolutionary biology. Fourteen distinct definitions are described in detail.

Understanding of the role of fitness in evolutionary biology requires that historical and cultural factors be appraised. Particular focus is given to the interdisciplinary structure of evolutionary biology. Relative merits of different kinds of theories for representing evolutionary phenomena are examined. A comparison is drawn between hypothetical-deductive explanations, semantic models, schematic arguments and schematic abstractions. The schematic abstraction of Darden and Cain (1989) is concluded to be most appropriate for representing the process of natural selection, when the role of fitness in that process requires clarification.

Evolutionary explanations are also analysed from a causal perspective. Although quantum physics casts doubt on whether the world can be described as causal, all explanations identify relevant causes. Hence, a causal analysis of evolutionary processes is used as an "heuristic device" for identifying confusing and contradicting aspects of processes in which fitness features.

Inquiry into what constitutes fitness clarifies the relationship of fitness to concepts of adaptation, adaptedness and adaptability. A vital aspect is the unique and complex relationship of adaptive traits to the organism’s life history. Because certain traits are traded off at the level of the life history, and because the life history forms a constraining matrix for organismic selection, natural selection must be regarded as acting on organisms and their life history. Bock and von Wahlert’s (1965) classification of adaptive traits is utilised for characterising the relationship between fitness-bearing traits and the environment. Fitness, as the sum of all fitness-bearing traits, is not causal in determining survival and reproductive success. Causal processes operate at the level of individual traits: in certain situations some traits may, on their own, determine survival and reproductive success and others may not be utilised at all. Since fitness is a potential for traits to realise survival and reproductive success - a potential unique for each organism and, in a wider comparison, unique for each species - it must be regarded as a supervenient property. This enables it to be related to other supervenient properties, invalidating the contention that relevant generalisation can only be drawn from the consequences of fitness, i.e. from relative survival and reproductive success.

Particular focus is placed on the process of which fitness is part: natural selection. An analysis of fitness in the Darden and Cain (1989) scheme concludes fitness must incorporate three attributes which are difficult to reconcile: interaction, unit of selection and replication. It clarifies the essential reason many different interpretations of the concept exist: more weight is often given to one attribute in favour of another, or sometimes two attributes combine, leaving the third under-represented.

Different interactors and units of selection are found at different levels of selection. The concept of the interactive plane is developed to designate the state-space for a selection process where unit(s) of selection, interactors and environmental factor(s) interact. Discussion of Wimsatt’s (1981) context-independence criterion and Lloyd’s (1988) additivity standard further specifies how levels of selection can be identified. It is followed by an investigation of the role of fitness in selection processes occurring at different levels of biological organisation.

When the role of fitness in natural selection is analysed, the central premise of the thesis becomes apparent: procurement, by the interactor, of an ecological benefit as a necessary prerequisite to survival and reproductive success. This recognised, it becomes possible to "map" several complex routes through which benefit can be translated into survival and reproductive success.

Fitness can be maximised by other processes operative within the wider parameters of natural selection. Two processes are identified: niche construction and phenocopy. Similar to natural selection, both have a benefit occurring as a prerequisite for differential survival and reproductive success. Since niche construction and phenocopy processes only occur at the organismal level, they reassert the central role of the organism in microevolution. In addition, these processes broaden the perspective on the definition of a selective environment.

The role of fitness in microevolution is dissimilar to that in macroevolution. Fitness’s of different evolutionary entities are non-transitive, so the fitness of a species cannot be extrapolated from the fitness’s of its constituent organisms and populations. It is argued that genealogical entities only possess fitness if they can be placed in a relevant ecological context. As genealogical entities above the species have no coherent ecological role to play, they cannot be allocated a fitness value. The role of fitness in the process of species selection is explored. It is concluded that fitness-bearing traits of species in the macroevolutionary context must be regarded as predispositions for fitness bearing traits in microevolution.

When the concept of fitness as regards adaptation, natural selection and macroevolutionary processes is clarified, the fourteen biological fitness definitions are re-evaluated in this expanded frame of reference. Such a reappraisal leads to the conclusion that the concept of fitness embodies two incompatible roles of interaction and replication, either in a micro- or macroevolutionary context. In the last chapter, a further analysis of different forms of reasoning in evolutionary biology concludes that the predominant focus on replication can be attributed to the demand in genetics to explain hereditary patterns, resulting in the utilisation of compositional forms of reasoning. These stand in contrast to evolutionary forms for which a temporal and directional element is a necessary part of the explanation. A proper assessment of interaction requires evolutionary forms of reasoning, whereas replication is best suited to compositional ones. This distinction underpins the subsequent proposal that the role of fitness can best be represented by a two-stage model. The first stage would investigate causes for fitness differences and how these causes translate into ecological benefits for interactors. The second stage would focus on evolutionary consequences of fitness: how benefits obtained by interactors translate into the survival and reproductive success of evolutionary replicators. Of central importance to this two-stage model is the ability to translate ecological benefits obtained by individual interactors in a contingent ecological setting into a measure of longer-term survival of less individual replicators in the wider evolutionary context.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Bradley, Stuart and Barns, Ian
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