Traits invariants and theories of everything

The last two chapters were concerned with how natural selection can change the way a lineage looks (its phenotype). These changes have had profound effects, first on the complexity of organisms (Chapter 2), and second on the complexity of the ecology of the planet (Chapter 3). This chapter continues the theme, but deals with traits that are individually far more ordinary: life history traits. Nonetheless, they account for much of the diversity in form across species, thus collectively are of immense interest.

A life history is a description of the major characteristics of an organism from its birth to its death. The traits are variables that can be measured or categorized across individuals and that collectively make up the description (Figure 4.1). In the past, most work on life history evolution proceeded on a trait-by-trait basis. Each of the traits is associated with a large body of literature asking the following question: what selective forces have driven the

Time

Fig. 4.1 A cartoon of a life history, showing size of an individual against time, and various life history traits highlighted. Semelparity means reproducing only once, iteroparity many times. Determinate growth is when growth stops at reproductive maturity, indeterminate growth when it continues.

Time

Fig. 4.1 A cartoon of a life history, showing size of an individual against time, and various life history traits highlighted. Semelparity means reproducing only once, iteroparity many times. Determinate growth is when growth stops at reproductive maturity, indeterminate growth when it continues.

trait into its present states? Typically, theoretical models are constructed for each trait, describing the kinds of environment that favour different values of the trait. The models are then tested by seeing whether their assumptions and predictions are met in nature. If the answer is yes in both cases, our model's assumptions provide us with the understanding we are looking for. If the answer is no, either the model or the data are inadequate and must be re-examined.

In general we might expect certain values of a trait to be favoured rather universally. For example, it is clearly best, in a Darwinian sense, to produce many offspring, all other things being equal. Happily for those who like diversity, all other things are not equal: high fecundity must come at a cost to some other trait of importance to the organism's fitness, such as offspring size. This phenomenon is known as a trade-off. The organism is thus faced with the more complex choice of finding the value of the trait that maximizes fitness in the face of trade-offs with other traits of importance. In the above example, evolution would essentially select between organisms that have large offspring but few of them, and organisms that have many but small offspring. Following this, our evolutionary model will generally make some assumptions about the nature of any trade-offs involved (a constraint), and about the consequences of each value of the trait for fitness (a currency). In general, it is assumed that the value of the trait (the strategy) that leads to greatest fitness in a given environment will be that which is selected for, and should, all being well, represent that observed in nature (Figure 4.2). Such models are commonplace in evolutionary ecology and are known as

Optimum

Optimum

Strategy set

Fig. 4.2 The characteristics of an optimization model. The arrow shows peak predicted fitness and therefore what would be expected in nature.

Strategy set

Fig. 4.2 The characteristics of an optimization model. The arrow shows peak predicted fitness and therefore what would be expected in nature.

optimization or optimality models (Parker and Maynard Smith 1990).Let us start by seeing how this approach works.

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