1.2.4 The evolutionary level
Life histories and trade-offs
In this section, the emphasis switches from proximate (molecular, cellular, physiological and behavioural) types of explanation to ultimate types of explanation. In order to proceed, we need to understand two key concepts: life history and trade-off. Both of these concepts are important tools in organising thoughts about why organisms are so diverse. An organism's life history is the set of key biological events in its life, including birth, growth, reproduction, sometimes migration, and death. Life histories are distinguished from life cycles, which are a detailed description of the sequence of stages in an organism's life. Life histories can be subjected to quantitative analysis. A familiar example of a life history is that of an annual plant which, following birth (germination), grows, flowers (reproduces), sets many seed and then dies. A contrasting life history is that of large-bodied mammals such as African elephants or blue whales. Following birth, these animals grow to a very large size before giving birth to (usually) one offspring at a time. There is then a period of recovery before new young can be produced. Death may follow 20-30 years after the first birth, during which time perhaps 8-10 young may have been born.
Between the extremes of an elephant and an annual plant lie the life histories of many species. If these life histories are subjected to quantitative analysis, for example by plotting fecundity (number of offspring) against chances of survival, the results indicate that trade-offs are at work. For instance, organisms with a high fecundity have a lower chance of future survival. What are the reasons for these results? The patterns between species reflect processes that have occurred within species and can be given proximate explanations. In proximate terms, each behavioural, physiological or biochemical activity has an energetic cost. For a given energy input (food or light intake) an organism cannot afford to indulge in many different costly activities. Therefore trade-offs have to occur. Let us take the example of a short-lived plant in which there is genetic variation in the amount of photosynthetic product (e.g. starch) that can be stored in the root. Some individuals can store more in the roots than others. Those individuals that store less in the roots divert the energy into seed production. The probability of plants surviving after flowering is inversely related to the amount of seed produced. Thus plants that put less stores into roots have a higher probability of dying after flowering, in contrast to plants that store more in the roots. The latter produce fewer or smaller seeds and hence have a higher probability of survival after flowering.
Natural selection can operate on this genetic variation. It may be that under certain environmental conditions the storers are favoured, and under other environmental conditions, those that produce many seeds are favoured. It may also be that both strategies are favoured by the same environmental conditions. Thus both seed producers and storers do well in winter, i.e. there are two equally adaptive solutions to the same environmental problem. Hence we can see how genetic variation underpinning life history variation due to trade-offs in individuals can, through natural selection and subsequent speciation, be represented as life history variation between species. In conclusion, it is possible to move from proximate explanations of trade-offs and life history variation in individuals, to ultimate explanations of life history variation between species. This course will cover many examples of trade-offs, which act as important constraints on organism structure.
Plant life histories
To review ideas on life histories, let us consider some plant examples. Among plants, there are three broad categories of life history, each of which has different implications for how they respond to the onset of winter:
Annuals As described above, these plants complete their life cycle in a single year. Most annuals in temperate regions set seed during the summer and then die before winter (Table 1.1, strategy 3). They represent the extreme version of the high seed-producers discussed above.
Biennials or short-lived herbaceous perennials These plants have the capacity for storage of photosynthetic product(s). In this particular category are plants that live for two or a few years, flowering and setting seed only in the second or final year. Since biennials that did not survive their first winter would fail to reproduce, they must be adapted for winter survival. They allocate some of their resources to growth in the first year, and some to accumulating stored reserves in their roots. Their foliage may die back with the onset of winter (strategy 2) but their stored nutrients and energy enable them to grow quickly in the second year, prior to flowering. Thus, whereas growth (vegetative) and reproductive phases of the life history occur in the same year in annuals, they are separated into different years in biennials.
Long-lived perennials These plants may persist for many years, typically reproducing many times. Perennial plants include herbaceous plants that die back each year (strategy 2), and woody plants, which possess a number of adaptations for surviving the winter (strategies 1 and 2). There may be a long juvenile period before the first reproduction event.
For smaller biennials and perennials, the typical overwintering strategy is for those parts of the plant above ground to die back, leaving an underground storage organ, such as a tap root, a bulb or a rhizome, to survive the winter. For larger plants such as trees, this option is not viable, because reconstructing the above-ground parts of the plant every year would preclude any long-term growth. Trees have two main strategies for surviving the winter, belonging to either the deciduous or the evergreen category (strategies 2 and 1 respectively). Deciduous trees are those that shed all their leaves in a particular season, usually the autumn. Evergreen trees retain leaves all year round; they do drop and replace their leaves, but only some at a time. While trees do not possess discrete storage organs, they store energy-releasing compounds and other nutrients during the winter, in their trunks and roots.
Storage during the winter involves risks because, among temperate animals, there are herbivores that follow strategy 1, remaining active through the winter. Plant storage organs are a vital food source for such animals and many individual plants fail to survive the winter because their storage materials have been consumed by animals or fungi.
We will now discuss in more detail the means by which selected groups of organisms survive the winter, with reference to the four strategies in Table 1.1. Throughout the discussion of these strategies, we will move between different types and levels of explanation. It will also be clear that certain strategies are only open to certain taxonomic groups.