restricted access I.10 Life History
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I.10 Life History William F. Morris OUTLINE 1. Variation in life history among species and the notion of trade-offs 2. Key life history patterns and associated trade-offs The term life history summarizes the timing and magnitude of growth, reproduction, and mortality over the lifetime of an individual organism. Important features of an individual’s life history include the age or size at which reproduction begins, the relationship between size and age, the number of reproductive events over the individual’s lifetime, the size and number of offspring produced at each reproductive event, the sex ratio of offspring, the chance that the individual dies as a function of age or size, and the individual’s lifespan or longevity (the time elapsed between the birth and death of the individual). Although all of these features (so-called life history traits) describe individuals, some are more easily understood when viewed as aggregate properties of a population of individuals. This is particularly true of mortality and lifespan. Each individual dies once, at a certain age. But in a population of identical individuals, some may die at a young age and some at an old age. By imagining that the fraction of this population that is still alive at a given age also represents the probability that an average individual survives to that age, we see that the chance of survival to a given age, which is the converse of the chance of dying, or mortality, is a property of an individual. Similarly, we can envision the average lifespan (or ‘‘life expectancy’’) even though each individual has a single age at death. All sunflowers and the vast majority of sequoia seedlings die before reaching one year of age. Yet in a sequoia population, individuals have the potential to live for several millennia, which distinguishes sequoias from sunflowers. GLOSSARY fertility. The number of daughters to which a female gives birth during a specified age interval geometric mean. The nth root of the product of n numbers iteroparity. A reproductive pattern in which individuals reproduce more than once in their lives life table. A table summarizing age-specific survivorship and fertility used to calculate the net reproductive rate net reproductive rate. The average number of daughters to which a newborn female gives birth over her entire life semelparity. A reproductive pattern in which individuals reproduce only once in their lives survivorship. The probability that a newborn survives to or beyond a specified age 1. VARIATION IN LIFE HISTORY AMONG SPECIES AND THE NOTION OF TRADE-OFFS As for the difference in life expectancy between sun- flowers and sequoias, each of the key life history traits varies 1000-fold or more among species, as illustrated in figure 1. Life history traits also vary among individuals of the same species. The fundamental question in ecological and evolutionary studies of life history is: why is there so much variation in life history traits among and within species? To answer this question, we start by recognizing that life history features are traits just like any other (e.g., coloration, bill shape, cold tolerance, body size, etc.) that can be acted on by natural selection. Moreover , variation in life history traits among individuals in a population often has a genetic basis, so genotypes favored by natural selection can potentially increase in frequency from one generation to the next. If life history traits are genetically based and subject to selection , evolution of life history might be expected to lead to an organism that begins reproducing immediately after birth and produces a large number of wellprovisioned offspring in a series of reproductive events throughout an infinitely long life (such an organism has been termed a ‘‘Darwinian monster’’ because it would quickly displace all other species from Earth). The reason we do not see Darwinian monsters even though life history traits evolve is that different life history traits are not independent. Because the resources that an organism has available to invest in maintenance and survival, in growth, and in reproduction are always limited, life history evolution is constrained by trade-offs: a greater investment in one life history trait must come at the expense of a smaller investment in one or more other life history traits. Trade-offs between many different pairs of life history traits have been documented, and we will see several examples in the following section of this article. In recognizing trade-offs, we no longer expect that evolution will produce Darwinian monsters...


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