recap

54.3 recap

An organism’s life history is characterized by its lifetime pattern of growth, reproduction, and survival. All species have a characteristic life history strategy, which arises from genetic and environmental constraints on life history traits. Two extremes of life history strategies include species with high population growth rates (r-strategists) and species that persist at or near their carrying capacity (K-strategists). Most species fall along a continuum between these two strategies.

learning outcomes

You should be able to:

  • Distinguish between the relative contributions of environment and genetics to life history traits in specific situations.

  • Describe how and why life history trade-offs might arise in a species or population.

  • Distinguish between r-strategists and K-strategists, and identify specific situations in which evolution might favor one strategy over the other.

Question 1

Refer to Figure 54.14.

  1. Does the change in life expectancy for humans over the past 175 years suggest a genetic or an environmental cause?

  2. Given your answer, are the large differences in human life span for Japan versus Angola, shown in Figure 54.13, likely the result of genetic or environmental factors? Explain.

a. Life expectancy has increased steadily over the past 175 years, suggesting that advances in food and health security (i.e., environmental factors) have allowed humans to live longer. Changes in genetically determined life span would occur over evolutionary time.

b. The differences in life span for citizens of Japan versus Angola are also mostly the result of environmental factors. Citizens of Japan generally have a better quality of life than those of Angola, due to Angola’s high rates of HIV infection and civil unrest.

Question 2

Why are species unable to maximize growth, reproduction, and survival? In other words, why are there life history trade-offs?

There are life history trade-offs because constraints on optimality include genetic variation for evolution to act on as well as the mediating effects of the physical and biological environments. Examples of trade-offs are allocation of resources, high growth versus high reproduction, and high reproduction versus survival.

Question 3

Refer to Figures 54.11 and 54.12. What life history strategies characterize Lobelia telekii versus L. keniensis? How about guppies in high-predation versus low-predation stream sites?

Lobelia telekii reproduces just once, produces many seeds, and has a short life span, which falls on the side of an r-strategist. L. keniensis reproduces multiple times, produces fewer but larger seeds, and lives longer, suggesting a K-strategist life history. In terms of the guppy example, those living in high-predation streams have an r-strategist life history whereas those living in low-predation streams have a more K-strategist life history.

1182

For millennia, humans have tried to increase populations of desirable or useful species and reduce populations of species they consider undesirable. Such efforts are most successful if they are based on knowledge of how those populations grow and what determines their densities.