Chapter LIFE HISTORIES

What you need to know! How life histories are shaped by natural selection How to read an age structure graph

Life Histories The series of events from birth, to reproduction, to death Shaped by natural selective forces to maximize success of offspring Key traits: 1.Earliest reproductive age 2.Frequency of reproduction 3.Number of offspring 4.Amount of parental care

Evolution of Life Histories Life Histories are composed of heritable traits (genes) Life history traits are evolutionary outcomes, not conscious decisions by organisms Example:

Types of Life Histories 1.Life histories that are sensitive to population density and carrying capacity are know as K-selection Populations that live close to carrying capacity Modeled using logistic growth 2.Life histories that maximize reproductive success are called r- selection Populations with large intrinsic rates of growth (r) Modeled using exponential growth (boom-and-bust)

Human Population

Countries can stabilize their populations (zero population growth) in two ways 1.ZPG = High birth rates – high death rates 2.ZPG = Low birth rates – low death rates When countries develop they tend to move from #1 to #2 (demographic transition)

Age Structures The proportion of individuals in different age-groups are amazing indicators of the stability of a population Pyramids = rapid growth; rectangles = no growth; reverse pyramids = rapid decline