Bio 221, Chapter 7 PDF

CHAPTER 7: Life Histories and Evolutionary Fitness From an evolutionary point of view, individuals exist to produce as many successful progeny as possible. The only question is how to best accomplish this. Life history – The pattern of a species’ development, growth,...

CHAPTER 7: Life Histories and Evolutionary Fitness From an evolutionary point of view, individuals exist to produce as many successful progeny as possible. The only question is how to best accomplish this. Life history – The pattern of a species’ development, growth, life span, and reproduction. Includes age at maturity, number of offspring, and other aspects. Like all other traits, life histories are subject to natural selection and therefore adapted to the environment. Average number is 7 Hogstedt, Science 210:1148-1150(1980). Life history components include: 1.Age at maturity – Age at which reproduction begins. 2.Parity – Number of times an organism reproduces. 3.Fecundity – Number of offspring produced per reproductive episode. 4.Longevity – Life span. Life history components are adapted to the organism’s environment Number of eggs in a clutch size is correlated with hours of light available for food gathering. Alaska Panama  Life histories vary along a continuum Slow Fast Large mammals Insects Large reptiles Many fish Birds of prey Small mammals Relationships are observed between survival and fecundity. Survivorship Curves Number of offspring produced annually is correlated with adult annual mortality rate Births and deaths must balance “Investment In offsprIng draIn’s a parents resources…”  Life histories balance trade-offs between current and future reproduction Survival of parents and number of offspring produced must be balanced Breeding takes time, energy, and involves risks Parents may hunt harder European kestrel Hover overhead Two chicks added 81% vs 98% survival 0.8 more chicks per nest Impacted parental survival  Primary trade-offs organisms face: 1.Age at first reproduction 2.Fecundity and survival 3.Growth (of parents) and fecundity 1. At what age should reproduction begin? Natural selection favors the age that results in the greatest offspring production over the life of the animal. Many organisms exhibit determinate growth, therefore the longer they delay sexual maturity the greater their fecundity. 2. Trade-off between fecundity and survival  2. Trade-off between fecundity and survival (cont.) When life span is long and many offspring survive, adult survival is favored over fecundity. When life span is short and few offspring survive, fecundity is favored over survival. S0 = Offspring survival SN = Adult survival 3. Trade-off between growth and fecundity. Many species exhibit indeterminate growth Long lived species favor growth over fecundity. Short lived species favor fecundity over growth. Experiment examining natural selection and life history trade- offs: Trinidad guppies  Semelparity and Iteroparity Semelparity– Organisms that reproduce once and die. - Rare among long lived plants and animals. - Sometimes favored in habitats with variable conditions (typically plants) or when preparation for reproduction is extremely costly. - Not the same as an annual life history. Iteroparity- Organisms that reproduce repeatedly.  Periodical Cicadas – Semelparity Emerge every 13 or 17 years. Likely a mechanism to overwhelm potential predators. 13 and 17 year intervals make it difficult for predators to synchronize emergences to coincide. Sensitive to reproductive cycles of trees. Calling Calling  Senescence Decline in function with age. Occurs in all animals but rates vary widely. - Storm petrel may live 40 years but rodent of equivalent size and metabolism rarely lives past 5 years. In populations with low survival all effort goes toward reproducing. In populations with higher survival, older organisms may produce offspring, more effort goes to maintain body. Life Histories Respond to the Environment  Storage of food and buildup of reserves Plants - Cacti – water - Chaparral plants – fire resistant root crowns Animals - Internal and external storage Subject to natural selection  Dormancy Inactive state that allows an organism to survive a temporarily inhospitable environment. Hibernation – Slowed metabolic rate to avoid scarcity of winter food. Diapause - Utilized by insects to endure harsh conditions - prevents winter freezing and summer dessication. Endospores Aestivation - African and Australian lungfish. File:Australian-Lungfish.jpg  Stimuli for Change Proximate factors – Cues by which the organisms sense the state of the environment Ultimate factors – Features that directly impact fitness Response varies according to location: -Michigan – Daphnia enter diapause in mid-September (12 hours or less light) - Alaska – Daphnia enter diapause in mid-August (20 hours or less light) Life Histories are Sensitive to Environmental Influences Individuals may reach maturity at a certain age, certain mass, or at some point between the two. Predation may also influence time to maturity. - Mayflies and presence or absence of trout.  Animals Forage to Maximize Fitness Central place foraging – Birds bringing food to a nest. European starlings and leatherjackets. European starling and mealworm experiment.  Animals Forage to Maximize Fitness (cont.) Risk sensitive foraging - Reward must be worth the risk – creek chub experiment. Gilliam and Fraser. 1987. Ecology 68:1856-1862. Gilliam and Fraser. 1987. Ecology 68:1856-1862. Gilliam and Fraser. 1987. Ecology 68:1856-1862. 1.00 0.17 1.00 0.17 0.50 0.17 0.65 0.17 0.33 0.17 0.17 0.17