Chapter 7

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Biological Sciences
Marc Cadotte

Lecture 6, Chapter 7 2/2/2013 2:54:00 PM Life History Diversity  Life history  Record of events relating to an organism’s growth, reproduction & survival  Life history strategy  Overall pattern of timing & nature of life history events averaged across all individuals in the species (7.3)  Shaped by division of energy & resources between growth, reproduction and survival  Natural selection determines the effects of these strategies, not the individual’s choice  Environmental differences  Phenotypic plasticity – one genotype may produce different phenotypes under different environmental conditions (7.4) o Example A: ponderosa pine trees in cool, moist climates allocate more resources to leaf production than do trees in desert climates o Example B: desert trees are shorter than those grown in cool climates but they do have thicker trunks  Phenotypic plasticity may produce a continuous range of growth rates OR discrete types (morphs with no intermediate forms) o Example: Spadefoot toad tadpoles contain two morphs  Omnivore morphs – feed on detritus & algae,  Carnivore morphs – feed on fairy sheep, bigger mouths, stronger jaw muscles, accelerated growth  Omnivores can turn into carnivores if they switch their diet to fairy shrimps  Proportion of omnivores: carnivores depends on food supply o Can be a physiological or an adaptive response Mode of reproduction is a basic life history trait  Asexual reproduction  Simple cell division  Occurs in all prokaryotes & many protists  However, some multicellular organisms reproduce both asexually and sexually (e.g. corals, study fig 7.6) o In corals, their colonies grow asexually which form individual polyps o A multicellular bud splits off from a parent polyp to form a new one o Each polyp is a clone of the found polyp o Once colony has grown to certain state, polyps reproduce sexually  producing offspring that develop into polyps  The cost of sex  Pros: recombination leads to genetic variation  increase capacity of pop to evolve in response to environmental challenges  Cons: only half of one parent’s genetic material is transferred, growth is 50% slower than that of asexually reproducing ones Life cycles are often complex  Complex life cycles involve at least two distinct stages that may have different body forms and live in different habitats  Transition between stages may be abrupt  E.g. metamorphosis – abrupt transition inform from the larval to the juvenile stage (80% of all animals experience it)  Often result when offspring and parents are under very different selection pressures  Why? To exploit different environments, competition and better dispersal  Simple life cycles have no abrupt transitions  also called “direct development”  no free-living larval stage  Alternation of generations  Diploid sporophyte alternates with a haploid gametophyte  Sporophyte produces haploid spores  disperse & grow into gametophytes which produces haploid gametes  gametes combine to fertilize into sporophytes Life history continua  Reproduction can be categorized along several continua  Semelparous – species reproduce only once  Iteroparous – species can reproduce multiple times  Examples of semelparous species  Annual plants  Agave – vegetative growth can last up to 25 years, produces clones asexually  Giant pacific octopus – female lays single clutch of eggs, broods them for 6 months, dies after they hatch  Examples of iteroparous species  Trees such as pines and spruces  Most large mammals  R and K selection describe two ends of a continuum of reproductive patterns  dN/dt = rN(1-N/K)  R-selection (Live fast, die young)  Intrinsic rate of increase of pop  Measured how fast a pop can grow  Selection for high pop growth rates  Can occur if there is o low pop density o uncrowded environments o newly disturbed habitats  Organisms have o short life spans o rapid development o early maturation o low parental investment o high rates of reproduction  K-selection (Slow and steady)  Carrying capacity for a pop  Selection for slower growth rates in populations that are at or near K  Favors crowded conditions and efficient reproduction  Organisms have o long lives o slow development o delayed maturation o heavy parental investment o low rates of production  tend to be larger species than r-selection  high rate of survival of offspring  Plant life histories  Based on stress and disturbance (Grimes)  Stress – any factor that reduces vegetative growth o Examples: extreme temperature, shading, low nutrient levels  Disturbance – any process that destroys plant biomass o Examples: biotic sources (herbivorous) & abiotic sources (fire)  Three possible habitat types for plants (extreme end) (see 7.12)  Low S-high D o Ruderal plants o Short life spans, rapid growth, heavy investment in seed production, seeds that survive for a long time, rapid germination o Ex. dandelions  Low S-Low D o Comp
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