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Lecture 7

Lecture 7: "Life History"

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Biology 2483A
Hugh Henry

Ecology Lecture No.7: Life History nd Tuesday October 2 , 2012 Introduction: -An organism’s life history is a record of events relating to its growth, development, reproduction, and survival. Life history characteristics include: Age and size at sexual maturity, amount and timing of reproduction, and survival and mortality rates. Clownfish Case Study: -In real life, two to six clownfish spend their entire adult lives within one sea anemone, but are not usually related. The largest fish is a female; the next largest is the breeding male. The remaining fish are immature nonbreeders. There is a strict pecking order in the group, based on body size. If the female dies, the breeding male becomes a female, and the next largest fish becomes the breeding male. Hatchlings move out of the anemone, and juveniles must find a new anemone to inhabit. -Experiments with clownfish show that hierarchy is maintained by regulating growth rates. If two fish become similar in size, a fight results and one is expelled from the anemone. They are completely dependent on protection by the sea anemone. They are easy prey outside the anemone. Conflicts result in expulsion and death, probably without having reproduced. -So there is strong selection pressure to avoid conflict. Sea anemones are a scarce resource for clownfish. Growth regulation mechanisms have evolved because individuals that avoid growing to a size that necessitates conflict are more likely to survive and reproduce. Life History Diversity: -Individuals within a species show variation in life history traits due to genetic variation or environmental conditions. The life history strategy of a species is the overall pattern in average timing and nature of life history events. It is shaped by the way the organism divides its time and energy between growth, reproduction, and survival (e.g. the metamorphosis of a frog). -Some life history traits are determined genetically. Natural selection favors individuals whose life history traits result in their having a better chance of surviving and reproducing. Ideal or optimal life histories maximize fitness (genetic contribution to future generations). But none are perfect; all organisms face constraints and ecological trade-offs. Phenotypic Plasticity: -Phenotypic plasticity is where one genotype may produce different phenotypes under different environmental conditions. For example, growth and development may be faster in higher temperatures. Phenotypic plasticity may result in a continuous range of sizes or discrete types called morphs. Polyphenism: -Polyphenism describes a single genotype that produces several distinct morphs. Spadefoot toad tadpoles have small omnivore morphs and larger carnivore morphs. Carnivore tadpoles grow faster and metamorphose earlier. They are favored in ephemeral ponds that dry up quickly. Omnivores grow more slowly and are favored in ponds that last longer; they metamorphose in more favorable conditions and have more chance of survival. -Different body morphology results from different growth rates of body parts in both the Ponderosa pines and spadefoot toads. Allometry describes different body parts grow at different rates, resulting in differences in shape or proportion. Modes Of Reproduction: Asexual reproduction is also known as simple cell division (binary fission) — all prokaryotes and many protists. Some multicellular organisms like coral reproduce both sexually and asexually. The Cost Of Sexual Reproduction: -The benefits of sexual reproduction include recombination as it promotes genetic variation and increased ability to respond to environmental challenges. Disadvantages include how an individual transmits only half of its genome to the next generation; population growth rate is also slower. Isogamy & Anisogamy: -Isogamy refers to gametes that are equal in size (e.g. the green alga Chlamydomonas reinhardii) Anisogamy refers to gametes of different sizes. In humans, the egg is usually much larger and contains nutritional material. Most multicellular organisms produce anisogametes. Development Of Species: -Some species have direct development where the fertilized egg develops into a juvenile without passing through a larval stage. Complex life cycles have at least two stages, with different body forms and that live in different habitats. Metamorphosis is the abrupt transition in form between the larval and juvenile stages. Life History Continua: -Classification schemes for reproductive patterns place the patterns on continua with extremes at each end. The number of reproductive events per lifetime forms to categories under which species fall. Semelparous species reproduce only once (e.g. annual plants). Iteroparous species can reproduce multiple times (e.g. trees and most large mammals). Reproductive Strategies: -r-selection and K-selection describe two ends of a reproductive strategy continuum. r is the intrinsic rate of increase of a population. For high population growth rates there is an advantage to being an r- selected species in newly disturbed habitats and uncrowded conditions. r-selected species have short life spans, rapid development, early maturation, low parental investment, and high reproduction rates. Most insects, small vertebrates such as mice, and weedy plant species are r-selected. -K is the carrying capacity for a populatio
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