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Lecture 7

Lecture 7 - Life History

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Western University
Biology 2483A
Hugh Henry

LECTURE 7: LIFE HISTORY Introduction  An organism’s life history is a record of events relating to its growth, development, reproduction, and survival  Life history characteristics include: o Age and size at sexual maturity o Amount and timing of reproduction o Survival and mortality rates Nemo Grows Up: A 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 non-breeders.  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.  Why do clownfish maintain the hierarchy? o They are completely dependent on protection by the sea anemone. They are easy prey outside the anemone. o Conflicts result in expulsion and death, probably without having reproduced.  So there is a 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  “Decisions” made in a life history are dictated by natural selection  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: One genotype may produce different phenotypes under different environmental conditions  For example, growth and development may be faster in higher temperatures  Changes in life history traits can cause change in adult morphology  Phenotypic plasticity may result in a continuous range of sizes; or discrete types called morphs  Polyphenism—a single genotype 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 – advantageous to get out of the pond as quick as possible  Omnivores grow more slowly and are favored in ponds that last longer; they metamorphose in more favorable conditions and have more chance of survival – tradeoff here is that although they grow slower, once they are mature, their chances of survival are higher in the end  Different body morphology results from different growth rates of body parts in both the Ponderosa pines and spadefoot toads  Allometry – different body parts grow at different rates, resulting in differences in shape or proportion – eventually determines body form/size Modes of Reproduction  Asexual reproduction – simple cell division (binary fission) – all prokaryotes and many protists  Some multicellular organisms reproduce both sexually and asexually (e.g., corals)  Benefits of sexual reproduction: recombination promotes genetic variation and increased ability to respond to environmental challenges  Disadvantages of sexual reproduction: an individual transmits only half of its genome to the next generation; population growth rate is slower  Isogamy – gametes are equal in size o e.g. the green alga Chlamydomonas reinhardii  Anisogamy – gametes of different sizes. Usually the egg is much larger and contains nutritional material. Most multicellular organisms produce anisogametes  Some species have direct development – the fertilized egg develops into a juvenile without passing through a larval stage  Most vertebrates have simple life cycles without abrupt transitions. But complex life cycles are common in insects, marine invertebrates, amphibians, and in some plants, algae, protists and fish  Complex life cycles have at least two stages, with different body forms and that live in different habitats  Metamorphosis – abrupt transition in form between the larval and juvenile stages Classification of Life History Strategies  Number of reproductive events per lifetime: o Semelparous species reproduce only once o Iteroparous species can reproduce multiple times  Semelparous species include: o Annual plants o Agave—vegetative growth can last up to 25 years (but also produces clones asexually)  Iteroparous species include: o Trees such as pines and spruces o Most large mammals  r-selection and K-selection describe two ends of a reproductive strategy continuum.  r is the intrinsic rate of increase of a population  r-selection: For high population growth rates; an advantage in newly disturbed habitats and uncrowded condition  r-selected (“live fast, die young”): o Short life spans, rapid development, early maturation, low parental investment, high reproduction rates o Most insects, small vertebrates such as mice, weedy plant species  K is the carrying capacity for a population  K-selection: For slower growth rates in populations that are at or near K; this is an advantage in crowded conditions; efficient reproduction is favored  K-selected (“slow and steady”): o Long-lived, develop slowly, late maturation, invest heavily in each offspring, low reproduction rates o Large mammals, reptiles such as tortoises and crocodiles, and long-lived plants such as oak and maple trees  One classification scheme for plant life histories is based on stress and disturbance (Grime 1977)  Stress – any abiotic factor that limits growth  Disturbance – any process that destroys plant biomass  Four habitat types possible: o Low stress, low disturbance o High stress, low disturbance
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