01_LifeCycles - Life Cycles page 1.01 RR Lew Growth...

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Life Cycles – page 1.01 – RR Lew Biological organisms normally exhibit some type of life cycle –growth, development and reproduction– which has evolved to enhance survivability and adaptability. Sur- vival implies successful reproduction to continue the species; adaptability implies the ability of the organisms to adapt to environmental (and other) challenges during its reproductive lifetime. One of the simplest life cycles is that of a prokaryote. Prokaryotes usually divide by binary fission: The lifecycle, from one to two to four cells, etc., is usually described by the generation time (the doubling time –the time required for a cell division to occur). The doubling time depends on nutrient supply, tempera- ture and other conditions. Under optimal conditions, doubling is about 20–80 min- utes. In the absence of any constraints on continued growth — ad infinitum — popula- tion growth will follow the relation: N T = N 0 • 2 (T/g) N T is the number of cells at time T N 0 is the number of cells at time T = 0 g is the generation time 0 50 100 150 200 250 300 350 400 0 50 100 150 200 g(20) g(40) g(80) N T T Implicit in the process of binary fission is the concept that DNA will be duplicated and passed on from the ‘mother’ cell to the two ‘daugh- ter’ cells. Mutations can occur during DNA replication. Mutations will increase genomic variability through the many bacterial genera- tions. The age of the bacteria is remarkable: The progenitor of any bacteria existing now appeared 3 to 4 billion years ago. as time increases, t/g = 1, 2, 3 . .., thus 2 1 , 2 2 , 2 3 , etc. Growth
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It is important to emphasize the complexity of growth. The simple exponential growth process can and does occur under some conditions, but constraints on growth are many. Growth of the individual may or may not be determinant (stopping at a well- defined endpoint, a good example is human (animal) development). Growth of a population will decline as resources are exhausted (humanity may become a good example . ..). Below is just one example of population growth over time: These are population growth curves for the organism Moina macrocopa . The data are from Terao, A. and T. Tanaka (1928) Population growth of the water- flea, Moina macrocopa Strauss. Proc. Imper. Acad. Jpn. Vol. 4, pp. 550–552. It’s an example of how temperature affects the doubling time (fastest at 33.5 degrees) and the population size (maximal at 24.8 degrees [1] . [1] Data were redrawn by Seiichi Sakanoue in a recent publication: Sakanoue, S. (2007) Extended logistic model for growth of single-species populations. Ecological Modelling 205(1-2):159-168. The article describes the multitude of equations which are used to predict not only the sigmoidal characteristic of growth of organisms and populations, but also many economic processes. The complexity of the mathematical models can be daunting, especially when the multi-variate
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01_LifeCycles - Life Cycles page 1.01 RR Lew Growth...

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