Growth is a universal energetic phenomenon for all living organisms, and is undertaken by meristemetic and stem cells through energy metabolism. Most growth processes can be modeled with a sigmoidal function, meaning that early growth is near exponential and later growth declines with age or time until at a certain point (i.e., maturity), when evolutionary optimization is realized and a species should turn all or part of its growth energy into reproduction and then death occurs, consequently. This is usually called a life history process. Evolutionary optimization means that fitness should be supposedly maximized during this process for all species, but how it is realized for all species is still a challenge or gap in our knowledge. This dissertation is dedicated to fill this gap.First, a general bio-energetic growth theory is presented here that assumes a dynamic universal energy pattern: a constant energy income rate deceleration with each cell reproduction, and a species- or clade-constant maintenance cost of cells. This pattern can produce the observed power relationships between energy metabolism and body size as described in Kleiber's law and the von Bertalanffy growth function. Secondly, the new growth theory and a related new bio-energetic definition of mortality are combined with a typical evolutionary optimization process to predict that maturity should begin when net energy income rate equals to the maintenance cost for all living activities and that maintenance cost per mass integrated till maturity should be a defined constant. Two fish life history datasets were used to test the new theory, and the result supported the predicted relationships well. Thirdly, because leaves are the basic functional modules of plants, a comparable evolutionary life history theory for the leaf is developed. The theory predicted a similar energy equilibrium status between energy income (photosynthesis) and expenditure (maintenance) for mature leaves. A global leaf physiological trait dataset (GLOPNET) was used to test this prediction, and a large proportion of variation in photosynthesis of diverse mature leaves is explained (R2 = 87% on mass-based metrics). Lastly, it is suggested that through an established relationship between leaf photosynthesis and seed energy income, there should also be an allometric relationship between leaf photosynthesis and seed mass. Using a large database of leaf traits and matched seed mass for the same species, it was shown that though a relatively small proportion (20%) of seed mass variation was explained by leaf photosynthesis, the predicted relationship is still generally supported.
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