What Is Evolution

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What Is Evolution

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 1 Current Lecture What is evolution? • Descent with modification • Outcome is an organism matched (adapted) to its environment • Matching optimizes reproductive success (fitness) • Reproductive success – the probability that one’s genes will be passed on to the next generation. This is the concept of fitness. • Evolution does not optimize lifespan or general health. It does not optimize happiness or well-being • Within a population, evolution optimizes fitness in the environment. If the environment changes, the population’s fitness will suffer. • Driving force is natural selection Mechanisms of Evolution Evolution is based on 3 main observations: 1. Species produce more offspring than resources can support. 2. Individuals vary in ability to compete for resources, survive, and reproduce (variation) 3. Some of this variation is heritable (inheritance): Inherited variability is due to 3 main sources: mutations, recombination during meiosis, and epigenetic modifications (modifications of DNA that affect gene expression and are inherited but do not involve changes in nucleotide sequences – e.g., histones binding to DNA or methylation of DNA). A fourth source is developmental plasticity – the environment during embryonic and postnatal development that affects the phenotype, which is especially important in the development of the CNS in utero and post natal. Darwin took these observations, and made them into the following two conclusions: 1. Differences in survival and reproduction lead to over-representation of more successful forms in next generation (selection). 2. Selection over generations leads to change in composition of the population (evolution). And how long it takes depends on generation time and selective pressures. But over time it will lead to changes in composition of the population. Can be small (microevolution) or large (macroevolution – includes production of a new species). Natural Selection  Survival of the Fittest • Selection maximizes reproductive fitness, not health or longevity Ex. There are insects that hatch, mature, breed, and die in a day, and they are very reproductively fit. Natural selection doesn’t CARE about the well-being or survival of the individual. It only cares about the successful transmission of genes to the next generation. • Evolutionary success of humans did not require a “long” life as part of the human reproductive strategy. Looking at the graph on the left, before the marked increase in human longevity starting in the 1800’s, the human life span was estimated to be ~30 years. In the past, most people didn’t live long enough to contract many of the popular/chronic diseases that occur during middle or advanced age Historical life span of humans also makes sense if you look at human fertility as a function of age(next page). Fertility peaks in the 20’s and falls precipitously after that. This is a good match with the past lifespan. Reproductive fitness is maximized in humans in their 20’s, and therefore, diseases that affect people in their 40’s and beyond have little to no effect on our reproductive fitness: Natural selection cannot optimize things that occur AFTER reproductive period. Ex. ...

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