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USC BISC 307L - 2. Evolutionary Perspectives
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Bisc307 Lecture 2 (1-11-12)Topics covered: Evolutionary PerspectivesWhat 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 selectionMechanisms of EvolutionEvolution 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 ¹ Survival of the Fittest• Selection maximizes reproductive fitness, not health or longevityEx. 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 ageHistorical life span of humans also makes sense if you look at human fertility as a function of age(nextpage). Fertility peaks in the 20’s and falls precipitously after that. This is agood match with the past lifespan. Reproductive fitness is maximized inhumans in their 20’s, and therefore, diseases that affect people in their 40’sand beyond have little to no effect on our reproductive fitness: Naturalselection cannot optimize things that occur AFTER reproductive period.Ex. Heart failure. Why hasn’t evolution fixed this? In youngindividuals, the mechanisms that heart and kidney use to compensatefor heart failure work very well in a young, healthy, reproducingperson. But heart failure is a disease of middle and old age, past thereproductive fitness period in which evolution can take effect. Theseflaws can’t be removed by natural selection because they do not affectreproduction. Humans Are Bipedal and Have Large BrainsThese are the two most important characteristics that set humans apart. Being bipedal means we can stand up on hind limbs. And our brains, specifically our neocortexes, are huge. Graph on left – brain weight vs. body weight. Most primates fit on the line. But you can see that humans are above the line (red circle). We have unusually large brains relative to our bodyweight. • Evolution results in compromises: Ex 1. Having our spines upright is a bad design because allof our weight and pressure is on our spine, so our kneesand ankles are easily injured, but being upright is a bigenough evolutionary advantage to compromise for thevulnerable design of the human knee • (Bipedalism ® narrow pelvis) vs. (newborn survival ®large head): This is another example of an evolutionarytradeoff. Our pelvises need to be narrow so we can walkupright and balance efficiently. However, babies need to beborn as developed as possible, with big brains to maximizechances of survival. But it is difficult to have a huge braincoming out of a narrow pelvis (right). The human headcan only barely fit through the pelvis, and only if it is in the correct orientation. Without assistance, the success rate is only 85%. How could evolution have resulted in a system that is 15% faulty? It is all about compromise. The female pelvis is as wide as it can be to allow efficient walking and running. And the baby’s brain is as large as it can be, but can still barely fit through the pelvis. Having a large brain allowed for the evolution of language, culture, society. Moms giving birth had the assistance of older members who could help them. This tradeoff for a big brain and bipedalism was good enough to ensure the survival of our species. • Babies are altricial; they need extended parental care: Large brain size has other ramifications – the head cannot be fully developed at birth. We are not a precocial species like zebras (anewborn zebra can be walking with the herd within an hour). Babies are altricial (opposite of precoccial). They need a lot of help because their brains are born very immature. Humans cannotsurvive independently before 8 years after birth. Humans had to invest a lot of effort into caring for their young. As a result, women could not have very many children, and humans tended to live in groups. Anthropologists looked at sizes of groups. It turns out that the ideal group size for humans is ~150. Beyond that, we lose ability to keep track of social structure. Evolving this ability to work in social group is one of the strategies to deal with evolutionary compromises liketaking care of babies. Human Body is Full of “Design” FlawsEvolution only works by modifying what was there before. It can’t start over. Characteristics are


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