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Chapter 12

Chapter 12 - Humans, Human Brains, and Evolution.doc

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Ted Petit

Chapter 12: Humans, Human Brain and Evolution Module 12.1 Evolution of Humans • Darwin’s theories tell us all living creatures been and continue subject to selection. • Evolutionary theory provides important insights into behaviours that also been subject to selection pressures. • Few researchers applied principles of evolution to human behaviour. This new research is very controversial. • Level of acceptance not yet achieved for evolutionary psychology, which attempts apply principles of adaptation and selection to human behaviour. Lack of acceptance of evolutionary psychology attributed to relative newness and difficulties that are inherent whenever new theory challenges accepted social views. We do not like thinking of ourselves as animals and being subject to laws of nature. Evolutionary Theory • Discovery of evolutionary theory generally credited both Charles Darwin and Alfred Russel Wallace. Development of theory was made independently. • Darwin became intrigued with understanding how species emerged and how geographic isolation on islands produced such variety of features in what he presumed were same species. • Wallace explored Amazon River became intrigued with the relationships between geography of particular location and its effects on specific characteristics of species that inhabited that niche. • Important insights that allowed them produce the theory evolution. Ideas include the following: o Carolus Linnaeus observed commonalities in structure among related species. o Charles Lyell and William Smith supported notion earth was far older than previously thought. Primitive forms occurring in older strata of the earth. Lyell argued that length of time required for these event occur suggested the earth much older than previously been thought. o Malthus suggested populations grow exponentially until they surpass food supply, leads to struggle for existence. Historical Theory Evolution • Historical theory of evolution can be summarized by three terms: variation (all individuals vary, resulting in differences in morphology), inheritance (differences in morphology can be passed from one generation to next), and differential reproduction (these individual differences morphology result variations in success in environment, terms of survival and reproduction). • Darwin suggested mechanism underlying these changes natural selection. Natural selection requires all individuals unique and that characteristics give an animal reproductive advantage result in magnification of these traits in population. When trait results reproductive advantage and selected for, called adaptation. Trait to be adaptation must be inherited from one generation to another. • Darwin eventually became dissatisfied w/ natural selection as only possible cause or (force) evolution therefore proposed another type of selection: sexual selection. Natural selection – competition among individuals for survival to reproduce determines which genes remain in gene pool and which genes disappear from face of planet. Sexual selection - competition among individuals for reproduction determine gene’s fate. - Intersexual selection – one sex chooses a mate from among members of other sex on basis of specific traits. - Intrasexual selection – members of same sex compete for partners of opposite sex. o Original theory of evolution referred as historical theory of evolution because of changes to theory that has occurred since inception. o Traits are passed on by genes and the original source of variation is random mutation of these genes. Current version of evolutionary theory referred as modern synthetic theory of evolution or modern synthesis. Modern Synthesis o Modern synthesis based on what known about genes DNA (deoxyribonucleic acid), chromosomes, and population biology. o Genes, made of DNA, assort in pairs and located on chromosomes, which simply strings of genes. Human have twenty-three pairs of chromosomes. When egg is fertilized, sperm contributes twenty-three chromosomes to egg’s twenty-three chromosomes, resulting in twenty-three pairs of chromosomes. o There is more the same in genetic makeup of individuals than different. These similarities allow for there to be common features such as eyes and bipedal gait (walking upright on two legs), among members of our species. o Your genotype is entirely your genetic composition, and barring exposure certain chemicals or radiation, genotype is invariant during lifetime. Phenotype is interaction of your genotype w/ environment which you develop. o Only genotype passed on to your children. o Natural selection operates on phenotypes, only genotypes transmitted to generation to generation. o Genes make proteins not traits or diseases. o There is often more than one form of given gene; different forms called alleles. o When say found gene for Alzheimer’s disease, mean is we found one allele of gene that, in interaction w/ environment, produces some of symptoms associated w/ Alzheimer’s disease. o As genes assort in pairs, dominant genes always expressed whenever present. Recessive genes expressed only when there no dominant genes present. o Both alleles are same, say that an individual is homozygous for that trait. o When two alleles diff., say that individual is heterozygous for that trait. o Very few traits completely determined by single gene. When multiple genes affect a trait, say trait is polygenic. Eye color is affected by at least three different genes on two different chromosomes. o Any population, there is genetic variation. Variation comes about by random mutation and/or recombination of DNA. Mutation occurs when there is change in genotype due to an error in replication of DNA. Rate of mutation is typically slow, most mutations are harmful to organism, resulting in reducing ability to survive. o Evolution works by selecting individuals who are better able to survive and reproduce, thus passing their genes on to offspring. o Central tenet of modern synthesis is certain environments select certain phenotypes and phenotypes are expression of genotype interacting w/ environment. o Environment cannot typically change the genotype. Environment factors may affect rates of mutation, do not provide direction for mutation to occur. Environment does not produce adaptations. o Genes occur at level of individual, evolutionary change occurs level of populations. Populations evolve by changes in gene frequency that are brought about by natural selection, random genetic drift, and gene flow. Small diff. in genes can result large changes in genetic makeup of population in relatively short period of time if provide tangible survival and/or reproductive advantages for those who possess the gene. o Genetic drift, tendency for isolated populations to depart from original genetic composition of population. Drift away from original genetic composition of population most likely result of extensive interbreeding (offsprings resulting from related parents) and adaptation to isolated environment. Gene flow movement of genes through population results form mating. o Species represent diff. gene pools rather than fundamentally unique groups. Species judged as such by genotypes, not their phenotype. When judge speciation basis of phenotype, can often be misled because there is often great variation in phenotype among individual of population. o Three major diff. between modern synthetic theory of evolution and historical evolutionary theory. First, modern theory recognizes traits are result of genes that are inherited from one’s parents and interact w/ environment. Second, recognition that there are mechanisms other than natural selection that effect evolutionary change. Third, recognition that what we call species only diff. in gene pools of populations. Evolution of the Nervous System o Brain tissue soft and quickly deteriorates under natural conditions, thus leaving no fossilized remains. o Researchers examine fossilized skulls to obtain clues regarding brains of our ancestors. o Direct observation of skulls of our fossil ancestors, can see an increase in skull size and in size of brain. o Must be cautious about direct implicating brain size w/ increased intelligence. o Tend to have one of larger brains for size of our body. o Human brain has almost 3.2 times amount of cortex than other species. o Another way examine skull of animals make endocast, or mol
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