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Chapter 12

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Ted Petit

Chapter 12 - Humans, Human Brain, and Evolution 12.1 Evolution of Humans - The Origins of Species applies to more than just physical changes; evolutionary theory provides important insights into behaviours that also have been subject to selection pressures - This theory provides us with a means by which we can integrate our behaviour with our prehistory and gain profound insights into what it means to be human - As a species, we have difficulty applying the principles of evolution to ourselves. It appears that we do not like thinking of ourselves as animals and as being subject to the laws of nature Evolutionary Theory - The discovery of evolutionary theory is credited to both Charles Darwin and Alfred Russel Wallace, both of whom were naturalists with interests in geology - Although Darwin and Wallace are generally credited with discovering the principles of natural selection, they credited others with providing important insights that allowed them to produce the theory of evolution, such as the classification of organisms based on structure, the notion that the earth was far older than was previously thought and that populations grow exponentially until they surpass their food supply, which leads to a struggle for existence Historical Theory of Evolution - The historical theory of evolution can be summarized by three terms; variation, inheritance and differential reproduction - Individual differences in morphology result in variations in success in the environment, in terms of survival and reproduction - Natural selection requires that all individuals are unique and that characteristics that give an animal a reproductive advantage will result in the magnification of these traits in population. When a trait results in a reproductive advantage and is selected for, it is called an adaption. For a trait to be an adaption, it must be inherited from one generation to the next - Darwin himself was puzzled by some of the physical traits observed in species and eventually became dissatisfied with natural selection as the only possible cause of evolution. In Darwin’s second book, The Descent of Man, he proposed another type of selection; sexual selection. In sexual selection, it is competition among individuals for reproduction that determines a gene’s fate. - We refer to the original theory of evolution as the historical theory of evolution because of the changes to the theory that have occurred since its inception. The current version of evolutionary theory is referred to as the modern synthetic theory of evolution or the modern synthesis Modern Synthesis - The modern synthesis combines information from such diverse areas as molecular biology and paleontology - Your genotype is the entirety of your genetic composition and is invariant during your lifetime. Your phenotype is the interaction of your genotype with the environment in which you develop. Regardless of your phenotype, only your genotype will be passed on to your children. In essence, it can be said that while natural selection operates on phenotypes, only genotypes are transmitted from generation to generation - Genes do not make traits or diseases, they make proteins. In addition, there is often more than one form of a given gene; these different forms are called alleles. When we say that there is a gene for blue eyes and one for brown, what we really mean us that there is an allele for eye color - As genes assort in pairs, dominant genes are always expressed whenever they are present. In contrast, recessive genes are expressed only when there are no dominant genes present. When both alleles are different, we say that an individual is homozygous for that trait. When both alleles are different, we say that an individual is heterozygous for that trait - When multiple genes affect a trait, we say that the trait is polygenic. Many polygenic traits associate; that is, they behave like best friends, and although they are individual genes, they go everywhere which explains that people with brown hair usually have brown eyes and people with blonde hair usually have blue eyes - In any population, there is genetic variation. These variations may come about by random mutation or recombination of DNA. The variation results in differing ability to adapt to the environment - Genetic drift is the tendency for isolated populations to depart from the original genetic composition of the population which is the result of extensive inbreeding. Gene flow is the movement of genes through a population that results from mating. - Species are judged as such by their genotype, not their phenotype - The three major differences between the modern synthetic theory of evolution and the historical evolutionary theory is that modern theory recognizes that traits are the result of genes
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