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Western University
Psychology 3229A/B
Scott Mac Dougall- Shackleton

Chapter 2: Mechanisms of Evolutionary Change Darwin’s Theory of Evolution • Artificial Selection • Herbert Simon once argued that domestic animals have been selectively bred fro features that humans can make use of • Therefore, by choosing individuals with a particular feature you can, over a number of generations, create a change in the direction that you have selected • Selective breeding has also been responsible for more radical changes in organism’s physical form • Therefore, many organisms are the way they are today because their traits have been selected not by nature but by human beings, a process known as artificial selection • Natural Selection • In evolution, there is no omnipotent being choosing which organisms should survive and which should be consigned to oblivion, and there is no ultimate goal that the selection process is trying to achieve • Natural Selection and Survival of the Fittest • Natural selection is based on differential reproductive success of heritable characteristics that vary in a population • Since individuals that survive to reproductive age are generally viewed as being physically fitter than those that don’t, this led to the term ‘survival of the fittest’ • ‘Survival of the fittest’is an unfortunate term since fitness means different things to different people • Evolutionists began to use the term as a measure of how successful an individual is at reproducing, that is its lifetime reproductive success Mendel and Post-Mendelian Genetics • Mendel’s Findings • The results of Mendel's experiments gave rise to three important findings • First, they showed that traits are caused not by a single gene, rather that genes operate in pairs • Second, his work revealed that the relationship between the genes that an individual possess and that individual’s physical structure is rather more complex than it might appear • Their phenotype is the same but their genotype is different • Finally his experiments showed that inheritance is particulate rather than the result of a process of blending • Modifications of Mendel’s Laws • Mendel had no idea of how these units of inheritance were physically realized, or where they were located • Second, it soon became clear that there are exceptions to most of his original suggestions • Genes and Chromosomes • Genes are located in the nucleus of an organism’s cells at specific locations on larger bodies called chromosomes • The discovery of chromosome pairs provided a physical home for Mendel’s paired genes Chapter 2: Mechanisms of Evolutionary Change • Agene for a particular characteristic occurs at a specific point on the chromosome called it locus • The purpose of the Human Genome Project is to identify the loci of all the genes in our species • Generally a locus is home to more than one alternative form of a gene; when this occurs the alternate forms are called alleles • Chromosome pairs become separated during sexual reproduction so that each sperm or ovum produced has half of the normal complement of genes • Meiosis ensures that when sperm and ovum fuse to form a fertilized egg or zygote the number of genes an offspring has is restored • FactorsAffecting the Transmission of Genes • Different genes may be linked together when they are passed on to offspring • If two genes are found on the same chromosome they are far more likely to be passed on together to offspring than if they appear on separate chromosomes • Genes may be linked together and a chromosome may be considered as a ‘linkage group’of genes • This does not however, mean that all of the genes on parental chromosomes will be passed on together to the offspring • Prior to gamete formation, homologous chromosomes pair up together and exchange genes at specific points • This exchange of genes is knows as crossing over • In this way genes become recombined and the genes that are passed on to offspring are said to have undergone recombination • The closer together two genes are found on a chromosome the more likely they are to be passed on together during recombination • Other Exceptions to Mendel’s Laws • Crossing over is a form of chromosome mutation that is common because it occurs every time a gamete is formed • Individual genes may also be subject to mutations but these are rare compared with chromosome mutations • Individual organisms will have quite a large number of mutations, however, since the vast majority of these are neutral, that is, they have no phenotypic effect • When a gene mutation does have a phenotypic effect it is more likely to be detrimental than beneficial • Modern day genetics there are considered to be three sources of variation in a population that natural selection can work on: • Mendelian variation due to the mixing together of parental genes • Recombination • Gene mutations • Many genes have more than one phenotypic effect; this is known as pleiotropy and many characteristics depend on more than one gene, that is, they are polygenic • Finally, the way that a gene is expressed in the phenotype may be altered by a modifier gene Chapter 2: Mechanisms of Evolutionary Change Modern Genetics • Genes and the Structure of DNA • Chromosome consists of a double strand of the chemical DNA • Each link of sugar, acid and base is called a nucleotide • The bases code for the production of amino acids that are the building blocks of protein • Each amino acid is coded for by a triplet or codon of bases • The main purpose of DNAis to produce proteins • Free-floating bases within the cell attach to the exposed bases, forming a second type of molecule - mRNA • This mRNA, once formed, detaches and travels to cell ‘organs’called ribosomes where, via another form of RNAit forms the template for amino acid and hence protein production • The formation of proteins form DNAis called transcription • DNAhas another main function - replication • Many behavioural biologists believe that difference in personality and intellectual ability may, in part, be traced back to difference in the genetic code that we inherit since the various levels of neu
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