© 2008 McGraw-Hill Higher Education. All right reserved. Window on Humanity Conrad Phillip Kottak Third Edition Chapter 3 Evolution, Genetics, and Human Variation

© 2008 McGraw-Hill Higher Education. All right reserved. Overview Evolution and natural selection Genetics Mechanisms of genetic evolution Human biological adaptation

© 2008 McGraw-Hill Higher Education. All right reserved. The Origin of Species –Creationism Divine creation of life forms as described in Genesis Biological characteristics of life forms immutable –Carolus Linnaeus – first biological classification (taxonomy) of plants and animals –Fossil record – raised doubts about creationism –Catastrophism Modified version of creationism Ancient life forms destroyed by catastrophes (e.g., fires, floods) then replaced by new, divinely created species

© 2008 McGraw-Hill Higher Education. All right reserved. Theory and fact Alternative to creationism and catastrophism was transformism, also called evolution –Charles Darwin Influenced by Lyell’s concept of uniformitarianism –Explanations for past events should be sought in the long-term action of ordinary forces observable today –Current geological features are the result of long-term natural forces Darwin applied the notions of uniformitarianism and long-term transformation to living things –All life forms are ultimately related –Number of species has increased over time

© 2008 McGraw-Hill Higher Education. All right reserved. Natural selection Process by which nature selects the forms most fit to survive and reproduce in a given environment Necessary conditions for selection to occur: Variation within a population Competition among individuals for strategic resources –Individuals whose attributes render them fitter to survive and reproduce in a given environment will do so in greater numbers than individuals that are less fit –Differential reproductive success (rather than survival of the fittest) Evolutionary theory is used to explain, to show how and why

© 2008 McGraw-Hill Higher Education. All right reserved. Genetics Helps us understand the causes of the biological variation upon which natural selection operates –Mendelian genetics – studies how chromosomes transmit genes across generations –Biochemical genetics – examines structure, function, and changes in DNA –Population genetics – investigates natural selection and other causes of genetic variation, stability, and change in breeding populations

© 2008 McGraw-Hill Higher Education. All right reserved. Genetics Mendel’s experiments –Gregor Mendel – discovered that heredity is determined by discrete particles or units –Two basic forms of traits: dominant and recessive Dominant forms mask recessive forms in hybrid (mixed) individuals Reappearance of recessive traits in later generations – “Dominance produces a distinction between genotype, or hereditary make up and phenotype, expressed physical characteristics.”

© 2008 McGraw-Hill Higher Education. All right reserved. Genetics: –Basic genetic units (genes or alleles) are located on chromosomes Humans have 46 chromosomes (23 pairs) Genes – positions on chromosomes that wholly or partially determine biological traits Alleles – biochemically different forms of a given gene –Individuals may be homozygous or heterozygous with respect to a particular gene Homozygous – possessing two identical alleles (TT) (tt) Heterozygous – possessing different alleles (Tt)

© 2008 McGraw-Hill Higher Education. All right reserved. Genetics Dominance produces a distinction between genotype and phenotype –Genotype – hereditary makeup –Phenotype – expressed physical characteristics Some traits have only dominant and recessive forms Other traits (e.g., human blood type) are determined by more than two alleles –Alleles may be codominant

© 2008 McGraw-Hill Higher Education. All right reserved. Independent assortment Law of independent assortment –Formulated by Mendel –Traits are inherited independently of one another The fact that traits are inherited independently and are recombined is one of the ways that variety is produced in a population. Create new types on which natural selection can operate –Mitosis – ordinary cell division –Meiosis – production of sex cells, each with only 23 chromosomes –Fertilization – combination of 23 chromosomes from each parent Parents’ genotypes can be assorted in more than 8 million (2 23 ) different ways

© 2008 McGraw-Hill Higher Education. All right reserved. Population genetics Studies stability and change in gene frequencies within breeding populations Gene pool – all the alleles, genes, chromosomes, and genotypes within a breeding population Genetic evolution – change in gene frequency (the frequency of alleles) in a breeding population from generation to generation

© 2008 McGraw-Hill Higher Education. All right reserved. Four mechanisms of genetic evolution: 1.Natural selection 2.Mutation 3.Genetic drift 4.Gene flow

© 2008 McGraw-Hill Higher Education. All right reserved. 1. Natural Selection Genotype vs. phenotype –Phenotype is influenced by both genotype and the environment –Human biological plasticity Natural selection operates only on phenotype (not on genotype) Natural selection affects gene frequencies within a population Traits that are adaptive in a given environment will be selected for Individuals possessing the alleles responsible for those traits will reproduce more frequently

© 2008 McGraw-Hill Higher Education. All right reserved. 1. Natural Selection, cont. –Directional selection Long-term selection of the same adaptive trait(s) from generation to generation Maladaptive alleles may be removed from a gene pool Continues as long as environmental forces stay the same –Sexual selection – the process by which certain traits of one sex are selected because of advantages they confer in winning mates

© 2008 McGraw-Hill Higher Education. All right reserved. 1. Natural Selection, Cont. Stabilizing selection Selective forces may favor a balanced polymorphism in which the frequencies of two or more alleles remain constant between generations –Example: two alleles (Hb A and Hb S ) that affect the production of human hemoglobin Homozygous for Hb A = normal hemoglobin Homozygous for Hb S = sickle-cell anemia High frequencies of the Hb S allele exist among certain populations in Africa, India, and the Mediterranean Hb S allele persists because the heterozygote is the fittest phenotype in malarial environments –Adaptation and fitness are in relation to specific environments

© 2008 McGraw-Hill Higher Education. All right reserved. 2. Mutation Mutation –Changes in the DNA molecules of which genes and chromosomes are built –Occurs spontaneously and regularly –Produces variety on which natural selection may operate

© 2008 McGraw-Hill Higher Education. All right reserved. 3. Random genetic drift Change in allele frequency that results from chance rather than from natural selection May operate in any population, large or small, but more rapid in small populations

© 2008 McGraw-Hill Higher Education. All right reserved. 4. Gene flow Exchange of genetic material between populations of the same species Produces variety on which natural selection can operate Tends to prevent speciation (the formation of new species) –Species – groups of related organisms whose members can interbreed to produce offspring that can live and reproduce –Speciation occurs when populations of the same species are isolated from each other (lack of gene flow) and, as a result of genetic change, become incapable of interbreeding

© 2008 McGraw-Hill Higher Education. All right reserved. Human biological adaptation –Because of extensive gene flow and interbreeding, Homo sapiens has not evolved subspecies or distinct races –Biological variation between human populations involves gradual shifts (clines) in gene frequencies and other biological features

© 2008 McGraw-Hill Higher Education. All right reserved. Human biological adaptation Genes and disease –Infectious diseases posed an increasing threat to human populations beginning with food production (10,000 years ago) –Tropical diseases (e.g., malaria, schistosomiasis, and filariasis ) affect more than 10 percent of the world’s population today

© 2008 McGraw-Hill Higher Education. All right reserved. CHAPTER 3 Evolution, Genetics, and Human Variation Genes and Disease –Human blood types play an important role in resistance to some diseases Various alleles producing human blood types interact with infectious and noninfectious ailments For example, type A or AB blood cells seem to make a person more susceptible to smallpox, while the presence of O or B blood cells appears to increase smallpox resistance

© 2008 McGraw-Hill Higher Education. All right reserved. Human biological adaptation Lactose tolerance –Phenotypical adaptation – adaptive changes which occur during an individual’s lifetime –Lactose tolerance (ability to digest milk) determined by both genes and phenotypical adaptation People from herding populations remain lactose tolerant into adulthood, while those from nonherding populations do not Lactose tolerance among descendents of herding populations – genetic adaptation to a milk-rich diet Lactose tolerance can vary during an individual's life, depending on amount of milk consumed – phenotypical adaptation