Human Variation Part 1 Ways Humans Vary Biologically Sources of Human Biological Variation Within Group Variation in Mendelian Traits Between Group Variation in Mendelian Traits In what ways do humans vary biologically o Skin color stature hair type blood group genetic diseases lactase persistence etc Sources of Human Biological Variation o 2 sources of human biological variation Genetic Variation phenotypic differences between individuals caused by variation in the genes they inherited from their parents e g SNPs in DNA Environmental variation phenotypic differences between individuals caused by the effect of environmental factors e g climate habitat competing species culture o Genetic and environmental sources often interact in Ex body type complicated ways to influence human phenotypic variation o It is often difficult to determine the relative importance of genetic vs environmental impacts on a phenotypic trait Children and parents might look similar because of shared DNA or because of similar environments or both Which is it o Human variation is partitioned within and between groups Within group variation refers to differences between people within the same group e g variation in height in NBA Within group variation in Mendelian traits Between group variation refers to differences between people in different groups e g mean height in NBA vs mean height of jockeys groups can be defined in many ways cultural geography age gender class etc the causes of variation within groups can be very different from the causes of variation between groups alleles at single locus present AA Aa homozygotes aa homozygous recessive condition o Mendelian traits are controlled by the combination of two o The dominant allele is expressed in the phenotype when it is o The recessive allele is expressed only in recessive o Many heritable ie Genetic diseases are caused by the If these serious diseases are caused by recessive alleles then why hasn t natural selection gotten rid of them entirely Why is there ANY within group variation in the Part of the answer is MUTATION Mutation will frequencies of these alleles maintain a small proportion of recessive alleles in any population o Balanced polymorphism A steady state in which both alleles dominant and Natural selection can maintain a balanced recessive persist in a population and the dangerous recessive allele is at frequency greater than predicted by mutation alone polymorphism when the heterozygous form has a higher fitness than either of the homozygotes In some W African populations the recessive allele for Hemoglobin S that causes sickle cell anemia exists at a frequency of 0 1 Case study sickle cell anemia 1 10 people have his normal RBC AA sickle cell RBC SS the heterozygous condition offers partial protection against the most deadly form of malaria increasing the fitness of AS these RBCs offer some resistance to malaria the parasite cannot reproduce as easily Normal RBC AS normal RBC AA sickle cell RBC SS Between group genetic variation can be maintained by o Natural selection Selection that favors different genes in different environments maintains variation among regional populations case study 1 sickle cell anemia case study 2 lactase persistence Lactase persistence the ability to synthesize the enzyme lactase which is required for the hydrolysis of lactose as an adult appears to be a simple Mendelian trait In Europe high frequencies of lactase persistence are found in human populations with a long history of dairying but doesn t natural selection take a very long tie much longer than the 10 000 years that people have been herders o Drift o Demographic history Genetic drift can cause differences between populations with little gene flow Ex Old Order Amish founded by 200 people within limited gene flow since Neutral genetic differences between populations reflect the spread and subsequent geographic isolation of modern human groups Pairwise genetic distances vs geographic distance between populations Human variation Part II Polygenic Traits Within Group variation in polygenic traits Between group variation in polygenic traits The Race Concept Ethnicity Summary Within group variation in polygenic traits o Polygenic many genes traits vary continuously Genotype environment phenotype Heritability the proportion of variation observed in offspring generation that is due to the effects of genes not the environment Higher heritability less within group variation Between group variation in polygenic traits o Most of the observable phenotypic traits that vary among geographically defined groups are polygenic stature hair type skin color eye color etc o These traits are heritable to varying degrees o For some traits the variation is adaptive to local conditions o Case study 1 between group variation in body type size shape Some of the variation in body size and shape appears to Bergmann s Rule size there is an inverse relationship Allen s Rule shape individuals in warmer climates be adaptive between body size and mean annual temperature have a leaner more linear body shape longer thinner limbs than those living in colder climates Ex longer trunk limbs decreases mass to surface area ratio helps dissipate body heat Ex shorter trunk limbs increases mass to surface area helps retain body heat o Differences in stature between groups may be not explained o Skin color as an adaptation entirely by genes environmental conditions play a role The amount size of melanin granules is genetically Melanin provides protection against UV radiation which Geographic distribution of variation in skin pigmentation determined and varies among people can lead to cancer Vitamin D synthesis UV radiation is highest near the equator Vitamin D is synthesized in the body partly as a result of exposure to UV radiation Vitamin D deficiency leads to rickets deformation of long bones and pelvis Natural selection may have favored less melanin production in populations that moved into northern latitudes marked by less UV radiation so as to allow for Vitamin D synthesis The Race Concept o Human phenotypes vary greatly Do human phenotypes vary continuously or How closely are different phenotypic characters categorically correlated in individuals or in groups the common concept of race dividing human variation into discrete categorical groups is not based in biology Why o Race as a social construct Most traits used to define races are polygenic Human phenotypes vary continuously over space