Presentation on theme: "Agents of Evolutionary Change"— Presentation transcript:
1Agents of Evolutionary Change The Five Forces of Evolution and How We Measure Them
2I. What is Evolution? First, remember that… Evolution is a two-stage process:Production and redistribution of variation (inherited difference between individuals).Natural selection (whereby genetic differences in some individuals lead to their higher reproductive success.
3I. What is Evolution?A change in the frequency of alleles in a population from one generation to the next.An allele frequency is the percentage of all the alleles at a specific locus accounted for by one specific allele.For evolution to occur, a new allele must spread through the population and change in frequency.Still, the frequencies of all alleles ill add to 1.0 (100 %)Evolution is driven by five known factors or agents.
4II. Agents of Evolutionary Change MutationGene FlowNonrandom (Assortative) MatingGenetic DriftSelectionFor the next few slides we review the 5 main agents of evolutionary change. Most general biology texts develop definitions and examples.Assigning a text reading on agents of evolutionary change in advance is a good idea. Students should have access to their texts or a handout addressing the topic during the activity.Call upon class members to define and give examples for each category.
5II A. Mutation Mutation is a molecular alteration in genetic material: For a mutation to have evolutionary significance it must occur in a gamete (sex cell).Such mutations will be carried on one of the individual's chromosomes.During meiosis the chromosome carrying the mutation will assort giving a 50 percent chance of passing the allele to an offspring.
6II A. Mutation, continued Mutation is the only way new variations can be produced.Since mutation occurs so infrequently at any particular locus, it would rarely have an effect on allele frequencies.Most mutations are "hidden" as recessive alleles.example: About 1 in in 12,000 babies carry the homozygous form of the allele for PKU (phenylketonuria). Without early detection and treatment, this disorder will lead to mental retardation.
7II B. Gene FlowGene flow is the exchange of alleles between populations. Also called migration.Long-term patterns of mate selection can cause significant changes in gene frequency.e.g. African Americans are largely of West African descent, but there has been considerable influx of alleles from non-African populations.
8II C. Assortative Mating Purposeful selection of mates based on the presence or absence of certain traits in potential mating partners.Also called non-random mating or sexual selectionUndesirable traits will diminish over time as the organisms having those traits are less reproductively successful.One example of this in history was choice of antler rack size among Red Deer in the British isles.
9II D. Genetic Drift Random fluctuation in allele frequencies Can change in either direction.Associated with small populationsTwo variants.Founder Effect occurs when a small migrant band of "founders" colonizes a new area away from the parent group.Bottleneck Effect occurs when circumstances dictate that only a select number of individuals will survive to reproduce.
10Tristan de Cunha: Example of Founder Effect An Island in the South Atlantic was settled in early 1800s by one Scottish family of about 15 people.Population was added to by shipwrecks and birthsThe 1987 population numbered 296 people with 7 family names, and mtDNA studies have shown that they are descended from five female founders.The population has a high frequency of a hereditary eye disorder (retinitis pigmentosa). Four islanders have the disease and nine are carriers at last count.
11Northern Elephant Seal: Example of Bottleneck Hunted down to 20 individuals in 1890’sPopulation has recovered to over 30,000No genetic diversity at 20 loci.Concern about resistance to pathogens
12II E. Natural SelectionDifferential reproductive success in a population due to heritable variants among the populationMalaria is found in areas where the sickle-cell allele is found and it was discovered that heterozygous carriers of sickle-cell anemia are more resistant to malarial infection than the homozygous "normals".Natural selection did not cause the sickle cell allele to appear; it merely acted to increase its frequency in some populations.
13III. Evolution in Population Genetics Some things to remember:Allele frequencies are the proportions of alleles in a population.Each population has a particular complement of genes, a level of variability, called its gene pool.A change in variability in the gene or allele pool across generations is thus microevolutionAs a species changes over time, macroevolution (speciation) may occur
14III. Evolution in Population Genetics Physical anthropologists a formula called the Hardy Weinberg Equilibrium to determine whether allele frequencies in an isolated human population are changing.The Hardy Weinberg is good for two things:Estimating allele frequencies for a population in which actual counts are impracticalActing as a model against which to test assumptions about the evolutionary status of a known population.The formula is: p2+2pq+q2=1.0
15III. Evolution in Population Genetics For the Hardy Weinberg to be true…NONE OF THE FOLLOWING CAN OCCUR:New variation (mutation).Redistributed variation through gene flowRedistributed variation through genetic driftSelect "advantageous" allele combinations that promote reproductive success (natural selection)Assortative mating
16I. Types of Human Variation Humans vary in their:PhysiologyBody proportionsSkin color
17I A. Physiologic Variation Lactose ToleranceAbility to produce enzyme lactase throughout lifeFound in populations with a pastoral heritageVasoconstriction and VasodilationAbility to control restriction of blood vesselsFound in populations in extreme environments
18I B. Body Proportions Bergmann’s Rule Climate controls body mass to surface ratioAllens’ RuleClimate controls limb lengths
19I C. Skin Color Factors affecting skin color Sunlight Nutrition Vitamin D needs, cancer, and folateNutritionExcess dietary pigment can change colorEnvironmentShort term responses (I.e. tanning)GenesWhat your parents pre-determined about you
20II. Historical Views of Human Variation Biological determinismcultural variations are inherited in the same manner as biological variationsEugenicsGalton’s "race improvement" through forced sterilization & encouraged reproductionMono and PolygenismOne or many origins for humanity (A&E?)
21III. The Concept of RaceAll Modern Humans Belong to the Same polytypic species, Homo sapiens.A polytypic species consists of local populations that differ from one another in the expression of one or more traits.
22III. The Concept of Race, cont. Variation (race) is geographic in natureVariation can be plotted on clines
23IV. The Traditional Race Concept Since the 1600s race has been used to refer to various culturally defined groups.While race is usually used as a biological term, it is also one with enormous social significance.In the 1950's the use of the term "race" was replaced with the term "ethnicity "
24V. Race and Intelligence Both genetic and environmental factors contribute to intelligence.Many psychologists say that IQ scores measure life experience.Innate differences in abilities reflect individual variation within populations, not inherent differences between groups.There is no convincing evidence that populations vary with regard to cognitive abilities.
25VI. Adaptive Significance of Variation Human variation is the result of past and present adaptations to environmental conditions.Physiological response to environmental change is under genetic control and environmental influence and operates at two levels:Long-term (i.e. genetic) evolutionary changes characterize all individuals within a population or species.Short-term physiological response to environmental change is called acclimatization; such physiological change is temporary.