2 Gene PoolsPopulation – collection of individuals of the same species in a given area and is the smallest biological unit that can evolve (individuals can’t evolve)Ex. All the students in SHS is an example of a populationAll members of a population can interbreed and they share a common group of genesGene Pool – combined genetic information of all the members of a particular population / total collection of allelesGene pool contains two or more forms of a certain gene for each traitEx. Brown or black fur in mice
3 Relative FrequencyRelative Frequency – the number of times an allele occurs in a gene pool compared with the number of times the other allele(s) occurWhen the relative frequency of alleles changes over a number of generations, evolution is occurring on its smallest scale.Sample PopulationFrequency of Allelesallele for brown furallele for black fur48% heterozygous black16% homozygous black36% homozygous brown
4 Sources of Genetic Variation Darwin couldn’t explain how variations occurred – he didn’t know about geneticsWe know now that mutations and genetic shuffling that results from sexual reproduction are sources of variations
5 MutationsChange in a sequence of DNA that occurs due to mistakes in DNA replication or from radiation/chemicals in the environmentProduce changes in an organism’s phenotype (physical characteristics) – can affect fitness (ability to survive and reproduce in the environment)
6 Gene ShufflingChromosomes “move around” the cell during meiosis (formation of sperm and egg cells)Crossing over of the chromosomes occurs during meiosis creating different genetic combinationsSexual reproduction is a major source of variation within a population (even more so than mutations)
7 Single-Gene and Polygenic Traits Number of phenotypes produced for a given trait depends on how many genes control that traitVariable traits in a population may bepolygenic, resulting from the combined effects of several genes, ordetermined by a single gene.Polygenic traits tend to produce phenotypes that vary more or less continuously.Single-gene traits tend to produce only a few distinct phenotypes
8 Single-Gene TraitTrait controlled by one gene with two alleles (Ww) – variation leads to only TWO phenotypesEx. Widow’s Peak or no Widow’s Peak10080604020Frequency of Phenotype(%)Widow’s peakNo widow’s peakPhenotype
9 Frequency of Phenotype Polygenic TraitsTraits controlled by two or more genes. Each gene of a polygenic trait has two or more alleles = many phenotypes possible.Ex. Height in humans – short, tall, and everything inbetween. When the average is graphed, a “bell-curve” is created for that traitFrequency of PhenotypePhenotype (height)
10 Natural Selection & Genetics ****Natural Selection acts directly on phenotypes**** - affects which individuals having different phenotypes will survive and which will notNatural selection does change relative frequencies of alleles in a population over timeAny factor that causes alleles to be added or removed from a population will change relative frequencies of alleles. If an individual dies without reproducing, its genes are removed from the population. If an individual reproduces a lot, that individual’s genes increase in the gene pool
11 Natural Selection & Genetics (continued) Evolution is any change in the relative frequencies of alleles in a population’s gene pool****Evolution acts on populations, NOT individuals
12 Natural Selection on Single-Gene Traits Can lead to changes in allele frequencies and thus to evolutionEx. Lizard color – brown color is normal, whereas red and black are mutations. These lizards live on dark soil. The brown and black lizards are hard for birds to see, but the red is easy for birds to see. Predict what will happen to the 3 variations of color…
13 Analyzing Gene Pools A gene pool consists of all the alleles in a population at any one time andis a reservoir from which the next generation draws its alleles.Alleles in a gene pool occur in certain frequencies.
14 Analyzing Gene Pools (continued) Alleles can be symbolized byp for the relative frequency of the dominant allele in the population (ex. R),q for the frequency of the recessive allele in the population (ex. r), andp + q = 1.Note that if we know the frequency of either allele in the gene pool, we can subtract it from 1 to calculate the frequency of the other allele.
15 Analyzing Gene Pools (continued) Genotype frequencies can be calculated from allele frequencies (if the gene pool is stable = not evolving).The Hardy-Weinberg formulap2 + 2pq + q2 = 1can be used to calculate the frequencies of genotypes in a gene pool from the frequencies of alleles.p2 = frequency of homozygote for one allele (ex. RR)2pq = frequency of heterozygotes (ex. Rr)q2 = frequency of other homozygote (ex. rr)
16 Analyzing Gene Pools (continued) Alleles can be symbolized byp for the relative frequency of the dominant allele in the population (ex. R),q for the frequency of the recessive allele in the population (ex. r), andp + q = 1.Note that if we know the frequency of either allele in the gene pool, we can subtract it from 1 to calculate the frequency of the other allele.