Population Genetics Chapter 23

Levels of Organization Atoms - CHNOPS Molecules – Carbs, Proteins, Lipids, Nucleic Acids Organelles – Nucleus, Ribsomes, ER, Golgi, vessicles etc Cells – smallest level for life Tissues Organs Organ Systems Organism - Individual Population – smallest unit of evolution Community Ecosystem Biosphere

Population Group of organisms that can interbreed to produce fertile offspring Gene pool – total alleles in a population

Hardy-Weinberg Equilibrium Evolution does NOT occur if the gene pool remains constant from one generation to the next. Outside forces must act on a population for there to be change

Conditions NO Evolution = genetic equilibrium No Mutations Extremely Large population size – no genetic drift No gene flow – no migration Random Mating No Natural selection Evolution = no genetic equilibrium Mutations Small populations – genetic drift Gene flow – migration Non Random mating Natural Selection

Equation - Hardy-Weinberg Equilibrium p 2 + 2pq + q 2 = 1 p + q = 1 p = dominant allele q = recessive allele p 2 = homozygous dominant 2pq = heterozygous q 2 = homozygous recessive

Example Problem #1 In a population the frequency of the recessive allele is 0.4. – What is the frequency of the dominant allele? – What frequency of the population will be homozygous dominant, heterozygous, and homozygous recessive? – What frequency of the population will demonstrate the dominant phenotype?

Example problem #2 In a population 25% of the individuals demonstrate the recessive phenotype. – What is the frequency of the recessive allele? – What is the frequency of the dominant allele? – What frequency will be homozygous dominant in the population? – What frequency will demonstrate the dominant phenotype?

Violations to H-W Equilibrium – cause evolution Mutations Small populations – genetic drift Non random mating Natural Selection Migration – gene flow 23_08EvolutionaryChanges_A.swf

1. Mutations Change in DNA’s nucleotide sequence. Raw source for new genes and alleles Most mutations are somatic cell mutations and do not affect offspring Only gametic mutations affect a gene pool. Mutation rates – Lower in organisms with a longer generation span Plants and animals – 1/ genes – Higher in organisms with a shorter generation span Bacteria and viruses

1. Mutations Point Mutations – alter one nucleotide base only – Usually neutral – Sickle cell anemia Chromosomal Mutations – alter many regions or loci of the entire chromosome – Gene duplication Usually harmful, but when beneficial act as an important source of variation in a population

2. Nonrandom Mating – sexual selection Creates sexual recombination – joining of different alleles in a gene pool – Huge source of variation in a population Gametes from different organisms contribute different alleles to the next generation.

3. Natural Selection Differential success in reproduction based on variation in a population. Better suited organisms in an environment tend to produce more offspring than less suited organisms. – Fitness Types – Directional – Disruptive – Directional – Sexual – Artificial

4. Genetic Drift Random fluctuation of allele frequencies from one generation to the next. Greater occurrence in smaller populations b/c one organism who is homozygous recessive breeding more than expected can create a large increase in the frequency of the recessive allele, if the pop is small Figure 23.7 CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW CRCWCRCW CRCRCRCR CRCRCRCR Only 5 of 10 plants leave offspring CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW Only 2 of 10 plants leave offspring CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3

4. Genetic Drift Bottleneck effect – Sudden environmental change can drastically reduce the size of a population – only some survive Fire, flood, human influence etc. – Reduces the genetic variation in a population – Ex: An example of a bottleneck: Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

4. Genetic Drift Founder effect – Occurs when a few individuals become isolated from the original population – The smaller population may not have the same gene pool as the original and speciation is likely

5. Migration – Gene Flow Addition or loss of alleles to or from a gene pool. Caused by the movement of fertile individuals to or from a population – migration, a deer goes to live with another population Scientists are studying the effects of cultivated crop’s (artificially selected crops) gene flow on wild populations

Genetic Variation Differences in phenotypes between members of a population Inherited in genotype The raw source for natural selection within a population Figure 23.1

Sources of Genetic Variation Mutations Sexual Reproduction – Crossing over Crossing over – Independent assortment Independent assortment – Random fusion of gametes Random fusion of gametes Diploidy – recessive allele does not show Diploidy – recessive allele does not show 23_04SexualRecombination_A.swf 23_04SexualRecombination_A.swf

Sources of Genetic Variation Outbreeding – mating with unrelated partners Balanced polymorphism – Heterozygote advantage – sickle cell carriers – Hybrid vigor – plant species – Frequency – dependent selection – least common phenotypes have an advantage