Evolution of Populations Chapter 17

Chapter 17.1 Genes and Variation Objectives Define evolution in genetic terms. Identify the main sources of genetic variation in a population. State what determines the number of phenotypes for a trait.

Genetics in Evolution Review: Allele: Genotype: Phenotype: Natural selection acts on organism’s phenotype. Individuals with more suitable phenotypes reproduce and pass on traits. Form of a gene Organism’s genetic makeup Organism’s physical appearance

Genetics in Evolution Population: group of individuals of same species that mate and produce offspring Gene pool: all genes (including all different alleles for each gene) present in a population

Genetics in Evolution Allele frequency: number of times an allele occurs in gene pool compared to total number of alleles in that pool for same gene Evolution, in genetic terms, involves a change in frequency of alleles in a population over time. Allele frequency: in mouse example: 50 alleles – 20 alleles are B (black) and 30 alleles are b (brown) allele frequencey of black (B) is 40% (20/50) and of brown (b) is 60% (30/50) Has nothing to do with whether the allele is dominant or recessive – in mouse example, the recessive allele occurs more frequently than the dominant allele

Sources of Genetic Variation Mutations: change in genetic material of cell Neutral – do not change phenotype Negative – lethal or lower organism’s fitness Beneficial – improve individual’s ability to survive and reproduce Only important if they can be passed to offspring (in germ line cells) How common are mutations? each person born with about 300 mutations – most are neutral one or two potentially harmful a few may be beneficial Mutation that occurs in a skin cell (example – skin cancer) will not be passed on to offspring

Sources of Genetic Variation Genetic Recombination in sexual reproduction Homologous chromosomes separate independently during meiosis Crossing over also recombines genes In humans, with 23 pairs of chromosomes, the process of independent assortment can produce 8.4 million gene combinations This is why no two sibling (except identical twins) look alike

Sources of Genetic Variation Lateral Gene Transfer: Passing of genes from one organism to another organism that is not its offspring Important in evolution of antibiotic resistance in bacteria Most eukaryotic organisms pass genes from parent to offspring

Single vs. Polygenic Traits Single-gene trait: trait controlled by one gene May have just 2-3 possible phenotypes For example, presence/absence of shell banding is a single-gene trait gene that controls it has two alleles allele for no bands is dominant over allele for bands In this population, the frequency of individuals with bands (recessive phenotype) is higher than freq of indiv w/o bands (dominant pheno) Bands No bands

Single vs. Polygenic Traits Polygenic trait: trait controlled by two or more genes Results in many phenotypes For example, height in humans range from very short to very tall and everywhere in between Can sample this by measuring height of students in class and could then calculate the average height if you plot this, you would get a graph that looked something like the one shown here many students just little bit taller or shorter than average, but some would be very short or very tall symmetric bell-shaped curve is very common of polygenic trait (this curve also called normal distribution) Bell-shaped curve

Check-in Natural selection acts on the organism’s ___________________. Name 3 sources of genetic variability What type of trait results in many phenotypes? phenotype Mutations, genetic recombination, lateral transfer Polygenic trait

Section 17.2 Evolution as Genetic Change in Populations Objectives Explain how natural selection affects single-gene and polygenic traits. Describe genetic drift. How many have every taken an antibiotic? Antibiotics are medications that can kill bacteria (but have no effect on viruses). However, even though they kill bacteria, there can be some bacteria in the population (due to natural variability) that have are resistant (just by chance) to the antibiotic. After the bacteria that are not resistance are killed, the resistant bacteria are able to reproduce and pass on their resistance traits to future generations of bacteria. This is why doctors are starting to be careful when they prescribe antibiotics to you – an antibiotic will not cure your cold and could in fact result in an increase in antibiotic resistance bacteria in your body. Same is true for resistance to pesticides among insects

How Natural Selection Works Single-gene traits Can lead to changes in allele frequencies and thus changes in phenotype frequencies Initial Population Generation 10 Generation 20 Generation 30 90% 80% 70% 40% 10% 20% 30% 60% Mice have a single gene that determines fur color (gray or black) – I know it is a single-gene trait because there are only two phenotypes Black fur individuals are more fit because it allows them to hide from predators better. This over 30 generations we see more black mice and fewer gray mice. The black mice are able to reproduce and pass on their alleles to future generations, but the gray mice are less able to pass on their alleles (because they are more likely to be caught and eaten).

How Natural Selection Works Polygenic traits result in 3 types of selection: Directional selection: Individuals at one end of curve have higher fitness than in middle or at other end. Example: among seed eating birds, birds with bigger thicker beaks can feed on harder thicker shelled seeds if supply of small and medium sized seeds runs low (leaving only larger seeds), birds with larger beaks would have easier time feeding over time, beak size would likely increase on average

How Natural Selection Works Stabalizing selection: Disruptive selection: Individuals near center have higher fitness than individuals at either end. Individuals at outer ends of curve have higher fitness than individuals in middle of curve. Stabalizing: mass of human infants at birth too little – likely to be less healthy and less likely to survive too big – likely to have difficulty being born Disruptive: can create two different phenotypes medium sized seeds become less common – only large and small seeds available birds with extra large or extra small beaks would have advantage

Genetic Drift Genetic drift: random change in allele frequency in small populations Bottleneck effect: change in allele frequency following dramatic reduction in size of population Genetic drift – in small populations, when chance occurrences cause an allele to become more or less common in a population Bottleneck – disaster (like disease) can kill many individuals in population just by chance, smaller gene pool may have different allele frequencies than original gene pool severe bottleneck effect can sharply reduce a population’s genetic diversity can happen to organisms on the verge of extinction

Genetic Drift Founder effect: allele frequencies change as result of migration of small subgroup of population Founder effect – can occur when a few individuals colonize a new habitat founding indiv may have alleles that differ in relative freq compared to main population (just by chance) ex. Fruit flies on the Hawaiian islands – all descended from same mainland fruit fly pop, but species on different islands have allele freq different from those of original species

Check-in Match pictures to definitions Stabalizing selection Founder effect Disruptive selection Directional selection Bottle neck effect