GENETIC VARIATION SC STANDARD B-5: Students will demonstrate an understanding of biological evolution and the diversity of life.
Inquiry Activity • Page 392 in textbook • Page 47 in notebook • Work in table groups
CN page 44 • Topic: Genetic Variation • EQ: What factors contribute to genetic variation in a population?
Genetic Variation : How common is it? Most mammals are heterozygous for 4 to 8% of genes
Gene Pools • Consists of all the different alleles present in population. • Population: group of individuals of same species that live in same area
Gene Pools • The # of times an allele occurs in a gene pool compared with the # of times other alleles occur is called the allele’s relative frequency • In genetic terms, evolution is any change in the relative frequency of alleles in a population
Sources of Genetic Variation 1. MUTATIONS 2. Gene Shuffling • May have a change in DNA sequence that does/does not result in a change in phenotype • Some mutations alter the individual’s fitness, some do not • Cause of most heritable differences • Due to meiosis • Crossing over increases genetic variation • Homologous chromosomes assort independently in anaphase I • Random gamete fertilized • Does not change gene frequency
The # phenotypes produced for given trait depends on # genes that control the trait. Single-Gene Trait Polygenic Traits • Example: Widow’s Peak is dominant over no widow’s peak • >2 genes control trait & there are often >2 forms of alleles
Natural Selection on Single-Gene Traits • can lead to changes in allele frequencies and so to evolution • Example: whitish tree bark fed on by moths with variations in color white-gray to gray-black
Natural Selection on Polygenic Traits • can affect distribution of phenotypes in 3 ways • 1. Directional Selection • occurs when individuals at one end of curve have higher fitness than individuals at other end or in middle of curve
How Natural Selection Affects Phenotypes 2. Stabilizing Selection 3. Disruptive Selection • Individuals near center of curve have higher fitness than individuals at either end • Individuals at both ends have higher fitness than those near middle of curve
Genetic Drift • is the random change in allele frequencies that occurs in small population • In small populations individuals that carry a particular allele may leave more offspring than other individuals, just by chance. • Over time, a series of chance occurrences of this type can cause an allele to become common in a population.
Genetic Drift • seen when a small population colonizes a new habitat • these individuals may carry different relative frequencies than did the larger population they came from
Founder Effect • the change in allele frequencies as a result of migration of a small subgroup of a population • Example: in 1814 15 British subjects colonized an uninhabited island in the Atlantic • 1 individual was heterozygous for a recessive allele for retinitis pigmentosa • by 1960 4 of 240 citizens afflicted with R.P. a much greater incidence than in Britain
Hardy-Weinberg Principle • states that allele frequencies in a population will remain constant unless 1 or more factors cause those frequencies to change • genetic equilibrium: situation in which allele frequencies remain constant
5 conditions to maintain genetic equilibrium • 1. random mating • ensures each individual has equal opportunity to pass on its alleles • 2. large population • genetic drift has less effect on larger #s • 3. no immigration or emigration • allele frequencies remain same • 4. no new mutations • allele frequencies stay the same • 5. no natural selection • All genotypes have equal chance of surviving & reproducing
p = dominant allele • q = recessive allele • pq = heterozygotes p² + 2pq + q² = 1