CH 13
Genotype vs Phenotype
Genotype refers to the genetic makeup of an organism (the alleles it carries AA, Aa, aa) while phenotype is the observable physical or biochemical characteristics
Definition of Evolution
Evolution is the change in the genetic composition of a population over time, leading to the development of new species.
Convergent/divergent/co evolution
- Convergent Evolution: Different species evolve similar traits due to similar environmental pressures.
- Divergent Evolution: Related species evolve different traits due to different environments or ecological niches.
- Coevolution: Two or more species evolve in response to each other’s adaptations.
In a population of 1000 moths, dark coloration (D) is dominant to light coloration (d). Initially, 640 moths are dark. After a period of industrial pollution, 810 moths are dark. a) Calculate the initial frequencies of alleles D and d. b) Calculate the new frequencies of alleles D and d after selection. c) What type of selection is occurring, and why?
a) Initial frequencies: Dark moths (DD + Dd) = 640/1000 = 0.64 Light moths (dd) = 360/1000 = 0.36 q² = 0.36 q = √0.36 = 0.6 p = 1 - q = 0.4 b) New frequencies: Dark moths = 810/1000 = 0.81 Light moths = 190/1000 = 0.19 q² = 0.19 q = √0.19 ≈ 0.436 p = 1 - q ≈ 0.564 c) It is directional selection. The frequency of the dark allele (D) is increasing over time, likely because dark moths have a selective advantage in the polluted environment.
In a population of 1000 rabbits, brown fur (B) is dominant to white fur (b). 360 rabbits have white fur. Questions: a) Calculate the frequencies of alleles B and b. b) Calculate the frequencies of genotypes BB, Bb, and bb. c) Is this population in Hardy-Weinberg equilibrium?
a) Frequency of white rabbits (bb) = 360/1000 = 0.36 q² = 0.36 q = √0.36 = 0.6 p = 1 - q = 1 - 0.6 = 0.4 b) BB = p² = 0.4² = 0.16 Bb = 2pq = 2(0.4) (0.6) = 0.48 bb = q² = 0.6² = 0.36 c) Yes, the population is in Hardy-Weinberg equilibrium because the frequencies of genotypes match the expected frequencies based on p and q.
Homozygote, Heterozygote
A homozygote has two identical alleles for a gene (e.g., AA or aa), while a heterozygote has two different alleles (e.g., Aa).
Evidence for Evolution
Evidence supporting evolution includes the fossil record , comparative anatomy (similarities in the structure of different organisms), embryology (similar developmental stages), and molecular biology (DNA comparisons).
Systematics and Taxonomy
Systematics is the study of the diversity of life and its evolutionary relationships. Taxonomy is the classification of organisms.
A small population of 50 butterflies experiences a bottleneck event, reducing it to 10 individuals. The frequency of a wing pattern allele (A) was initially 0.4. a) Calculate the possible range of the new allele frequency after the bottleneck. b) If 7 of the surviving butterflies carry the A allele, what is the new allele frequency? c) How does this demonstrate genetic drift?
a) Possible range: 0 to 1 The allele could be lost (0) or fixed (1) due to chance in such a small population. b) New allele frequency: A alleles = 7 Total alleles = 10 × 2 = 20 New frequency = 7/20 = 0.35 c) It demonstrates genetic drift because the change in allele frequency (from 0.4 to 0.35) is due to random chance in a small population, not selection. The frequency could have easily been very different depending on which individuals survived the bottleneck.
In a population of 500 flowers, red (R) and white (W) are codominant alleles for petal color. 120 flowers are pink (RW). Questions: a) Calculate the frequencies of alleles R and W. b) Calculate the frequencies of genotypes RR, RW, and WW. c) How many white flowers would you expect in this population?
a) Frequency of RW = 120/500 = 0.24 2pq = 0.24 p + q = 1 p = q = 0.5 (because codominant alleles are often at equal frequencies) b) RR = p² = 0.5² = 0.25 RW = 2pq = 2(0.5)(0.5) = 0.5 WW = q² = 0.5² = 0.25 c) Expected white flowers = WW * total population = 0.25 * 500 = 125 flowers
Dominant-Recessive
Inheritance where a dominant allele masks the expression of a recessive allele (e.g., A is dominant to a)
5 Mechanisms of Evolutionary Change:
Key processes driving evolution include natural selection, genetic drift, mutation, gene flow, and non-random mating
Phylogenetics
The study of evolutionary relationships among species, often represented by phylogenetic trees.
Two populations of fish, A and B, live in adjacent lakes. In population A, the frequency of a scale color allele (S) is 0.3. In population B, it's 0.7. After a flooding event, 20% of population A migrates to population B, which originally had 1000 individuals. a) Calculate the new allele frequency in population B after gene flow. b) How many individuals migrated from population A to B? c) How does this gene flow affect the genetic variation between the populations?
a) New allele frequency in B: Original B individuals: 1000 × 2 × 0.7 = 1400 S alleles Migrants from A: 1000 × 0.2 = 200 migrants Migrant alleles: 200 × 2 × 0.3 = 120 S alleles New total individuals: 1000 + 200 = 1200 New S allele frequency = (1400 + 120) / (1200 × 2) = 0.633b) 200 individuals migrated (20% of 1000) c) Gene flow decreases genetic variation between the populations by making them more similar. The allele frequency in population B has moved closer to that of population A.
In a population of 800 fruit flies, red eye color (R) is dominant to white eye color (r) and is X-linked. 15% of males have white eyes. Questions: a) Calculate the frequencies of alleles R and r. b) Calculate the frequencies of genotypes in females (XRXR, XRXr, XrXr). c) What percentage of females would you expect to have white eyes?
a) In males, the frequency of r = frequency of white-eyed males = 0.15 q = 0.15 p = 1 - q = 1 - 0.15 = 0.85 b) Females: XRXR = p² = 0.85² = 0.7225 XRXr = 2pq = 2(0.85)(0.15) = 0.255 XrXr = q² = 0.15² = 0.0225 c) Females with white eyes = XrXr = 0.0225 * 100 = 2.25%
Codominance vs Incomplete Dominance
Both alleles in a heterozygote are expressed equally (e.g., AB blood type) Incomplete Dominance : Neither allele is fully dominant, leading to an intermediate phenotype (e.g., red and white flowers producing pink offspring).
Macro-evolution vs Micro-evolution:
- Micro-evolution refers to small evolutionary changes within a population (e.g., changes in allele frequencies).
- Macro-evolution refers to larger-scale changes that occur at or above the species level, potentially leading to the formation of new species.
The concept that species are groups of interbreeding populations with offspring that can reproduce. Speciation can occur through allopatric (geographic) or sympatric (non-geographic) means.
In a population of 5000 sunflowers, plant height follows a normal distribution with a mean of 150 cm. Plants between 140-160 cm have the highest fitness (1.0), while plants outside this range have a fitness of 0.8. a) If 30% of plants are between 140-160 cm, calculate the proportion of plants with reduced fitness. b) In the next generation, assuming only height affects fitness, what proportion of offspring would you expect to come from plants between 140-160 cm?
a) Proportion with reduced fitness: 1 - 0.30 = 0.70 or 70% b) Proportion of offspring from 140-160 cm plants: (0.30 × 1.0) / (0.30 × 1.0 + 0.70 × 0.8) ≈ 0.349 or 34.9% c) This demonstrates stabilizing selection because individuals with intermediate values of the trait (height) have higher fitness. Over time, this will reduce variation in the population by selecting against extreme values, maintaining an optimal middle range
In a population of 3000 butterflies, wing pattern is controlled by two alleles: spotted (S) and plain (s). Currently, 540 butterflies have plain wings. Questions: a) Calculate the current frequencies of alleles S and s. b) Calculate the current frequencies of genotypes SS, Ss, and ss. c) If natural selection favors heterozygotes (Ss) and their fitness is 10% higher than homozygotes, predict how allele frequencies might change in the next generation.
a) Frequency of plain wings (ss) = 540/3000 = 0.18 q² = 0.18 q = √0.18 ≈ 0.424 p = 1 - q ≈ 0.576 b) SS = p² ≈ 0.332 Ss = 2pq ≈ 0.488 ss = q² = 0.18 c) With heterozygote advantage, we expect the frequency of the Ss genotype to increase in the next generation. It will lead to a more balanced frequency of the S and s alleles, as neither allele can become fixed in the population. Over time, the allele frequencies will move towards an equilibrium where heterozygotes are most common, and the frequencies of S and s will be closer to 0.5 each.
X-linked Dominant and X linked recessive
X-linked dominant traits are expressed in individuals with just one copy of the allele on the X chromosome, while X-linked recessive traits require two copies (one on each X chromosome in females or one in males) for expression
Patterns of Evolution :
- Disruptive Selection: Both extremes of a trait are favored, leading to a bimodal distribution.
- Stabilizing Selection: Intermediate forms are favored, and extremes are selected against.
- Directional Selection: One extreme of a trait is favored, causing a shift in the population’s distribution.
Who is Carl Linnaeus
Father of binomial nomenclature how we name species
In a population of 10,000 bacteria, a beneficial mutation occurs at a rate of 1 × 10⁻⁶ per generation per individual. a) How many new mutants would you expect in one generation? b) If the mutation provides a 5% fitness advantage, calculate the selection coefficient. c) Assuming the population size remains constant, how might this mutation affect the population over time?
a) Expected new mutants: 10,000 × (1 × 10⁻⁶) = 0.01 mutants per generation b) Selection coefficient: s = (w_mutant - w_wildtype) / w_wildtype s = (1.05 - 1) / 1 = 0.05c) Over time, if the mutation provides a consistent fitness advantage, its frequency in the population would likely increase due to natural selection. However, the initial increase would be slow due to the low mutation rate and the effects of genetic drift on rare alleles.
In a population of 2000 pea plants, 1200 have yellow seeds (dominant trait, Y) and 800 have green seeds (recessive trait, y). Questions: a) Calculate the frequencies of alleles Y and y. b) Calculate the expected frequencies of genotypes YY, Yy, and yy if the population were in Hardy-Weinberg equilibrium. c) Is this population in Hardy-Weinberg equilibrium? Explain why or why not.
a) Frequency of green seeds (yy) = 800/2000 = 0.4 q² = 0.4 q = √0.4 ≈ 0.632 p = 1 - q ≈ 0.368 b) Expected frequencies if in equilibrium: YY = p² ≈ 0.135 Yy = 2pq ≈ 0.465 yy = q² = 0.4 c) No, the population is not in Hardy-Weinberg equilibrium. The observed frequency of the dominant phenotype (Y_) is 1200/2000 = 0.6, which is different from the expected frequency of YY + Yy = 0.135 + 0.465 = 0.6. It suggests that one or more of the Hardy-Weinberg assumptions are being violated.
Complex Inheritance Patterns
Traits that do not follow simple Mendelian inheritance include multiple alleles (more than two forms of a gene, e.g., blood types), polygenic inheritance (traits controlled by multiple genes, like skin color), and epistasis (where one gene affects the expression of another)
Natural Selection vs Artificial Selection vs Sexual selection
Natural Selection : A process where individuals with traits better suited to the environment have higher reproductive success, leading to the increase of those traits in the population over generations.
Artificial Selection : Humans intentionally select desirable traits in organisms, such as in breeding pets or crops.
Sexual Selection: A form of natural selection where certain traits increase an individual’s chances of mating, even if they may be disadvantageous for survival (e.g., peacock's tail).
Explain Genetic Drift, Gene flow, Mutation
Genetic Drift: The random fluctuation of allele frequencies in a population due to chance events, more pronounced in small populations. Founder Effect: A type of genetic drift that occurs when a small group of individuals forms a new population Bottleneck Effect: A drastic reduction in population size due to an event
Flow: Movement of alleles between populations (immigration or emigration)
Mutation and Variation: Mutations create genetic variation, which can be acted upon by natural selection, contributing to evolution.