The Evolution of Populations and Population Genetics

Foundations of Population Genetics

• Definition of Evolution: The diversity of life results from evolution.

• Level of Evolutionary Operation: Evolutionary forces operate at the Population level (Choice C in the multiple-choice selection), rather than at the organ system, organism, or community level.

• Modern Synthesis:     - Population genetics was formally incorporated into the Theory of Evolution following the Modern Synthesis.     - This synthesis represents the reconciliation of Charles Darwin's theory of natural selection with Gregor Mendel's newer, population-oriented view of genetics.

• Microevolution:     - Defined as the change in allele frequencies in a population through time.     - Lab activity examples: The Gizmos Natural Selection and Microevolution Labs serve as models for microevolutionary processes.     - Clinical example: The evolution of methicillin-resistant Staphylococcus aureus (MRSA), which represents staph bacteria that have become resistant to multiple antibiotics typically used for treatment.

• Macroevolution:     - Represents large-scale evolutionary changes over long periods of time.     - Examples: The evolution of mammals, the radiation of honey creepers in Hawaii, and the evolution of modern humans from early hominins.

Review of Mendelian Genetics

• Alleles and Inheritance:     - A gene may have several alleles coding for different traits.     - Individuals in a population of diploid organisms carry two alleles for a particular gene.     - One allele is inherited from the father and the other from the mother.     - More than two alleles may be present across the individuals within a population gene pool.

• Homozygous vs. Heterozygous:     - Homozygous: An individual possesses two identical alleles for a gene.     - Heterozygous: An individual possesses two different alleles for a gene.

• Dominance and Recessiveness:     - A dominant allele masks the expression of a recessive allele.     - An individual who is heterozygous will exhibit the trait specified by the dominant allele.     - Misconception Alert: The dominant allele is not necessarily the most common allele in a population.

• Genotype vs. Phenotype:     - Genotype: The internal genetic makeup of an individual.     - Phenotype: The observable physical trait or traits seen in an individual.

Concepts in Population Genetics

• Allele Frequency:     - The frequency with which a specific allele appears within a population.     - It is expressed as a decimal fraction or a percentage.     - Evolution is defined in this context as a change in the allele's frequency in a population (Microevolution).

• Gene Pool: The sum of all the alleles in a population.

• Polymorphic: Describes the state of having two or more variations of particular characteristics within a population.

• Selection Pressure: A driving selective force that influences allele frequency. Examples include better camouflage or a stronger resistance to drought.

• Environmental Influence: The environment influences allele frequency; natural selection causes favorable alleles to spread, while detrimental mutations decrease in frequency or are eliminated.

• Genetic Variance:     - Heritability: The fraction of phenotype variation that can be attributed to genetic differences (genetic variance) among individuals.     - Importance: A population with greater genetic variance possesses more "raw material" for evolution.

• Inbreeding and Genetic Health:     - Inbreeding: The mating of closely related individuals.     - Deleterious Effects: It can bring together deleterious (harmful) recessive mutations, causing abnormalities and disease.     - Carrier Probability: In large, healthy populations with unrestricted habitats, the chance of two carriers of a rare disease allele mating is low (and even then, only $25\%$ of offspring would inherit the disease from both parents).     - Inbreeding Depression: Occurs when interbreeding increases homozygosity, leading to more individuals exhibiting deleterious phenotypes.

The Hardy-Weinberg Principle of Equilibrium

• Core Concept: A population’s allele and genotype frequencies are inherently stable. Unless an evolutionary force acts upon the population, neither frequencies would change.

• History: Named after English mathematician Godfrey Hardy and German physician Wilhelm Weinberg, who demonstrated this mathematically in the early 20th century.

• Assumptions of Hardy-Weinberg Equilibrium:     1. No mutations.     2. No migration or emigration (no gene flow).     3. No selective pressure for or against any genotype.     4. An infinite (very large) population size.

• Scientific Utility: While no real-world population satisfies all these conditions perfectly, the principle provides a useful null model against which to compare real population changes. Differences between field measurements and predicted values allow scientists to infer which evolutionary forces are at play.

Mathematical Framework of Hardy-Weinberg

• Allelic Frequency Equations:     - Let $p$ be the frequency of the dominant allele.     - Let $q$ be the frequency of the recessive allele.     - Standard Equation: $p + q = 1$

• Genotypic Frequency Equation:     - $p^2 + 2pq + q^2 = 1$

• Calculated Proportions:     - $p^2$: Frequency of homozygous dominant individuals.     - $2pq$: Frequency of heterozygous individuals.     - $q^2$: Frequency of homozygous recessive individuals.

Decimal and Percentage Conversions

• Conversion to Decimal: Divide the percentage by $100\%$.     - Example: $\frac{19\%}{100\%} = 0.19$

• Multiplication of Decimals:     - Multiply as if there is no decimal.     - Count the total digits after the decimal in each factor.     - Place the same number of digits behind the decimal in the final product.     - Example: $0.1 \times 0.1 = 0.01$ (The decimal moves two places left).

Practice Problem Walkthroughs

Problem 1: Violet Flower Color

• Conditions: Violet ($V$) is dominant over white ($v$). $p = 0.8$, $q = 0.2$. Population size $n = 500$.

• Homozygous Dominant ($VV$):     - Calculation: $p^2 \times n = (0.8)(0.8)(500) = 0.64 \times 500 = 320$

• Heterozygous ($Vv$):     - Calculation: $2pq \times n = 2(0.8)(0.2)(500) = 0.32 \times 500 = 160$

• Homozygous Recessive ($vv$):     - Calculation: $q^2 \times n = (0.2)(0.2)(500) = 0.04 \times 500 = 20$

• Phenotype counts:     - Violet flowers: $320 + 160 = 480$     - White flowers: $20$

• Verification: $320 + 160 + 20 = 500$

Problem 2: Chin Dimpling

• Conditions: Dimpling ($D$) is dominant over undimpled ($d$). In a population, $36\%$ have undimpled chins.

• Finding Allele Frequency:     - $q^2 = 0.36$ (frequency of homozygous recessive phenotype).     - $q = \sqrt{0.36} = 0.6$     - Since $p + q = 1$, then $p = 1 - 0.6 = 0.4$     - Frequency of the dimpling allele ($D$) is $0.4$.     - Note from Transcript Page 18: There is a discrepancy in the transcript prompt and solution text regarding which % represents which phenotype; however, the math dictates that if the recessive phenotype frequency is identified, the square root provides $q$.

Problem 3: Broccoli Moths and Beetles (Assigned)

• Assigned Problem 1: $70\%$ of alleles are "$b$" for hating broccoli (recessive).     - $q = 0.7$     - $p = 1 - 0.7 = 0.3$     - Genotypes: $BB = 0.09$, $Bb = 0.42$, $bb = 0.49$     - Phenotypes: Love broccoli = $51\%$ ($0.09 + 0.42$), Hate broccoli = $49\%$

• Assigned Problem 2: 1000 beetles, 750 green legs ($G$ is dominant).     - Recessive phenotype (blue legs) = $1000 - 750 = 250$, or $0.25$.     - $q^2 = 0.25$, so $q = 0.5$.     - $p = 1 - 0.5 = 0.5$.     - Genotypes: $GG = 0.25$, $Gg = 0.50$, $gg = 0.25$

• Assigned Problem 4: $19\%$ of moths have orange spots (dominant).     - This implies the recessive phenotype (yellow spots) is $1 - 0.19 = 0.81$.     - $q^2 = 0.81$, so $q = 0.9$.     - $p = 1 - 0.9 = 0.1$.     - Genotype frequencies: $AA = 0.01$, $Aa = 0.18$, $aa = 0.81$