The Evolution of Populations and Population Genetics

Introduction to Population Evolution and Microevolution

• Levels of Evolutionary Force: Evolution is responsible for the diversity of life on Earth. Evolutionary forces operate specifically at the level of the Population. They do not operate at the level of the organ system, the individual organism, or the community.

• Modern Synthesis: Population genetics was formally integrated into the Theory of Evolution through the "Modern Synthesis." This framework reconciles Darwin's theory of natural selection with Mendel's principles of genetics (Modern Synthesis = Darwin + Mendel).

• Population Genetics Defined: This field studies what factors and forces cause changes in allele frequencies within populations over periods of time.

• Microevolution: This term refers to the change in an allele's frequency within a population over time. Examples of activities that model microevolution include the Gizmos Natural Selection and Microevolution Labs.

• Macroevolution: This refers to larger-scale evolutionary changes. Examples include:     * The evolution of mammals.     * The radiation of honey creepers in Hawaii.     * The evolution of modern humans from early hominins.

Fundamentals of genetics in Populations

• Genes and Alleles: A gene may possess several alleles, which are alternate versions of a gene coding for different traits.

• Diploid Organisms: In populations of diploid organisms, each individual carries two alleles for a specific gene. One allele is inherited from the biological father and one from the biological mother.

• Allele Presence: While an individual carries only two alleles, more than two alleles for a particular gene may exist across the various individuals within the broader population.

• Mendelian Review:     * Homozygous: Possessing two identical alleles for a gene.     * Heterozygous: Possessing two different alleles for a gene.     * Dominance: A dominant allele masks the phenotypic expression of a recessive allele. A heterozygous individual will express the trait associated with the dominant allele.     * Genotype: The specific genetic makeup of an individual organism.     * Phenotype: The observable trait or traits seen in an individual resulting from their genotype.

Allele Frequency and the Gene Pool

• Allele Frequency: The frequency at which a specific allele appears in a population, expressed as either a percentage or a decimal fraction.

• Influence of Environment: The environment significantly influences allele frequencies. Natural selection causes favorable alleles to spread, while alleles with detrimental mutations decrease in frequency or are eliminated.

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

• Polymorphic: A term describing populations that have two or more variations of particular characteristics.

• Selection Pressure: Also known as the driving selective force. Examples include factors like better camouflage or a stronger resistance to drought, which favor certain alleles over others.

The Hardy-Weinberg Principle of Equilibrium

• Definition: This principle states that a population’s allele and genotype frequencies are inherently stable. Unless an evolutionary force acts upon the population, neither the allele nor the genotypic frequencies will change.

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

• Ideal Conditions (Assumptions): For a population to be in Hardy-Weinberg (H-W) equilibrium, it must satisfy the following:     * No mutations.     * No migration or emigration.     * No selective pressure for or against any specific genotype.     * An infinite population size.

• Utility: While no real-world population perfectly satisfies these conditions, the principle serves as a baseline or "useful model" to compare against real population changes. If field-measured frequencies differ from predicted values, scientists can infer which evolutionary forces are impacting the population.

Hardy-Weinberg Equations and Calculations

• The Basic Equations:     * $p + q = 1$     * $p^2 + 2pq + q^2 = 1$

• Variable Definitions:     * $p$ = frequency of the dominant allele.     * $q$ = frequency of the recessive allele.     * $p^2$ = frequency of individuals that are homozygous dominant.     * $2pq$ = frequency of individuals that are heterozygous.     * $q^2$ = frequency of individuals that are homozygous recessive.

• Decimal Calculation Refresher:     * Percent to Decimal: Divide by $100\%$. (Example: $19\% / 100\% = 0.19$).     * Multiplying Decimals: Multiply the numbers as if there were no decimal. Count the total digits after the decimal in both factors, then move the decimal in the product that many places to the left. (Example: $0.1 \times 0.1 = 0.01$).

Practice Problems: Hardy-Weinberg Application

• Problem 1: Violet Flowers: In plants, violet ($V$) is dominant over white ($v$). In a population of $500$ plants, $p = 0.8$ and $q = 0.2$.     * Homozygous Dominant ($VV$): $p^2 \times 500 = (0.8)(0.8)(500) = 320$ plants.     * Heterozygous ($Vv$): $2pq \times 500 = 2(0.8)(0.2)(500) = 160$ plants.     * Homozygous Recessive ($vv$): $q^2 \times 500 = (0.2)(0.2)(500) = 20$ plants.     * Phenotypes: Violet flowers = $320 + 160 = 480$; White flowers = $20$.

• Problem 2: Chin Dimpling: Dimpling ($D$) is dominant over undimpled ($d$). If a population has $36\%$ with dimpled chins, what is the frequency of the dimpling allele ($p$)?     * If $36\%$ are dimpled, then $64\%$ are undimpled ($q^2 = 0.64$).     * $q = \sqrt{0.64} = 0.8$.     * $p = 1 - 0.8 = 0.2$.     * Frequency of dimpling allele is $0.2$.

Genetic Variance and Inbreeding

• Heritability: The fraction of phenotypic variation observed in a population that can be attributed to genetic variance (differences) among individuals.

• Genetic Variance: The diversity of alleles and genotypes within a population. High genetic variance provides more "raw material" for evolution to act upon.

• Inbreeding: The mating of closely related individuals.     * Risks: It can bring together deleterious (harmful) recessive mutations, causing abnormalities and disease.     * Inbreeding Depression: A phenomenon where inbreeding increases homozygosity, leading to a higher frequency of individuals with deleterious phenotypes.     * Carrier Dynamics: In healthy populations with unrestricted habitats, the chance of two rare carriers mating is low. If they do, only $25\%$ of offspring inherit the disease from both. Interbreeding among family carriers dramatically increases these odds.

Assigned Lecture Activity Problems

• Problem 1: Broccoli Taste: The allele for hating broccoli ($b$) is recessive to the allele for loving broccoli ($B$). $70\%$ of alleles in the population are $b$.     * Allele Frequencies: $q = 0.7$, $p = 0.3$.     * Genotype Frequencies: $p^2 (BB) = 0.09$, $2pq (Bb) = 0.42$, $q^2 (bb) = 0.49$.     * Phenotype Frequencies: Love broccoli ($BB + Bb$) = $0.51 (51\%)$, Hate broccoli ($bb$) = $0.49 (49\%)$.

• Problem 2: Beetle Legs: In a population of $1000$ beetles, $750$ have green legs (Dominant, $G$) and $250$ have blue legs (Recessive, $g$).     * Determination: Phenotype frequency is given. Recessive phenotype $q^2 = \frac{250}{1000} = 0.25$.     * Allele Frequencies: $q = \sqrt{0.25} = 0.5$, $p = 1 - 0.5 = 0.5$.     * Genotype Frequencies: $GG = 0.25$, $Gg = 0.50$, $gg = 0.25$.     * Phenotype Frequencies: Green = $75\%$, Blue = $25\%$.

• Problem 3: Mouse Whiskers: Long whiskers ($T$) is dominant; short whiskers ($t$) is recessive. $60\%$ of alleles are for long whiskers ($T = 0.6$).     * Allele Frequencies: $T = 0.6$, $t = 0.4$.     * Genotype Frequencies: $TT = 0.36$, $Tt = 0.48$, $tt = 0.16$.     * Phenotype Frequencies: Long = $84\%$, Short = $16\%$.

• Problem 4: Moth Spots: Orange spots ($A$) is dominant; yellow spots ($a$) is recessive. $19\%$ of moths have orange spots ($AA + Aa = 0.19$).     * Approach: If $19\%$ are orange, $81\%$ are yellow ($q^2 = 0.81$).     * Allele Frequencies: $a (q) = 0.9$, $A (p) = 0.1$.     * Genotype Frequencies: $AA = 0.01$, $Aa = 0.18$, $aa = 0.81$.     * Phenotype Frequencies: Orange = $19\%$, Yellow = $81\%$.

• Problem 5: Tongue Rolling: Rolling ($R$) is dominant over non-rolling ($r$). On an island of $100$ people, $75\%$ can roll their tongues.     * Tasks: Calculate allele frequencies, number of homozygous dominant ($RR$), heterozygous ($Rr$), and homozygous recessive ($rr$) individuals.

• Problem 6: Tobiano Horse Pattern: Tobiano ($T$) is dominant; solid coat ($t$) is recessive. A Mustang population has $20\%$ $T$ alleles and $80\%$ $t$ alleles.     * Tasks: Calculate allele frequencies as decimals, frequencies of homozygous dominant, heterozygous, and homozygous recessive individuals, and the frequencies of the two phenotypes.