Evolution of Populations Lecture Notes

Speciation and Isolation Mechanisms

• Speciation Overview: Based on Alexandra Kukova's notes for Chapter 23, speciation has been a frequent topic on the FRQ (Free Response Questions) for the last 3 years.
• Allopatric Speciation (Geographical Separation):     - Occurs when a physical barrier stops populations from interbreeding.     - Examples of barriers include a mountain range separating Species A and Species B.     - Over time, separated groups change drastically enough through evolution that they can no longer interbreed even if reunited.
• Sympatric Speciation (Ecological and Behavioral Separation):     - Known as reproductive isolation at the genetic level.     - Polyploidy: A common phenomenon in plants where an organism possesses more than two complete sets of chromosomes.     - Tetraploidy: An organism that ends up with four sets of chromosomes due to a random mutation.         - While a tetraploid plant may be sterile in some contexts, it can still grow and divide via mitosis.         - Because a tetraploid organism cannot interbreed with its diploid parent organism, it theoretically creates a new species immediately.         - Diploids can reproduce with other diploids, and tetraploids can reproduce with other tetraploids.
• Zygotic Isolation Mechanisms:     - Prezygotic Isolation (Before a zygote is formed; no zygote is created):         - Mating Behavior: Individuals do not recognize each other as potential mates or fail to recognize mating behaviors, leading to no reproduction.         - Physical Incompatibility: Animals being physically unable to mate (e.g., a shark and a giraffe).         - Plant Incompatibility: Incompatibility between pollen and the stigma in plants.         - Gametic Incompatibility: Inability of a male gamete to fuse with a female gamete.     - Postzygotic Isolation (After a zygote is formed):         - Cell Division Failure: Failure of cell division in the fertilized egg (zygote).         - Non-viable Offspring: Offspring are born but die soon after.         - Viable but Sterile Offspring: The offspring is born and survives but cannot reproduce. A classic example is a mule.

Chapter 23: Evolution of Populations

• Marzano Learning Scale:     - 0: No understanding of the evolution of populations.     - 1: With help, a partial understanding of the evolution of populations.     - 2: A general understanding of the evolution of populations.     - 3: An understanding of the evolution of populations.     - 4: A complete understanding of the evolution of populations with the ability to apply these concepts to new situations.
• Grading and Scale Correlation:     - A: $89.5\% - 100\%$ (Marzano Scale: $3.0 - 4.0$)     - B: $79.5\% - 89.4\%$ (Marzano Scale: $2.5 - 2.99$)     - C: $69.5\% - 79.4\%$ (Marzano Scale: $2.0$)     - D: $59.5\% - 69.4\%$ (Marzano Scale: $1.0 - 1.99$)     - F: $0\% - 59.4\%$ (Marzano Scale: $0 - 0.99$)
• Definitions and Core Concepts:     - Population: A localized group of individuals that is capable of interbreeding and producing fertile offspring.     - Example Localities: Porcupine herd range and Fortymile herd range across Alaska, Canada (Fairbanks, Whitehorse) and the Beaufort Sea.     - Microevolution: Evolution occurring over a few generations. It is defined as the change in genetic makeup from generation to generation. Note: Individuals are selected, but populations evolve.     - Modern Synthesis: The integration of many other fields of study, such as statistics and botany, into the study of evolution.     - Gene Pool: The collection of all alleles at all loci within a population; represents all genes of a given population.

The Hardy-Weinberg Theorem

• Hardy-Weinberg Theorem: States that the frequency of alleles and genotypes in a population's gene pool will remain constant from generation to generation if only Mendelian segregation and recombination of alleles are at work. This describes a non-evolving population.
• Purpose: It acts as a negative control in biological studies to see if evolution in a specific population is significantly different from a theoretical baseline.
• Allele Frequency Calculation (Wildflower Example):     - Consider a population of $500$ wildflowers with $1000$ total genes for color.     - Red Flowers ($C^R C^R$): $320$ flowers, contributing $640$ $C^R$ genes.     - White Flowers ($C^W C^W$): $20$ flowers, contributing $40$ $C^W$ genes.     - Pink Flowers ($C^R C^W$): $160$ flowers, contributing $160$ $C^R$ genes and $160$ $C^W$ genes.     - Total $C^R$ allele count: $640 + 160 = 800$.     - Frequency of dominant allele ($p$): $\frac{800}{1000} = 0.8$ ($80\%$).     - Total $C^W$ allele count: $40 + 160 = 200$.     - Frequency of recessive allele ($q$): $\frac{200}{1000} = 0.2$ ($20\%$).
• Determining Genotype Frequencies:     - Chance of $C^R C^R$ ($p^2$): $0.8 \times 0.8 = 0.64$ ($64\%$).     - Chance of $C^W C^W$ ($q^2$): $0.2 \times 0.2 = 0.04$ ($4\%$).     - Chance of $C^W C^R$ or $C^R C^W$ ($2pq$): $0.16 + 0.16 = 0.32$ ($32\%$).
• Hardy-Weinberg Equations:     - $p + q = 1$     - $p^2 + 2pq + q^2 = 1$     - $p = \text{dominant allele (A)}$     - $q = \text{recessive allele (a)}$     - $p^2 = \text{homozygous dominant (AA)}$     - $q^2 = \text{homozygous recessive (aa)}$     - $2pq = \text{heterozygous (Aa or aA)}$
• Conditions Required for Hardy-Weinberg Equilibrium:     1. Large population size.     2. No gene flow between populations.     3. No mutations.     4. Random mating.     5. No natural selection.
• Practical Application (PKU Case Study):     - Phenylketonuria (PKU) affects $1$ in $10,000$ people (homozygous recessive).     - $q^2 = \frac{1}{10000} = 0.0001$     - $q = \sqrt{0.0001} = 0.01$ ($1\%$ of the population carries the PKU allele).     - $p = 1 - 0.01 = 0.99$ ($99\%$ of the population carries the dominant allele).     - $2pq = 2 \times 0.99 \times 0.01 = 0.0198$ (about $2\%$ of the population are carriers/heterozygous).

Factors Altering Allele Frequency and Evolution

• Mutations:     - Point Mutation: A change at a specific point of DNA.     - Chromosomal Mutations: Includes translocation, inversion, deletion, and duplication.     - We can predict mutation rates in general, but we cannot predict the specific actual mutation that will occur.
• Sexual Recombination: Contributes to genetic diversity.
• The Three Principal Factors of Evolutionary Change:     1. Natural Selection: Selection based on traits; an individual either possesses advantageous traits or does not.     2. Genetic Drift: Fluctuation in allele frequency based on finite population size and chance. This can cause a phenotype to drift toward fixation regardless of superiority.         - Bottleneck Effect: A sudden environmental change reduces population size. Surviving individuals are few and may not reflect the original gene pool's genetic diversity.         - Founder Effect: A few individuals become isolated from a larger population and establish a new one. The new gene pool is limited to the traits of the founders.     3. Gene Flow: Genetic additions or subtractions from a population resulting in the "movement" of a trait. Example: Pollen from a flower on one island moving to another island, spreading genes into a pre-existing population.

Genetic Variation and Environmental Interaction

• Genetic Variation Components:     - Phenotypic Polymorphism: Two or more distinct morphs represented in highly notable frequencies.     - Average Heterozygosity: The average count of heterozygous loci in a population. Example: If $1920$ out of $13700$ loci are heterozygous, the average heterozygosity is $17\%$.     - Geographic Variation: Differences in the gene pools of separated populations (e.g., populations separated by a mountain range).     - Clines: A graduated change in a trait along a geographic axis, often observed along vertical axes like altitude.
• Case Study: Yarrow Plants:     - In the Sierra Nevada Range and Great Basin Plateau, plant height is dependent on altitude.     - Samples taken from various elevations (from $1000\,m$ to $3000\,m$) and grown in a common garden show that the higher the elevation of origin, the shorter the plant usually is.

Fitness and Selection Types

• Fitness: Reproductive success, measured by the contribution an individual makes to the gene pool of the next generation.     - Fitness ranges from $0$ to $1$.     - A fitness of $0$ means the individual does not pass on any traits (no reproduction).     - A fitness of $1$ means the entire next generation is composed of that individual's genes/traits.
• Modes of Selection:     1. Directional Selection: Shifting the frequency curve toward one favored phenotype due to environmental change or migration.     2. Disruptive Selection: Favors variants at both extremes and removes intermediates. This can lead to speciation.     3. Stabilizing Selection: Favors intermediate variants and removes extreme phenotypes.
• Sexual Selection: Natural selection for mating success.     - Intrasexual: Competition within one sex for the right to breed (e.g., males fighting males).     - Intersexual: Mate choice; typically the female choosing the "best" male.
• Reproduction Efficiency: Asexual reproduction is technically "superior" to sexual reproduction in terms of speed and population growth capacity; an asexual population will outgrow a sexual one because every individual can produce offspring.

Practice Problems

• Question 1: Tongue Rolling:     - In a population of $10,000$, there are $8,365$ rollers (dominant) and $1,635$ non-rollers. Calculate heterozygous rollers ($2pq$).     - $q^2 = \frac{1635}{10000} = 0.1635$     - $q = \sqrt{0.1635} \approx 0.404$     - $p = 1 - 0.404 = 0.596$     - $2pq = 2 \times 0.404 \times 0.596 = 0.4816$ ($48.16\%$     - Number of individuals: $10000 \times 0.4816 = 4816$ individuals.
• Question 2: Sickle-Cell Anemia:     - If $9\%$ of a population is born with sickle-cell anemia ($ss$), what percentage is heterozygous ($Ss$) and resistant to malaria?     - $q^2 = 0.09$     - $q = \sqrt{0.09} = 0.3$     - $p = 1 - 0.3 = 0.7$     - $2pq = 2 \times 0.3 \times 0.7 = 0.42 = 42\%$.