Unit 3 Genetic Variation and Evolution Study Guide

The Basics of Genetic Variation * The Ultimate Source: The fundamental origin of all new genetic traits and variation in any population is mutations. Every deviation from a standard genetic sequence begins with a mutational event. * Main Drivers of Variation: Variation in a population arises primarily from two specific mechanisms: mutations and gene shuffling. Gene shuffling occurs specifically during the process of sexual reproduction, which reorganizes alleles into new combinations. * Gene Pools: In biological terms, a "gene pool" is defined as the collection of all the different genes and alleles that are present within a specific, defined population. * Defining Evolution: Biologists identify that a population is evolving if the allele frequencies (the percentage or relative proportion of a specific trait) change over time. * Genetic Equilibrium: If the allele frequencies in a population stay the same over time without changing, the population is said to be in a state of genetic equilibrium. # Natural Selection and "Fitness" * Darwinian Fitness: Within the study of evolution, "fitness" is not defined by physical strength or speed. Instead, it is measured exclusively by the number of fertile offspring an organism leaves behind for the next generation. * Adaptation: Individuals that survive for a sufficient duration to reproduce are usually the ones best adapted to the specific conditions and constraints of their environment. * Common Ancestry: Charles Darwin noted that while different species might look different in the present day, similar physical characteristics often strongly suggest they descended from a common ancestor. * Modern Theory of Evolution: The contemporary theory of evolution includes natural selection, competition, and common ancestry. It notably does NOT include the "transmission of acquired characteristics," which is the debunked idea that traits acquired during an individual's life (such as a parent lifting weights to develop big muscles) will be inherited by their offspring. # Types of Natural Selection Students are required to identify three specific selection "curves" on a graph: 1. Directional Selection: This occurs when the entire distribution curve shifts toward one extreme of a trait. An example is the evolution of horses, which transitioned from small ancestors to the much larger bodies seen in modern horses. 2. Stabilizing Selection: In this scenario, the "average" or middle trait is favored, and both extreme versions of the trait are eliminated. For instance, if a parasite kills both very large and very small horses, only the mid-sized individuals are left to survive. 3. Disruptive Selection: Here, both extreme versions of a trait are favored while the "average" middle trait is selected against. A bird population where only very small or very large beaks are useful creates this effect, resulting in a distinctive "M" shaped graph. # Genetic Drift and Isolation In some cases, populations change purely by chance rather than because one trait is "better" or confers a survival advantage; this process is known as Genetic Drift. * Bottleneck Effect: A dramatic reduction in population size caused by sudden events like an earthquake or human activities like over-hunting. This leaves the surviving population with very little genetic variation. * Founder Effect: This occurs when a small group of individuals migrates to a new habitat and establishes a new population, with a gene pool reflecting only the alleles of the original migrants. * Reproductive Barriers: These mechanisms prevent different species from breeding and are categorized into: * Geographic Isolation: Separation caused by physical barriers such as rivers or floods. * Temporal Isolation: Species reproduce at different times, such as flowers that open in the morning versus those that open in the afternoon. * Behavioral Isolation: Differences in courtship rituals, mating dances, or specific animal "calls." # Practice Calculations for Allele Frequency To determine the allele frequency, one must count the total number of individual alleles ("letters") within the population. Consider this example involving a group of $50$ mice: * Genotype Data: 1. $12$ mice are $BB$ (Black), which totals $12 \times 2 = 24$ '$B$' alleles. 2. $24$ mice are $Bb$ (Black), which totals $24$ '$B$' alleles and $24$ '$b$' alleles. 3. $14$ mice are $bb$ (Brown), which totals $14 \times 2 = 28$ '$b$' alleles. * Calculating Totals: The total number of alleles in the population is $100$. The total number of '$B$' alleles is $24 + 24 = 48$. The total number of '$b$' alleles is $24 + 28 = 52$. * Calculating Frequencies: The frequency of the '$B$' allele is $\frac{48}{100} = 0.48$. The frequency of the '$b$' allele is $\frac{52}{100} = 0.52$. Students will be required to calculate allele frequencies on the quiz. # Real-World Scenarios * Pesticide Resistance: If a group of mosquitoes survives being sprayed with pesticide, it is because natural variation already existed within the population allowing some individuals to resist the chemical. These resistant survivors then reproduce, causing the next generation to be significantly harder to kill. * Camouflage: In a green environment, green grasshoppers survive better because they are less visible to predators. Over time, the allele frequency for the green color trait will increase because those individuals are more likely to survive and pass on their genes.