Comprehensive Guide to Evolution, Genetics, and Natural Selection

Foundations of Evolution: Natural Selection and Microevolution

Evolution Defined: The process of biological change in populations over time that causes descendants to be genetically different from their ancestors.
Scale of Evolution: * Microevolution: Occurs on a small scale affecting a single population. * Macroevolution: Occurs on a large scale affecting changes in species across multiple populations.
Charles Darwin: Developed the theory of natural selection to serve as the mechanism for how evolution occurs.
The Concept of Natural Selection: Organisms with more suitable traits (adaptations) will live longer and reproduce more than others, causing those traits to become more common in the population. This is often referred to as "Survival of the Fittest." * Fitness: The ability of an individual to survive to a reproductive age and to produce offspring.

Principles of Natural Selection

Overproduction of Offspring: Producing a high number of offspring with limited resources leads to competition for those resources.
Variation: Differences in the phenotypes of individuals in a population. * Sources of Variation: 1. Random Mutations: Deemed the ultimate source of all genetic variation. 2. Genetic Recombination: Occurs during meiosis through crossing over. 3. Migration (Gene Flow): The movement of genes in or out of a population.
Adaptation: A feature that allows an organism to better survive in its environment. Beneficial traits (adaptations) become more common over time because organisms possessing them live longer and reproduce more. This changes the gene pool, which is the combined alleles of all individuals in a population.
Descent with Modification: A change in gene/allele frequency over time. The natural environment acts on phenotypes. Individuals with adaptations to their environment are more likely to survive and reproduce, leading to populations with new phenotypes adapted to new situations.
Evolutionary Rule: Individuals do not evolve; only populations do.

Modes of Natural Selection

Directional Selection: Increases the expression of an extreme version of a trait in a population. * Example: If tree bark becomes darker, light moths become more visible and decrease, while dark moths become more camouflaged and increase.
Disruptive Selection: A process that splits a population into two groups; it removes individuals with average traits and favors both extremes. * Example: In seedcracker finches, the availability of only small or large seeds favors birds with small or large beaks, while those with medium beaks are selected against.
Stabilizing Selection: Favors intermediate traits and reduces variation at the extremes. * Example: Human babies with birth weights far below or above normal have lower survival chances than average-weight babies; thus, birth weight varies little.

Allele Frequencies and Calculations

Definitions: * Allele Frequency: The percentage or proportion of an allele in the population's gene pool. * Gene Pool: The combined alleles of all individuals in a population.
Fundamental Equations: * $p = \text{Frequency of the dominant allele}$ * $q = \text{Frequency of the recessive allele}$ * $p + q = 1$
Example Case #1: Wildflowers * Population: $500$ total plants. * Phenotypes: $20$ white ( $rr$ ), $320$ homozygous red ( $RR$ ), $160$ heterozygous red ( $Rr$ ). * Total alleles: $500 \times 2 = 1000$ . * Dominant alleles ( $R$ ): $(320 \times 2) + 160 = 800$ . * Recessive alleles ( $r$ ): $(20 \times 2) + 160 = 200$ . * Dominant Frequency ( $p$ ): $\frac{800}{1000} = 0.8$ . * Recessive Frequency ( $q$ ): $\frac{200}{1000} = 0.2$ .
Example Case #2: Pea Plants * Total: $400$ plants. * Phenotypes: $50$ short ( $tt$ ), $200$ homozygous tall ( $TT$ ), $150$ heterozygous tall ( $Tt$ ). * Total Alleles: $800$ . * Dominant Alleles ( $T$ ): $550$ . * Recessive Alleles ( $t$ ): $250$ . * Allele Frequencies: $p = 0.6875 \approx 0.7$ ; $q = 0.3125 \approx 0.3$ . * Genotype Frequencies: * $TT = \frac{200}{400} = 0.5$ * $Tt = \frac{150}{400} = 0.375 \approx 0.38$ * $tt = \frac{50}{400} = 0.125 \approx 0.13$

Mechanisms of Microevolution

Mutations: Any change in a DNA sequence. Creates new alleles and changes allele frequency. Increases the raw material for evolution; can be harmful, beneficial, or neutral.
Natural Selection: Organisms more fit for their environment survive and reproduce more, making beneficial adaptations common.
Genetic Drift: A random change in the frequency of alleles over time. It often results in a loss of genetic variation and is more apparent in smaller populations.
Gene Flow: Movement of genes into or out of a population during migration, resulting in increased genetic variation.
Sexual Selection (Non-random Mating): Favors traits that are necessary for reproduction (attracting mates) even if they do not necessarily improve survival.

Hardy-Weinberg Equilibrium (HWE)

Definition: A state of genetic equilibrium where no changes in allele frequencies occur over time.
Conditions for Neutrality (Evolution will NOT occur if): 1. Population is very large. 2. Mating is random. 3. No migration occurs. 4. No mutations occur. 5. No natural selection occurs.
Hardy-Weinberg Equation: Used to calculate genotypic frequencies when a population is in equilibrium: * $p^2 + 2pq + q^2 = 1$ * $p^2$ = Frequency of homozygous dominant ( $RR$ ). * $2pq$ = Frequency of heterozygous ( $Rr$ ). * $q^2$ = Frequency of homozygous recessive ( $rr$ ).

Patterns of Evolution: Speciation and Extinction

Speciation: The formation of a new species from a pre-existing species. Requires geographic isolation. Gene pools become different until the two groups can no longer produce viable, fertile offspring.
Extinction: The elimination of a species. * Gradual Extinction: Occurs at a slow rate due to climate changes or natural disasters. * Mass Extinction: Occurs suddenly because of catastrophic events (meteor, volcano, tsunami).
Gradualism: Slow, constant changes over a long period (e.g., peppered moths darkening after the Industrial Revolution).
Punctuated Equilibrium: Bursts of change followed by periods of stability (e.g., mammal population changes during the Mesozoic/Paleozoic eras).

Macroevolutionary Patterns

Divergent Evolution: Multiple species arise from one common ancestor. Differences evolve as populations adapt to new environments (Adaptive radiation). * Example: Common mammal ancestor giving rise to bears, monkeys, and koalas.
Convergent Evolution: When unrelated species evolve similar characteristics due to living in similar environments. * Example: Ancestral birds, mammals (bats), and reptiles (pterosaurs) all evolving flight independently.
Coevolution: Two populations form a specialized relationship and change in response to each other (e.g., flowers and their specific insect pollinators).

Evidence of Evolution

Paleontology: The study of prehistoric life through the fossil record. * Transitional Fossils: Fossils that link ancestral species to descendants (e.g., Archaeopteryx, the link between birds and dinosaurs).
Morphology: The study of the form of living things. * Homologous Structures: Similar structures suggesting common ancestry; same structure, different function (result of divergent evolution). * Vestigial Structures: Structures with little or no function, leftover from ancestors (result of divergent evolution). * Analogous Structures: Different structure, same function. Due to similar environments, not common ancestry (result of convergent evolution).
Biogeography: The study of the geographic distribution of plants and animals. Island species resemble mainland species more than distant island species. Includes endemic species found only in one region (e.g., Galapagos tortoises).
Embryology: Similarities in vertebrate embryos early in development suggest common ancestry.
Biochemistry: Comparison of DNA and protein sequences. * Pseudogenes: Nonfunctional genes that act as "vestigial structures" of DNA.
Direct Observation: Microevolution observed in short-life-cycle populations (e.g., peppered moths, pesticide-resistant mosquitoes, and antibiotic-resistant bacteria like MRSA).

Phylogeny and Taxonomy

Taxonomy: Field of biology classifying organisms by similar characteristics. * Three Domains: Eubacteria (prokaryotes, pathogens), Archaebacteria (prokaryotes in extreme environments), Eukarya (eukaryotes). * Hierarchy: Domain -> Kingdom -> Phylum -> Class -> Order -> Family -> Genus -> Species. * Carolus Linnaeus: Known as the "Father of Taxonomy." * Binomial Nomenclature: Two-name system (e.g., Genus species).
Phylogeny: Evolutionary history of a species. * Endosymbiotic Theory: Suggests eukaryotes evolved through one prokaryote living inside another. First life (prokaryotes) dated back $3.5 \times 10^9$ years ago; first eukaryotes evolved around $2.1 \times 10^9$ years ago.
Phylogenetic Trees: Diagrams predicting evolutionary relationships. * Branch Points: Show new species diverging from a common ancestor. * Nodes: Represent a more recent common ancestor. * Maximum Parsimony: Using the simplest explanation when creating a tree.

Mendelian Genetics

Genetic Basics: Most body cells are diploid, containing two sets of chromosomes (one from mother, one from father). * Gene: Section of DNA providing instructions for a protein. * Allele: Different versions of the same gene. * Homologous Chromosomes: Matching chromosomes from mom and dad with same genes but potentially different alleles.
Gregor Mendel: Austrian monk who used pea plants to develop the 3 Laws of Inheritance.
Law of Dominance: A dominant allele will express itself over a recessive allele. * Genotype: The actual alleles inherited (e.g., $FF, Ff, ff$ ). * Phenotype: The physical traits seen (e.g., purple flowers).
Law of Segregation: During Meiosis I, homologous chromosomes separate, so each gamete receives only one chromosome from each pair.
Law of Independent Assortment: Alleles for different genes separate independently. Chromosomes line up randomly during metaphase, allowing any combination of alleles.

Complex Inheritance Patterns

Incomplete Dominance: The heterozygous phenotype is a blend between the two homozygous phenotypes (e.g., Red and White flowers making Pink).
Codominance: Both traits are fully and separately expressed (e.g., Red and White speckled flowers; AB blood type).
Multiple Alleles: Having more than two alleles for a gene (e.g., Blood types $A$ , $B$ , and $i$ ).
Polygenic Inheritance: Traits produced by two or more genes, usually showing a range of phenotypes (e.g., skin color, height).
Epistasis: When one gene overshadows others (e.g., albinism).
Linked Genes: Genes on the same chromosome inherited together (e.g., red hair and freckles). They only separate during crossing over.
Sex-Linked Traits: Genes located on sex chromosomes ( $X$ or $Y$ ). * Females ( $XX$ ): Inherit X-linked genes normally; dominance applies. * Males ( $XY$ ): Inherit the X, but the Y carries few genes. Males express whatever trait is on their single X. * Carrier: A female who carries a recessive allele but does not show it because of a dominant allele on the other X.

Mutations and Genetic Analysis

Mutations: Changes in the order of nucleotide bases ( $A$ , $T$ , $C$ , $G$ ). * Somatic Mutations: Occur in body cells; passed by mitosis; may cause cancer. * Germ Line Mutations: Occur in gametes; passed by meiosis; lead to mutated offspring.
Gene Mutations: * Point Mutations: Substitute one nucleotide for another (e.g., GAG to GUG in Sickle Cell Anemia). * Frameshift Mutations: Insertion or deletion of a nucleotide, affecting all subsequent codons.
Chromosome Mutations: * Duplication: Multiple copies of a single gene. * Translocation: Segments of non-homologous chromosomes exchange pieces. * Nondisjunction: Chromosomes fail to separate correctly during anaphase, resulting in abnormal numbers (e.g., Down Syndrome).
Pedigrees: Charts tracing phenotypes/genotypes in a family. * Autosomal Recessive: Rare, skips generations, affects males and females equally. * Autosomal Dominant: Common, never skips generations, affects both sexes. * Sex-Linked Recessive: More common in males, often skips generations, fathers cannot pass X-linked traits to sons.