AP Bio Evolution (Unit 7) Mega-Review (25–26): From Natural Selection to Speciation to Phylogeny

This section introduces AP Bio Unit 7, focusing on the vastness of biological evolution and the mechanisms driving change over time. It outlines the main topics to be covered: selection (natural, artificial, sexual), population genetics and Hardy-Weinberg equilibrium, evidence for evolution, speciation, variation in populations, phylogeny, and the origin of life.

This part details the three types of selection. Artificial selection (selective breeding) is explained with examples like Brassica oleracea varieties and dog breeds. Natural selection is described as a process involving inherited variation, differential reproduction, and adaptation, exemplified by bat wings and camouflage. Sexual selection, driving traits for reproductive success and sexual dimorphism (intersexual and intrasexual selection), is also discussed. Different modes of selection (directional, stabilizing, disruptive) and the concept of adaptive melanism (e.g., rock pocket mouse) are explored. Evolutionary fitness is defined as reproductive success over generations, highlighted by penguin life stages. The peppered moth serves as a directly observed example of directional selection and adaptive melanism.

This segment introduces population genetics as the study of gene distribution and change in populations, focusing on allele frequency and the gene pool. It clarifies the misconception about dominant alleles always being more common, using achondroplasia as an example. The Hardy-Weinberg equations (p + q = 1 and p² + 2pq + q² = 1) are explained for calculating allele and genotype frequencies. The Hardy-Weinberg principle is presented with its five conditions for maintaining constant allele frequencies. Factors that cause evolution by violating these conditions (genetic drift, natural selection, sexual selection, gene flow, mutation) are then detailed. Genetic drift is illustrated by population bottlenecks (cheetahs) and the founder effect. The impact of gene flow and mutation on allele frequencies is also covered. Sickle cell disease is used as an example of heterozygote advantage in population genetics, linked to malaria prevalence.

This section explores various lines of evidence supporting evolution. Homologous traits (e.g., vertebrate forelimbs) demonstrate descent with modification from common ancestors, resulting from adaptive radiation (e.g., Galápagos finches). Vestigial structures (e.g., whale pelvis, human coccyx) further support common ancestry. Analogous features (e.g., shark, ichthyosaur, dolphin forms) are distinguished from homologous ones, arising from convergent evolution. Molecular homologies (e.g., hemoglobin sequences in vertebrates, pseudogenes like GULO) provide evidence at a molecular level. Shared universal features across all life (DNA, ATP, genetic code, ribosomes, metabolic pathways) indicate a common origin of life. Features shared by all eukaryotes (nucleus, mitochondria, endomembrane system, introns, linear chromosomes, sexual reproduction) point to their common ancestry. Embryological development, with early vertebrate embryos showing similarities and vestigial features (e.g., human tail and gill slits), is another line of evidence. The conservation of developmental genes (e.g., Eyeless gene, homeotic genes) across diverse animal species highlights deep evolutionary relationships. Biogeography, the study of species distribution (e.g., marsupials in Australia, parallel evolution), supports the idea of populations evolving and spreading. Fossils (petrified remains, transitional forms like whale ancestors) demonstrate change over time. Relative dating (superposition) and absolute dating (radioactive isotopes and half-life, e.g., Carbon-14) of fossils are explained. Finally, the evolution of resistance to DDT in mosquitoes is presented as a contemporary example of ongoing evolution.

This part defines the biological species concept and its limitations (e.g., hybrids, asexual species). Reproductive isolating mechanisms, which keep gene pools separate, are categorized into prezygotic (behavioral, temporal, mechanical, habitat, gametic isolation) and postzygotic barriers (hybrid inviability, hybrid sterility, hybrid breakdown). The two main modes of speciation, allopatric (requiring a geographic barrier, leading to genetic and reproductive isolation) and sympatric (occurring without a geographic barrier, as seen in plant polyploidy or animal microhabitat adaptation like cichlid fishes and bird lice) are contrasted. Adaptive radiation is revisited as a process where a single parent species diversifies into multiple descendant species, each adapted to a different ecological niche, exemplified by Galápagos finches.

This section emphasizes the importance of phenotypic variation as the raw material for natural selection. It illustrates how variations in phospholipid structure provide adaptive function in browsing mammals by maintaining membrane fluidity in different temperatures. It also explains how variations in hemoglobin production (fetal vs. adult hemoglobin) maximize oxygen absorption at different life stages in humans and other placental mammals. Finally, variations in chlorophyll types (A and B) are shown to increase photosynthetic efficiency in plants by absorbing different light wavelengths (red, blue) and adapting to various light conditions.

This segment introduces phylogeny as evolutionary history and phylogenetic trees as branching diagrams illustrating evolutionary relationships. Trees are constructed using morphological, molecular, and genetic evidence, making claims about relatedness (e.g., hippos and whales). Key terms like 'clade' (common ancestor and all descendants, e.g., large ground finch, entire groups of finches), 'shared derived character' (trait evolved in a common ancestor distinguishing a clade, e.g., lungs and four limbs for amphibians and mammals), 'nodes' (points of divergence representing common ancestors), and 'sister groups' (descendants from the same node) are defined and explained with examples. The concept of an 'outgroup' for comparison is presented. A critical mistake to avoid in phylogenetic analysis is highlighted: interpreting vertical closeness on a tree as evolutionary closeness; instead, the recency of common ancestry is the sole determinant of relatedness. 'Ancestral features' are defined as traits shared by a clade and larger, more inclusive clades (e.g., claws/nails in mammals). The evolution of methods used for constructing phylogenetic trees (from morphological similarities to nucleotide and amino acid sequences) is discussed. Molecular clocks, which use constant mutation rates to estimate divergence times (e.g., hemoglobin mutation rate to date species splits), are also explained.

This part addresses the fundamental question of how life emerged naturally, reconciling it with the cell theory and the role of enzymes. The five key steps for the origin of life are outlined: a habitable and stable Earth, abiotic synthesis of monomers, abiotic synthesis of polymers and vesicle formation, assembly into protocells, and the emergence of self-replicating cells (the last universal common ancestor). The Miller-Urey experiment (1950s) is described in detail as proof of concept for abiotic synthesis of organic monomers like amino acids in an early Earth environment. The significance of RNA, not DNA, as potentially the first hereditary molecule is explained, citing its ability to store genetic information and act as a catalyst (ribozyme). The 'RNA world' hypothesis is presented, detailing the progression from inorganic precursors to RNA monomers, then polymers, complex folded RNAs with enzymatic properties, self-replicating RNA systems, encapsulation in lipid bilayers to form protocells, and finally, the last universal common ancestor. The visual representation of the last universal common ancestor is used to review essential cellular biology components (lipid bilayer, DNA, RNA, ribosomes, membrane channels, enzymes, ATP synthase).