Crush AP Bio Unit 7: Evolution

The video introduces AP Bio Unit 7: Evolution, highlighting topics like selection (natural, artificial, sexual), population genetics, evidence for evolution, speciation, extinction, and the origin of life. Artificial selection, or selective breeding, is explained as the process where breeders select organisms with desired traits over generations, leading to specific gene pools (e.g., Brassica oleracea varieties, dog breeds). Natural selection involves inherited variation, reproductive rates exceeding survival, survival of those with beneficial traits, and continuous adaptation. Sexual selection focuses on traits increasing reproductive success, leading to sexual dimorphism (intersexual selection involves mate choice, intrasexual selection involves competition). Selection can alter phenotype distribution in a population through directional, stabilizing, and disruptive selection.

Adaptive melanism is discussed as the darkening of body coloration in response to environmental darkening, driven by predation (e.g., rock pocket mice). Evolutionary fitness is defined as the number of offspring that survive to reproduce, emphasizing its impact across all life stages. The peppered moth serves as a directly observed example of directional selection and adaptive melanism in response to industrial pollution and subsequent environmental recovery, demonstrating rapid evolutionary change.

Population genetics is introduced as the study of gene distribution and change over time, with allele frequency as a key measure. Evolution is defined as a change in allele frequencies within a gene pool. The common misconception that dominant alleles are always more common is addressed. The Hardy-Weinberg equilibrium principle is explained using two equations (p+q=1 and p^2+2pq+q^2=1) to calculate allele and genotype frequencies. The five conditions for Hardy-Weinberg equilibrium are outlined (infinite population size, no selection, random mating, no gene flow, no mutation); violation of these conditions leads to evolutionary change.

Factors causing evolution are detailed: small populations leading to genetic drift (random changes in allele frequencies), natural selection, sexual selection, gene flow, and directional mutation. Genetic drift includes population bottlenecks (e.g., cheetahs) and the founder effect. Gene flow is the movement of alleles between populations, reducing differences. Mutation is the ultimate source of genetic variation. Sickle cell disease is used as an illustrative example of heterozygote advantage, where the recessive allele provides protection against malaria, maintaining its frequency in certain populations.

Abundant evidence for evolution is presented, starting with homologous traits: features sharing a common underlying structure and embryological origin due to descent with modification from a common ancestor (e.g., vertebrate forelimbs). This results from adaptive radiation, where one species diversifies into many. Vestigial structures are non-functional homologous features inherited from an ancestor where they had a function (e.g., whale pelvis, human coccyx). Analogous features have similar functions but different underlying structures, arising from convergent evolution (e.g., shark, ichthyosaur, and dolphin bodies, bird and bat wings).

Molecular homologies, such as shared hemoglobin structure and amino acid sequences in vertebrates, provide evidence of common ancestry, with closer morphological relationships correlating with greater molecular similarity. Pseudogenes, nonfunctional gene variants (e.g., the human GULO pseudogene for vitamin C synthesis), are molecular vestigial features indicating shared ancestry with organisms that have functional versions.

Universal homologies, shared by all life, indicate a common ancestor at the origin of life (e.g., DNA as genetic material, ATP, universal genetic code, ribosomes, shared metabolic pathways). Homologies among eukaryotes (nucleus, mitochondria, endomembrane system, linear chromosomes, sexual reproduction) indicate common ancestry. Embryological development also provides evidence, with similar early vertebrate embryos indicating a common ancestor, and vestigial embryonic features (e.g., human tail, pharyngeal gill slits) showing descent with modification.

Shared developmental genes (e.g., 'eyeless' gene for eye development, homeotic genes for body plan) across diverse animal phyla are profound homologies, indicating a deep common ancestry for animals. Biogeography, the study of species distribution, supports evolution by showing patterns of evolution through space (e.g., marsupials in Australia) and parallel evolution in similar niches.

Fossils, petrified remains of living things, demonstrate evolutionary change over time through transitional forms (e.g., whale ancestors). Relative dating uses superposition (deeper layers are older) to determine the age sequence of fossils. Absolute dating uses the decay of radioactive isotopes (e.g., carbon-14 half-life) in remains or nearby strata to determine numerical ages, providing a timeline for evolutionary events.

Evolution continues to this day, as seen in the evolution of resistance to DDT in mosquitoes. Initial pesticide application kills most mosquitoes, but resistant individuals survive, reproduce, and pass on their resistance genes, leading to a rapid increase in resistant populations over generations. This phenomenon is also observed with antibiotic resistance in bacteria, herbicide resistance in weeds, and chemotherapy resistance in cancer cells.

The biological species concept defines a species as a group that can naturally interbreed to produce viable, fertile offspring and is reproductively isolated from other groups. Limitations include closely related species hybridizing, and issues with extinct or asexual species. Reproductive isolating mechanisms keep gene pools separate, categorized as prezygotic (preventing zygote formation: behavioral, temporal, mechanical, habitat, gametic isolation) and postzygotic (zygote forms but offspring are unsuccessful: hybrid inviability, hybrid sterility, hybrid breakdown).

Allopatric speciation involves a geographic barrier leading to genetic differentiation and then reproductive isolation. Sympatric speciation occurs without a geographic barrier, such as through polyploidy in plants or adaptation to microhabitats (e.g., cichlid fish, bird lice) or sexual selection in animals. Adaptive radiation, where one parent species diversifies into many descendant species each filling a different ecological niche (e.g., Galapagos finches), is connected to speciation and reflected in phylogenetic trees.

Phenotypic variation is crucial for evolution, providing the raw material for natural selection to act upon. Without variation, there's no natural selection or adaptation, leading to increased risk of extinction. Examples include phospholipid variations in mammals for cold adaptation and hemoglobin variations in humans for oxygen absorption at different life stages, and different chlorophyll types in plants for efficient photosynthesis. Extinction is a normal part of life, with over 99% of species having gone extinct.

The extinction vortex describes the process of species decline: population decline leads to loss of genetic diversity (due to genetic drift), which reduces variability and fitness, accelerating further decline. Mass extinctions are widespread, rapid decreases in biodiversity, often caused by geological or astronomical events (e.g., Cretaceous extinction). Mass extinctions leave ecological niches vacant, often followed by extensive adaptive radiation of surviving species (e.g., diversification of placental mammals after the dinosaur extinction). Human activity is currently causing the 'sixth extinction' through habitat destruction, overhunting, and invasive species.

Phylogeny is evolutionary history, represented by phylogenetic trees built from morphological, molecular, or genetic evidence. A clade is a group of organisms consisting of a common ancestor and all its descendants. A shared derived character is a trait that defines a clade. Nodes represent common ancestors where lineages diverge, and sister groups are descendants that split from the same node. An outgroup is a distantly related group used for comparison.

A common mistake in phylogenetic analysis is assuming evolutionary closeness based on vertical proximity on a horizontal tree; only recency of common ancestry matters. Ancestral features are traits shared by a clade and larger, more inclusive clades. Modern phylogenetic trees primarily use molecular evidence (nucleotide and amino acid sequences). Molecular clocks use the constant rate of accumulated mutations in a gene or protein to estimate divergence times between species, calibrated against the fossil record.

The origin of life addresses how life emerged naturally from non-living matter, considering the challenges of cell theory (cells from pre-existing cells) and the importance of enzymes for complex chemistry. Key steps include Earth becoming habitable, abiotic synthesis of monomers, abiotic synthesis of polymers and vesicle formation, formation of protocells, and the emergence of self-replicating cells. The Miller-Urey experiment (1950s) demonstrated that amino acids could be synthesized abiotically in a simulated early Earth environment, providing a proof of concept for the origin of life's monomers, despite some historical inaccuracies in atmospheric composition assumptions.

The RNA world hypothesis proposes that RNA, not DNA, was the first hereditary molecule due to its ability to store genetic information and act as an enzyme (ribozyme). Self-replicating RNA systems likely grew in complexity, eventually becoming encapsulated in lipid bilayers to form protocells. Further evolution led to the last universal common ancestor (LUCA), characterized by a lipid bilayer, DNA as genetic material, RNA for information transfer, ribosomes, membrane channels, complex protein enzymes, and ATP synthesis. This LUCA gave rise to the three domains of life: Archaea, Bacteria, and Eukarya.