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Chapters 24 & 25 – Speciation and the History of Life - Coggle Diagram
Chapters 24 & 25 – Speciation and the History of Life
Speciation
Speciation
→ process of forming new species
→ connects microevolution (allele frequency change) to macroevolution (broad patterns over time)
Species Concepts
Biological Species Concept (Mayr)
→ species = group of populations that interbreed and produce fertile offspring
→ separated by reproductive isolation
Limitations
→ cannot be applied to fossils or asexual organisms
→ emphasizes absence of gene flow (but gene flow can occur between distinct species)
Other Concepts
Morphological → based on structural features
Ecological → based on ecological niche
Phylogenetic → smallest group sharing common ancestor (one branch on tree of life)
Reproductive Isolation
Prezygotic Barriers (prevent fertilization)
Habitat isolation → different environments
Temporal isolation → different mating seasons/times
Behavioral isolation → different courtship behaviors
Mechanical isolation → incompatible reproductive structures
Gametic isolation → sperm/egg cannot fuse
Postzygotic Barriers (after fertilization)
Reduced hybrid viability → hybrids don’t develop or survive well
Reduced hybrid fertility → hybrids sterile (e.g., mule)
Hybrid breakdown → next generations weak or sterile
Modes of Speciation
Allopatric Speciation
→ population divided by geographic barrier
→ gene flow interrupted → divergence
Supported by island studies (e.g., Darwin’s finches)
Barriers depend on species mobility (a canyon may isolate small rodents but not birds)
Sympatric Speciation
occurs in same geographic area
Polyploidy → extra sets of chromosomes (esp. plants)
autopolyploid (same species)
allopolyploid (hybridization between species)
Habitat differentiation → subpopulations exploit different niches
Sexual selection → mate choice reinforces divergence
Patterns & Rates of Speciation
Punctuated equilibrium → rapid change in short bursts, followed by stasis
Gradualism → slow, steady accumulation of differences
Average time for speciation: ~6.5 million years (varies widely)
Macroevolution & History of Life
Macroevolution
→ evolutionary change above species level
→ broad patterns: emergence of terrestrial vertebrates, mass extinctions, origin of key features
Conditions for Life’s Origin
Abiotic synthesis of small organic molecules (Miller–Urey-type conditions)
Joining into macromolecules (proteins, nucleic acids)
Packaging into protocells (lipid membranes form spontaneously)
Self-replicating molecules (RNA world hypothesis, ribozymes)
Fossil Record & Dating
Fossils document emergence, dominance, extinction of species
Radiometric dating → isotopes (C-14, U-238)
Stratigraphy → rock layers (relative dating)
Key Events in Life’s History
3.5 bya: First prokaryotes (stromatolites)
2.4 bya: Oxygen revolution
→ caused by cyanobacteria photosynthesis
→ O₂ accumulation → extinction of many anaerobes → aerobic respiration evolved
1.8 bya: First eukaryotes
→ endosymbiont theory (mitochondria, plastids derived from prokaryotes)
1.2 bya: First multicellular eukaryotes (algae)
600 mya: Larger, more complex multicellular life
535–525 mya: Cambrian explosion → sudden diversification of animal phyla
500 mya: Colonization of land → plants, fungi, then animals
→ adaptations: vascular tissue in plants, arthropod exoskeletons, vertebrate limbs
Plate Tectonics & Earth’s History
Continents drift → alter habitats, climate, ocean currents
Explains biogeography (e.g., similarity of South American & African fossils)
Mass extinctions often linked to tectonic activity
Mass Extinctions (5 Major Ones)
Permian (252 mya) → ~96% marine species lost
→ massive volcanism → global warming → ocean anoxia
Cretaceous (66 mya) → asteroid impact → extinction of dinosaurs (except birds)
→ opened niches for mammalian radiation
Consequences
Recovery took millions of years
Triggered adaptive radiations → diversification into new niches
Adaptive Radiations
→ periods of evolutionary change where groups diversify rapidly
After mass extinctions (e.g., mammals post-dinosaurs)
After colonization of new regions (e.g., Hawaiian islands)
After evolution of novel traits (e.g., flight in insects, seeds in plants)
Evolutionary Novelties
Exaptations → structures evolved for one function but used for another (e.g., feathers)
Changes in developmental genes (Hox genes) → major impact on body plans
Evo-devo connects small genetic changes to large morphological effects