Rucete ✏ AP Biology In a Nutshell
19. Phylogeny, Speciation, and Extinction
This chapter explains how scientists trace the evolutionary history of species using phylogenetic trees, how new species evolve through speciation, and how extinction affects biodiversity. It also reviews modern-day examples of continuing evolution.
Phylogeny and Common Ancestry
• Phylogeny is the evolutionary history of a species or group.
• Phylogenetic trees and cladograms are hypotheses based on shared traits and molecular data.
• Trees show lines of descent and indicate common ancestors (nodes).
• Molecular clocks (DNA/protein sequence changes) are more accurate than morphology for building trees.
• Shared derived characteristics (synapomorphies) define clades and support common ancestry.
• Outgroups are the least closely related species on a tree.
• The root represents the most recent common ancestor of all species in the tree.
Speciation and Extinction Events
• Phylogenetic trees can show speciation (formation of new species) and extinction (loss of species) events.
• LUCA (Last Universal Common Ancestor) is the proposed shared ancestor of all life ~3.5 billion years ago.
Theories for the Origin of Life
1. Inorganic molecules on early Earth formed biological molecules (supported by Miller-Urey experiment).
2. Organic molecules were delivered by meteorites (e.g., Murchison meteorite with amino acids and sugars).
Evidence for a Common Eukaryotic Ancestor
• Shared traits in all eukaryotes: – Membrane-bound organelles – Linear chromosomes – Genes with introns
• These features are not found in prokaryotes → supports common ancestry for eukaryotic life (~2.7 billion years ago).
Speciation
• Speciation = the process by which one species splits into two.
• Occurs when populations become reproductively isolated and diverge genetically.
Allopatric Speciation
• Caused by geographic separation (e.g., rivers, mountains).
• Gene flow stops → populations evolve independently.
• Example: squirrels on opposite sides of the Grand Canyon.
Sympatric Speciation
• Occurs without physical separation—often due to polyploidy, sexual selection, or niche differentiation.
• Common in plants through genome duplication (polyploidy).
Reproductive Isolation Mechanisms
Prezygotic barriers: prevent fertilization
– Temporal isolation (different mating times)
– Behavioral isolation (different courtship behaviors)
– Mechanical isolation (incompatible anatomy)
– Gametic isolation (sperm and egg do not fuse)
Postzygotic barriers: prevent viable or fertile offspring
– Hybrid inviability (offspring do not survive)
– Hybrid sterility (e.g., mule)
– Hybrid breakdown (offspring of hybrids are weak or sterile)
Extinction and Biodiversity
• Extinction is a natural part of evolution and opens ecological niches.
• Mass extinctions (e.g., asteroid impact 65 MYA) reduce diversity but also trigger adaptive radiation.
• Current rates of extinction are higher due to human activity (6th mass extinction).
Adaptive Radiation
• Rapid diversification of a single lineage into many species.
• Often follows mass extinction, colonization of new environments, or evolution of new traits.
• Example: Darwin’s finches evolved beak types to match different food sources.
Continued Evolution
• Evolution continues as environments change and new mutations arise.
• Examples: – Antibiotic resistance in bacteria – Insecticide resistance in mosquitoes – Evolution of new viruses (e.g., influenza, SARS-CoV-2 variants)
In a Nutshell
Phylogenetic trees illustrate evolutionary relationships and support the concept of common ancestry. Speciation occurs through genetic divergence, with reproductive barriers maintaining separate lineages. While extinction reduces biodiversity, it also paves the way for adaptive radiation. Evolution continues today in response to environmental pressures and genetic change.