4.6 Evidence for Evolution and Phylogeny
Key Takeaways
- Multiple independent lines of evidence converge on common descent: the fossil record, comparative anatomy, embryology, molecular sequence data, and biogeography.
- Homologous structures share a common evolutionary origin (e.g., the tetrapod forelimb), analogous structures evolved independently for similar function (e.g., bird and insect wings), and vestigial structures are reduced remnants of ancestral organs.
- Phylogenetic trees should be read by tracing shared derived characters (synapomorphies); a monophyletic clade contains an ancestor and ALL of its descendants, while paraphyletic groups exclude some descendants and polyphyletic groups exclude the most recent common ancestor.
- Molecular clocks estimate divergence times by assuming a roughly constant rate of neutral mutations, calibrated with fossils, and are most accurate when applied to large data sets across many genes.
- The endosymbiotic theory explains mitochondria and chloroplasts as descendants of free-living bacteria engulfed by an ancestral eukaryote; both organelles still have their own circular DNA and 70S ribosomes.
Five Lines of Evidence
On the Praxis, you should be able to give an example for each major line of evidence for evolution.
| Evidence | What It Shows | Example |
|---|---|---|
| Fossil record | Documents the sequence and timing of past life | Tiktaalik (375 Mya) shows a tetrapod-like fish, supporting the fish-to-tetrapod transition |
| Comparative anatomy | Reveals shared body plans modified by descent | Forelimbs of humans, bats, and whales share the same bone arrangement but serve different functions |
| Embryology | Embryos of related species share features lost in adults | Pharyngeal arches in vertebrate embryos, including humans |
| Molecular biology | DNA and protein sequence similarity tracks relatedness | Cytochrome c sequences differ by a small predictable amount among closely related species |
| Biogeography | Geographic distribution of species fits historical isolation | Marsupial diversity in Australia after its separation from other landmasses |
Comparative Anatomy in Detail
- Homologous structures: same evolutionary origin, possibly different function. The pentadactyl limb in mammals, birds, and reptiles is the textbook example.
- Analogous structures: similar function, independent evolutionary origin (convergent evolution). Bird wings and insect wings are analogous - both are wings, but they arose from completely different ancestors.
- Vestigial structures: reduced or non-functional remnants of features that were functional in ancestors. Examples: the human appendix, whale pelvic bones, and snake hindlimb buds.
Reading Phylogenetic Trees and Cladograms
A phylogenetic tree depicts evolutionary relationships. The key reading skill is identifying synapomorphies, the shared derived characters that define each clade.
- Branch points (nodes) represent common ancestors.
- Sister taxa share the most recent common ancestor on the tree.
- The order taxa appear on the tips is arbitrary; only the branching pattern matters.
Three Clade Types
| Term | Definition | Status |
|---|---|---|
| Monophyletic | Common ancestor + ALL descendants | Valid clade (e.g., Mammalia) |
| Paraphyletic | Common ancestor + SOME descendants | Not a valid clade (e.g., 'reptiles' if birds are excluded) |
| Polyphyletic | Group whose members do NOT share an immediate common ancestor with the others | Not a valid clade (e.g., 'warm-blooded animals' = birds + mammals) |
A cladogram uses only branching order; a phylogram or chronogram also scales branch lengths to represent change or time.
Molecular Clocks
A molecular clock uses the relatively steady accumulation of neutral mutations in DNA or protein sequences to estimate divergence times. It must be calibrated against the fossil record. Clocks run faster in mitochondrial DNA than nuclear DNA, faster in fast-mutating genes, and faster in lineages with short generation times - so multiple-gene approaches and fossil calibration are needed for reliable dates.
Endosymbiotic Theory
Lynn Margulis popularized the endosymbiotic theory: ancestral eukaryotes engulfed free-living bacteria that became today's organelles.
| Organelle | Likely Ancestor | Surviving Evidence |
|---|---|---|
| Mitochondria | Aerobic alpha-proteobacteria | Own circular DNA, 70S ribosomes, double membrane, binary-fission division |
| Chloroplasts | Photosynthetic cyanobacteria | Same features as above; multiple membranes consistent with secondary endosymbiosis in some lineages |
The 70S ribosome size, circular DNA, and double membrane are tested repeatedly because they distinguish organelle ribosomes from eukaryotic 80S cytoplasmic ribosomes - and explain why antibiotics targeting 70S ribosomes (like streptomycin) don't poison human cytoplasmic translation.
Major Events in Life's History
| Era / Event | When | What Happened |
|---|---|---|
| Origin of life | ~3.8 Bya | First prokaryotic cells in the Archean |
| Great Oxygenation Event | ~2.4 Bya | Cyanobacterial photosynthesis raises atmospheric O2, enabling aerobic respiration |
| First eukaryotes | ~1.8-2.1 Bya | Endosymbiotic origin of mitochondria and later chloroplasts |
| Cambrian explosion | ~541 Mya | Rapid diversification of most major animal phyla over ~25 million years |
| Colonization of land | ~470 Mya (plants) and ~430 Mya (arthropods) | Adaptations for desiccation resistance and structural support |
| Permian-Triassic extinction | ~252 Mya | Largest extinction in Earth's history; ~96% of marine species lost |
| K-Pg (formerly K-T) extinction | ~66 Mya | Asteroid impact at Chicxulub; non-avian dinosaurs go extinct; mammals diversify |
| Genus Homo | ~2.5 Mya | Stone tool culture; later expansion of brain size |
The Cambrian explosion matters because it shows that the major animal body plans were established early in animal history. The K-Pg extinction matters because the mammalian radiation that followed it - filling vacated niches - is a textbook case of adaptive radiation.
A student examines a phylogenetic tree and notices that the group 'reptiles' (lizards, snakes, turtles, crocodilians) is highlighted, but birds are placed outside the group even though they share a common ancestor with crocodilians inside it. What is the BEST description of 'reptiles' as drawn?
Which observation provides the STRONGEST direct support for the endosymbiotic theory of mitochondrial origin?