Macroevolution and Phylogenetics

Main notes

Macroevolution is the study of the grand patterns of life's history: how new species arise, how lineages diversify, and how major groups rise and fall over millions of years. Phylogenetics supplies the framework, an evolutionary family tree that shows which species are related to which. These trees organize everything the rest of this course covers, from the three domains down to individual species, and they connect the heredity studied in earlier units to the survey of diversity that follows.

Cladistics and Character States

Cladistics is a method of classification that groups organisms strictly by common ancestry. The evidence comes from characters, heritable features of organisms such as a bone shape, a gene sequence, or a metabolic pathway. Every character exists in different character states: a species either has a backbone or it lacks one, a beak or no beak. Cladists compare states across many species to discover which ones share evolutionary innovations, and DNA sequence data now dominates the practice.

The critical step is telling ancestral states, present in the ancestor before a group split, from derived states, new in one or more descendant lineages. The standard tool is outgroup comparison: pick a species outside the group under study that split off earlier, and the state it shows is the ancestral one. A synapomorphy is a shared derived character, a new feature passed from a common ancestor to two or more lineages, and it is the only kind of evidence that supports a group. A symplesiomorphy is a shared ancestral character, a trait that existed long before the split, and it says nothing about who is most closely related to whom.

Homology and analogy matter here. Homologous characters are similar because of inheritance from a common ancestor: the forelimb bones of bats, whales, and humans all descend from the same original limb. Analogous characters are similar because of convergent evolution, independent adaptation to the same way of life: bird wings and insect wings, or the streamlined bodies of dolphins and sharks. Only homology carries information about ancestry, and similarities that evolved independently are called homoplasy. When several trees fit the data, a rule called parsimony chooses: prefer the tree that requires the fewest character-state changes.

Common Mistake: Treating any shared trait as evidence of close kinship. If two species share a tail because their distant ancestor also had one, the tail is a symplesiomorphy and proves nothing about closeness. Only shared derived characters, synapomorphies, place species in the same clade.

ELI-10

Imagine a family where almost everyone has brown hair, but grandmother dyed hers purple and her two children copied her. If two strangers both have purple hair, you might guess they share a recent family tie. Brown hair tells you nothing, because nearly everyone in that family has it. Cladistics works exactly this way: only the new, shared inventions reveal who is closely related.

Tree Reading

A phylogenetic tree is a hypothesis of evolutionary relationships, drawn as branching lines of descent. When branch lengths carry no meaning, the picture is called a cladogram. Every tree has named parts: the root, the deepest point, stands for the common ancestor of all organisms on the tree; branches are the lines of descent; nodes are branch points where a single lineage splits into two; tips are the species at the ends of the lines. Each node represents the most recent common ancestor of everything above it, and the two lineages that emerge from one node are sister taxa, each other's closest relatives.

Relationships come only from the most recent common ancestor, never from how the tips are arranged on the page. Two tips drawn side by side can be distant cousins, and two tips far apart can be close kin. Branch rotation changes nothing: you can spin any branch like a mobile, and the tree still states the same relationships. A time-calibrated tree is a variant in which branch lengths do carry meaning, with longer branches representing more time or more evolutionary change.

1. Find the root, the oldest point on the tree.
2. Pick two tips and trace each lineage back toward the root.
3. The first node where the two lineages meet is their most recent common ancestor.
4. The closer that node sits to the tips, the more closely the two species are related.
5. Compare several pairs, and the whole pattern of kinship appears.

Common Mistake: Reading the tree as a ladder in which neighboring tips are closest relatives. Humans did not evolve from chimpanzees; the two lineages share a common ancestor and are sister taxa, since their branches meet at a single node with no other species between them.

ELI-10

A family reunion photo can be turned into a tree of everyone present. You and your cousin are related through the grandparents you both climb up to meet. You and a stranger from another town are related too, but only through an ancestor who lived many generations ago. The higher you must climb before your lines join, the older the connection is. Species trees work the same way.

Monophyly

A monophyletic group, also called a clade, contains an ancestor and all of its descendants, and nothing else. These are the only groups that reflect real evolutionary history, because they cut the tree cleanly: every lineage that arose from one founding species belongs together. Birds, for example, descend from dinosaurs, so the group that unites reptiles with birds is monophyletic, whereas the traditional group "reptiles" with birds removed is not.

Two broken arrangements fail the test. A paraphyletic group contains an ancestor and some but not all of its descendants: fish are paraphyletic because tetrapods, including humans, descend from lobe-finned fishes yet are excluded from "fish". A polyphyletic group has no single common ancestor at all, such as "flying animals" gathering birds, bats, and insects whose wings evolved independently.

Modern classification names only monophyletic groups wherever the evidence allows. This is why the three-domain system recognizes Bacteria, Archaea, and Eukarya as the three great clades of life, and why phylogenetic classifications place birds within the same group as crocodilians.

Common Mistake: Assuming that every familiar name is a natural group. Traditional "reptiles" is paraphyletic because birds, the living descendants of dinosaurs, are left out. A group is a real clade only when it contains every lineage descended from its founding ancestor.

ELI-10

Picture grandmother, her two daughters, and the grandchildren. "Everyone in grandmother's family" is a whole group that includes all of them. "Grandmother, one daughter, and only her sons" is a broken group, because you left some people out. "Everyone who owns a red car" is a nonsense group with no family meaning at all. Those three pictures are monophyletic, paraphyletic, and polyphyletic.

Taxonomic Ranks

Biologists since Linnaeus have sorted species into a nested series of taxonomic ranks, from broad to narrow: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. Any named group at any rank is a taxon. Humans slot into the scheme as follows.

Ranks are a convenient filing system, but they are human inventions, not facts of nature. A genus in one group is not equivalent in age or distinctness to a genus in another, and two species need not resemble each other just because they share a rank. Modern practice tries to make every named rank a monophyletic group, and when a traditional rank conflicts with the tree, the tree wins.

Each species carries a unique two-part label under binomial nomenclature: the genus name capitalized, followed by the species epithet, both italicized, as in Homo sapiens. Because the name is the same in every language, biologists worldwide share one unambiguous label for each species, and the same naming rules apply across all three domains.

Common Mistake: Treating ranks as comparable across groups, as if one "order" equaled another. Ranks are just labels pinned onto a tree. Also, the species epithet is never capitalized: the name is Homo sapiens, never Homo Sapiens.

ELI-10

Think of a mailing address. The country covers many states, each state holds many cities, and the house number names one exact home. Taxonomic ranks run the same way, from the enormous domain down to a single species. A plain common name like "robin" is like saying "the red house", which could mean many places. The binomial name is the full house number, and it fits one species only.

Molecular Clocks

The molecular clock turns DNA into dates. In any lineage, differences accumulate at a roughly steady pace, so the number of differences between two species' DNA is proportional to the time since their lineages separated. The rate is expressed in substitutions per site per year: animal mitochondrial DNA typically changes at about one substitution per site per 100 million years, while many nuclear genomes change at about one per site per billion years.

A clock must be calibrated with independent dates, almost always from the fossil record. If fossils show that two lineages split 50 million years ago and their DNA differs at one position in 20, the rate is one substitution per site per billion years, and that rate can then date splits that left no fossils at all.

1. Align the DNA sequences of two lineages and count the differences.
2. Divide the differences by the sequence length to get divergence per site.
3. Date at least one known split on the tree with fossils.
4. Divide the divergence by the fossil age to obtain the clock rate.
5. Apply that rate to undated nodes to estimate their ages.

Clocks are rough hourglasses, not stopwatches. Rates vary between genes and between lineages, and very old splits suffer saturation: so many changes accumulate that the sequences no longer track time. This is why every clock needs fossil calibration and why molecular dates always carry a range of uncertainty.

Common Mistake: Treating the molecular clock as perfectly regular. Rates vary among genes and among lineages, and deep divergences saturate, so raw sequence differences stop tracking time. Reliable molecular dates require calibration with fossils and an honest error range.

ELI-10

You copy the same long document by hand once a year, always making about the same number of typos per page. Years later you find two old copies of the document, one with more typos than the other. The copy with more typos was probably started earlier and copied more times. Counting the typo gap tells you roughly how far apart the two copies are in time. Counting DNA differences does the same for species.

High-Yield:

• Shared derived characters, synapomorphies, are the only evidence that builds clades.
• Read a tree by the most recent common ancestor, never by the order of the tips.
• Only monophyletic groups, ancestor plus all descendants, are natural units.
• The three domains are Bacteria, Archaea, and Eukarya, and every species carries a binomial name.
• Molecular clocks date divergences from DNA differences, but only after fossil calibration.

Quick Review

• Cladistics groups organisms by shared derived characters; ancestral characters carry no grouping power.
• Homologous traits reflect common ancestry, while analogous traits reflect convergent evolution.
• In a tree, sister taxa share the most recent common ancestor, and rotating branches never changes the meaning.
• A clade is monophyletic; paraphyletic groups omit descendants, and polyphyletic groups lack any common ancestor.
• The Linnaean hierarchy runs Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
• Binomial nomenclature gives every species a unique italicized two-part name such as Homo sapiens.
• Molecular clocks convert DNA divergence into time estimates after calibration with the fossil record.

Key terms

Key terms are emphasized and defined within the main notes.

Important formulas or processes

See the formulas, procedures, and process blocks in the main notes where applicable.

Common mistakes

See the labeled common-mistake callouts in the main notes where present.