Phylogeny, Common Ancestry, and Speciation

Reading phylogenetic trees

A phylogenetic tree is a diagram that shows hypothesized evolutionary relationships among organisms. A cladogram is a tree built from shared traits. Time runs from the root (the common ancestor) toward the tips (the organisms we study today).

Key vocabulary:

• Node (branch point): where one lineage splits in two. Each node represents the most recent common ancestor of everything beyond it.

• Clade: an ancestor and all of its descendants (shaded above). Think of cutting one branch off the tree with a single snip.

• Outgroup: a lineage that branched off earliest, used as a reference point for the others (the ingroup).

• Sister taxa: two lineages that share a node just after splitting. They are each other's closest relatives.

Homology, analogy, and molecular evidence

How do biologists decide where the nodes go? They compare traits and molecules.

Homologous structures have the same underlying structure because they were inherited from a common ancestor, even if they now do different jobs. A bat wing, a whale flipper, and a human arm all have the same bones (humerus, radius, ulna, carpals, and fingers). Homology is evidence of common ancestry.

Analogous structures have a similar function but evolved independently, usually in different environments facing similar selection pressures (convergent evolution). A bird wing and an insect wing both allow flight, but their structure and origin are completely different. Analogy does not show close relationship, so it can mislead when building a tree.

Molecular evidence is often the strongest. All living things use DNA, the same genetic code, and ribosomes. The more recently two species shared an ancestor, the fewer differences you expect in their DNA or protein sequences, because mutations accumulate over time.

Building a cladogram from traits

A cladogram groups organisms by shared derived traits: traits that appeared in a common ancestor and are passed to all of its descendants. Each new trait marks a node.

Species and reproductive isolation

The biological species concept defines a species as a group of populations whose members can interbreed in nature and produce viable, fertile offspring. Members of different species are reproductively isolated by barriers that keep their gene pools separate.

A mule shows why the concept matters: a horse and a donkey can mate and produce offspring, but the mule is sterile, so genes do not flow between the two species through mules.

How new species form

Speciation is the formation of new species. It needs reproductive isolation, which means the gene pools stop mixing and then drift apart through mutation, genetic drift, and natural selection.

Allopatric speciation ("other country"): a physical barrier splits one population into two.

1. A population is divided by a geographic barrier (a canyon, a river, a rising mountain range).

2. Gene flow stops.

3. Each population faces different selection pressures and accumulates different mutations and chance changes.

4. Over time they diverge so much that, even if they meet again, they cannot interbreed. Reproductive isolation is a by-product of divergence.

The Grand Canyon is a classic case: the Abert's squirrel on the south rim and the Kaibab squirrel on the north rim are thought to share an ancestor that the canyon divided.

Sympatric speciation ("same country"): new species arise in the same area, with no physical barrier. Examples: polyploidy in plants (an error in cell division doubles the chromosome number, producing instant isolation), or disruptive selection where two food or habitat niches favor different forms that then mate mostly with their own type.

Gene flow (movement of alleles between populations) works against divergence by mixing gene pools. If gene flow increases between two forms, it spreads alleles in both directions and makes them more alike.

Other patterns you may see:

• Adaptive radiation: one ancestor gives rise to many species that fill different niches (Hawaiian honeycreepers).

• Divergent evolution: related lineages become more different.

• Convergent evolution: unrelated lineages become more similar under similar pressures.

• Co-evolution: two species evolve in response to each other (flowers and pollinators).

• Gradualism vs. punctuated equilibrium: slow, steady change versus long stable periods broken by rapid bursts of change.

Back to the start: the lizard is a closer relative of the mouse than the frog is, because the lizard and mouse share the amniotic egg, a derived trait inherited from a more recent common ancestor.

Summary

• A phylogenetic tree or cladogram shows relationships. Nodes are common ancestors, a clade is an ancestor plus all its descendants, and the outgroup branches off first.

• Homologous structures (shared ancestry) support relatedness. Analogous structures (convergent evolution) do not.

• Molecular data: fewer DNA or protein differences means a more recent common ancestor.

• Cladograms are built from shared derived traits.

• Biological species concept: interbreeding with fertile offspring. Prezygotic and postzygotic barriers keep species apart.

• Allopatric speciation needs a physical barrier. Sympatric speciation happens without one. Gene flow reduces divergence.