Speciation and Phylogeny

Objective: Explain how reduced gene flow can contribute to speciation.

Speciation is the formation of distinct evolutionary lineages. In one common pathway, geographic separation reduces gene flow, allowing mutation, selection, and drift to accumulate differences. Reproductive isolation can also evolve with ecological, behavioral, or other barriers, including in some cases without geographic separation. The biological species concept emphasizes reproductive compatibility and isolation but is limited for asexual organisms and fossils, so other species concepts may be useful. There is no universal amount of genetic difference or fixed time required before two populations count as distinct species.

Objective: Interpret nodes, sister groups, and shared ancestry.

A phylogenetic tree models hypothesized relationships among lineages. A node represents a common ancestor of descendant branches; sister groups share an immediate common node relative to the included sample. Rotating branches around a node does not change relationships, and the left-to-right placement of tips is not a ladder of progress. Branch lengths show time or amount of change only when a scale is explicitly supplied; otherwise treat them as topology. An outgroup can help root a tree. Trees are hypotheses supported by character data, not photographs of the exact ancestors.

Objective: Use shared derived characters and distinguish trees from certainty.

A clade includes a common ancestor and all its descendants in the modeled tree. Shared derived traits, or synapomorphies, can help identify clades when they are homologous and properly polarized using an outgroup or other evidence. Convergent traits and reversals can mislead a one-character tree. Researchers compare multiple characters and often DNA sequences using explicit inference methods, then assess uncertainty. A tree that changes after new evidence is not a failure of science; it is a revised hypothesis. Different genes may yield different trees because gene histories can differ from species histories.