Through firsthand observations in the field, examination of museum specimens, and reading the scientific literature on manakins, Kim assembled a data set of the mating display elements and traits of the Pipra and related species. In phylogenetics, such a data set is known as a character matrix. Since Kim suspected that complex mating rituals evolve in a piecemeal fashion, she selected characters that represent specific constituent parts of the manakin mating ritual — sound type, movements, etc. — instead of an overall measure of ritual complexity. In addition to these behavioral traits, Kim also included anatomical characters that allow the birds to produce wing noises, such as feather shape and bone thickness. A portion of her character matrix is shown below.
(6th and 7th rachi)
|Club winged Manakin||Yes||Big hops||Enlarged and club-shaped||Very stocky and solid|
|Fiery-capped Manakin||Yes||?||Enlarged||Very stocky with thick walls|
|Striped Manakin||Yes||?||Enlarged||Very stocky with thick walls|
|Red-capped Manakin||Yes||Short flights||Normal||Very stocky|
|Golden-headed Manakin||Yes||Medium flights||Normal||Very stocky|
|Red-headed Manakin||Yes||Short flights||Normal||Very stocky|
|Wire-tailed Manakin||Yes||Small hops||Normal||Somewhat stocky|
|Band-tailed Manakin||Yes||Small hops||Normal||Somewhat stocky|
|Crimson-hooded Manakin||Yes||Small hops||Normal||Somewhat stocky|
By mapping these traits onto the phylogeny, Kim could reconstruct the evolution of different elements of the mating ritual in different lineages of manakins. She applied the principle of parsimony to determine which character states are homologous. If two species share a trait and are each other’s closest relatives, it is much more likely that they both inherited that trait from a recent common ancestor than that they each independently evolved the same trait. Under the principle of parsimony, mappings that minimize the number of character state changes are more likely to represent the true evolutionary history of the lineage than mappings that involve more character state changes. The example below shows two ways that the presence of quill knobs (bumps on bones where feathers attach) might be mapped on a portion of the manakin phylogeny. The club-winged, fiery-capped, and striped manakins all have quill knobs. The other manakins in this clade do not have quill knobs. Both mappings show a pattern of evolution that is consistent with the traits we observe in manakins, but mapping B requires two evolutionary changes, while mapping A requires only one. Hence, mapping A is more likely to accurately represent evolutionary changes in quill knobs. The quill knobs in the club-winged, fiery-capped, and striped manakins are likely to be inherited from a common ancestor, and thus homologous to one another.
Kim was dealing with a larger portion of the manakin phylogeny and many more characters and character states, but she applied the principle of parsimony as in the simple example above. The most parsimonious mappings for four aspects of the manakin mating ritual are shown below.
This phylogenetic analysis makes it clear that neither the intricate mating ritual of the club-winded manakin, nor its crazy wing-sound and corresponding morphological oddities, evolved in one fell swoop. By starting at the root of the tree and following the lineage that leads to the club-winged manakin, one can see the history of changes that ultimately produced this boogieing bird:
- as its mating display began to gain behavioral complexity (for example, incorporating a backwards element and side-to-side display), its bones became heavier;
- its bones became even stockier, facilitating the production of simple wing noises that may have begun to evolve earlier;
- its wings became further modified with thicker bone walls and enlarged feather tips;
- finally, its bones became fully solid, the feather tip enlarged into a club-shaped, and the original side-to-side display evolved into a display with a longer hop.
Because many elements of the club-winged manakin’s display are homologous to display elements in other manakins, Kim was able to use phylogenetics to untangle how different pieces of the display were assembled over evolutionary time.
It’s noteworthy that, as revealed by Kim’s reconstruction, bone structure, feather morphology, and mating behavior evolved in concert, with changes in one followed by changes in the others.