“What use is half a wing?”

As a rebuttal to Darwin’s (1859) explanation of the origin and diversification of life, St. George Jackson Mivart (1871) posed a challenge. With this simple question, Mivart challenged Darwin to explain the adaptive role of intermediate forms within an evolutionary continuum, prompting Darwin to expand on the concept of functional shifts within structural continuity (Gould 1985).

Evolution wing theories

A response to Mivart’s question is that if the wing of a flying bird is a product of small, gradual structural changes, these transitional forms must have had some function during the evolution of powered flight.

But how do we assign and test a hypothetical function or propose an adaptive value for a transitional form that we find preserved only in the fossil record?

This dilemma has spurred volumes of publications on the origin of flight, which have characteristically centered around two well-entrenched schools of thought.

The first, known as the arboreal theory, proposes that flight evolved from tree-dwelling ancestors and predicts a gliding intermediate phase (Marsh 1880, Bock 1965, 1985, Feduccia 1996, 2005, Xu et al. 2003).

The other, known as the cursorial theory, considers ancestral birds to be terrestrial dinosaurs that developed powered flight “from the ground up” (Williston 1879, Nopsca 1907, Ostrom 1979, Caple et al. 1983, Chatterjee 1997).

Historical theories

However, none of the historical theories regarding the evolution of avian flight adequately explains the functional value of a transitional wing to a protobird!

Animals locomote to acquire food, locate mates, migrate, defend a territory, seek shelter, and escape predators.

Locomotor performance during predator avoidance is relevant to all age groups, but the period from hatching to locomotor proficiency is an especially vulnerable life stage.

Susceptibility to predation is amplified for birds that hatch on the ground, requiring that the chicks be sufficiently cryptic or competent to flee, or both.

 

See a video here :

https://www.youtube.com/watch?v=JMuzlEQz3uo

Read more here : https://academic.oup.com/bioscience/article/56/5/437/234719

What Use Is Half a Wing in the Ecology and Evolution of Birds?
Kenneth P. Dial Ross J. Randall Terry R. Dial
BioScience, Volume 56, Issue 5, 1 May 2006, Pages 437–445
Published: 01 May 2006