This Week's Citation Classic

One of the first science books I ever read I bought at a yard sale in my early teens. Mixed among various Sidney Sheldons and Jackie Collins was a dog-eared copy of David Lack's Darwin's Finches. Up to that point, evolutionary biology was not covered in any of my science classes in junior high, nor was it discussed later when I was in high school. Focus, instead, was largely directed to anatomy and physiology, particularly of humans. Lack's book made me aware of a vast world of biology outside the strict confines of what my instructors considered "relevant." Thus, in his honor, I submit as this week's citation classic:

Lack, D. (1947) The significance of clutch size I-II. Ibis, 89, 302-352. (Not available online).

Here Lack applied evolutionary theory to the question of "Why don't birds lay more eggs?" At the time, many thinkers (e.g. Wynne-Edwards) suggested that animals limited the number of offspring for the good of the species. As Lack later wrote in "The Significance of Litter Size,"

"The popular view, repeated in explicit and implicit forms in many text-books, is that the reproductive rate of each species has been adjusted by natural selection so as to equal the total mortality, thus keeping the population balanced. It is claimed that such an arrangement has survival value through preventing over-population."

Lack's key insight was that clutch size is ultimately determined by the number of offspring parents can provide with food, not the number of eggs the mother can lay, or by any consideration of the "good of the species." This applies not just in the course of a single year, but over the parent's lifetime. As a considerable number of studies have shown, birds can feed more young than eggs they lay in a single season. Lack realized that natural selection will act to maximise lifetime reproductive success, not reproduction per brood. If birds maximised reproduction per brood, the energy expenditures required would limit their ability to survive harsh winters and provision future clutches. This hypothesis can be generalised to include, with some exceptions, all organisms that provide parental care to offspring.

Interesting evidence supporting Lack's hypothesis comes from the observation that clutch size and latitude are positively correlated. Summer day length increases with latitude providing birds with more time to find food to provision clutches thus increasing the number of offspring they can rear (however, this is of course dependent upon resource availability in the habitat).

The paper is important for several reasons. It was one of the first to suggest that life history traits of organisms are evolutionary adaptations. Second, Lack was one of the first to link evolutionary theory with population biology. Third it sharpened the focus of the study of adaptation to dispute the notion that adaptations can provide for "the good of the species", an argument later thoroughly disabused by George Williams.

Lack is also renown for studying Darwin's Finches, the iconic Galapagos birds first observed scientifically by Darwin and later studied to great effect by Peter and Rosemary Grant. Lack's influence is seen widely in the study of life history theory, sex allocation, bird behavior, population biology, speciation, and evolutionary ecology. He was a Fellow of the Royal Society and received their Darwin medal 1972 before passing away March 12, 1973.

Great Egret and chicks in Morro Bay, CA Heron Rookery, photo by Mike Baird, bairdphotos.com