J. theor. Biol. 196, 197-209 (1999)
A Quantitative Model of the Simpson-Baldwin Effect
LAUREN W. ANCEL
Department of Biological Sciences, Stanford University, Stanford, CA 94305, U.S.A.

G. G. Simpson was the first to explain the Baldwin Effect completely in terms of the theory of natural selection. A genetic version of a seemingly non-hereditary adaptation may arise when natural selection acts on the likelihood of having an adaptive trait not just on the trait itself. We present a quantitative model of the Simpson-Baldwin Effect. Organisms in the model have mutable ranges of phenotypic plasticity. The distribution of phenotypes in a population depends largely on the extent of environmental stochasticity. When the environment undergoes intermediate rates of fluctuation, the Simpson-Baldwin effect arises through the interaction of natural selection and mutation on norms of reaction. In a highly volatile environment, organisms benefit from plasticity, and consequently do not experience a Simpson-Baldwin channeling of phenotypic possibility.

Introduction

Just over a century ago J. Mark Baldwin postulated a conciliatory alternative to both Neo-Lamarckism and Neo-Darwinism. His influential treatise, A New Factor in Evolution, introduced organic selection as the evolution of hereditary adaptations by way of non-hereditary innovations (Baldwin, 1896). Organisms that make non-hereditary physical or behavioral modifications to survive environmental stresses will have better representation in future generations than less versatile organisms. Through natural selection then, the capacity for such adaptation along with the beneficial acquired traits will become universal. Although it initially received much attention (Morgan, 1896; Osborn, 1896), this line of theory faded with the rediscovery of Mendel and the subsequent development of the modern evolutionary synthesis.

In 1953, G. G. Simpson reintroduced the Baldwin Effect as genetical reinforcement of advantageous but initially non-hereditary traits. Until Simpson, the Baldwin Effect was seen as an evolutionary process distinct from natural selection. Simpson's critical insight was that the Baldwin Effect can be explained by the theory of natural selection. A genetic version of a seemingly non-hereditary adaptation may arise when natural selection acts on the likelihood of having an adaptive trait, not just on the trait itself.

Inspired by Schmalhausen (1949), Simpson outlined the following three conditions under which natural selection may produce a Baldwin Effect.

[S1] The ability to acquire a character has in itself a genetical basis.

[S2] ... selection for the ability to acquire an adaptive character so narrowed the developmental range that the character would usually or invariably appear.

[S3] There is ... a certain balance between lability and stability of developmental ranges and norms in evolution (Simpson, 1953).

This rendering generalizes Baldwin's original focus on traits learned within an individual life. A genetically determined norm of reaction is, broadly speaking, a range of phenotypic possibility. The actual trait or traits that an individual possesses may be acquired via learning throughout life, or they may be determined through physical interaction with the environment during early development.

Simpson's ideas brought the Baldwin Effect back to the forefront of evolutionary thought. In particular, it inspired the Hinton and Nolan legacy of models in which learning guides the genetical evolutionary trajectory (Hinton & Nolan, 1980). For various modes of environmental interaction and response, learning is tantamount to a search of phenotype space, and the population evolves towards those phenotypes that find higher fitness faster. Learning initially finds optimal phenotypes, and genetical selection follows with reinforcement.

In this paper, we present a quantitative model of Simpson's simple formulation of the Baldwin Effect. Individuals have genetically determined degrees of phenotypic plasticity which undergo mutation and selection. The tension between intrinsic costs of phenotypic plasticity and the benefits of flexibility in a dynamic environment steers evolution. Sometimes the balance of these forces narrows individual norms of reaction. This is what we call the Simpson-Baldwin effect. On the other hand, in volatile environments we observe the evolution of greater plasticity from ancestral inflexibility. Through the model, we explore the relationship between environmental fluctuation, fitness regimes, mutation rates and the occurrence of the Simpson-Baldwin effect.

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References


Baldwin Effect / Genotype-Phenotype Map