and functions; for example, physical constraints on evolution stem from
properties or laws of the physical world that do not depend on properties
of living organisms. For example, hydrodynamic forces have a profound
influence on the shape of aquatic organisms. The functions relating pressure drag and friction drag to velocity lead to an ideal length/width ratio of
4.7 for a streamlined body (Hildebrand 1974). Although this ratio is equally
valid for a swimming fish and an inert object dragged through the water, it
is clearly no accident that streamlined body forms of this aspect ratio have
evolved repeatedly in fast-swimming organisms (fish, penguins, dolphins,
and ichthyosaurs; Carroll 1997). The study of such physical constraints is an
important component of the discipline of functional morphology.
Phylogenetic constraints, on the other hand, result from developmental
and historical factors. They stem from the gradualistic principles that underlie the generation of variation in evolution: recombination and mutation
can only explore that small portion of adaptive space that is adjacent to a
particular species’ current position. The fact that insects have six legs is not
due to any physical constraint (as the existence of quadrupeds, spiders,
crabs, and millipedes clearly demonstrates) but to the strong canalization
of the developmental pathways that generate adult insect forms. Although
small mutations can have drastic phenotypic effects (e.g., mutations of the
Antennipedia gene lead to flies with legs in place of their antennae), the
chance of such macromutational changes leading to increased fitness is vanishingly small. Another example of a phylogenetic constraint is the genetic
code itself: there is no reason in principle for the nucleotide-to-amino acid
code to be shared by all life on this planet but, practically speaking, any
mutant with a deviant code would be eliminated very early in development.
Such constraints are in some sense arbitrary results of the particular evolutionary history of a species but nonetheless create extremely powerful
limitations on the viable genotypes available by mutational changes from
a given parental lineage.
A major goal of this chapter is to explore the role of physical, physiological, and cognitive constraints in shaping vertebrate communication
systems over time. Because many relevant constraints are shared by all vertebrates (and physical constraints are shared by all organisms), sampling a
wide range of species may allow us to uncover and understand such constraints, and outline their influence on the shape of adaptive space, with
some accuracy. Thus, we will cast a broad net, considering the communication systems of most terrestrial vertebrates at least briefly, although focusing on tetrapods, and birds and mammals in particular (see Ryan and Kime,
Chapter 5, for a fuller discussion of anuran communication, Bass and Clark,
Chapter 2, for discussion of underwater organisms, and Tyack and Miller,
in press, for more on marine mammals). In Section 2, we will outline a set
of relatively well-understood constraints that follow from the physics of
sound, relating body size to signal frequencies. Using the comparative
method, we show how these constraints have led to the evolution of cheap,
3. Unpacking “Honesty”
69
properties or laws of the physical world that do not depend on properties
of living organisms. For example, hydrodynamic forces have a profound
influence on the shape of aquatic organisms. The functions relating pressure drag and friction drag to velocity lead to an ideal length/width ratio of
4.7 for a streamlined body (Hildebrand 1974). Although this ratio is equally
valid for a swimming fish and an inert object dragged through the water, it
is clearly no accident that streamlined body forms of this aspect ratio have
evolved repeatedly in fast-swimming organisms (fish, penguins, dolphins,
and ichthyosaurs; Carroll 1997). The study of such physical constraints is an
important component of the discipline of functional morphology.
Phylogenetic constraints, on the other hand, result from developmental
and historical factors. They stem from the gradualistic principles that underlie the generation of variation in evolution: recombination and mutation
can only explore that small portion of adaptive space that is adjacent to a
particular species’ current position. The fact that insects have six legs is not
due to any physical constraint (as the existence of quadrupeds, spiders,
crabs, and millipedes clearly demonstrates) but to the strong canalization
of the developmental pathways that generate adult insect forms. Although
small mutations can have drastic phenotypic effects (e.g., mutations of the
Antennipedia gene lead to flies with legs in place of their antennae), the
chance of such macromutational changes leading to increased fitness is vanishingly small. Another example of a phylogenetic constraint is the genetic
code itself: there is no reason in principle for the nucleotide-to-amino acid
code to be shared by all life on this planet but, practically speaking, any
mutant with a deviant code would be eliminated very early in development.
Such constraints are in some sense arbitrary results of the particular evolutionary history of a species but nonetheless create extremely powerful
limitations on the viable genotypes available by mutational changes from
a given parental lineage.
A major goal of this chapter is to explore the role of physical, physiological, and cognitive constraints in shaping vertebrate communication
systems over time. Because many relevant constraints are shared by all vertebrates (and physical constraints are shared by all organisms), sampling a
wide range of species may allow us to uncover and understand such constraints, and outline their influence on the shape of adaptive space, with
some accuracy. Thus, we will cast a broad net, considering the communication systems of most terrestrial vertebrates at least briefly, although focusing on tetrapods, and birds and mammals in particular (see Ryan and Kime,
Chapter 5, for a fuller discussion of anuran communication, Bass and Clark,
Chapter 2, for discussion of underwater organisms, and Tyack and Miller,
in press, for more on marine mammals). In Section 2, we will outline a set
of relatively well-understood constraints that follow from the physics of
sound, relating body size to signal frequencies. Using the comparative
method, we show how these constraints have led to the evolution of cheap,
3. Unpacking “Honesty”
69