EVOLUTION AND DEVELOPMENT I: SIZE AND SHAPE
First, some general background to the study of development and evolution.
Evolution of organisms involves a change in the developmental program,
a change in a series of developmental processes. We often refer to evolution
as "descent with modification" and the modification we often
notice first is the overall appearance of the organism. This appearance
is a result of the development of the organism, thus evolution is intricately
involved with development.
Embryology played a major role in evolutionary theory in the 19th century,
but was largely ignored in the 20th. Development never really became part
of the modern synthesis. Some argue that this is due to the lack of communication
between geneticists and developmental biologists. The geneticists
were concerned with the rules of transmission of genetic material between
generations and the developmentalists were concerned with cellular changes
that led to the transformation of an egg into an adult organism. Mutations
in adult phenotype were readily available for the study of genetics, but
there were precious few "developmental mutants" that bridged
the gap between development and genetics (such mutants were discovered
in growing numbers during the formulation of the "Modern Synthesis",
and many more discovered later).
The general approach is the same as we have taken with the evolution
of other traits: development has a genetic basis, if there is genetic variation
for the developmental program then development can evolve. We will first
take a descriptive approach to evolution and development and next lecture
look at some of the more genetic and cellular mechanisms of development.
Early embryologists noticed similarities between ontogeny (the
development of an organism) and phylogeny (ancestor descendant relationships
in a group). The common phrase "ontogeny recapitulates phylogeny"
was put forward by Haeckel as his biogenetic law (see fig. 21.3,
pg. 588). Haeckel held that descendants, during their ontogeny, passed
through stages that resembled the adults of their ancestors. Before this,
Cuvier (1812) held that there were four major classes of organisms:
vertebrates, mollusks, articulates and radiates. Cuvier noticed that there
was nothing in the ontogeny of a vertebrate that resembled the adult stages
of, say, a mollusk. This is because evolution is a bush or a tree
not a "ladder" of the great chain of beings. This "branch-like"
pattern to phylogeny was apparent to Haeckel, but he still claimed there
von Baer made observations about ontogeny and phylogeny that
seem obvious to us today, but they are important in development and evolution
as they run counter to recapitulation: 1) more general
characters appear early in development, 2) less general forms
develop from the more general forms, 3) embryos do not
pass through other forms they diverge from them, 4) embryos
of higher forms only resemble embryos of other forms (human, calf,
chick and fish look similar at embryo stage but diverge quickly). See section
17.8.2, and fig. 17.11, pgs. 478-479.
Putting these two views together, we see that there can be a sort of
recapitulation within a lineage (i.e., within an evolutionary sequence
of ontogenies) but there are many examples that refute the notion that
phylogeny is reviewed during ontogeny.
First efforts to place development and evolution in a quantitative,
descriptive context were provided by d'Arcy Thompson in On Growth and
Form. Using simple rules of geometric transformation he showed that
one could obtain the varied forms of organisms by "warping" or
"bending" the relative positions of their body parts (see fig.
21.10, pg. 599). These types of diagrams are helpful in identifying what
changes of form have taken place, but they do not identify how developmental
mechanisms have evolved (the same criticism might be leveled towards
Raup's computer snails (see figures 13.7 and 13.8, pgs. 356-357),
but mechanism was not the intention of these approaches).
One thing Thompson and Raup's diagrams did contribute was to focus attention
on the notion of size and shape. These two very simple words are
deceptively complex in the context of the evolution of development. A general
paleontological pattern is Cope's rule which states that the body
sizes of species in a lineage of organisms tend to get bigger through
time. Horse evolution is a classic example. But what happens when you get
bigger? In most cases body parts do not grow at the same rate, thus we
Allometric growth is the differential rates of growth of two measurable traits of an organism (often it is described as size-correlated changes in shape). It is quantified as y = bxa where x is the measure of one trait, b is a constant, a is the allometric coefficient and y is the other trait. In this form it describes a logarithmic relationship. It can be made into a linear relationship by taking the logs of the values measured for each trait (or by plotting on log x log graph paper):
log y = log b + a log x. This is the equation for a strait line with
a being the slope of the line. When a<1 we have negative allometry
which means that as x gets bigger, y gets bigger at a smaller rate.
When a >1 we have positive allometry which means that as x gets
bigger, y gets bigger at a faster rate. When a=1 we have isometry
(or isometric growth) which means that there is no change in shape (i.e.,
the relative sizes of body parts) during growth. See fig. 21.9, pg. 597.
We can describe different kinds of allometry: 1) interspecific
allometry where traits of individuals of the same age (usually adults)
are compared between different species, 2) intraspecific
allometry where a) traits of individuals of all ages are compared within
a species (also called ontogenetic allometry), or b) traits of individuals
of the same age are compared within a species (also called static
Some examples: interspecific=the Irish elk example (more below), intraspecific
(static)=measurements of body height and arm length in class, intraspecific
(ontogenetic)=measurements of body height and arm length with my daughter's
day-care measurements included. See figure demonstrating ontogenetic and
interspecific allometry of brain and body weight in the same graph.
Intraspecific allometry just describes growth, and alone is not an evolutionary
comparison. It is of interest that the allometric coefficient of Bio 48
males and females is ~ 1.0, but if the toddler data are included the allometric
coefficient goes up to ~ 1.3. This means that as adults we have about the
same proportions (a=1) but as we grow from infant to adult, our arms get
proportionally longer (a=1.3).
Allometry is useful in describing the evolution of size and shape. Different
species attain different morphologies by virtue of different timing
of various developmental processes. This change in timing is called heterochrony.
Figures 21.5 - 21.8 and table 21.1, pgs. 590-594 review some of the typical
examples of heterochrony. Using the figure below, we can group these into
two general classes: in figs B and C the ancestor (dotted) and descendant
(solid, but hard to see in C) have the same slope but the descendant
stops growing (=adult) at a different time; in figure D and E, the
descendant grows for the same amount of time (in these cases same amount
of x but different amount of y) but at a different slope. Both are
heterochronic changes because some aspect of timing (relative or
absolute) has changed in evolution.
Notice that each axis of these graphs include both a measurement
component and a time component simply because growth by definition
is both a temporal and a dimensional phenomenon. Note that only one
of these examples of different growth plans (graph B) demonstrates "ontogeny
recapitulates phylogeny": hypermorphosis. Therefore, shape changes
can be observed as 1) changes in the slope of an allometric relationship,
or 2) changes in the y intercept of an allometric relationship.
Recalling high school algebra, a change in the slope will change
the intercept, but the intercept can be changed without changing the slope
(keep the line parallel and move it up or down). All of these changes
result in change in shape. Even with the same slope but different
intercepts the relative sizes of x and y will be different so there will
be a change in shape. The only case where there is a change in size with
no change in shape is when the allometric slope = 1.0 and
growth continues (or retards) relative to the ancestor.
Classic examples of allometry are neoteny in human evolution:
as adults we look like the juvenile stages of chimps; and neoteny in salamanders:
the adult of descendant retains gills (a juvenile morphology in the ancestor).
Peramorphosis in the evolution of deer: the "Irish elk"
(actually a deer) has phenomenally large antlers and are "disproportionately"
large because there is an allometric relationship between body size and
antler size. In fact the Irish elk falls right on the line of allometry
for other species in the family (interspecific allometry). Thus, the antlers
are larger than usual, but they follow precisely the developmental program
that seems to be a part of its phylogenetic group. Previous adaptive (and
maladaptive) stories had been told about these huge antlers and how they
probably drove the elk to extinction, thus a challenge for "adaptive"
evolution, but the allometry shows that they are not really "abnormal"
(probably went extinct due to climatic changes and hunting). See discussion
on pg. 356-358 of adaptive, vs. non-adaptive explanations of morphology.
Allometry is also important in the context of the criticisms to the "adaptationist program". If you looked at a Titanothere with its bizarre horns pointing out of its snout, you might say "what are those things for" as if they evolved for some function. They may not be "for" anything but simply the result of a positive allometric relationship between body size and horn size during evolution. Now the question becomes: what causes Cope's rule? Since allometry is so common, changes in size will produce changes in shape.