>3. Body plans are extremely stable. Early stages of individual development
>are remarkably resistant to evolutionary change, either because they are
>resistant to mutations or have extremely efficient repair mechanisms. Only
>later appearing, superficial features of the animal, such as coloration, are
>subject to adaptive change. Evolution thus does not change the body plan and
>other early-appearing features of the embryo. Phyletic body plans have not
>changed in more than 500 million years, with the possible exception of some
>reformulation of sea urchins' body plans.
Larvae which are free-living face assorted selective pressures and certain
changes are quite common. For example, many gastropod lineages have gone
from planktotrophic larvae to lecithotropy or direct development throughout
geologic time. In bivalves, evolution of small adult size is strongly
correlated with a change from planktotrophic larvae to brooding. Walter
Garstang's poems on larval forms give several examples.
>
>4. Development proceeds in characteristic time sequences and directions.
> This is the top-down direction of development. According to basic laws of
>development formulated by von Baer almost two centuries ago, (a) the general
>features, which the embryo shares in common with all members of its phylum,
>appear before more specific features; and (b) that specific features emerge
>out of general ones, These constitute two of the most fundamental principles
>of individual development.
>
Again, selection on the larvae must be taken into consideration. Also,
some species are distinctive from very early stages of their growth.
>
>5. The shape of the developmental life span is curvilinear. The entire life
>span of individual animals is a manifestation of development. All organisms
>start small and morphologically simple at conception; rise rapidly through
>the prenatal and juve nile stages; grow large, robust, and complex; reach a
>rounded maximum on many vari ables in maturity; decline in old age, and
>eventually die. The rise and subsequent decline is an invariant
>characteristic of lifelong individual development.
>
>There is also an unvarying succession of changes in ancestral lineages of
>higher complex animals, starting with a few, small, insignificant animals,
>that eventually increase in size, complexity, population density, and on many
>other dimensions; reach a rounded maximum, and then decline to fewer,
>smaller, less robust groups. This orderly sequence is isomorphic to life-long
>development in individual organisms.
>
The increase in size ("Cope's Law") has, in many cases, been shown to be an
artifact of our perception. Dan Miller (working on muricid snails) and
Clay Kelly (working on planktic forams) have found that the actual pattern
is an increase in variation. A given kind of organism will have a lower
size limit. If the ancestral species was small, random variation will
generate an increase in average size.
>6. Eventual decline. All individual organisms show signs of senescence if
>they live long enough. Phyletic lineages of higher, complex animals begin to
>show signs of decline and aging after they reach and pass their maximum
>growth and development, and will eventually die if they do not first become
>extinct.
A lot of lineages seem to be doing just fine, and others seem to plod along
without diversifying or declining (many "living fossils"). Still others
fluctuate widely but survive. Additionally, the "decline" of many lineages
is a taxonomic artifact. If one lineage evolves from another and the old
version disappears while the new one thrives, the old lineage has not
declined in a cladistic sense.