Lesson 5:
Life's History: The Great Adventure
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5.2
Clues From Development
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Figure 5.2.1
A comparison of vertebrate embryos at the same stage of
development.
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"It is notorious that the wings of birds and bats, and the legs of horses and other
quadrupeds, are undistinguishable at an early embryonic period, and that they become
differentiated by insensibly fine steps." Thus writes Darwin (6th ed., p. 226),
using this observation regarding individual development of organisms in defense of
gradual evolution along ancestral lines.
Individual development of an organism is called ontogeny. Its line of ancestors
defines its phylogeny. Is there a connection between the two? Does ontogeny
ndeed provide clues for ancestry, as claimed by Darwin?
The most successful group of organisms on this planet, when species diversity
is used as criterion is the insects. They have invaded every nook and cranny
in the terrestrial realm, including the freshwater bodies. We might think that
beetles are abundant, but their larvae "grubs" are even more abundant, except
that they are hidden in the ground. Many other multicellular organisms have
immature stages that look very different from mature ones. Besides the larvae
of insects, the tadpoles of frogs are perhaps the best known examples, to us land
dwellers.
In the ocean, the larvae of nearshore organisms populate the coastal ocean as microscopic
"meroplankton". The meroplankton has larvae from corals, sea stars, polychaete worms,
barnacles and crabs, mussels and snails, and many others. It is from such larvae
(which look nothing like the adult stages) that one can tell quite readily that
barnacles and crabs belong to the same general tribe of animals. The same is true
for mussels and snails.
In terms of function, the large differences between larval stage and mature
animal are readily appreciated. There are two strategies involved, addressing two
different needs. One is the search for resources and the other is
dispersion. In insects with a distinct larval development, the larvae
are responsible for feeding and putting on weight, while the main
task for the winged stages is to seek mates and look after dispersal.
(Additional food input is used for maintenance and to make more
eggs.) In the coastal ocean, the adult sessile (permanently attached)
stages are responsible for growth and reproduction, while the main
function of the larvae is dispersal, that is, finding good places to
settle. (Of course, the larvae also feed, to maintain their
metabolism and to get a good head start when settling.) In frogs,
both larvae and mature animals feed and grow, but they exploit
different habitats, one within the water, the other at its edge or
outside of it.
The fact that the larvae of many distantly related organisms resemble each other
more closely than do the mature animals (e.g., barnacles and crabs; tunicates,
tadpoles and fish), in addition to observations regarding vertebrate
embryos (quoted above) drew attention to the possibility that there
are clues, within development, to relatedness and hence evolution.
This idea, fundamentally sound, gave rise to the catchy phrase
"ontogeny recapitulates phylogeny" which implies that
development repeats aspects of the stages of evolution within a
single organism. When taken literally this is nonsense; neither
human embryos nor those of poultry have a fish stage. However, when
taken as an invitation to look for clues of the course of evolution
it does make sense. Human embryos, chick embryos and fish embryos have much in
common in their early developmental stages as was seen in 5.2.1.
The German biologist Ernst Haeckel (1834-1919), more than anyone else, insisted on the
importance of ontogeny as a clue to the "Tree Of Life". This is his best-known
contribution to the life sciences. (He also was the world's expert on radiolarians,
calcareous sponges and coelenterates in his time. A powerful writer and lecturer,
he urged the theory of evolution on his colleagues and the public.) In his
view, the reason that ontogeny proceeds the way it does is that each
organism remembers its evolution. Early development is driven by
ancient programs; the earlier the development, the more ancient the program.
Whether this is true or not for any given organism cannot be decided by disputation. The
answer will come from the study of the genetic code (a concept not
available at Haeckel's time). Inasmuch as evolutionary pressures act
on larvae as well as on adults (larvae have to find food and escape
predation) we should expect to find modifications in the genetic code
to improve adaptations of larvae. Embryos have other problems: they
must convince their parent that they need more resources. The
corresponding signals presumably would be different for sharks, birds
and mammals, leading to a divergence of developmental programs.
Because of these caveats, the concept that "ontogeny recapitulates phylogeny"
cannot stand as a general rule.
In any case, the patterns of development in animals, plants and fungi have
been widely used to generate classifications that are thought to reflect relationship in
terms of common ancestry. Clearly, whenever two different organisms
have complex but similar developmental programs, it must be assumed
that they inherited their programs from a common ancestor, rather
than that the programs arose independently, by chance.
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