Faculty of Biological Sciences, University of Leeds

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Evolutionary Developmental Biology Lecture 9

Embryology, classification and evolution.

Nielson C & Nørrevang A (1985) The trochlea theory: an example of life cycle Phylogeny In The origin and relationship of Lower Invertebrate Groups. Ed Conway Morris et al. pp28-41. Oxford University Press, Oxford.(OVERHEAD)

So, where are we?

The 34 types or Bauplans that we have today are a subset, possibly a random subset, of at least twice that number of Bauplans that seem to have been generated fairly rapidly after the origin of the metazoa. We can reasonably suppose that these did not arise from 70 or so events leading to the formation of 70 different metazoan types. It is most likely that most of these (except possibly the sponges) were the product of one transition from single celled animal to multicelled animal. It is likely that if the metazoans arose only once there is some sort of relationship between extinct and extant phyla. It is also likely that the different phyla arose due to modifications of a new property of metazoans - their embryology - as exemplified by their larval forms.

So the important questions are: (OVERHEAD)

1. How are phyla related?

2. How are phyla (and the lesser divisions down to species) converted i.e. how is one changed into another?

3. Evolution and development - Is there a link?

If we can answer these we can also answer some other difficult questions a bit nearer home: (OVERHEAD)

1. How does an egg turn into an insect/frog/mouse/man?

2. Why does a particular sort of egg always become a frog?

3. What is the difference between a frog's egg and a hen's?

or What are the similarities between a frog's egg and a hen's.

We look at relationships between animals in terms of similarities. We have already said that the most similar animals are identical twins, then brother and sister etc. We were, strictly speaking, talking about genes, but the effects of genes, phenotype can be substituted. Identical twins are not recognised by their karyotypes but by their appearance. Two cows are probably similar if they look alike. A cow is more like a sheep than it is like a fish. So phenotypic appearance is important i.e. animals are classified by their morphology.

Similarities between extant and extinct creatures are more difficult: we are unlikely to see a modern human foot on a fossil human ancestor: but it is still recognisably a foot, and so are the feet of dinosaurs. The foot is an homologous structure i.e. the same, probably descended from a common ancestral foot. We have said that the wing of a bird is not homologous with the wing of an insect. These do the same thing, but not in the same way: they are analogous. Often a similar feature arises more than once: the wing of a bat is not descended from the wing of a bird: are they analogous? No. Are they homologous No, not quite because one did not descend from the other. They are examples of convergent evolution, nearly homologous because each is homologous with the forelimb of some non-flying ancestor.

Sometimes homologous structures are so modified that it is difficult to see the similarity. What do we do then? When talking about different animal groups I gave you a clue: some animals have similar larvae, i.e. similar embryonic stages.

We now reach a point where we have to define terms. First of all lets look at evolution and development. In fact these terms are quite mixed up in laymen's language. Going to the O.E.D. we find:

(OVERHEAD) Evolution

1. opening out (of roll, bud etc.), appearance (of events etc.) in due succession

2. development (of organism, human society, the Universe, design, argument etc.): origination of species by development from earlier forms, not by special creation.

(OVERHEAD) Development

(of organism, design, argument etc.) Theory of E (that the embryo is not created by fecundation, but developed from a pre-existing form); origination of species by development from earliest forms

So evolution, in non scientific terms, means both the development of an individual and the development of a species. And development covers evolution too. Are these two the same thing? If not how are they related? Why are they considered to be related at all?

Well, earlier, pre-evolutionary views certainly fit well with this idea of evolution as unfolding, like a scroll. There was a widespread view that all human bodies were created fully formed but rolled up, as it were, in the ovaries of Eve. A complementary theory described the homunculus, perfectly formed, rolled up in every sperm. (OVERHEAD)

The other view of evolution, as progressive change through time is usually attributed to Darwin, but he, interestingly, only uses the word once in the Origin of the Species - then it is the last word in the book

There is a grandeur ...evolved (OVERHEAD)

Elsewhere he speaks of descent with modification.

There are perfectly good reasons for distinguishing the two developments, or the two evolutions, one of the organism the other of the species. For instance the eggs of many (but not all) animals are separated into soma and germa. The soma develops into the body of the individual: the germa goes on to form the gametes, future generations.

Modifications in the soma (somatic mutation, teratogens) will not affect the species but will affect the operation of the genetic program during an animal's lifetime: mutations in the germa will not influence the development of the current generation but may influence the future of the species.

These two processes have, of course long been distinguished. The development of an individual from fertilisation to maturity is ontogeny: the development of a species or lineage is phylogeny.

The idea of a relationship between phylogeny and ontogeny is an old one, but was probably crystallised by looking at embryos of different species. These all look suspiciously alike, and the younger the embryos the more similar they look.

This was first noticed by Meckel (1781-1833) who studied human embryos and saw them passing through what he considered to be a hierarchy of forms: fish, reptile, mammal, human. Coincidentally this was the same order that the fossils that were being discovered at the same time occupied in geological strata. Meckel very clearly saw that the same laws covered both 'development' of the individual and 'evolution' of the species. By evolution, of course he meant unfolding - he was pre-Darwinian. Did the developing embryo recapitulate the past of its species? Did it climb up its own family tree?

Von Baer (1792-1876), another good comparative embryologist, took over from there. His problem was that he found structures in higher animals - such as the yolk sac in birds - that were not present in lower animals. Also birds did not, at any stage, have fish tails. Therefore recapitulation as such was out. What was happening was a gradual specialisation: all vertebrate embryos first developed characteristics of the phylum vertebrata, then fishes developed characteristics of the class Pisces, birds of the class Aves, rabbits of the class Mammalia etc. A little later they developed more specific details, like fins or floppy ears.

Now we have a problem. Do the Phyla, which we have defined by going from more similar to less similar have fundamental similarities as well as differences? Von Baer would expect similarity in cow embryos, and in mammalian embryos, but how about an earthworm embryo and a sea urchin. Are they similar?

We have already looked at the ideas of Nielson He noticed similarity between the ciliary feeding structures of adult Rotifers and several types of Spiralian (animals with spiral cleavage, see later) and other protostomian larvae, and the difference between these and deuterostomes. (OVERHEAD). Essentially, remember, protostomes and deuterostomes differ in collecting food upstream or downstream of their cilia. Some years it was proposed that animals might be related via different types of free living, planktonic ancestors, three of which gave rise to creeping descendants of various types.

The early part of this is nothing new: in fact it corresponds with the gastraea theory of Haeckel (1834-1919) which we talked about last week. Haekel reasoned, remember, that all multicellular organisms arose from unicellular organisms, and that this probably happened only once. The different bodyplans of the different phyla arose from a hypothetical, archaetypical Blastaea a hollow ball of cells. We have already talked about this, and said it had to be an animal not a plant, because further progress depends on the fact that not all cells feed - meaningless in plants - and have connections: we identified a likely candidate in the choanoflagellate.

Once settled on the bottom, the Gastraea was also seen by Haekel to be a common ancestral type, although there are gastraea and gastraea.

Not everyone agrees with this: (OVERHEAD)

A giddy little Gastrula, gyrating round and round,

Was thought to show the way we get our enteron profound:

A little whirlpool in its wake maintained by a tasty store,

A pocket sank to lodge it all and left a blastopore.

Invagination surely is a thing of later date-

Procedure speeded up to suit the embryonic state:

The cells as loose irregulars build up the lower grades,

And yield but slowly, step by step, to organised brigades.

Walter Garstang, Larval Forms, Blackwell, Oxford 1951
:Feature Protostome Deuterostome
1. blastopore becomes divided into mouth and anus by fusion of its lips becomes anus: mouth a new hole
2. larva

trochophore:

compound ciliary bands form downstream collection system around mouth

single cilia forming upstream collecting system around mouth
3. nervous system apical organ + paired ventral nerves dorsal longitudinal nerve cord

 

(OVERHEAD) Not all animals will conform to this scheme, but most do.

It has been argued that small eggs and pelagic larvae are nowadays a feature only of rather large marine animals which produce large numbers of eggs. Small animals produce few eggs and protect their brood. But ancestral forms are likely to be small: increase in size is a well known evolutionary trend. (Known as Cope's law or Cope's rule - but is it true? Some of you are writing an essay about this.). Therefore (small) ancestors are not likely to have had pelagic larvae. This, of course, applies now, but not necessarily 600mya. when there were presumably less multicellular predators in the plankton.

We should perhaps look a little more closely at the concept of a larval form. We are quite accustomed to organisms which change their proportions as they grow (kittens, puppies, children OVERHEAD,OVERHEAD - the phenomenon of allometry) but perhaps not so used to the idea of specific adaptations at different stages of life (OVERHEAD,OVERHEAD) perhaps because we do not perceive that mammals have them, although insects and many other animals do.. We do, in fact, have them: but the non self-feeding part of our life cycle, the part where we live in an aquatic environment is passed either in mum. There is in fact no reason why any stage of a life cycle should not develop specialisations and, say feed in a different way or live in a different place. There is also no reason why changes in lifestyle should not be accompanied by changes in shape - metamorphoses. And there is really no difference in kind between a metamorphosis, which occurs suddenly and maturation which occurs rather more slowly, except the time scale. There is also no reason why development shouldn't get out of step with itself to such an extent that one of these juvenile forms should not develop gonads and breed. If that happens, as with the axolotl, a Mexican salamander, the 'adult' stage can be missed out altogether.

This suggests that there must be mechanisms which can change the relative rates of growth during embryology: the next question is what are they?

So we have a working hypothesis on classification which derives three sorts of animalia from larval forms swimming in the sea. Haeckel required that new evolutionary stages be added to the end of the life cycle of ancestral forms. As well as a phylogenetic tree we have also described embryology: the blastula, the gastrula are the same as the blastaea and the gastraea. Abolition of free swimming larval forms (brood protection again) could generate an embryology which resembles our phylogenetic tree, but in which some stages are not free living, so don't really need to work and could be compressed or not finished - in this way some features might become rudimentary like the appendix or the tail in man.

Von Baer extended his studies by demonstrating that all vertebrate embryos had three germ layers and that the outer and inner ones corresponded to the two layers of coelenterates, and Lankester extended the triple layer of cells to all animal phyla except the single celled animals and the two layered coelenterates.

So, the embryo is the ancestor, and all phyla are related.


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