Recapitulation is a bold and influential idea that is particularly associated with Ernst Haeckel though many other biologists also supported it in the nineteenth and early twentieth centuries.
According to the theory of recapitulation, the stages of an organism’s development correspond to the species’ phylogenetic history- in a phrase, “ontogeny recapitulates phylogeny.” Each stage in development corresponds to (that is, “recapitulates”) an ancestral stage in the evolutionary history of the species.
The transitory appearance of structures resembling gill slits in the development of humans, and other mammals, is a striking example. Mammals evolved from an ancestral fish stage and their embryonic gill slits recapitulate the piscine ancestry.
Another example, often quoted in the nineteenth century, is seen in the tail shapes of fish. During the development of an individual, evolutionarily advanced fish species, such as the flatfish Pleuronectes, the tail has a diphycercal stage in the larva.
It then develops through a heterocercal stage, to the homocercal form of the adult. However, not all fish have homocercal tails in the adult. Indeed fish species can be found with all three kinds of tail in the adult. The lungfish, sturgeon, and salmon are the examples. The lungfish is thought to most resemble an early fish, the sturgeon to be a later stage, and the salmon to be the most recently evolved form. Thus evolution has proceeded by adding on successive new stages to the end of development.
We can symbolise the diphycercal, heterocercal, and homocercal tails by a, b, and c respectively. The development of the early fish advanced to stage A and then stopped. Then, in evolution, a new stage was added on to the end- the development of the fish at the second stage was A → B. The final type of development was A → B → C. Gould named this mode of evolution terminal addition (Figure 2a).
When evolution proceeds by terminal addition, recapitulation is the result. An individual at the final evolutionary stage in Figure 2a grows up through stages a, b, and c. recapitulating the evolutionary history of the ancestral adult forms. However, evolution does not always proceed by terminal additions. We can distinguish two kinds of exception. One is that new, or modified, characters can be intruded at earlier developmental stages (Figure 2b).
Many specialized larval forms are not recapitulated ancestral stages (for example, the zoea of crabs, the Muller’s larva of echinoderms, and the caterpillar of Lepidoptera). They probably evolved by modification of the larva, rather than by adding on a new stage in the adult.
The second kind of exception arises when the members of a species evolve to reproduce at an earlier developmental stage. We need to distinguish the rate of reproductive development from the rate of somatic development. Somatic development proceeds through a series of stages, from egg to adult.
If the organism becomes reproductively mature at an earlier stage, then its development will not fully recapitulate its ancestry. Its ancestral adult form has been lost. Reproduction in what was ancestrally a juvenile form is called pedomorphosis. Pedomorphosis can arise in two ways.
One is neoteny, where somatic development slows down in absolute time, while reproduction development proceeds at the same rate. The other is progenesis, where reproductive development accelerates while somatic development proceeds at a constant rate.
Among modern species, the classic example of neoteny is the Mexican axolotl, Ambystoma mexicanum. The axolotl is an aquatic salamander. Most salamanders have an aquatic larval stage that breathes through gills; the larva later emerges from the water as a metamorphosed terrestrial adult form, with lungs instead of gills.
The Mexican axolotl, however, remains in the water all its life and retains its external gills for respiration. It reproduces while it has this juvenile morphology. However, a Mexican axolotl can be made to grow up into a conventional adult salamander by a simple treatment (it can be done, for instance, by injection of thyroid extract).
This strongly suggests that the timing of reproduction has moved earlier in development during the axolotl’s evolution. Otherwise there would be no reason for it to possess all the unexpressed adaptive information of the terrestrial adult. So the Mexican axolotl is pedomorphic abut is it neotenous or progenetic? Its age of breeding (and the body size at which it breeds) is not abnormally early (or small) for a salamander. Its time of reproduction has therefore probably stayed roughly constant, while somatic development has slowed down. The axolotl is an example of neoteny.
Humans have also been argued to be neotenous. As adults, we are morphologically similar to the juvenile forms of great apes. This pedomorphosis, if it is real (and there is a serious argument that it is not), would be neotenous rather than progenetic because our age of breeding has not shifted earlier relative to other apes.
Our age of first breeding is actually later than other apes. Our somatic development has not simply slowed down while reproductive development has stayed the same. What might have happened was that our somatic development slowed down even more than our reproductive development.
Changes are made only in the adult, and new stages are added on to the end of the existing developmental sequence. Through the 1920s, biologists come to accept a broader view. Evolution does often proceed by terminal addition, and recapitulation results.
But other developmental stages can also be modified, and the timing of reproductive and somatic development may be altered in any way a some of which result in recapitulation, and others which result in pedomorphosis.
Categories of Heterochrony:
1. Accelerated:
i. Reproductive Organs- Unchanged
ii. Name of Evolutionary Result- Acceleration
iii. Morphological Process- Recapitulation (by acceleration).
2. Unchanged:
i. Reproductive Organs- Accelerated
ii. Name of Evolutionary Result- Progenesis
iii. Morphological Process- Pedomorphosis (by truncation).
3. Retarded:
i. Reproductive Organs- Unchanged
ii. Name of Evolutionary Result- Neoteny
iii. Morphological Process- Pedomorphosis (by retardation).
4. Unchanged:
i. Reproductive Organs- Retarded
ii. Name of Evolutionary Result- Hypermorphosis
iii. Morphological Process- Recapitulation (by prolongation).
The changes that we have been considering in the relative rate of somatic and reproductive development are one example of an important general concept- heterochrony. Heterochrony refers to all cases in which the timing or rate of one developmental process in the body changes during evolution relative to the rate of another developmental process. In progenesis, neoteny, and so on, the rate of reproductive development is speed up or slowed down relative to the rate of somatic development.
Heterochrony is a more general concept, however. It also refers to changes in the development of one somatic cell line relative to another. Consider, for example, a D’ Arcy Thompson transformation (Figure 4). D’Arcy Thompson found that related species superficially looking very different could in some cases be represented as simple Cartesian transformations of one another.
We met the most thoroughly worked out modern example. With some simplification, the axes on the fish grids in Figure 4 can be thought of as growth gradients. The evolutionary change between the species would then have been produced by a genetic change in the rates of growth in different parts of the fish’s body.
One general point is that evolutionary changes between species may be simpler than we might at first think. If we looked at, for example, Scarus and Pomacanthus without the grids of Figure 4 we might think that an evolutionary change from one into the other would be at least moderately complicated.
The interest of D’ Arcy Thompson’s diagrams is then to show that shape changes could have been produced by simple regulatory changes in growth gradients. The more specific point here is that changes in the growth gradients of different parts of the body are further examples of heterochrony. Evolutionary changes in morphology are often produced by changes in the relative rates of different developmental processes- that is by heterochrony.
Regulatory Gene and Structural Genes:
Biologists distinguish between regulatory genes and structural genes.
Structural genes code for enzymes, building block proteins, and transport and defensive proteins. Regulatory genes code for molecules that regulate the expression of other genes (whether structural or regulatory). The distinction is imperfect, but can be used to make a point about evolution.
Human bodies have been redesigned for upright walking, human jaws have become shorter and weaker, and human brains have expanded, and we have acquired the use of language. In human evolution, a large phenotypic change appears to have been produced by a small genetic change.
King and Wilson hypothesised that most of the genetic changes of human evolution were in regulatory genes. A small change in gene regulation might achieve a large phenotypic effect. We shall not know what genetic changes occurred in human evolution until we have (and understand) the genome sequences for chimpanzees and some other apes, as well as for human beings.
Genes that Regulate Development:
A long list of genes that operate during development is now known, and the list is rapidly expanding. The genes fall into two main categories- genes that code for transcription factors and genes that code for signaling proteins.
Transcription factors are molecules that bind enhancers. An enhancer is a stretch of DNA that can switch on a specific gene. Signaling proteins function in the cell’s control pathways for switching specific genes on and off. For instance, a receptor protein in the cell membrane might change shape when bound by a hormone. The shape change might trigger further molecular changes in the cell, ultimately leading to the release of a transcription factor that switches on a specific gene.
The protein in the cell membrane, or any other problem in the chain of reactions, would be an example of a signaling protein. The Hox genes, for example, as well as such genes in fruitflies as distal-less, eyeless, and engrailed all code for transcription factors.
However, other developmental genes, such as the genes in fruitflies called hedgehog, notch, and wingless, are signaling proteins, and most of the points of principle that we look at for transcription factors would also apply for signaling proteins.
The genes that regulate development are best understood in two species, the mouse and the fruitfly. However, geneticists have looked for the same genes in other species and their findings have led to an important generalisation.
All animals seem to use much the same set of genes to control development. For example, the Hox genes were first studied in fruitflies. After the genes were cloned it was possible to look for them in other species too, and they were duly found in every other animal taxon.
The Hox genes have similar functions in all animals. They act as region-specific selector genes. The basic map coordinates of the early embryo are set out by another set of genes. Then, during development, specific sets of genes are switched on to cause the correct structures to develop in each region of the body.
The genes for building a head have to be switched on at the top of the body, for example. Different Hox genes are expressed in different body regions, and act to switch on other genes that code for appropriate structures. The Hox genes mediate between the basic body map information and the genes that code for the structures in each body region.
The finding that all animals use much the same set of developmental genes might not have been predicted. The main groups of animals at the Protostoma and Deuterostoma were initially defined by basic differences in how the animals develop. In the protostomes, cleavage in the egg is spiral; in the deuterostomes it is radial.
In protostomes the embryonic structure called the blastopore develops into the mouth; in deuterostomes the blastopore develops into the anus. And so on. It might have been expected that these deep differences in development would reflect different genes regulating development. But in fact the same set of genes is at work in both taxa. The genes that regulate development presumably evolved once, when animals with development first originated, and has been conserved ever since.