Goals: Learn about heterochrony and proposed relationships between embryonic development (ontogeny) and evolutionary history (phylogeny) proposed by von Baer and Haeckel. Learn to evaluate different hypotheses of heterochrony using phylogenies.
Related Textbook Material: Freeman and Herron (2001) Chapter 17
Lab Manual Questions over this material are in Lab Manual Chapter XVIII
One kind of mutation could occur and be unlikely to seriously harm the phenotype is a mutation that changes the rate at which the body, or a structure in the body, develops. Such a mutation would keep the same basic developmental steps, so that the overall developmental process would be less likely to be harmed, but they would occur either faster or slower than they did in individuals without the mutations. Heterochrony refers to a change in the rate of development of an organism or a structure of that organism. Mutations that cause heterochrony may be more likely to evolve since they can produce new forms that may turn out to have high fitness without disrupting the process of development.
Heterochrony can occur in two ways: either development can be speeded up so that new forms are added at the end of the faster developmental process, or development can be slowed down so that the organism, or structure, does not develop to the final stages it did in the original species. Terminal addition is the name for heterochrony that speeds up development. Paedomorphosis is the name for development that slows down development.
Developmental contingencies, and heterochrony, can result in relationships between the process of embryonic development and evolutionary history. Interestingly, relationships between either organismic diversity or evolutionary history and embryonic development had already been observed, but not clearly explained, long before people knew enough about causes of embryonic development to propose the hypotheses of developmental contingency and heterochrony to explain them. We will first describe the proposed relationships between development and evolution, or organismic diversity, and then look at how they might be explained by developmental contingency and heterochrony. Then we will consider how to test for the phylogenetic patterns predicted by hypotheses of heterochrony.
An embryologist named von Baer, who worked before Darwin, described the first relationship between the embryonic development of organisms and variation among different animal species. He proposed what is now called von Baer's Law. von Baer's Law states that structures that are present early in development are widely distributed among animals, while structures that are present late in development are less widely distributed. In other words, the features that distinguish between different species tend to arise late in development; earlier developmental stages are common to many different species. There are many examples of von Baer's Law. One example is seen in the traits common to vertebrates: all vertebrates develop a cartilage rod down the back called a notochord, and gill pouches in the throat region, early in development -- these are traits of early development and are found in many species. Later in development, however, these traits do not exist in most vertebrates and the traits that are distinctive to the different vertebrates develop. It is actually quite remarkable how similar early embryos of different vertebrate species look. Your textbook has a picture showing this on page 28; you should look at this picture and see the similarities of early embryos of fish, salamanders, tortoises, chickens, pigs, cows, rabbits, and humans, and some pictures of the process through which these species change, embryonically, to develop the distinctive, different traits of their own species.
von Baer's Law is not, in itself, evolutionary; it simply describes how structures develop. One of the goals of evolutionary biology is to explain how such general patterns have evolved. An early attempt at this was made by a biologist named Haeckel. Haeckel proposed what he considered to be a law of evolution (but as we will see, it isn't -- it is true in some cases but not others); he proposed this law as a very catchy phrase: "ontogeny recapitulates phylogeny." So let's see what this catchy phrase means. Ontogeny is another name for embryonic development. "Recapitulate" means to repeat the pattern of something, or restate something. Phylogeny is, of course, evolutionary history. So Haeckel was claiming that embryonic development repeats evolutionary history -- he thought that the stages of embryonic development repeated the adult stages of the ancestors of a species. So, for example, in our embryonic development we go through a stage of having gill pouches like a fish, and in the distant past our ancestors were fish and had gills as an adult stage. This is an example of ontogeny recapitulating phylogeny. Note that it also shows von Baer's Law; when ontogeny recapitulates phylogeny, von Baer's Law is also observed. Haeckel was trying to put von Baer's Law in an evolutionary perspective.
It turns out that for some characterstics the ontogeny of the trait does go through stages much like the adult forms of the ancestors of the species with the trait-- that is, ontogeny does recapitulate phylogeny. But there are also many examples where this does not occur. This pattern is nowhere near as widespread as von Baer's Law. Also note that while it describes a possible evolutionary pattern, stating that ontogeny recapitulates phylogeny does not explain WHY this pattern might exist.
The idea of a developmental contingeny can, however, explain von Baer's Law. If mutations that change early development are usually lethal because they alter subsequent development too drastically, then early developmental stages will be less likely to change through evolution, so species will have the same stages early in development. Differences among species will arise only later in development.
Hypotheses of terminal addition, which is, remember, one of the forms of heterochrony, can explain why the ontogeny of some traits recapitulates phylogeny. If development is speeded up, and new forms are added at the end of development, then individuals of a species will go through developmental stages that are like the adult forms of their ancestors. However, note that the other form of heterochrony, paedomorphosis, does NOT result in ontogeny recapitulating phylogeny. Consider an example: primitively, salamanders go through a stage of having gills and then develop into forms that lose the gills. One salamander species, the axolotl, has evolved through paedomorphosis so that it keeps the gills throughout life. Its ontogeny does NOT recapitulate its phylogeny -- it never goes through a stage of not having gills that would be like the adult form of its ancestors. Note that for both forms of heterochrony, von Baer's Law is observed -- earlier developmental stages of species are similar; later stages are different because some develop to later stages than do others.
Hypotheses of the two forms of heterochrony, paedomorphosis and terminal addition, can be tested by studying the phylogeny to determine the likely pattern of evolution of traits. If a trait has evolved through terminal addition, then the primitive form should be to stop at an earlier developmental stage and the derived form should be to continue to a later developmental stage. In contrast, if a trait has evolved through paedomorphosis, then the primitive form should be to continue to a later development al stage and the more derived form should be to stop at an earlier developmental stage. Primitive and derived forms can be determined, as we have done in the past, through outgroup comparison. If we have a group of species in which some progress to a later developmental stage than do others, then we can look at the outgroup to this group of species. If the outgroup stops at the earlier developmental stage, then developing to the later stage is derived, and a hypothesis of terminal addition is supported. In contrast, if the outgroup develops to the later developmental stage, then stopping at the earlier stage is derived, and a hypothesis of paedomorphosis is supported. The following figure illustrates how to conduct phylogenetic tests of these hypotheses of heterochrony.
Study Tips: