Ontogeny is development. We generally think of it as the series of stages multicellular organisms go through, (usually) beginning with the fusion of egg and sperm to form a zygote and continuing (at least) to adulthood. As an organism completes development, it takes on the characteristics of whatever species it may be. A pattern of relationships among species, a phylogeny, can be traced by examining the characteristics that one species shares with another species, or that groups of species share with each other. It is the individual organisms that express species membership – they can interbreed and make more of themselves, they are anatomically and behaviorally similar. Individuals can be catalogued into species, and species can be grouped at higher levels, tracing a pattern of evolutionary relationships.
Mishtu Banerjee and I were both students of Jack Maze’s at the University of B.C. Mishtu travels in an analytical and programming universe that is far beyond me (there’s a link to his web site on my home page), but I love his description of ontogeny and quote it here: “Development is characterized by increasing specification. Vague patterns early in development become distinct categories later in development. To make a measure on a developing system assumes we can first identify the category of what we are about to measure – and that the category even exists at the point of observation. That is, in the course of development, measurable variables emerge, and the ability to measure emerges.”
Cy Finnegan offers some historical perspective on the notion that “ontogeny recapitulates phylogeny” or that each individual goes through a series of “ancestral states” as it develops: “Early in the 19th century Karl von Baer observed a comparatively large group of various developing Vertebrate embryos and reached certain conclusions (principles). The general characteristics of the groups were the first to appear in development and were followed in time by the less general. Subsequently, the more specific (features) appeared in development until finally the individual characteristics were present in the developed forms. At no time did the forms resemble extant adults, but rather approximated the embryos of associated forms. Later, Ernst Haeckel, an expert in the Protista, asserted that he observed a recapitulation in the Protists and extrapolated it to all development. Von Baer fought this aphorism mightily noting it was embryos resembling embryos, if at all, and not adults. Von Baer knew his Vertebrates; Haeckel his Protists. As Barbara McClintock wisely admonished us all, know your organism!”
The idea that ontogeny recapitulates phylogeny, that as an individual, multicellular organism develops it passes through many of the earlier evolutionary stages of its ancestry, lacks accuracy. Though some ancient patterns are carried along in parts of development, they disappear as organisms reach their more mature forms. Cy warns, “Embryos resemble only embryos,” and not adult forms. It seems that a developing organism reads and translates genetic and other information, changing form and manifesting measurable features as it moves along a developmental trajectory, “which to a considerable extent is predetermined, given no external interferences,” Cy adds.
Perhaps the original saying, “ontogeny recapitulates phylogeny” should be amended to read, “phylogenesis is expressed through ontogenesis.” For multicellular organisms, species membership can only be expressed through the phenomenon of development. Individual organisms manifest at least two levels of emergence (where the whole is more than the sum of the parts; the organism has properties not possessed by the collective parts). The first is the individual itself – an entity that expresses uniqueness as it develops. The second is the expression of an individual’s membership in a species. Here a species can be considered a group of individuals that shares a developmental program characteristic to that group and only to that group (characteristics that define a single species are called autapomorphies). Species not only manifest individuality, but they also express a deeper pattern of ancestral relationships. Humans share many characteristics with other species on Earth. We are vertebrates – we have backbones just like bony fishes, birds, lizards and lions. We are the subset of vertebrates called mammals, all of which have hair and whose females produce milk for their young. We share these two features with other mammals such as lions, mice and whales, but not with non-mammals such as fish or lizards. We are primates, sharing an ancestry with chimps and gorillas, a much smaller subset of Earth’s creatures. It has been suggested by Noam Chomsky (need a reference…this quote came originally from Jeremy Campbell’s book Grammatical Man) that complex language ability is a uniquely defining feature, (an autapomorphy), for Homo sapiens, a specific character that appears very late in individual development.
Systematics is the branch of biology that seeks to understand evolutionary relationships among species and higher groups, generating branched diagrams that look something like family trees. Different methodologies have been used in elucidating relationships among species, but phylogenetic systematics (known as cladistics) is the most rigorous. An autapomorphy is a characteristic shared by all of the individuals in a species. An autapomorphy can be physical or behavioral. It helps to define the species, but doesn’t clarify one species relationship to other species. A synapomorphy is a character that is uniquely shared by a group of species and defines that group. When we look at all of the animals, only mammals have hair. Hair is a synapomorphy for mammals and helps to define the group. A plesiomorphy is a character that is too ancient to assist in defining relationships within the group of interest. Mammals have backbones, but so do other animals that are not mammals. If we are looking at all of the animals, the presence of a backbone is the defining synapomorphy for a subgroup, the vertebrates. But if we are looking only at mammals, the presence of a backbone is a plesiomorphy because it gives us no new information. We already know that all mammals are vertebrates.
This way of looking at relationships among species, where evidence of membership in a group of species consists of characters (synapomorphies) shared only by the members of the group is the central approach of phylogenetic systematics or cladistics. We end up with something like a family tree defined by nested sets of related species that are grouped by their uniquely shared characteristics. Remember set theory in algebra? This is very similar – the set of primates is nested within the larger group of mammals, nested within the still larger set of vertebrates. At each level there are criteria, specific characteristics like the synapomorphies described above that define membership. Perhaps complex language is a synapomorphy for the entire genus Homo. Unfortunately, it would be difficult to find evidence for this, as the other species of Homo are apparently extinct. Some level of language ability may be a defining characteristic for a larger group of primates. It’s not so surprising that researchers have been able to teach hundreds of words of sign language to chimps. It would be far more surprising if our primate cousins were incapable of learning words and expressing themselves through some level of language ability. Human abilities are remarkable and demonstrate emergence, but they did not appear miraculously out of the ether, fully formed from the fingertip of a deity.
A fellow named Willi Hennig first developed phylogenetic systematics as a method of clarifying evolutionary relationships in the 1950s (reference…). The great strength of this method is that it relies entirely on evidence, limiting assumptions (especially those relying on perceived “adaptations”) to 1) the existence of homology (the evidence tells us the wings of birds evolved only once) and 2) the occurrence of evolution (speciation or lineage splitting happens). Cladistics generates trees (cladograms) that illustrate relationships among species or larger groups. The method requires that the uniting characters be shown on the tree and that any characters that provide falsifying evidence also be shown. When properly done, cladistics demands that synapomorphies be identified by comparison with outgroups, more distantly related species. Outgroup comparison allows systematists to discover which characters or character states are most ancient or more recent. Comparison also helps systematists identify variables that are irrelevant. The clearest characters are those that obviously define a group because they don’t exist elsewhere in living things – no outgroup has the characters. Animals with backbones are a good group, as are the mammals and the flowering plants. Each group is defined by clear and unique features. (Did John Lynch coin “universal synapomorphy”?)
Cladistics, does not automatically label as “advanced” what might look to us like clever “adaptations” because this invites unnecessary subjectivity. The evidence gained from comparative studies is what matters.
One of the most important concepts in biology is the idea of homology, structures or other features that arise from a single source, though they may have different functions. Conversely, analogs share a common function, but they come from different sources. The classic example is wings. The wings of insects and bats are analogous as both function in flight, but they are not homologous as they have a different evolutionary source. The arms of humans are homologous to the wings of bats or birds (both are tetrapod forelimbs), but they perform different functions and are not analogs. With the elucidation of DNA we thought we would be able to fully pinpoint and understand homologs. We hoped that each character would be coded for by one gene or maybe a simple set of genes. It’s a lot more complicated than that, and the DNA “Rosetta Stone” turned out to be more like alphabet soup. We see the letters, but can’t read the language, perhaps because, as with all languages, there’s more to the language than the letters.
This is one of the most basic, most studiously ignored problems in biology – we don’t know how we get from DNA to organisms. We need a good theory of morphogenesis, with clear, testable conjectures about how developing organisms translate the information in their DNA and other molecules into structured forms and behaviors. Certainly, genetics and molecular biology have discovered many things, but the methods frequently consist of shooting in the dark. Tweak this gene and see what happens. Compare a group of humans that share a particular disease and look for a genetic correlation. We are only pretending that we know what we’re doing. This is the most important argument against the use of genetically modified organisms: we don’t have much of a clue. The good scientist is willing to admit this and continue the search beyond the hype and the marketing, but industry’s buying of science has made this very difficult.