In embryology phylotype refers to the concept that there is a conserved stage during embryogenesis (the phylotypic stage), during which the developing embryos of species of the same phylum appear to be similar.
In taxonomy, phylotype refers to an observed similarity that classifies a group of organisms by their phenetic relationship. This phenetic similarity, particularly in the case of asexual organisms, may reflect the evolutionary relationships. The term is rank-neutral, thus one can choose the rank at which the phylotype is described, e.g. species, class, 97% genetic similarity or homology. The term is often used in microbiology since the genomes of prokaryotes do not lend themselves to classification via Linnean taxonomy as easily as do many eukaryotes such as plant and animals.
Embryological phylotype concept
The original idea of animals being similar in development can be traced back to Aristotle. In his text The Generation of Animals, Aristotle observed the embryos of a number of vertebrate animals, such as chickens, fish and snakes, and noted that the differences that make each species separate from one another arose late in development.
Karl Ernst von Baer proposed something similar to Aristotle’s idea as one of his laws of embryology in his work of 1828 Über Entwickelungsgeschichte der Thiere. His third law of embryology states that the form of an embryo does not converge upon other forms, but separates from them. In other words, the juvenile stages of animals embryos are shared, but they then diverge.
In his 1866 work Generelle Morphologie der Organismen, Ernst Haeckel attempted to explain Von Baer’s ideas in the context of his own recapitulation theory – the notion that throughout embryogenesis, each stage of development represents a stage of the evolution of the organism’s ancestors. According to Haeckel, embryos would indeed diverge later in development and have similar early stages if evolution worked, as he claimed, by the addition of new stages to the end of development. Haeckel’s theory is no longer considered to be entirely correct, although modern evolutionary and developmental biology built on these ideas to prove the existence of the phylotypic stage.
The Funnel and Hourglass models
Von Baer's third law stated that divergence between species increased with the age of the embryo. This pattern of divergence became known as the funnel model. However, it was eventually noticed that early embryos can be very diverse, with a more conserved stage occurring afterwards (the proposed phylotypic stage), followed by the embryos diverging again afterwards. This model came to be known as the hourglass model, owing to its shape, with the 'waist' marking the period of least divergence. For many years, the two models were subject to controversy, as they depended on physical comparisons of animal embryos, and what could be considered similar is subjective.
To confirm which model was correct, molecular methods were used. If the physical characteristics of the stages of development were determined by the expression of genes, then identifying the patterns of gene expression in different organisms and comparing them would reveal which stages of development were most similar. This experiment was done using six different species of the fruitfly Drosophila. In the experiment, fly embryogenesis was divided into stages, and the expression patterns at each stage was compared across species. The stages that showed the most similar gene expression were those in the middle stages-the 'waist' of the hourglass- seeming to confrirm the hourglass model.
A related study attempted to estimate the age of the genes in Drosophila, attempting to locate when they arose in the history of animals. The stage of development in which these genes were expressed was then identified, and the average age of the genes in each stage calculated. The study found that the genes at the beginning and end of development were much younger than those in the middle stages, suggesting that the middle stages of development are more conserved among species and that the hourglass model is correct.
- John S. Wilkins (2006). "Microbial species 2: recombination".
- Richardson, Michael K.; Keuck, Gerhard (2002). "Haeckel's ABC of evolution and development". Biological Reviews of the Cambridge Philosophical Society. 77 (4): 495–528. doi:10.1017/S1464793102005948.
- Prud'homme, Benjamin; Gompel, Nicholas (2010). "Evolutionary biology: Genomic hourglass". Nature. 468: 768–769. doi:10.1038/468768a.
- Domazet-Lošo, Tomislav; Tautz, Diethard (2010). "A phylogenetically based transcriptome age index mirrors ontogenetic divergence patterns". Nature. 468: 815–818. doi:10.1038/nature09632.
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