Molecular studies show how creatures are related to one-another and allow taxonomists to arrange them in groups that reflect their kinship. A ‘Phylum’ contains all animals that are thought to share a common ancestor.
Microscopes and Modern Taxonomy
Before there were useful microscopes it was only possible to study the larger animals, and then only their anatomy and behaviour. When microscopes became readily available a whole new world opened up. Biologists soon realised that all large animals were composed of millions of individual cells, and that there were many organisms composed either of a small number of cells or a single unit.
Two Different Cell Types
- Under the microscope the cells of bacteria were seen to be fundamentally different from those of all other organisms, so the bacteria (and all their relations) were put into a their own ‘Kingdom’ – the ‘Prokaryota’.
- All other living things share a common pattern at the cellular level, but differences are sufficient to group them into four additional kingdoms (Fungi, Plantae, Animalia, and Protoctista).
Within the animals (Animalia) there are al least 35 separate phyla, or groups each thought to share a common ancestor. Over 95% of all animals can be allocated to one of the 9 ‘major’ phyla, while the rest are dispersed among the remaining 26 or so ‘minor animal phyla’.
Major Animal Phyla
The major phyla - Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata – all consist of animals that are large enough to allow their members to contain specialised cells and well-developed organ systems. Anatomy alone has made it possible to distinguish many species within each phylum - molluscs with over 100,000 species, sponges 5,000, cnidarians 11,000, flat worms 25,000, nematodes 80,000, annelids 15,300, arthropods 1,000,000, echinoderms 7,000, and chordates 100,000.
Minor Animal Phyla
The 26 (or maybe more) minor phyla consist of animals that are too small to display much variation in cell types or anatomy. Different species within each phylum often look very similar (or identical) to one-another under the microscope, so further study requires molecular work.
Molecular Studies and Modern Classification
Techniques are now available to investigate the molecular details of the way cells work. For example their proteins can be analysed, and DNA can be sequenced. When this is done it is possible to distinguish between species that might look identical, and to infer relationships between different groups. For example:
- The phylum Cycliophora was originally thought to contain only one species (Symbion pandora) but has now been shown to consist of several.
- Looking at the details of DNA and RNA has allowed relationships between phyla themselves to be inferred. For example the mesozoans are possibly related to the flatworms (Platyhelminthes) in some way, and the placozoans are maybe related to the sponges (Porifera).
Darwin’s idea that all animals are related and share common ancestors (the ‘tree of life’ idea – see article image) seems to hold true, although commentators have recently pointed out that things can get very confusing when DNA is passed between species (as it is in the bacteria).
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