Unity and diversity of life Flashcards
Discuss Carl Linnaeus and his contributions to classifying organisms
Carl Linnaeus was an 18th-century Swedish naturalist that 1st organized the characteristics of living species into a logical framework. He introduced the system of nomenclature that is still used today. For macroscopic organisms, the Linnean classification is reinterpretable as a phylogenetic tree- a set of ancestral-descendent relationships between species. A thread of continuous family history unites all organisms. There is a continuous evolutionary pathway from a common ancestor to daughter species and species are fundamentally discrete. Despite the difficulty in extending these ideas to prokaryotes, they remain a cornerstone of biological thought.
What is the best way to define a species
Genome sequences provide the most general, detailed, and consistent approach to the definition of species. Sequences rule microbial taxonomy, but they battle for power with the morphological methods in the classification of plants and animals.
Distinguish between the traditional methods and the new methods for bacterial classification
The traditional methods were based on features of morphology(cell size and shape), biochemistry(the uptake of stains, C and N sources, fermentation products), physiology (growth temperature range, optimum temp, osmotic tolerance), and immunological cross-reactivity(especially among infectious diseases). Before sequencing hybridization of DNA from two different bacteria was a criterion for similarity. Most bacterial DNAs will form hybrid double-helical structures if the similarity in their base sequences is > 80%.
Later, following work that was done by C.Woese, prokaryotic species were defined in terms of variations in the 16S rRNA and other sequences. Bacteria for which the 16S rRNA sequences are more than about 2.5-3% different are considered different species. Typically, this corresponds to no more than 70% similarity in the overall genome sequence.
Protein, RNA, and DNA sequences have illuminated relationships between species, both for macroscopic organisms and microbes. The sequences have clarified some relationships, but have exposed others as simplistic. What are the major results
> All life on Earth has enough general similarity to show that all life forms had a common origin. Evidence includes the universality of the basic chemical structures of DNA, RNA, and proteins, the universality of their general biological roles, and the near-universality of the genetic code.
On the basis of 16SrRNAs, C.Woese divided living things into bacteria, archaea, and eukarya. The major divisions of the tree of life were shown. The ends of the eukaryotic branch are the metazoa- yeast and all other multicellular organisms- plants, fungi, and animals. We and our closest relatives are in the vertebrate branch of the deuterostomes. Although archaea and bacteria are both unicellular organisms that lack a nucleus, at the molecular level archaea are in some ways more closely related to eukarya than to bacteria. The recent discovery of Lokiarchaeum (the most complex known prokaryote) bridges the gap between archaea and eukaryotes. It is also likely that archaea are the closest living organisms to the root of the tree of life.
Dating of historical events from sequence differences. As species diverge, their sequences diverge. Researchers suggested that if sequence divergence occurred at a constant rate, it would provide a ‘molecular clock’ that would allow the dating of the splits in the lineage between species.
Although the clock is not universal, judicious calibration of rates of sequence change with palaeontological data permits the dating of events in the history of life.
The importance of horizontal gene transfer. This is the acquisition of genetic material by one organism from another by natural rather than laboratory procedures through some means other than descent from a parent during replication or mating. Several mechanisms of horizontal gene transfer are known, including direct uptake, as in Griffith’s pneumococcal transformation experiments, or via a viral carrier- eh. E.coli has about 25% of its genes that appear to have been acquired from other species. Horizontal gene transfer among different species has affected most genes in prokaryotes. It requires a change in our thinking from ordinary ‘clonal’ or parental models of heredity. Remember that arrangements of species into phylogenetic trees, in contrast, depend on strict ancestor-descendant relationships between different organisms during evolution. Microorganisms do not easily fit into the structure of the ‘tree’ of life but require a more complex organizational chart.
