What is the concept of " the big tree" in microbial diversity

Priya sarda

The big tree, The tree of life or universal tree of life is a metaphor, model and research tool used to explore the evolution of life and describe the relationships between organisms, both living and extinct, as described in a famous passage in Charles Darwin's On the Origin of Species (1859).Charles Darwin (1809) used the metaphor of a "tree of life" to conceptualize his theory of evolution. In the Origin of Species (1859) he presented an abstract diagram of a theoretical tree of life for species of an unnamed large (see figure). On the horizontal base line hypothetical species within this genus are labelled A to L and are spaced irregularly to indicate how distinct they are from each other, and are above broken lines at various angles suggesting that they have diverged from one or more common ancestors. On the vertical axis divisions labelled I to XIV each represent a thousand generations. From A, diverging lines show branching descent producing new varieties, some of which become extinct, so that after ten thousand generations descendants of A have become distinct new varieties or even sub-species a10, f10, and m10. Similarly, the descendants of I have diversified to become the new varieties w10 and z10. The process is extrapolated for a further four thousand generations so that the descendants of A and I become fourteen new species labelled a14 to z14. While F has continued for fourteen thousand generations relatively unchanged, species B,C,D,E,G,H,K and L have gone extinct.In Darwin's own words: Thus the small differences distinguishing varieties of the same species, will steadily tend to increase till they come to equal the greater differences between species of the same genus, or even of distinct genera.".This is a branching pattern with no names given to species, unlike the more linear tree Ernst Haeckel made years later which includes the names of species and shows a more linear development from " lower to higher species. In his summary to the section, Darwin put his concept in terms of the metaphor of the tree of life: