New Evolutionary Models

Models of evolutionary relationships are not fixed. They change when new evidence is found, especially molecular evidence.A famous example is the work of Carl Woese in the 1970s. Woese compared the base sequences of ribosomal RNA (rRNA) from many organisms. rRNA is found in all cells and changes slowly, so it is useful for comparing distantly related organisms. He found that prokaryotes, which had all been grouped together as bacteria, actually form two very different groups: Bacteria and Archaea. The Archaea were as different from Bacteria as they were from eukaryotes. This led to a new model of classification based on domains, above the level of kingdom.At first, many biologists rejected Woese's model. It became accepted as more evidence supported it, for example differences in cell membrane lipids and in the enzymes that copy and transcribe DNA.The scientific community validates new models through these steps:
• peer review: before publication, a paper is checked by independent experts in the same field, who assess whether the methods are valid and the conclusions are supported by the data.
• publication in scientific journals, so that other scientists can read the full methods and results.
• presentation at scientific conferences, where findings are discussed and challenged.
• other scientists repeat the work and test the model with new data. A model is accepted when independent results agree.This shows that scientific knowledge is tentative: our models of evolution are the best explanation of the current evidence.