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WHEN DID EVOLUTION CAUSE SPECIES TO DIVERGE? Evolution & Homeostasis 2011 Phylogeny • Over time, scientists have used different methods to determine how similar different organisms are to each other. • This is used to construct cladograms (also called phylogenetic trees), which show which organisms evolved from common ancestors, and when. • Cladograms can be constructed using various characteristics • Structural similarities- anatomy & physiology • DNA hybridisation • DNA sequences • Protein sequences • Immunoglobulins • Mitochondrial DNA • The cladogram may be different depending on which characteristic is used Biochemical evidence • The more differences between the DNA or proteins of two species, the more distantly related they are, and the further back in the past their ancestors diverged. DNA hybridisation 1. 2. 3. 4. 5. DNA is extracted from cells from 2 different species. The DNA is cut into fragments with enzymes. DNA is heated so that the 2 complementary strands separate. Strands from the 2 different species are hybridised together, but in some places the sequence is different so there are mismatches. The number of mismatches can be calculated by measuring the temperature required to separate the strands- the more mismatches, the lower the temperature required. DNA sequencing • Now molecular biology & computers can be used to find & compare DNA sequences directly. Comparisons of the bhaemoglobin gene Mitochondrial DNA • Mitochondria contain their own DNA, separate & different from the cell’s genomic DNA. • mtDNA is only passed down from the mother. • Hence it is possible to determine the maternal lineage. mtDNA as an evolutionary clock • mtDNA mutates at a known rate. • Hence differences in the sequence of mtDNA can be used to determine when species diverged. • Fewer differences between species indicates that they have a recent common ancestor. • More differences indicates that the species diverged earlier in their evolutionary history. Protein evidence • Studies of the amino acid sequence of proteins such as the globin family (myoglobin/ haemoglobin) show how long ago species diverged from a common ancestor. Immunoglobulin evidence • Relationships between living species can be determined by studying reactions between antibodies from one species & proteins from other species. • Example: • Protein from a human is injected into a rabbit. • The rabbit produces antibodies against the human protein. • Blood is taken from the rabbit, and the serum containing antibodies is separated from the blood (anti-human serum). • If the human protein is added to the serum, antibodies bind to the protein & a white precipitate forms. • If chimpanzee protein is added to the rabbit serum, only some of the antibodies recognise the protein enough for some precipitate to form. • The more distantly related the species are from humans, the more dissimilar the proteins are, and fewer antibodies bind to them. • Thus a family tree can be built up based on the structural differences between proteins.