![]() These weak bonds can be broken with just a little heat, while closer matches require more heat to separate the strands again.ĭNA hybridization can measure how similar the DNA of different species is - more similar DNA hybrids “melt” at higher temperatures. When the two species’ DNA bonds together, the match between the two strands will not be perfect since there are genetic differences between the species - and the more imperfect the match, the weaker the bond between the two strands. To compare different species, scientists cut the DNA of the species into small segments, separate the strands, and mix the DNA together. If the strands are heated, they will separate-and as they cool, the attraction of the nucleotides will make them bond back together again. Each DNA molecule is made of two strands of nucleotides. Charles Sibley (left) and Jon Ahlquist pioneered the use of DNA kinetics to investigate evolutionary relationships using a technique called DNA-DNA hybridization (see figure, right). Scientists studying the chemistry of DNA moved even closer to actual sequences. Testing similarity using DNA Image courtesy of Thayer Birding Software. Times of divergence and phylogeny of hominoids, as estimated from immunological data. Expanding upon the work of others and making the assumption that fewer protein differences corresponded to shorter times of separation, Vincent Sarich (above left) and Allan Wilson (above right) estimated that humans, chimpanzees, and gorillas shared a common ancestor only 5 million years ago - a much shorter length of time than was commonly accepted at the time. These more sensitive techniques revealed the remarkable similarity between the proteins of humans and those of other great apes. Although variations of this technique were being employed as early as 1904, more sensitive protocols were developed in the 1960s. The more similar the proteins from the two species (human and chimpanzee) are, the stronger this second attack will be. If those same rabbit antibodies are exposed to a similar protein - from a chimpanzee, perhaps - they will attack it as well. For example, the immune system of a rabbit will recognize a human protein as foreign and will mount an attack against it by making antibodies specific to that protein. One way that researchers assessed protein similarities was by harnessing the immune system’s ability to recognize foreign proteins. ![]() Image courtesy of the Allan Wilson Centre for Molecular Ecology and Evolution. Testing similarity using antibodies Image courtesy of UC Department of Anthropology, Berkeley. So before the 1970s, proteins were used as stand-ins for genes in studying evolution. After all, if two species are closely related, they should have similar gene sequences, which should then make similar proteins. Scientists first began to zoom in on gene sequences by studying the products of DNA: proteins. In small steps, scientists came closer to their target. However, reading the genomes of entire organisms did not fall immediately from the discovery of DNA in the 1950s. DNA sequences form the hereditary links between generations, so it is no surprise that scientists investigating evolutionary relationships have sought to get closer and closer to the DNA that underlies those relationships. But today, a scientist working on the same problem could also use the very instructions from which that anatomy was built: its genetic code. To investigate how birds are related to one another, a biologist of the 1950s would have carefully studied their anatomical similarities and differences.
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