The evolution of all modern proteins
can be traced back to a common ancestor. Figure 4.2 illustrates this concept. Around 500
million years ago suppose that there was a common ancestor for flounders and rabbits. Some
of the descendants of this ancestor evolved into rabbits. Others evolved into flounders.
Today the DNA of this common ancestor is not available, but the DNA of flounders and
rabbits is certainly available. When the DNA of a flounder and a rabbit are compared, most
of the information found in their DNA is the same. The insulin found in a flounder is very
similar to that found in a rabbit. But there are differences. The two species have had 500
million years to accumulate changes independently.
Figure 4.2: Common Ancestors
If a mutation modifies the insulin amino acid sequence, several fates exist for the
modified protein.
• If the modified protein is better than the original, natural
selection may encourage it to spread through the population. With time the new protein may
be fixed in the population. This means that every member of the population contains the
modified gene.
• If the modified protein provides no selective advantage, it may
still be fixed in the population. As long as the modified protein is as good as the
original, but no better, the probability of fixation is equal to the rate of change.3
Any protein that meets these criteria is termed neutral. So if a specific amino acid in
insulin mutates every 100 million years, then a modified insulin with the changed amino
acid is expected to be fixed in the population every 100 million years.
• If the mutation is slightly harmful, natural selection will most
likely eliminate it from the population but not always.3
Assume for a minute that the amino acid sequence of insulin is not
important and that almost any protein composed of 50 amino acids or more can signal cells
to take up sugar. In other words, the insulin hormone contains almost no useful
information. If this assumption is true, then one would expect the insulin amino acid
sequence in fish and in mammals to be completely different. The sequences have had 500
million years to change independently.
Analysis of insulin in fish and in mammals has revealed that this is
not the case. Many of the amino acids are the same or have similar chemical properties.
These amino acids are said to be conserved.
Conserved amino acids are a measure of information. To accurately
measure this information, a comparison of many diverse species is required. The more
diverse the species the better. The technique works best for proteins that are found in
all of the kingdoms of life.
next: Information in Insulin
home: Intelligent Design and the origin of Life
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