In part 1 and part 2 I argued that protein science has consistently shown that functional proteins are rare in sequence space. In part 3 I consider the major objections to the argument and find that they fail in providing an adequate response to the challenge that protein science poses for evolution.
A brief summary of the argument follows below:
1) Functional and folding proteins are rare in protein sequence space.
2) A blind search starting from a random protein sequence space is unlikely to find a folding, functional protein.
3) If protein evolution is true then novel proteins must come from modifying preexisting functional and folding proteins.
4) De novo genes that code for functional proteins have been discovered in the majority of organisms
5) It is postulated that these de novo proteins evolved from RNA coding sections of DNA and not from pre-existing proteins.
6) If so then it means random mutations would have to do a blind search in protein sequence space starting from a random sequence to find a folding, functional protein.
7) But because of (1) and (2) – 6 cannot be true
8) Therefore de novo protein evolution is not possible
Possible solutions for evolution
- A possible solution for evolution is that proteins are not distributed randomly in sequence space. It means all proteins used by all different species are concentrated within the area that organisms have searched. In other words – if evolution is true then proteins should not be distributed randomly throughout sequence space.
- Evolution does not have to build long complex proteins from scratch – but rather begins with smaller protein chains and increases in complexity. In other words random mutations do not have to search through sequence space – but rather build on existing short proteins. Think of the how the word “undertaker” is made up the word “under” and “taker”. Evolution finds the smaller words and combines them to make more complex ones. However there are complex words like rhythm, synonym, which are not made up of smaller simpler words. Similarly there are complex functional protein structures and protein domains not made up of simpler shorter protein structures and domains.
- “Suppose that one draws letters randomly from a box. There is little chance that the sequence will form an English word. But if one begins with a small word, and asks, instead, how much the word can be changed by a modification of its letters: the answer is quite a lot. Single letter switches are sufficient:” (T. Hampton)WORD → WORE → GORE → GONE → GENE. Small changes to existing proteins leads to proteins with new functions and therefore there is no need to begin from a random sequence in protein space. Protein evolution must begin with an existing protein.
The discovery of de novo proteins shows that they could not have come from previous proteins and must therefore have come from random sequences of DNA. Random mutations would have to search through large protein sequence space to find de novo genes.
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