The annoying thing is that reality - god's creation, if you like - shows you wrong.
Just a few examples:
Here is a link to a site that lists more than 700 variations of the human haemoglobin gene - all viable.
A very cool experiment, performed by Yuuki Hayashi et al. proves you also wrong.
Before the actual gene there is a genetic "switch" in the DNA before the DNA part that codes for a protein. This switch can be activated by a hormone, a the metabolic starting material or any other signal molecule. Yuuki Hayashi et al. performed a cool experiment: they stripped the coding part of the DNA of a virus that codes for the protein that grants access to a E. coli batcteria and replaced it with a random stretch of DNA. After 20 generations the virus had increased its infectivity by 1.7*10^7 compared with the starting generation.
The funny thing is that despite doing exactly the same function (allowing the virus to enter a cell) the sequence was completely different from the wild type.
source to the original papers:
The fitness landscape in sequence space determines the process of biomolecular evolution. To plot the fitness landscape of protein function, we carried out in vitro molecular evolution beginning with a defective fd phage carrying a random polypeptide ...
www.ncbi.nlm.nih.gov
https://www.sciencedirect.com/science/article/abs/pii/S0022519306005984
Another simple and elegant way to show multiple variations can perform the same function, was set up by professor Kishony and his team. A gigantic petri dish was divided in lanes with increasing concentration of antibiotics, from (0 , no antibiotics: 1 just enough to kill all bacteria, gradually up to 1000 x the concentration of 1). Different strains of
Escherichia Coli were spotted in the 0 lane. As this lane got filled and the places for new bacteria got depleted the bacteria were pushing against the boundary of the 10 lane. Only those bacteria and their descendants that got the suitable mutations for surviving in a higher concentration of antibiotics made it to the next lane. The experiment filmed over 11 days shows clearly that bacteria can evolve a resistance to a 1000 fold stronger concentration of antibiotics than the wild type bacteria.
Here you have the same experiment, but with professor Kishony explaining the experiment
It shows that evolution is cumulative. Each mutation increases the resistance to the antibiotics in an incremental way (see how the growth of the culture pauses at every boundary and how the growth always start at one tiny spot). But it also shows that different lineages with different DNA sequences can make it to the next concentration.
a technical paper published by the team
Spatiotemporal microbial evolution on antibiotic landscapes
the website of Roy Kishony's research institue:
Home - Kishony lab