The story of Rasputin's murder tells how he survived poison, multiple bullet wounds and an attempt to be drowned. Meanwhile, he was noted for having a very distinct physical appearance with some unique features such as "phosphorescent" eyes and a gargantuan manhood.
I'm curious if there were attempts to scientifically explain his condition through biology and genetics.
Okay, since many of the comments regarding your question have been deleted, I'll attempt to address them.
I am not aware of any attempts to explain Rasputin physical strengths through biology or the likes, especially in regards to his assassination. The reason being, he and the assassination was not as superhuman as legend seems to have taken.
First, in an earlier attempt on his life in 1914 in which he was stabbed by Khioniya Guseva, he displayed no super powers and required a surgeon, doctors and was quite weakened and bedridden for some time as result.
As far as his actual assassination and death, there are several factors to look at. First, even though Yusupov and Purishkevich poisoned the pastries they invited him to eat, Rasputin did not eat sweets. He followed a strict diet, according to his own daughter, he 'never ate sweets, meats or pastries'. Author Edvard Radzinksy backs this up with several more sources verifying his strict diet. (The Rasputin File - Radzinksy, pg 477-478). Also, it appears Yusupov and Purishkevich used too little potassium cyanide in the wine along with the fact Rasputin did not drink much of what was offered. (Also Rasputin File, pg 477) So really, the whole "poisoned" part can be explained easily. As far as his shooting, well, that's quite a bit to explain, but in the end, you had two amateur assassins that were quite nervous and not the greatest of shots. From the best accounts, he was most likely shot and injured within the home. When they went to check on him, he was able to make a run for it. At the best, they may have gotten two more shots in him (from a distance) as he ran across the courtyard. To make sure he was dead, they then threw him in the river.
When reading through sources, it is pretty easy to see that both Yusupov and Purishkevich initially embellished their stories. After all, they were trying to justify just what a demon Rasputin was and hopefully be seen as hero's for their feat and not just two inexperienced assassins. (these claims can be backed up by both their initial and later testimonies along with other witnesses of which can be found from Chapter 15 in "The Rasputin File" by Edvard Radzinsky. I used this as my source as the author relied and extensively used testimony and once classified government files and pieced together an excellent work on the subject).
This is probably a dumb question, since I know very little about Rasputin, but how believable are the stories of his endurance in the first place?
What does it mean that he had these "phosphorescent" eyes?