What advantages would extreme longevity need to provide for it to develop in a human subspecies/humanoid species without outside tampering? I ask because elves and the like in fantasy are often depicted as living far longer than humans do, and are healthier for longer into old age, and assuming that's not just because they're more magical than humans and magic leads to a longer lifespan, what would induce the pressure to cause a species to develop this kind of longevity?
There are some traits in the animal kingdom that tend to be reasonably correlated (although not necessarily linear). Among those are size, birth rate, and longevity. Anecdotally, the mouse is small, lives just a couple of years, and has a big litter of pups every few months. The elephant is large, lives up to 70 years, and have one calf every five or six years. Blue Whales are enormous, live 80 to 110 years, and have one calf every couple of years. Humans seem to fall somewhere in the middle of the size range, live 70-100 years, and can have one baby every year, with occasional multiples. Note that the bigger-older-lower birth rate relationship is not perfect, but an overall pattern emerges.
Now we have a problem, as elves and humans are comparable in size, for them to follow this pattern they should also have comparable longevity and birth rate. However, if there was some event in the ancient elven past that made it part of their mores to not have as many children (for example, due to reduced resources to have to share with an increasing population), then evolution would have gradually favored longevity as well. This same scarcity of resources would also have favored not growing larger.
Here, Will's answer comes in, as those elves who are best able stick around to care for the few children they do have, will fare better in the long-term survival of the society. Thus, the long-lived more frequently pass on their genes for longevity.
In conclusion, have the ancient split between elves and humans happen in such a way that the humans got the lush forest, and the elves got the scrub at the edge of a desert, or something like that.
What advantages would extreme longevity need to provide for it to develop in a human subspecies/humanoid species without outside tampering/
Fitness (often denoted w w or ω in population genetics models) is the quantitative representation of natural and sexual selection within evolutionary biology. It can be defined either with respect to a genotype or to a phenotype in a given environment. In either case, it describes individual reproductive success and is equal to the average contribution to the gene pool of the next generation that is made by individuals of the specified genotype or phenotype.
It is somewhat unusual that this has not evolved. Consider
Man A lives 65 years. He fathers 5 children between ages 20 and 40. He contributes to the welfare and survival of his children and grandchildren from ages 40 to 65.
Man B lives 195 years. He fathers 5 children between ages 20 and 40, 5 more from 40 to 60, 5 more from 60 to 80, 5 more from 80 to 100, 5 more from 100 to 120, 5 more from 120 to 140, 5 more from 140 to 160. For his last 35 years he contributes to the welfare and survival of his many descendants.
Man B lives 3 times longer and fathers 7 times as many children as man A. His genetic fitness is far superior.
In fact, you would need to build in some sort of low reproductive rate for the long lived elves or they would quickly overwhelm and out-compete the humans with their cumulative numbers.
ADDENDUM I am a little perplexed by the comments and horrified by the downvote, and so I thought I would add more mechanism to the fitness advantage of longetivity than just the weight of numbers.
Suppose there is a selective event - a plague, or a famine, or a toxin in the water. Man A and Man B are resistant and survive. Man A fathers one more child and then comes to the end of his natural life. Man B continues contributing his resistant genes to many subsequent generations. This is how it works for lots of lower animals like fish or sea urchins. Very long lived creatures who escape the mortality risks in their environments contribute offspring season after season.
When in doubt use sexual selection.
If you have already included magic then you can use that as a indirect selective pressure. Magic is how elves choose mates (or it is at least a huge part of it), based on power or displays with magic. since you set the rules for magic you can say the longer you live the stronger your magic becomes and the better you can use it, works even better if magical ability also takes a long time to develop. Now age is roughly equal magical power which is basically how sexy you are to an elf, and evolution will create all kinds of stupid conditions in the name of mate attraction. so pushing longevity as far as it can makes sense.
This is pretty easy to see with your set up if humans can also use magic, magic is ancestral. Elves split off early by focussing on magic, where as faster breeding humans focused more on new ideas, aka technological development. I imagine elves to also be highly reliant on magic with stone age technology otherwise. Humans may have less power but may be more creative with it. Perhaps Humans don't just push the rock with magic like elves with abundant magic do, they use magic like a lever to roll it accomplishing the same thing with far less raw magical power.
this could also explain why magic in animals is rare if you so desire, most are too short lived.
What advantages would extreme longevity need to provide for it to develop in a human subspecies/humanoid species without outside tampering?
This is a malformed question! Extreme longevity is the advantage! I will assume you asked:
What evolutionary pressures would lead to extreme longevity?
To the best of my knowledge our live spans is limited by our genes deteriorating and this inevitably leads to cancer.
In any case a mechanism that would select for longer life spams would be something like:
longer childhood periods
If the environment is very dangerous then development times will most likely increase due to the longer training a child will need to undergo to be able to handle themselves in the world.
unbalance in sexes
If there are more females then males or vice-versa then this would favor individuals that live longer because it gives them more time to find a mate and allows them to reproduce.
These conditions together would lead to greater health and longevity and it also fits a fantasy world with dragons, orcs, and generally more predators.
With humans it typically takes 10-15 years to learn enough about their environment to be independent. Traditionally the first half of that was the most dangerous, so most strategies aimed at having only one young child per couple at a time and sequentially raising several over the peak active decades.
If elves follow the same logic but need longer to learn the basics of their environment, say because they aren't as smart as us or because there are more interesting things trying to kill them (like demons and dragons and humans) or because learning magic is hard that might be a childhood of 10-20 years minded by a couple with no other children. With 100% survival that might mean they need 40 active years just for replacement, and accidents happen so more like 60 would be needed for safety. If their enemies are more effective that might not even be enough.
The logic of grandparents might still hold if they had fields of study that still allow interesting growth after a century of study. If magic has a learning curve with no known end any individual able to live longer to study it might bring benefits to their genes long after reproduction.
Perhaps continuity of long term effort is important for some reason, say the intricacies of tending stalagmites or tending some tree through a full life cycle is valuable, but better learned by starting fresh than risking interfering with an existing project. Say it turns out if you consistently pet a pine tree in exactly the same way every morning never deviating even a little for 80 years it produces pineapples or whatever. This would make having long lived individuals valuable.