Dawkins pointed out in The Selfish Gene, evolution is mathematically necessary when you have:
- Differential reproduction
Variation is managed via the phenotype-genotype difference, the large number of chromosomal variants wandering about, and by mutation.
That's easy stuff. Well, except the phenotype-genotype bit. Anyway...
Differential reproduction is the place people get confused.
Elephant seals and Gorillas are interesting creatures because in those groups, ONLY the alpha-male of the group reproduces. This makes the explanation easier.
Elephant seals may fail to reproduce for 3 reasons.
1. Died due to weakness. Seals' young deaths are, on average, the weaker seals. There is a statistical correlation between weaker/slower/etc. and dying young.
2. Died by accident. If harpooned by a hungry eskimo, the Seal really couldn't have done anything about it. Some orca/shark attacks are like this as well.
3. Male survived just fine, but didn't reproduce. I assume that MOST elephant seal males fall into this category. Survival isn't a problem, but they never reach AMOG (Alpha Male Of Group) status, and so they never reproduce.
Most people talking about evolution in an uneducated way massively underestimate the importance of #3, and often misunderstand the importance of #2.
Roughly...half of evolution is surviving in order to reproduce. The other half of evolution is successfully reproducing. In other words, something like half of evolution is surviving at all. The other half of evolution is out-competing the neighbor in generating great-grandchildren.
This means, by the way, that a huge chunk of animal-evolution is (in misleadingly teleological language) targetting NOT higher survivability, but higher likelihood of reproducing (sexiness). Also...the issue is specifically differential reproduction, not total reproduction...so a Gorilla's fitness is mostly his fitness as compared to other Gorillas.
Half of evolution is competing against other species, with herbivores and grass being the classic example. And parasite-games being the most interesting for humans. The other half is competing against other members of the same species (and gender).
If this is remembered, then most silly group-selection theories will die off. And the discourse on evolution will be better.