The most recent theory is that cro-magnon man, the closest hominid to modern humans simultaneous bred with neanderthals and out-competed them. Remember, the key factor driving evolution is simply the ability of one set of genes to survive into the next generation. Both cross-breeding and "survival of the fittest" are means by which one set of genes isn't carried on in the next generation. Neanderthal's initial advantage in physical abilities was eventually over-taken by cro-magnon man's superior cognitive abilities.
[QUOTE=finlee17;2423799]You’re putting the cart before the horse here. What you want to ask is, “Why do extinctions occur and how does evolutionary theory explain such phenomena?”
Extinctions are typically correlated with population size variance (reproductive potential) and with the extent of population isolations (dispersal ability and species-area relations).
I don’t know much about hominid ecology and evolution, but I would assume both intraspecific (within species) and interspecific (among species) competition resulted in the death of the Neanderthal.[/QUOTE]
Having an ecology background (M.Sc. in Ecology), I usually look at natural selection in terms of adaptedness and population density. The process of natural selection usually leads to an increase in species adaptedness to the environment by increasing per capita survival and reproduction of the carriers of a gene or genotype in a specific environment. Higher population densities of species are usually supported when species become more adapted to their environments. Carry capacity of specific species have been theoretically and empirically proven to evolve to higher levels with density-dependent selection. Naturally there are trade-offs between birth rates and carry capacity in combination with external forces keeping populations below carry capacity.
However, there isn’t always a strong correlation between adaptedness and natural selection. There is a strong degree of frequency dependence where fitness and adaptedness (i.e., increased population size) to evolve in opposite directions. For example, in predator-prey systems (my background), some genotypes of a predator species have higher prey encounter and capture rates than other genotypes. Theoretically, more efficient and effective search and handling of prey would be favored by Darwinian selection. This would occur despite that fact that it may lead to declines in both prey and predator populations and even the eventual extinction of both.
In reality, the interactions are complex and prey adapt to top-down pressure exerted by predators. Additionally, intraspecific and interspecific competition limits predator numbers because they compete for limited resources and so on.
I don’t want to bore people to sleep with basic ecological concepts. So… there is no real easy way to describe evolution through natural selection in one or two sentences. Survival of the fittest doesn’t due justice because the fittest don’t always produce survivable offspring in both absolute and relative terms.
Asking me to describe natural selection would be equivalent to asking an economist to describe market forces. Very complex phenomena.
In fact, since I have economics on my mind… a good question would be why do firms go extinct and how does current economic theory explain such phenomena. Considering there are more users with economic backgrounds and who started/own their own businesses than those of us in the sciences, this might be a more fruitful conversation.