1) That's not correct. natural selection doesn't have a narrow/expand bias one way or the other. Lineages simply survive if they manage to produce offspring. S

1) That's not correct. natural selection doesn't have a narrow/expand bias one way or the other. Lineages simply survive if they manage to produce offspring. Sometimes that's because they evolved some novel new thing (like birds learning to fly). Sometimes it's because they didn't do that, e.g. in the Cretaceous–Tertiary extinction event the boring mammals that were small and had a tendency to burrow to escape trouble survived. A variation that happens through a mutation doesn't need to be present in both breeding partners for it to survive. This is obvious if you look at common dominant[1] and non-dominant genes, like the tendency towards a certain haircolor. That trait might be passed on to the offspring, and would permeate if it was advantageous enough. A lot of variation also doesn't happen through mutation at all, but simply selection within a species. E.g. this happened to humanity [3] relatively recently. We lost a lot of biodiversity within Homo Sapiens simply because a lot of us died out. Most of the biomass on this planet never "expanded" in any significant sense. Multicellular life is still relatively rare, most of the biomass on earth is bacteria. 2) The notion that each species has an ecological niche is false. We live on a large planet, so there's a big opportunity for species that "do the same thing" (to simplify). Of course that's only true up to a point, consider the example of the Eastern Gray Squirrel displacing the Red Squirrel in the UK. 3) There's a lot of things that species could do but aren't doing. Not because that thing would be a "bad idea" as such, but simply because it hasn't evolved yet. E.g. insects could probably take up a lot more space than they do now if they grew larger, but that would require evolving a whole new breathing apparatus. Maybe monkeys would be better suited to trees if they had four hands, but two is all they (and the whole vertebrate line) had to work with. 4) I don't get this. Why do you think species should be much more inter-breed-able. They are to an extent, but once speciation occurs they aren't anymore. 1. http://en.wikipedia.org/wiki/Dominance_(genetics) http://en.wikipedia.org/wiki/Dominance_(genetics) 2. http://en.wikipedia.org/wiki/Eastern_Gray_Squirrel#Displacement_of_red_squirrels http://en.wikipedia.org/wiki/Eastern_Gray_Squirrel#Displacem... 3. http://en.wikipedia.org/wiki/Mitochondrial_Eve http://en.wikipedia.org/wiki/Mitochondrial_Eve