JerryClower said:
Not trying to "troll" or anything of that nature, but no human has ever seen any type of mammal give birth to a slightly different mammal.
Happens all the time. Where do new dog breeds come from? What about birth defects? What about disease immunities (including recent, and I believe novel, genetic resistance to HIV)?
JerryClower said:
How can an animal of one species produce an entirely different species? I know that it is gradual changes over time, that is why nobody has ever seen it happen. But what I'm asking is, how can an organism have offspring of a different species, like the theory of evolution states?
Of course speciation is not particularly well-defined, or rather there's not much agreement about what a good definition would be. But in short, change is incremental enough that A could mate with B, B could mate with C, ..., and Y could mate with Z, but A cannot mate with Z. (That is, "can mate with" or "is the same species as" are not actually transitive relations.)
The well-known example of a ring species gives an unrelated illustration of the concept. Letting <--> mean "is fertile with" or "is the same (biological) species as", we have
Herring Gull <--> American Herring Gull <--> East Siberian Herring Gull <--> Birula's Gull <--> Heuglin's gull <--> Siberian Lesser Black-backed Gull
but
Herring Gull <-/-> Siberian Lesser Black-backed Gull.
This is similar to the linguistic concept of a dialect continuum, if you're familiar with that.
JerryClower said:
Someone needs to tell me, don't most scientists believe that all land animals came from prehistoric life forms found in the ocean?
Sure, but that's not a hard transition. The 'recent' (~50 Ma) ancestors of dolphins were land-dwellers, whereas the transition from sea to land was much further back (~450 Ma).
JerryClower said:
Also a question about natural selection. In natural selection, does the DNA of the organism actually "recognize" it needs to make a change, and then change.
Never.
JerryClower said:
Or does natural selection just mean that an organism with the best sight and hearing (example) will be most likely to reproduce so then it will pass the 'best' sight and hearing onto its offspring. Eventually, over a lot of years, an organism will be produced that has great sight and hearing, since the organisms with the best sight and hearing are the ones that mated and reproduced offspring. Am I right?
Not particularly, but that's closer than the idea that DNA "wants" something and does it.
Natural selection is just this: what creatures live, live. If having eyes was a deadly disadvantage for some reason, then lots of creatures with eyes would die. Then most existing creatures would be eyeless, either because they survived the Great Eye Purge or because their parents (etc.) did.
If eyes provide no particular advantage but cost a lot (metabolically speaking), then there's some selection pressure away from eyes. When food is plentiful, creatures with eyes survive and reproduce just fine; when scarce, they're more likely to die off. Thus creatures in environments not friendly to eyes (e.g., naked mole rats) have, over a long course, selected against expensive (in terms of disease as well as development) large eyes. In environments where eyes are an advantage, the selection is in the opposite direction. A gazelle with poor eyesight might have trouble escaping predation, in which case it fails to pass along its genes.
Most novel developments (whether mutations or simple cross-breading) are either harmful or essentially neutral. Sometimes several apparently neutral developments, when combined, have an effect; in these cases that effect can be selected for or against.