If we accept Darwin’s theory of evolution, which I do, then we accept that we are the way we are as a result of a very period of gradual changes brought about by the pressures that our species has experienced through emergence and during process of its existence.
But let’s take a step back. All organisms have so called genetic material, stuff within them which encodes the way they are and the way that their offspring will be. The genetic material is copied as a part of the process of living, of growing and of repairing the organism if it sustains damage.
If that were all there was to the process, then organisms would be static, with no changes and no evolution. In fact the process is not perfect and both minor and major changes to the genetic material happen all the time, by all sorts of means.
Obviously, if too many changes or major changes occur in the genetic material, then the organism may not grow properly and may not repair itself properly when damaged. Also if the genetic material is passed to the organism’s descendants, they may they may not be viable or they may be disadvantaged and be unable to thrive and reproduce.
To counter this, our bodies have mechanisms to repair our genetic material, our DNA. If our bodies did not have this ability, it is unlikely that we would last long, as our body cells could experience millions of cases of damage to our genetic material per cell per day. That’s an awful lot of damage!
As described in the Wikipedia reference above, the errors in our genetic material could result in cell death or unregulated growth resulting in tumours. The DNA repair mechanism in our cells do a good job, but they are only effective if the DNA strands are broken or incomplete. If a change is minor, and is properly reflected on both strands of our double helices, then the repair system will not notice the change.
This allows small changes to slip through, and provided they don’t cause life threatening problems, they may get passed to our descendants. The same applies to organisms other than ourselves of course.
Some major changes do slip through and organisms may end up with extra chromosomes or with damaged chromosomes. Sometimes these issues may not cause too many problems for the organism, while in other cases the descendant organism may not survive long enough to breed.
The minor errors mentioned above may affect the descendant organism to some extent, making it more or less successful than its parent organisms. The theory of evolution suggests that if the change in the genetic material makes it more successful than its siblings who don’t have the small errors, then, over generations, organisms carrying the new DNA changes will eventually replace those who don’t carry the change.
This could lead to problems for an organism. If we consider a stable population with few pressures, that has plenty of resources, there is little that would cause any permanent changes to the population, and small genetic traits could appear and disappear over time and not have any measurable effect.
If the environment then changes, such that one trait provides a large benefit to those individuals who have this trait, then over time there will be a tendency for the trait to be found in more individuals and the number of individuals without it would fall.
If the environment changes back again, then those with the trait may be disadvantaged and those without the trait could then come to dominate the population. However if enough time had passed and all the individuals without the trait in their genetic material had died out, then the population would be stuck with the trait.
It would be extremely unlikely but not impossible for the change in the genetic material to be reversed by chance as this would require another minor error to exactly reverse the original error. In effect, evolution as reflected in the genetic material never (or astronomically rarely) reverses.
If a group of organisms gets isolated from the rest of its species, some of the genes that are present in the population at large will not be present. In addition, some of the genes in the isolated population will also die out, either by chance, or because the trait that they confer is not beneficial in the isolated environment.
This can cause problems for the population if the environment changes dramatically to the detriment of the organisms. While the population at large may have genes which would enable the population to survive the changes, but those genes may have died out in the isolated environment, and the population may fail.
Of course, a mutation may arise which would enable organisms to survive in the new conditions, but environmental changes would almost certainly be faster than the rate of evolution through mutation.
Some species have different behaviours and appearance while still remaining the same species. Some of Darwin’s finches are an example. At least two varieties of one of the species feed on the Opuntia cactus, but they have different ways of feeding on them. One variety has a long beak and can punch holes in the cacti, while the other variety, with a short beak, break open the cacti to feed.
The birds can and do interbreed, so they are indeed the same species. This is similar, I presume, to the variation in skin colour in humans or the various blood types in humans. Such species have the same genes, but have slightly different versions (alleles) of it. This is called genetic polymorphism.
A species, like the finches, has to adapt. If its environment changes and it is unable to respond, then it will die out as innumerable species have done and are still doing. However, a species needs time to respond to environmental changes. For instance, polar bears may die out because the sea is is not freezing over as it usually does, and as a result there are no seals for the bears to hunt.
Whether or not you attribute the warming to mankind’s actions or not, the lack of freezing is a fact, and the bears are so far unable to adapt to the new conditions, and are often becoming a nuisance to arctic communities.