I have 2 questions:
A: How come animals such as the eagle, that are so similar, have managed to evolve on multiple continents? Are the conditions that identical?
B:How did animals survive before they had evolved to their conditions? Did they barely scrape along, or did they find Ok place, then evolved so they were perfectly suited to it? If the latter, could a species become too evolved (specialised?)
Not sure what you mean here. IIRC, Eagles fly around a lot, and thus live in a lot of places.
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You have to remember that evolution is a constant process. Generally, if there is a natural equilibrium, things usually will stay at the status quo. When and if the environment changes (or they find themselves in a new environment) then whatever traits that are advantageous to survival will get passed along, and the ones who have inferior traits will die out, until a new equilibrium occurs.
If you look at the fossil record, you’ll see most things stay the same for a while, then change drastically all of a sudden, then stay the same for a while. This is called “punctuated equilibrium.”
In other words, things tend to evolve when the environment demands it.
If two closely related groups are isolated for a while, speciation can occur. That is, the two become so different that they are no longer compatable for mating.
Hope this helps. If it didn’t please be more specific with your question and I’ll try to answer (been a while since Biology class, tho)
All birds almost certainly descended from one species of bird. Most of the similarities can be explained by their common ancestors. For species that have no recent common ancestors, like birds and bats, the fact that similar environmental conditions will lead to similar solutions explains a lot. Conditions don’t have to be “identical”, a wing is awing, whether you’re in a jungle or a plain.
Keep in mind that evolution is relative. The driving force behind evolution is that some individuals reproduce more than otheres of that species. This may result in an overall benefit to the entire species; it may not. Consider a bunch of pine trees. If one tree is taller than another, it will shade the shorter tree. Shorter trees will therefore die out, leading to a higher average tree height. If there are a bunch of other species in the forest, this may allow pine trees to compete with them better. If there are only pine trees in the forest, then they are only competing with each other, and there will be no overall benefit for the pine trees as a species (in fact, they’ll all have to spend more resources growing taller, which is a disadvantage). Even if there are other trees, those other trees will also grow taller, and so pine trees will be in basically the same relative position. So if you look at trees now, you may wonder how any species of trees could possibly evolve from, say, a fern; any plant which tried to compete with all those other trees would have a heck of a time. But those trees weren’t always there.
I have two answers:
A. no animals have evolved on multiple continents, or multiple times, or anything like that. They appear once and spread. Evolution converges sometimes, in response to similar environmental pressures, so we have, for example, both birds that fly and mammals that fly, and we have birds that don’t fly and mammals that don’t fly.
B. no animal is perfectly suited to its environment, ever. There are always threats to it. Evolution is a continuous process driven by two things: a certain degree of randomness in the gene shuffling with every generation, which accounts for differences between individuals (mutations fall in here too), and natural selection, which determines the fitness for reproduction of any individual. A lot of creationists mock the basis of evolution by assuming it’s all driven by random chance, but that’s a fundamental misunderstanding of it. Natural selection is the mechanism, not random chance.
A: All eagles are pretty closely related. Rather than being examples of convergent evolution, where distantly related animals share similar features, they are examples of an adaptive radiation, where, a successful type of animal (in this case, the common ancestor of all species of eagle) colonizes many regions. As populations in the different regions drift apart and develop into different species of eagle. The different species of eagles share a great many features by descent, rather than convergence.
B: The Ryan covered this pretty well. Maybe an example will help. Let’s say that a group of eagles arrive at a place where their best bet for food is to snatch fish out of the water. Some of the eagles in the group will be good at snatching fish, others won’t be. The ones that are better at snatching fish will have more offspring, and in the next generation the average fish-snatching capability will have risen. That way, the eagles will get better and better at fish-snatching from generation to generation. I don’t really want to bring up favorable mutations or novel gene combinations right now, but they can contribute to increased fish-snatching ability too.
Also bear in mind that at one time, there weren’t multiple continents. There was one big continent that split apart, each chunk carrying its cargo of critters in another direction.
I think this has been well answered, but here’s my two additional cents…(I’m no evolutionary biologist, but the topic interests me)
A - A species evolves and spreads. Different regions have different environments/conditions, so different traits (those that are advantageous to that particular area) begin to come through over time. So, you start getting variations on the same theme when comparing species (like eagles) from different regions. There is usually limited room for a particular trait…so one species of super-raptor would outcompete others…so speciation tends to stay regionalized. The result? Multiple variations from a common ancestor.
B - As environments change, species with traits that work are able to produce more offspring. Those with traits that don’t work, die off. Now, if there is a gene mutation, more often than not, it would not be advantageous and would not succeed. But occasionally, a mutation works. There are several “forces” at work on the evolution of species (e.g., natural selection, sexual selection, gene mutation, gene flow, etc.)
In fact, competetion * within * a poopulation is an important part of Darwin’s theory, and it explains why animals can evolve without competition or forces from outside.
To take the pine tree example–each tree wants (well, I guess wants isn’t the right word, but you catch my drift) to pass on its traits. So even in a forest of only pine trees, there is competition and pressure to evolve. Presumably, there is only a limited amount of resources available, and you want you and your offspring to get as much as they want.
As I said, minor point…
Umm, that’s ** population ** 
Minorly tangential, but traits (not species) can be developed several times separately. Flight for example evolved in the Pterodons (sp?), the birds, and the bats completely separately. Vision also has evolved differently in different animals.
Your questions have been addressed except for the last one. The answer to: If the latter, could a species become too evolved (specialized?) . YES. The best example today is the cheetah. They have evolved to chase down and kill fast herbivores. They really can only hunt a small group of prey. If something happended to their prey??? Also, these cats are all very closely related from a DNA standpoint. Genetically speaking, a single virus could be harmful to the entire population. The saber tooth is a great example of evolution carried to far. Once the large herbivores ran out; the cat could not adjust and died out. A lot of stuff and time left out but you get my drift.