I get the whole "man didn't evolve from ape but they did evolve from a common ancestor". But is there a reason that under different circumstances that one species couldn't evolve from another where they both ended up existing at the same time?

For example, a group of species A is separated off to another location with different environmental influences and eventually evolves into species B. But the rest of species A stays where they were and doesn't evolve because of some other circumstances.

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A species in the same geographical location for 5 billion years will still undergo change. There's no final state the gene is "trying" to accomplish. The gene will never stop changing and, as a result, the species will continue changing. Random mutation also occurs along with natural selection so the species is undergoing a constant change.

This is the point I was getting at. That regardless of environment, the species will still change.

I apologize to Scott, I got a little off his question whilst trying to understand it but all interesting nevertheless.

No apology needed.  I had no idea this "I wonder if" question of mine would spark so much interesting discussion.

We have several issues here, of which four are:

1. Referring to George's excellent pic, we don't know when a procreating couple are Hominini and their offspring Homo. The change would require many generations.

2. At no point in any line of descent are there two individuals a myth maker might call Adam & Eve.

3. A large enough group will divide into two groups, one group leaving in one direction and the other probably staying but possibly going in another direction. If the offspring of one group ever met the offspring of the other, how many generations would have passed before they would be unable to mate successfully?

4. Would speciation (the inability to mate successfully) be signaled by an increasing number of miscarriages or early deaths?

Imagine you have a kid, and you take a photo of them every day of their life from birth to adulthood.  If you shuffled the photos and had not labelled them, could you get them in the exact correct order again?  Which picture shows the day the child became a toddler?  Which day were they first a teen? 

Now imagine that instead of these pictures being the record of one individual, they represent the history of an ancestral line, with each day representing a generation.  It's that gradual.  An ape doesn't give birth to a human one day, it gives birth to an ape that happens to walk on two legs slightly better than the other apes in its tribe.  Maybe most of them can wade through a stream a meter across, and the better-walking ape adds a few centimetres to the distance it can manage.  Everything that makes a species the way it is comes from a line of tiny changes that would seem insignificant if you compared one generation to the next, but which add up to obvious dsignificance when you're comparing them from distant generations. 

Another analogy would be to imagine you have a pile of 10,000 grains of black sand.  Each day you replace one grain with a grain of white sand.  At what point did the overall colour of the pile first turn grey?  What if there were two piles of sand, and one was getting white grains and the other red grains?  What if, at a point when the first pile had turned light grey and the other had turned a dark crimson, any other black sand piles were washed away?  Now you have two piles that shared a common ancestral colour, but since that ancestor has died away, they both become islands unto themselves.  They will continue to change and become increasingly different from the ancestor they shared, and different from one another. 

When talking about evolution, people (even biologists) use to forget how the mechanism of natural selection works at molecular level (what Darwin deduced from his long time observations in many natural phenomena: fossil record, similarities and disimilarities between geographically separated organisms, etc.) A species offspring F1 present phenotipical differences produced by small genetic differences at molecular level. Some of these F1 individuals survive because their phenotipe works in such an environment while other F1 individuals don't; this is natural selection. F1 individuals may resemble their parents but they have small differences in their genes at molecular level. Some times, natural transformations of environment separate some individuals, some times those individuals migrate to different environments where some of these migrants survive while. Those F1 survivors produce an F2 offspring presenting small phenotipical and genotipical differences as compared to their F1 parents... this is how speciation works driven by the accumulation of genetic mutations at molecular level and environmental selection of those phenotipic patterns better working in such environments; this is what Darwin named adaptation, just like many people and biologists call it today. Nevertheless, organisms do not adapt to environment; the environment selects those individuals whose prebirth mutations at molecular level allow them to better work in such an environment, it is "they were already adapted". My statement is that adaptation is an anachronism reminiscent from Lamarck's evolution model and used in a different way by Darwin. So, it should be better to stop using adapt and adaptation as part of Darwinian evolution by natural selection.

I am not sure I understand your question.  But there are some examples of organisms that have not evolved much over millions of years.   Examples are Limulus polyphemus (horseshoe crab) and the genera Triops and Lepidurus (Tadpole or shield shrimp).  Organisms that do not go extinct, are well adapted to their environment,  and are living in an environment that is stable, tend not to change (evolve).  If an organism is well adapted to it's environment, it does not pay to change.

William, I've seen horseshoe crabs and agree that they are creatures from an earlier time.

About your "..., it does not pay to change."

Are you saying an organism can look at its environment, evaluate its adaptation to what it sees, and decide whether changing is advantageous?

I've no doubt that a highly evolved organism, such as a homo sapiens organism that's sober and free of other constraints, can do that. (Homo sapiens that fail in spectacular ways and take their DNA out of action might win a Darwin Award.)

Saying it in a way that applies to a wider variety of organisms (including homo sapiens):

Organisms mutate in many ways and at various speeds.

Environments change in various ways and at various speeds.

Some mutations in some environments survive and reproduce.

(Our predecessors were among those who reproduced.)

This all got me to thinking that if we never really finish evolving, what will we as humans end up like in the future? If we indeed adapt to a changing environment, will our fingers become different from keyboard use?, will our eyes change from watching screens? I point these out in particular because this seems to be what most of us do most of the time.

It is interesting indeed that we have evolved not only because of the natural environment changing, but also because of what we have created and invented and our adaptation to material items.

I also have to wonder that if we don't increase education of the masses, how this will eventually affect  the development of our brains. (Please see the movie: Idiocrasy, very silly movie but the story behind it is very scary: http://www.imdb.com/title/tt0387808/).

We travel the world, keeping gene flow pretty continuous, and we take care of our sick and disabled.  We've largely removed ourselves from the filter known as "natural selection".  Any biological evolution we do from this point will likely be human-directed, through sexual selection or even direct editing of genomes to correct known genetic flaws like Huntington's Chorea.  "Gattica" may be the more apt cautionary tale. 
Michio Kaku's take.

"We travel the world, keeping gene flow pretty continuous, ...."

Bigoted people call this race-mixing and do everything they can to prevent it.

According material I read some months ago, the genetic diversity our travels and our mating will help our descendants' bodies survive the activities of all kinds of tiny organisms their bodies will carry.

Sandi, I will use "...will our fingers become different from keyboard use?" to illustrate my untrained understanding of evolution:

1. If keyboard use results in millions of people getting repetitive strain injury (RSI), and

2. If people with RSI produce more offspring than people without RSI, then

3. RSI will become widespread and keyboard use will have played a part in human evolution.

A more common explanation:

Giraffes did not grow long necks so they could eat high on trees. Instead, giraffes with long necks produced more offspring than giraffes with short necks.

But, the idea that human behavior can cause evolution might result in imaginative writers producing sci-fi stories that sell well. Those writers might then produce more offspring than less imaginative writers.

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