Why did evolution branch off into two different types of species?
Posted on: September 20, 2007 - 6:28pm
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Why did evolution branch off into two different types of species?
This is more of a general question that I've been wondering about for a while. Most evolutionists say that humans and monkeys evolved from the same ancestor. But why did the evolutionary process create two different lines of species from one animal?
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I'm going to jump in and guess that different selection pressures were imposed on different groups of that animal that were geographically separated.
I study molecular evolution, not pop dynamics, but I can still answer:
There are four different types of evolutionary speciative mechanisms, and the ones we need to focus on are cladogenesis and anagenesis. Speciative mechanisms occur when two populations diverge, and is usually preceded by geographical seperation. In biological taxonomy, a clade is an evolutionary classification based on the organism in question and all of its direct descendants. In anagenesis:
1) A population of organisms is outphased by a new organism, hence giving a straight line on a phylogenic tree.
In cladogenesis:
1) Two populations X and Y of the same species. X becomes geographically seperate from Y and hence undergoes a different evolutionary gradient, with different pressures and hence a different change in morphology. Hence, if the two meet up again many generations in the future, they will no longer produce offspring. In a phylogenic tree, a "branch" or a "split" has occured since a population of the initial organisms moved away causing different morphology. That is the reason that two different species were created from a common ancestor.
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And given our phylogenetic tree, the fossile record and genetics, it is pretty evident that both mechanisms have influenced the hominid line. Cladogenesis furthur down the line, giving way to anageneisis as populations became better established, as is to be expected.
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Would anybody care to explain in a bit more detail anagenesis and this outphasing of a population by a new organism?
Or, could you recommend a link/book?
From what I gleaned from Richard Dawkins' The ancestors' tale, the vast majority of what he was talking about was cladogenesis. I was under the impression that geographic separation (doesn't even need to be long distances, just far enough, or with a barrier to prevent overly frequent mixing of the populations) was quite important as sexual reproduction tends to stabalise the genetic pool. i.e. even an organism in a population with a beneficial mutation of some kind still has to be similar enough to the rest of the population to breed etc.
Mind you, I suppose, even looking at just one population, the selection pressures could change, which would result in this anagenesis deludedgod mentioned.
Sorry if I've shown too much ignorance on the matters, I'd like to learn more, so if anybody wants to expand on the topic that would be great.
Why does some of my text highlight like that I wonder...
If you are referring to how humans and chimpanzees split into two different species, I believe it is closely related to climate change.
From what I have read, I believe that millions of years ago Africa resembled South America, as far as that rainforests consumed most of the continent.
There was a burst of cooling caused by geological upheavals (tibetian plains being pushed up by India crashing into Asia. Acted like a CO2 filter that lowered that greenhouse gas from the atmosphere.) that caused the rainforests to shrink and open savannah to grow.
Some of our ape-like ancestors decided to stay within the shrinking jungle. Others decided to venture out and take their chance in the savannah. Those that stayed in the jungle evolved into what we call chimpanzees. We are descended from those that ventured out.
Eh, but who knows? Bonobos and Chimps are very closely related, distinctly different, and live in the same type of environment and areas. And they refuse to interbreed. So maybe it wasn't near that dramatic.
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I've read that one key difference between bonobos and chimpanzees is that the Chimpanzee side of the impassable river that divides their habitats is also inhabited by gorillas. The bonobo side of the river doesn't have any gorillas.
The theory goes that this difference is responsible for a lot of the behavioral differences between the two species. Since chimpanzees compete with gorillas for ground forage, chimpanzee females have to range further and can't be physically near each other while foraging, which makes them vulnerable to aggression and exploitation by males. Since bonobo females aren't competing with gorillas for ground forage, they forage in groups and can band together to discourage male aggression.
So it's a great example of how environment has caused a drift in the instinctively-hardwired behavioral differences between the two groups. The study I haven't seen yet (maybe it's out there) is the one that connects the physical differences to the geographic separation.
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Interesting. Have you looked into the "Bili Apes"? Technically they are still considered just plain chimpanzees but seem to me to be as different from common chimps as bonobos.
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Sexual reproduction means anagenesis is going on all the time as different gene combinations are more or less successful at reproducing. This happens even if the other pressures are stable, resulting in a slow "improvement" of the species over time. In non-social species, like sharks, the process must be even slower.
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Bingo! Watcher beat me to the brief answer. It's also been shown that the split was a very "messy" one - lots of subsequent hybridization and so forth yet divergence eventually took hold and produce separate lineages.