what drives evolution?

Factual Questions
1

simple question, although I doubt the same could be said for an answer-

what is thought to be a bigger factor in the evolution of a different species? Is it a mutation causing a second species to slowly arise alongside the original, in the same environment? Or one species breaking up into two different environments?

I guess, does the up-and-coming species find the niche, or the niche the species?
jb,
who had wholeheartedly thought the former, but whose mind is being broadened by too much thinking about pointless stuff

2

In a way, I’d have to fall a little more on the side of the importance of mutation. Without mutation, new alleles wouldn’t arise, and genetic variability would be nonexistent; hence no speciation.

In speciation, the separation of two populations (allopatric speciation) is generally accepted as a simpler model to understand, and a higher probability of being the correct explanation, than sympatric speciation (where species diverge from one population, alongside each other, in the same area). Both can occur, but most of the classic examples seem to follow allopatric speciation.

3

I would think that the allopatric model is found more frequently. After the genetic mutation is expressed, it will generally either overwhelm the older “model” by the advantage it confers on the individuals or it will be overwhelmed by the existing “model” because it does not confer a benefit on the individual. It might remain a recessive trait until a change in ecology allows it to confer a greater benefit, but at that time it will then overwhelm the older variation.

Events such as those found on the Galapagos are not common. There, an initial population found itself on a previously uninhabited island where multiple potential niches were available to be exploited. Mutations that allowed the various niches to be exploited would not be in direct competition with the older populations, so they could be exploited.

There are not that many uninhabited ecosystems in the world that would permit that sort of event. The niches in most locations have been filled by other species (from different genuses–or even families) so that there is pressure on any given species to “conquer or die” in its own niche.

4

Yes, allopatric specialtion (speciation of isolated populations) is definitely the more common of the two. For several reasons:

First, if a mutation appears in within a single population, it is unlikely to be able to prevent interbreeding by in and of itself. So this mutation will be free to spread throughout the population. This means that the single population will continue to evolve, but it will remain a united species.

(There are exceptions to this. If a mutation causes the mutants to breed at a different time, causes different mating behaviours, causes a mechanical change that prevents interbreeding, or something that similarly prevents the mutants from mixing with the gene pool at large, sympatric speciation (speciation within a population) can occur.)

On the flip side of the first reason, if two populations seperate, there is nothing tying their gene pools (and therefore their evolution) together. Novel mutations that appear and lead one population in one direction will be different from those that appear in the other population, leading it elsewhere.

Also, allopatric speciation is more likely as the subdivided populations are smaller, thereby making genetic drift a bigger factor. A mutation can more rapidly fluctuate to fixation due to random sampling factors in a small population than in a big one.

Furthermore, the environments of the two split populations are likely to be different (if only very slightly) from each other. This means that natural selection will put different pressures on each group, causing them to differntiate from each other. This is especially true in a situation called ‘peripheral isolates’: a small sub-population that breaks off the edge of the parent population. Picture a species that occupies all of a desert. The edges of that desert probably have more rain, more vegetation, etc. If a small population somehow gets seperated there, the selection pressures will be different from the harsh central-desert region.

I’m sure there are more reasons, but that is all I can think of for now.

-b

5

Ahhh. See, this is the kind of thing I wasn’t thinking about. So I’m guessing it’s usually the case that mutations often don’t show themselves immediately? I suppose this was a rather large misconception to labor under, thinking of mutations appearing -ping!- and the new species totally besting the old (grr, a vestige of the “evolution is geared to making better life forms” mindset that I thought I had gotten rid of long ago).

Lemme see if I got this straight. A species has a bunch of different genes floating about the pool, some recessive and some dominant. Any can mutate, or get shuffled into different genetic combos (that’s not so clear- uh, consider the poker hand: no good hand, but you can have a card “mutate” and become a different card, or you can draw a card, and separate genes can combine together to the organism’s advantage or disadvantage- glurg, still not clear).

Now where the hell was I. Okay, I guess what I was (roundaboutly) getting to is: of the mutations that occur, both in allotropic speciation and sympatric, are the more influential ones recessive, or dominant.

Seems that the dominant would have the more pronounced immediate effect, but the recessive should weigh in more over the longer time frame. Because dominant mutations either hit or miss in a few generations, but recessive genes simmer in the pool until a random environmental change brings them to the forefront?
jb

6

JB:Seems that the dominant would have the more pronounced immediate effect, but the recessive should weigh in more over the longer time frame. Because dominant mutations either hit or miss in a few generations, but recessive genes simmer in the pool until a random environmental change brings them to the forefront?

That sounds pretty good to me, but it wouldn’t have anything to do with speciation. A mutation or group of mutations wouldn’t get you to speciation for as long as there is breeding between the dominants and the recessives the will still be one species.

Some figure it takes a million years separation before the two groups would become separate species.

Jois

7

The cichlids of Lake Victoria have developed into hundreds of separate species in only about 12000 years, all while sharing the same lake. So speciation can happen amazingly fast, even when the animals are not effectively separated physically. (The same thing has happened in other African lakes in a very short time. I think this kind of thing probably starts out as behavioral isolation, which seems to lead to reproductive isolation pretty quickly.) Here are a couple of cites:

http://cas.bellarmine.edu/tietjen/Ecology/ecology_returns_to_speciation_st.htm

http://www.fonz.org/zoogoer/zg1998/27(3)cichlids.htm

There’s a lot more on the web if you search for “cichlid africa speciation”.

8

Just a couple of things:

First, dominant and recessive do not have anything to do with how common alleles are in a population. Polydactyly (extra digits) in humans is a dominant allele, but it is only found in a tiny percentage of the population. The recessive allele – that for only five digits per – is far more common. And genes do not “remain” recessive; they are either recessive to another allele or they are not. If they are, they will always remain so.

Also, all genes are not dominant/recessive systems. In fact, very few are. Widow’s peaks and attached earlobes are dominant/recessive traits in humans. Other gene systems have incomplete dominance. Snapdragons have red and white flower color alleles. Individuals with one of each have pink flowers: the red allele is incompletely dominant. Then there’s codominance where both alleles are dominant: i.e. if an individual has both of them, both are shown completely. Human MN blood groups are like this. If someone has both M and N allels, they express both M and N proteins on their blood cells. There are multiple allele systems – like the human ABO blood group where A and B are codominant to each other, but are both dominant to O. There is epistasis, where alleles at one locus are controlled by alleles at another: mice have a black fur allele dominant to a brown fur allele, but neither of them will have an effect and the mouse will be albino if a different gene has two recessive albino alleles. Then there are traits like height, weight, skin color, and many more where the appearance of the individual is determined both by a large number of genes and environmental factors.

The point of this whole rant is that genes only rarely work in a simple Mendellian dominant/recessive way. There is a whole continuum of different ways that genes can behave.

The great majority of mutations are harmful to the individual. As any life form is an incredibly complex interaction of chemicals, that is hardly surprising. Rarely, however, a mutation happens that actually makes something work better. If said mutation is significantly better suited to the environment, then – whether recessive, dominant, or other – it will soon make up a significant proportion of the population due to the increased success of the decendants of the bearer of the mutant allele.

And yes, it may take millions of years for significant changes due to evolution to occur. Specifically, if the population is large, well mixed, and not heavilly being selected for, evolution will be quite slow. For instance, pigeons, rats, dandelions, widespread tree species, things like that may not evolve much for eons. However, if populations are small, seperated, or facing strong selection pressure, evolution can happen very rapidly. For example, populations on young islands like the Galapagos or Hawaiian Islands, populations facing rapid environmental change, species split by mountain formation, rivers, canyons, etc.

OK, rant over. Y’all may continue. (In my defence, I hale from Kansas, so seldom is the chance I have to get a good evolution rant in.)

-b

9

If I can jump in with a recommendation…

There’s a book called The Beak of the Finch by Jonathan Weiner, which is an overview of the work of scientists that have spent years studying the finches on the Galapogos islands. They collected data on the physical changes of the finches and compared it with the changes in the weather patterns of the islands from year to year. Absolutely fascinating reading, and quite compelling evidence for how evolution “works,” albeit in an extreme and isolated environment.

I get the impression from the book that evolution is not exactly an active process, but rather one that occurs only as a reaction to the environment.

[sub]minor, teeny tiny hijack…[/sub]
[sub]not trying to start another endless evolution vs. creationism debate![/sub]
I tried recommending it to a cousin of mine, who is a fundamentalist Christian that takes the idea of the young-earth concept and creation science very seriously. He didn’t even attempt to read it.

Go figure.

10

When and how mutations express themselves is going to depend on a wide variety of things, which I’m going to skip right not in the interests of brevity and simplicity (we can always go into them later).

Suffice to say that mutations cause variation in a population. Let’s take the cichlids Bob mentioned as an example. Mutations and a variety of gene combinations produce variation in the size and shape of the jaws of these fish while they are all still part of one species. So some fish have a thicker jaw with sharper teeth, while others have a thinner jaw with more blunt teeth.

Now that there is some variation in the population, natural selection can act on that variation. Natural selection does not directly act upon mutations, but on differences between phenotypes (i.e. a mutation that has no effect on phenotype will have no selective advantage or disadvantage). The fish with sharper teeth find it easier to eat fish eggs and larvae, while the fish with blunt teeth find it easier to chomp on insects and detritus. Let’s say for the sake of arguement that fish with teeth that are neither particularly blunt nor sharp aren’t good at either, so they lose in competition for food.

Now we have: Sharp teeth = lots of successful offspring, blunt teeth = lots of successful offspring, and intermediate = not many offspring. Over time, the elimination of the intermediates and the lack of successful breeding between the sharp teeth and the blunt teeth causes reproductive isolation. Sharp teeth only breed with sharp teeth and blunt teeth only breed with blunt teeth. Now we have two species.

It took a long time for natural selection to eliminate the intermediates, and for the sharp and blunt to stop breeding with each other. That’s why the mutation did not immediately produce a new species. Instead it produced variation in the phenotypes in the original species, which natural selection acted upon. It is also possible that the mutation could have introduced variation that had no selective advantage or disadvantage. For example, let’s say a mutation introduced an extra fin ray on the cichlid’s dorsal fin. Having 11 instead of 10 fin rays probably won’t have a selective effect, and won’t cause speciation.

Also, let’s say the environment were different. Imagine the only type of food in the lake were molluscs, and both the sharp and blunt toothed fish were equally good at eating molluscs. Since there’s no functional difference between the two types of teeth, in this context the variation would not lead to speciation.

I’ve left out mention of how these fish only breed with their own type; they have different coloration patterns on their bodies and anal fins. Looking back at my example, I only hope that it wasn’t too confusing on its own anyway. If anyone can turn the above into something clear and readable, I will be eternally in your debt. :frowning:

BTW, there is a suggestion by a British biologist (P.H. Greenwood) that speciation in cichlid fishes was allopatric, but I’m a bit skeptical of that.

True, but natural selection only operates on expressed genes, and all else being equal, recessive alleles are less likely to be expressed.

Whew! my typing fingers are winded!

11

From my readings at http://www.talkorigins.org , the “forces” behind evolution are…

  • Natural selection
  • Sexual selection
  • Genetic drift
  • Mutation
  • Recombination
  • Gene Flow

How/if these forces come into play is probably most the debate within the scientific community about evolution.

From what I understand, Darwin espoused a gradual evolution resulting from natural selection and a smattering of other forces such as sexual selection. Modern day theory is “neo-Darwin” and has modified his view (although I cannot explain exactly how).

S.J. Gould’s theories about “punctuated equilibria” (a current debate) are that species’ gene pools remain mostly static during their tenure but that sudden changes in the environment (geologically speaking) that segregates a portion of the gene pool will cause a rapid evolution of that fraction in the new “niche”.

I’d say that species also affect their niches, so as to the OP’s question about the chicken-and-the-egg situation for species-evolutions-and-niches, I’d say there is some level of feedback.

But I’m no biologist.

12

Wvets: "Also, let’s say the environment were different. Imagine the only type of food in the lake were molluscs, and both the sharp and blunt toothed fish were equally good at eating molluscs. Since there’s no functional difference between the two types of teeth, in this context the variation would not lead to speciation.

I’ve left out mention of how these fish only breed with their own type; they have different coloration patterns on their bodies and anal fins. Looking back at my example, I only hope that it wasn’t too confusing on its own anyway. If anyone can turn the above into something clear and readable, I will be eternally in your debt."

Jois: I don’t think you’ll be eternally in my debt for this :slight_smile: but what seems to be missing or unemphasized in your description above is separation. If something like sharp or blunt teeth did appear in fish or chimps or cats, then the diets would change to adapt to the new hardware, as diets changed so would feeding grounds; those with blunt teeth would hunt out vegitation and those with sharp would hunt for smaller animals. With this kind of separation comes the opportunity for further changes that would lead to speciation.

But I don’t think this has much to do with JB’s question.

Jois

13

wevets, excellent post. Thank you for making clear some points I was struggling with.

Yes, absolutely true. I was merely trying to correct a common error; one which i thought might have been afflicting tomndebb when he/she said, “It might remain a recessive trait until…” Just trying to point out that if one allele is recessive to another, it will always be so. It does not become dominant upon becoming the most prevalent.

Jois: but the two types of cichlids do not have to separate for speciation to occur. True, they probably will since the two food sources are likely located in slightly different micro-habitats. But it is possible for speciation to occur, even if the food sources share the same habitat, due merely to “diversifying selection” (which is what wevets was describing) where sexual and/or natural selection favors the extremes of a trait and not the intermediates.

Finally, ShyGhost: Your post freaked the hell out of me, as The Beak of the Finch is literally sitting two feet from my left hand. Weird.

-b

14

Frankly, I could give half a rat’s ass. This stuff is great!

So What’s up with genetic drift? I read up on it last year, so I’ve got the basics covered, but foggily so. Does it tend to be more or less important than natural selection? Can we even tell?
jb

15

OK, genetic drift is basically due to sampling errors. Only some of the individuals in a population manage to survive to reproductive age, only some of them manage to find a mate, and only some of the offspring manage to grow up. So, out of all the alleles represented in a population, only a small proportion of them (a smaple) are passed to the next generation.

Say we have a bucket with a million marbles: 90% white and 10% black. This represents a large population with two alleles of a particular gene. Well, if we draw a sample of 1000 marbles, the percentages are likely to be fairly close to that in the parent population. But probably not exactly: if we get 901 white marbles and 99 black, that discrepancy is genetic drift. You can see how genetic drift will be a bigger factor in a smaller population. Say a very small population from the parent bucket moves off to colonize an island; just 10 marbles out of the million. Well, with such a small population, the chances that they would all be white are pretty good due to random sampling error. And once the allele is gone from the population, it cannot be retrieved (barring mutation). This is called “drifting to fixation”: the white allele has become fixed in the population.

With animals like cheetahs – which were hunted nearly to extinction at one point – many alleles get lost from a population due to the “bottleneck effect”: a large population gets squeezed through a period of only a very few individuals before again climbing in size, and thereby loses a large amount of genetic diversity in the process. The remaining individuals are more closely related to each other because there are fewer alleles in the population and inbreeding problems occur.

An example of this last point can be found on a small island in the Atlantic (I am too lazy at this point to look up the specifics) that was colonized by a small group of settlers a few hundred years ago. The natives of this population now have an unusually high incidence of a certain kind of blindness. Why? Because, due to random sampling error, an individual on the colonizing ship had a rare recessive allele. Once on the island, and with a limited mating stock, this allele became, in a few generations, much more prevalent in these islanders than in humanity at large.

So genetic drift is only really important in small populations, while natural selection is influential to populations of all sizes. However, I don’t no how to quantify “more or less important”, so I’ll leave that up to you to decide.

-b