Hi Randman,
I know you haven't had a chance to respond to
Message 111 yet, but I thought it would be helpful to add a little bit about how one species transitions in continuous fashion into another. I'll just focus on sexual species.
Every reproductive act is a tiny evolutionary step. We are all different from our parents, who are different from our grandparents, who are different from our great grandparents, and so forth. The genetic code of babies is never identical to either parent. Each new human being is a unique individual genetically (ignoring the special case of identical twins, of course).
This tiny change comes from two sources. One is unique allele combinations. An allele is one type of a gene. To explain this a little better, let's say there's an organism whose eye color is determined by a single gene. The different eye colors are the alleles of the gene. There would be a gene allele for blue, another for brown, another for green, and so forth. Each parent contributes an allele for eye color. If both alleles are the same then the organism's eye color will be whichever allele that is. If the two alleles are different then the dominant allele determines eye color. For example, in humans brown is generally dominant over blue, and so when one parent contributes a brown allele and the other contributes a blue, the child's eye color will be brown (I think eye color in humans is determined by more than one gene, and so it may not be as simple as I'm protraying here, but I'm only trying to get the principle across).
To continue with humans as an example, since humans have somewhere around 30,000 genes, if we assumed that each gene had only two alleles then the number of possible allele combinations is around 10
9000. Since only around 10
10 humans have ever lived, there's only one chance in 10
900 of an allele combination appearing that is not unique. 10
900 is a very large number. It's a 1 with 900 zeros after it. Or you could say that it's a billion billion billion billion billion...(keep this up until you've said billion 100 times). The likelihood that each newborn baby is genetically unique is extremely close to 1. In other words, the probability of any allele combination occuring more than once is very small, and keep in mind that many genes have more than one allele, which increases the odds of uniqueness even more. The odds are reduced somewhat because some allele combinations are hostile to fetus viability, and the baby is never born, often shed so early that the women is never even aware an egg was fertilized.
But I mentioned another factor that contributes to the tiny change associated with each reproductive event, and that's mutation. Mutations are caused by imperfect copying of the genetic material, DNA. The estimates I've seen say that on average around 10 mutations occur in the DNA of every new human being. This raises even further the likelihood that each newborn individual will be genetically unique. Most mutations are tiny, only a single base pair missing or out of order or replaced by the wrong base pair or matched up with the wrong base. But some mutations can be larger, such as a gene being inverted or moved to a different part of the chromosome or even to a different chromosome. There can even be extra or missing chromosomes. The more significant the mutation, the more likely it is to be fatal.
So we've established that every reproductive act creates a unique individual both in terms of allele combinations, and due to the addition of some usually harmless mutations. So if each child is different from the parents, and if this is true of the child's eventual children, and their children, and their children after them, and so on into the distant future, then it can be seen that change can only accumulate. There are forces acting to harness this change, but I'll discuss those later.
As this change accumulates, at which point does one species become another? That turns out to be a difficult question to answer, and is probably best explained using the example of ring species.
The most famous example of a ring species is the herring gull. You can find information about it from all over the Internet, but here's a short excerpt from this one:
Ring Species.
There are several ring species, but the most famous example is the herring gull. In Britain, these are white. They breed with the herring gulls of eastern America, which are also white. American herring gulls breed with those of Alaska, and Alaskan ones breed with those of Siberia. But as you go to Alaska and Siberia, you find that herring gulls are getting smaller, and picking up some black markings. And when you get all the way back to Britain, they have become Lesser Black-Backed Gulls.
So, the situation is that there is a big circle around the world. As you travel this circle, you find a series of gull populations, each of which interbreeds with the populations to each side. But in Britain, the two ends of the circle are two different species of bird. The two ends do not interbreed: they think that they are two different species.
(If you delve more deeply into herring gulls you'll find it isn't quite this simple or neat, but there's no need introduce extraneous details for this discussion.)
The point of introducing ring species is to help explain that the more genetically similar two species are, the more likely they are to be able to interbreed. There is no miraculous sudden cut-off point. Consider two fairly similar populations of the same species that are gradually drifting apart genetically. It isn't like one year they can interbreed and the next year they can't. What actually happens is that as they become more and more genetically different, fewer and fewer individuals of those populations are able to interbreed, until eventually it is so few that they should be considered different species.
So with the herring gull you can ask the question, at which point as you circle the world from Britain to eastern America to Alaska to Siberia and back to Britain does the herring gull become a different species than the Lesser Black-Backed gull. There's no unamibiguous answer to this question. When two populations interbreed then it is said that there is a gene flow between them, and As Don Linday accurately says in the link I provided, "Two species are the same if there is 'significant' gene flow between them. But there is no sharp dividing line between 'significant' and 'insignificant'."
I earlier mentioned that there are forces that harness or rein in the degree of change in a species. One is natural selection. For a well-adapted species in a stable habitat, any significant change may be maladaptive, and such individuals are less likely to survive to reproduce, or are less likely to produce as many offspring as the average.
Another force harnessing in change is the size of the population. This has been mentioned more than several times by more than one person here. A species with a large population (human beings come to mind again) will have a large range and hence experience a variety of environmental conditions. At the extremes of the range the evolutionary forces for change are greatest, but genetic change from these regions all flow into the large central population and are diluted.
It also takes much longer for change to propagate through a large population. Take a population of animals with a very broad range, perhaps the bison of the American plains, but before, say, 1800, which would predate when the herds began to be decimated. An individual with a beneficial mutation or allele combination that lived at one end of the range would produce offspring who possessed this genetic trait, and the question concerns how far those offspring might travel during their reproductive lifetimes. How far that range is governs how fast the trait can spread through the population. Reproductive rates are also a significant consideration. Creatures that stay relatively close to home during their lifetime and which do not reproduce frequently can spread their traits throughout the population's range only very slowly. Those who have the opposite qualities, those who range far and reproduce often, can spread their traits quickly through a very broad range.
I've hope I've shown that evolutionary change is continuous, and that the division of populations into distinct species is not an unambiguous task. I hope I've additionally shown that genetic changes can propagate much more quickly throughout a small population than a large.
--Percy
This message has been edited by Percy, 08-19-2005 09:23 AM
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