Honest And Open Question
I have always had a question for evolutionists of the Neo-Darwinian persuasion that I have yet to have cogently answered.
I thought of it when reading Coyne’s Why Evolution is True. Coyne states,
“if evolution meant only gradual genetic change within a species, we’d have only one species today—a single highly evolved descendant of the first species. Yet we have many… How does this diversity arise from one ancestral form?” It arises because of “splitting, or, more accurately, speciation,” which “simply means the evolution of different groups that can’t interbreed.” - Coyne, Why Evolution Is True, pp. 5-6.
Here is my question, if speciation involves mutation that creates a new species that can no longer interbreed (since otherwise all life really would all be one genetic species with only varied morphology), then how does an advantageous mutation get spread?
If parent species A births an offspring B with such a significant mutation as to be a new species and incapable of breeding with species A, then how would B reproduce? Would there need to be the EXACT same mutation at the same moment in history and in the same geographical location so that male-B would find female-B and copulate?
I really do hope that someone can explain this since this is one problem with evolution that just seems so fundamentally flawed that I doubt no one has addressed it. Any thoughts?
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Evolution doesn't happen by single mutations. Single mutations pile on top of each other until the genetic code of one group is no longer compatible with that of the original group.
ReplyDeletePeople often think that evolution is where one species just magically gets born of the previous species. It is not like that at all. Many generations pass between one species and another. Each generation can interbreed with the previous generation and even multiple generations previous. Eventually though, after many mutations passed down through many generations, the organism is no longer able to mate with distant relatives that have not been introduced to the same multiple mutations, or at least a certain number of them.
Hi Tyler,
ReplyDeleteI'm a little disappointed in you, buddy. Clearly you have an inquisitive mind, but you're missing links, either from the source material or your interpretation of it. Keep reading though, it's a jungle in there!
The subtle changes that occur over time occur to groups of species, not just one member of them. That said, when one member of a group has a mutation that makes it more well suited to it's environment, that individual is more likely to pass on its genetic material than one who is less well suited to its environment.
As for the difference in species, well, unlike the biblical genesis story, we (everything) did not start out in one small area of the earth. On top of this mutations are responses to external stimuli (or internal in the case of ingestion) and these are not always experienced by all living things at the same time, in the same place. Consider that there is photosynthetic (sunlight), chemosynthetic (chemical), thermosynthetic (heat), nuclear and electrostatic (and I think I'm missing a few and underrepresenting the others).
Nonetheless, you get the point. Different creatures respond differently given the stimuli.
Oh and on the inabilty to breed downwards, you are thinking of a human breeding with a chimpanzee, this was and would never have been the case. It was only ever extremely slow mutations that gave one an advantage strong enough to pass on their advantageous genetic material to the next generation (i.e. survival of the fittest).
Peace Jake
The slight, successive modifications do not answer the question because slight modification within a genetic pool will remain the same species. So Horse A, has Horse B, has Horse C, down to B-Horse A. Now, Horse Z(the last Horse) can mate with all other Horses. It is in the horse species. It must, at some point give birth to the first B-Horse which is a new species and no longer able to mate with the parent species. This is where the problem lies. The FIRST instance of a new species (regardless of how many successive modifications it took to get there) will at some point, NOT be able to mate with the parent species. But then, what does it mate with if it is the only kind of its kind?
ReplyDeleteThe common answer, and the ones you have provided allude that Horse Z is new in that it is far removed from Horse A and even though it can reproduce with Horse Y it is unable to reproduce with Horse A - thus making it a "new species." This seems to be the problem of the "heap." Imagine you have a heap of sand - how many grains must you remove for it to cease to be a heap? Now, if Horse Z cannot reproduce with A but it can with Y, then at what point did the species split? That would make EVERY animal a transitional animal which would either render all life the SAME species or NO life the same species.
This seems to be a fundamental problem of neo-darwinian evolution. The other is the INCREASE in information even though mutations are ALWAYS destructive - a mutation does not CREATE new information, but always rearranges or deconstructs previously existing information. Imagine a lego car - all the parts are there to make a pretty sweet lego car. Now modification means you can rearrange the parts to that lego car but that you cannot add NEW block - only rearrange the original blocks.This will either make a slightly different car, or destroy the car. But you cannot turn it into a Mansion, or a working space station. Complexity involves MORE information, not equal or less information.
This would mean that either ALL the code for ALL life would have been in the first living cell (something we know is not the case) or that information would have to have been ADDED.
(Part 1)
ReplyDeleteHi Tyler, I hope you don't mind if a curious student takes a stab at your intriguing question. I can see why the answers you've received so far have been somewhat unsatisfying; they don't seem to present anything that you probably aren't already aware of.
Perhaps the heart of your question would be answered best if I told you the circumstances that would have to be present in order for a population NOT to evolve. They are a set of circumstances that, if present, would allow a group of reproducing individuals to reach something known as 'genetic equilibrium,' or in the honor of the biologist and mathematician you authored the principle, 'Hardy-Weinberg equilibrium.' It simply states that the frequency of an allele (or the popularity of a peculiarity) will remain constant unless some disturbing factors are introduced. Among them, nonrandom reproduction (selection), overlapping generations, genetic drift, population limits (ideally equilibrium requires an infinite population size), and some other minor factors like meiotic drive. Surprisingly enough, none of these circumstances are ever found in nature!
While the benefit of this principle is that we can derive a binomial equation from it, it demonstrates the simple fact that any population of reproducing organisms in nature are in a constant state of evolution. Your question deals with how a population can not only diverge from its parent, but also become the founding population of a new species; an even that seems especially odd if we think of evolution as gradual stages of change. Shouldn't there be some punctuated event? And if so, what are the odds that such an event would be shared by enough members of the population to be inherited?
The answer is that there is no punctuated *genetic* event in speciation. The only major event is when a founder population (a new one) diverges from its parent, cutting off all interbreeding, thereby establish the ever so important 'reproductive barrier.' The important thing to remember is that populations don't split purely in half. As one might expect, founder populations are much smaller than parent populations, and smaller bodies are disturbed much more easily than larger bodies.
At its fundamental level, evolution is really a change (or a fluctuation) in allele frequency, which is quantitative relationship between the frequency of an allele and the number of members in a population who have the potential to inherit it. Let's not use the examples 'Horse A' or 'Horse B' as it is better to think in terms of alleles: 'Allele 1' and 'Allele 2,' perhaps?
For the sake of extreme simplicity, let's say that a founder population diverges from a parent population, and all members of the species are distinguished by the inheritance of one of two alleles: A1 and A2. Because both offer a relatively same level of reproductive benefit, both have a frequency of around .5.
(Part 2)
ReplyDeleteOf course, we know from the principle of genetic equilibrium that the frequencies are anything but constant: they are constantly shifting, but in large populations, this shift is incredibly minute if neither affects reproductive success. However, when the founder population (which is considerably smaller) diverges, the smaller sample size sports a different ratio of alleles due to pure chance: we'll say A1 (.2) and A2 (.8). You would imagine that the frequencies would balance each other out, but the small size of the population makes this a difficult task. Any change carries a much heavier influence on the dynamics of the population as a whole.
A few different things could happen in this example. One allele could become extinct, and the other could be shared throughout all members...become 'fixated,' as it is technically known. However, we must also remember that the environmental circumstances into which the founder population ventured may be different from those that its parent experiences. Perhaps A1 and A2 would be distinguished by their reproductive benefit in this case, or perhaps neither is satisfactory. A slight mutation may create A3 - an allele which would be unnecessary in the parent as the other two alleles were both stable and satisfactory. But considering the smaller size of the population and the different circumstances that had arisen, A3 may be a viable possibility for evolution.
Even in this incredibly simple example, there is a very great potential for evolution. When discussing species that reproduce sexually, the examples are much messier and impacts of population size are much more profound. What should be taken from this example is that a population is not a homogeneous mixture, where any new trait is eventually blended into the others. Genes are not drops of paint, to borrow a metaphor from Dawkins' "River out of Eden," but rather balls of different colors. Some grow, others shrink, but they hardly ever mix.
Also, I have to disagree with you on your last comment. Mutations are not only destructive. Any rearrangement of nucleotides in a gene does add additional information relative to the information content of the previous gene given the basic definition of 'information.' Beyond semantics, genes often duplicate, as do entire chromosomes and sometimes entire genomes. (I recall reading an article - perhaps you could find it on PNAS - about a study which posited the possibility of a new species of yeast arising from the duplication of an entire genome.) While a pure, unaltered copy would be redundant (and in the fine language of genetics, this redundancy may have some disadvantageous consequences), it also provides raw material for new material to originate. New 'legos' of DNA are being added constantly.
PS: Please ignore any spelling/grammatical mistakes. It is just after 1 in the morning of my writing this and I am far too exhausted to proofread.
Still waiting for part 2 and beyond before I comment. I appreciate your insights so far. :)
ReplyDelete