Evolution is a drastic physical change in many individuals of a population or species within one or two generations resulting in the birth of a new population or species and the extinction of the parent population or species by some unknown or unidentifiable mechanism.
Edited to fix misremembered definitions of mitosis and meiosis.
That depends on how we define reconstruct. Lets see if I have this right.
If I understand you correctly, I'm pretty sure you have this wrong. It looks like you're confusing the two halves of a chromosome pair with the two halves of the DNA double helix. You've got the formation of gametes (haploid sexual reproductive cells or meiosis) mixed up with mitosis (cell division). It looks this way to me because you go on to say:
The chromosomes split into two pieces. From each of these two pieces (really each piece seems to be multiple pieces, but the term piece refering to each complete half of the DNA should suffice) the entire and correct dna sequence can be constructed.
Sperm and egg are haploid cells, meaning they only have half the full chromosome complement. For humans, instead of having 23 chromosome pairs like all other cells, sperm and egg have only 23 individual chromosomes. A haploid cell is created during meiosis by selecting only one chromosome from each pair.
Every chromosome is a lengthy and tightly coiled strand of DNA. During mitosis every chromosome (all 46 in humans) has it's DNA split down the middle, and each half goes to one of the two cells created by the division. These DNA halves are used as templates to rebuild copies of the original DNA strands. These rebuilt strands form the chromosomes (again, 46 in humans) which then gather back together into the chromosome pairs (23 in humans).
This message has been edited by Percy, 11-18-2005 12:43 PM
Regardless, I'm still in the dark about how what happens after the bacteria are exposed to the antibiotic. What is the connection between exposure to the toxin and the surviving bacteria producing a beneficial mutation that is passed on to the next generation?
Bacterial populations are always producing mutations. An antibiotic does not change this. Adding an antibiotic to a bacterial population does not change the way it produces mutations, except perhaps the stress caused by the antibiotic might cause more copying errors (mutations) during reproduction.
Billions of reproducing bacteria produce billions of different mutations. If one of those mutations confers a defense against the antibiotic then since the descendant bacteria will be more likely to survive and produce offspring they will eventually dominate the bacterial population.
Since, as you say, the antibiotic doesn't change the way bacteria populations mutate, how is it possible to determine if particular post-antibiotic mutations are due to the antibiotic or due to "natural" mutations (that may have occurred without the antibiotic)?
Few reproductive events produce perfect copying of the genetic material, hence copying almost always produces mutations, and they are random with respect to adaptation. Mutations are always occurring in bacterial populations, including mutations that would protect against an antibiotic. When an antibiotic is added then bacteria with a protective mutation have an advantage.
Mutations continue to occur after the antibiotic is added, but I'm not aware of any evidence for whether they they are different in character from before the antibiotic was added. Mutations generally are random copying errors and can occur anywhere and be of any of a number of different types. Again, mutations are random with regard to adaptation. Likely the frequency and severity of mutations increases in populations under stress.