Monday, February 23, 2009

A Question to Ponder

Please reply on the blog: (optional, but those that do will benefit)

Read the following statement. Then, based on your knowledge of cell biology,genetics
and evolution,decide to agree or disagree with the statement. Whichever you decide,
write a short paragraph that provides solid evidence defending your choice.

“When you think about sexual reproduction,it makes no sense. After all,evolution
selects for organisms that are best fit. In a population of sexually reproducing organisms,
a mutant that reproduced asexually would increase its representation more quickly than
the wild type,that is,it would have a higher fitness. So we should expect asexual
reproduction to be much more widespread among eukaryotic species than it is.”

Mrs. Lyon


WhitHar said...

Ok, so I don't agree that asexual reproduction would be or should be more widespread. Sexual reproduction allows different individuals to combine genetic material to produce offspring so that offspring are not genetically identical to their parents. This means that not all of a parent's genes get passed on to an offspring like in asexual reproduction. Although in asexual reproduction, the best adapted individual would be able to directly pass on their genes, there wouldn't be a chance for two of the fittest parents to combine their genes and create offspring that are more adapt to the changing environment than both the parents individually. Sexual reproduction causes recombination of genes and therefore creates more combinations of genetic traits in a population, and therefore more genetic variation. A population of sexually reproducing organisms can go through more natural selection, and adapt better, than an asexual population. An individual cannot be perfectly adapt to every condition. There is a chance in sexual reproduction that the strongest traits from each parent will be passed along to the offspring. Sexual reproduction may also affect the rate of speciation. Since sexual reproduction causes recombination of genes, it produces new combinations of alleles at different genes much more quickly than does asexual reproduction (which would just rely on mutation). Based on this increase in genetic variability in sexual species, evolution can occur more rapidly, so speciation should also occur more rapidly. Sexual reproduction is thus subject to species selection. One reason there are so many species with sexual reproduction may be that sexual reproduction increases the rate of speciation, and so many species with sexual reproduction are produced. Therefore I feel that asexual reproduction would cause a disadvantage in a species rather than helping it out.

meghan said...

So I had to read the statement at least 25 times to make up my mind whether I disagreed or agreed with it. Maybe that isn’t a good thing, but I think it will help me understand all the concepts a little better so I’m going to try to answer this.

I DISAGREE and here is why:

Yes, asexual reproducing organisms reproduce more quickly and exponentially. In asexual reproduction the offspring bud off and contains many cells (eukaryotic) and through processes like binary fission (prokaryotic). Regardless of whether the organism is eukaryotic or prokaryotic it can get its DNA from only one parent and therefore does not contain genetic diversity like offspring produced sexually. The only genetic diversity in asexually reproducing organisms occurs when mutations happen and it takes approx. 2400 generations (I think ) for the mutation to be considered dominant or produce a new species. Some mutations are helpful and others are harmful. The viability of the mutated organism may depend on the affect of the mutation and whether it occurred on an intron in its DNA. In sexually reproducing organisms genetic variation occurs every time a new individual is produced. Genetic variation occurs when the sperm fertilizes the egg to form a zygote, during crossing over, and when mutations occur. The DNA of sexually reproducing organisms goes through many mutations however most of them are corrected during the checkpoints of mitosis. Although most mutations are bad, some of them are good. The statement above does not state whether or not the mutations that changed the organisms DNA were good or bad (aside from changing is method of reproduction).
According to Darwin’s theory of evolution, the animals that survive are the fittest and therefore are able to reproduce and pass on those desirable traits. With asexually reproducing organisms they generally do not live as long and produce much more often. Although the organisms with the less desirable traits may die sooner, they also reproduce quickly and therefore those traits are always passed on. If a good mutation occurs then yes, it will become more populated but it takes a very long time for the helpful mutation to show itself in majority of the organisms. I would not expect asexual reproduction to be more widespread because the two fittest parents cannot contribute the desirable genes so undesirable traits are constantly being passed down. I would expect there to be more sexually reproducing organisms than asexually reproducing organisms because genetic diversity is what controls speciation and these sexually reproducing organisms have more success for adaption.