ThePhoenix

Fixed.

Jet Black

thanks, it was a silly oversight on my part

Dean Anderson

And rather beautiful...





That one is from Sierra Safari Zoo in North Virginia, USA.

He's called 'Hobbs', by the way, and has twice the mass of a Siberian Tiger...

openeyes

I think this is relevant to this discussion so I'm going to ask: Can someone try to explain to me how a mutation is a single organism eventually becomes part of a population? I read a book a couple years ago that said (IIRC) to become dispersed the mutation would have to be recessive and spread in a population until a critical mass of members had the mutation and produced offspring with the two recessives genes thereby showing the new trait, which could then mate with each other. Natural selection would decide if this new trait was advantageous or not. I think this was an explantion for the more "punctuated" type of change versus the gradual type.

Did I get this all wrong?

openeyes

(Sorry for the terrible run-on sentence in the last post, but I'm at the library and my time is running out!)

SEF

There are quite a lot of options.

In addition to your simple recessive case, there are mixed cases where the mutation confers a disadvantage and an advantage but the environment in one area makes the trade-off worthwhile, eg sickle-cell anaemia versus malaria (where 2 good copies or 2 bad copies is often fatal but the mix is survivable). So the population in one area selects against the mutation while another selects for it.

Then there are the unnoticed mutations which only become apparent when combined with a further mutation. This can be the result of an amino acid change only after 2 base changes or a protein folding change only after another amino acid change or an insertion or deletion frame-shift leading to a completely new gene which happens to do something (usually best in a redundant copy of genetic material).

Or the mutation may only become significant if the environment changes. For example all individuals without the mutation die out due to a disease against which they have no immunity, leaving only those who accidentally had some resistance. That resistance may then be built on by further mutations. The black plague seems to have caused this sort of population shift.

MrDarwin

My work with plants has shown me that virtually all species (except perhaps those with the smallest overall populations and the most restricted geographic distributions) are quite variable in the wild. They also tend to vary in different ways in different parts of their geographic ranges. This variation has mutation has its source. Whether a particular mutation persists or not has to do with (1) how large an effect it has on the organism (and many mutations have little or no effect) and (2) whether it is advantageous, disadvantageous, or neutral with respect to local selective pressures. Whether they are dominant or recessive has little to do with it (in fact mutations are rarely strictly dominant or recessive). The inescapable conclusion of my own research is that a huge number of mutations occur that do not reduce the reproductive success of the individual, and are likely to be passed on to the next generation. Variation precedes speciation.

Also, mutations happen all the time and I'm willing to bet that in many cases the exact same mutation occurs numerous times in any given population during any given time. Again, whether that mutation's incidence in the population will increase or decrease depends on its effect with respect to local selective pressures.

By selection and by sheer chance certain mutations will become quite widespread in local populations. Sooner or later these local populations may have a genetic bottleneck--either due to near-extinction (perhaps due to climate change, a new disease, or a new predator) or a founder effect after a dispersal event--in either case, a small number of individuals is isolated from other members of the population, i.e., the number of individuals available to interbreed with each other becomes very small, which greatly increases the chances of any particular mutation to become fixed in all of their descendants.