The following is a comment I have put up at Just One Minute to what appears to be a pet subject there, namely short time frame genetic effects. It is so much fun to see how scientists cannot communicate clearly, the MSM can foul things up and the common man is really stuck as to what to do... or *not* as the case may be:
Well, geography is certainly *one* factor that plays a role, but when considering environment as a whole there are other things that are also geographically limited, but not to varieties of mankind, generally heaped into races.
A gene that offers resistance to malaria can occur in different populations and arise from separate genetic occurances. However, such changes also carry a price of having deficeits in the organism carrying them. So, widely variant populations exposed to the same endemic disease will show differing responses due to genetic variation. While the original disease will be resisted, the allele that confers resistance may also cause other problems in the body, thus lowering overall longevity. But if those carrying it are able to reproduce and pass on their genetic resistance while those without, proportionately, are unable to, the resultant generations will trend towards the gene expression as it is preferred due to the offering of resistance. So while envrionments are similar for two populations the response is different. But, it should be noted that the gene suite conferring the resistance naturally occurs in other populations and is usually selected against due to limited life span compared to the population. So the endemic disease presence is a selector. Thalassemia, then, is located within populations exposed to malaria that have a common suite of genes to provide resistance and that suite is not geographically centralized. Sickle cell disease, while offering a similar resistance to malaria, arises from a mutation and it confers resistance to malaria, but generally shortens life span.
This is a sieving process of which disease, genetic commonality of ancestral populations, geographic isolation and mutation will all play a role or *not* as the case may be.
Now island populations suffer two common problems: large creatures get smaller over generations and small creatures get larger with succeeding generations over time. This is a well known effect of limited environment and the Square-Cube Law playing out which is better codified under Bergmann's Rule. Here the limitation of habitat to body size is of prime importance along with efficient use of caloric intake. Also note that this effect speaks to island isolation in general, so that a series of connected lakes that lose their connections are considered to be 'islands' for the aquatic cultures within each one. This geographic isolation and change in dietary structure will preferentially select smaller and more efficient body sizes for larger creatures to adjust mass to limited diet and larger, more efficient body sizes for smaller creatures so that they use fewer calories by preserving heat due to larger mass but only moderate increase in surface area.
Once one begins to understand the role isolation plays, geography plays, disease plays and other factors such as general climactic condition for any geographic region, then reproductive success becomes a multivariate problem that needs to be examined in many different ways. Genetic heritage and mutation play their roles, and the ancestor effect via limiting an available gene pool due to number of ancestors to found a population, will also play its role.
Finally, extinction events mark extreme change in climactic and other conditions in which there is no real set rule for survival and it is often by happenstance that one genetically isolated part of a larger population will survive while the rest dies out. During the last major extinction event at 65 MYA foraminifera were of a widely divers population globally exhibiting a large number of different body types adapted to differing climates. The extinction event wiped all of them out, save for one type that had seasonal hibernation period due to its living in arctic conditions. This one body plan and lifestyle type in the foram population survived and is the sole founder of the modern foram population. A more interesting example is population diversity of brachiopods and bivalves before the Permo-Triassic extinction. Both body plans use similar feeding styles but have different overall characteristics. Before the P-T event brachiopods were highly diverse throughout the warm near shore waters and bivalves were a few paltry species existing in niche territories. After the extinction event the situation reversed over time as bivalves turned into many different populations while brachiopods became a few species in niche environmental zones. Something in that event favored the general reproductive rate of one over the other for a similar environment and allowed bivalves to proliferate and speciate while brachiopods could not compete to do so.
So, my understanding of evolution is that one may not consider just one factor in isolation, but must consider numerous factors and conditions to understand why populations drift within and amongst species. Once all the conditions are examined, along with all other competitors for similar ecozones and niches, can success be measured via differential survival according to genetic changes.
Studies such as those cited? Nice, but only a small part of any puzzle. Diversity of life is a difficult thing to understand, but has basic rules with which it can be understood, although they are complex in their enaction.
So endith my comment.
Are we having fun yet?