How can isolation and small population sizes cause rapid evolution?Watch
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In my textbook it says 'the islands' isolation and small population sizes provide optimal conditions for rapid evolutionary change'. But how can isolation and small population sizes cause rapid evolution?
If you weren't allowed to communicate or one scentence could mean death which covers in some places either I think high functioning autism may be the dominant gene under a certain selection pressure.
Others would be large overall size or adapted not to eat much, tough scars and rapidly healing scars. (Small islands get this all over the world)
Or some places are so remote we would never be able to understand their language so becoming isolated is a survival mechanism, they have less competitors.
Specifically, it's always a why question for most of biology.
This prevents the dilution of the islands unique allelic frequencies.
By genetic drift, we may end up having some alleles less expressed than others in successive generations, i.e. due to non random mating.
This results in allelic frequencies altering rapidly as there is a small population.
This factor is key: per the small population, genetic drift is strong, as if ten percent of individuals in the small population don't breed as much, and these ten percent happen to possess an allele for long hair, the long hair variant of hair length will be reduced in frequency.
This can happen several times, reducing its prevalence.
Overall, this changes the allele frequency, possibly eliminating it, and in turn changing the overall outlook of the population on the island.
Selective agents also act on a population, and their effects are quite strong in a small population. The number of individuals in the population who possess a particular trait that may be selected against will have less chances to breed, as there will be less chance of encountering or breeding with a female, or higher competition for mates, and with the diminished amount of these individuals and genetic drift, well, you get the picture.
Probably some other stuff.
In the first place, there is the "founder effect" - the small subset of the population that ends up on an island is unlikely to have the same allele frequencies as the whole population. This may be due to chance (imagine dipping your hand into a bowlful of M&Ms - by chance, you are quite likely to come out with no reds and a load of greens, for instance). Or it may be non-random (maybe only individuals able to tolerate immersion in salt water can reach the island, say)
Then, as Callicious points out, the effect of chance events such as localised flooding, fires etc can have a disproportionate impact on a small population in a small area - by chance, perhaps most of the individuals with red feathers just happened to be in the particular bit of forest that got hit by lightning that day.
Evolution due to non-random selection may also result from reduced gene flow (i.e. being cut off from the main population). Being isolated may allow an island population to become more specialised in response to the particular conditions on the island, rather than be diluted by gene inputs from the rest of the mainland species. This might be further enhanced if competitor species present on the mainland did not make it to the island - the lessening of interspecific competition may allow the island species to diversify into many more specialised niches. (The Galapagos finches are often cited as the classic example of this.)
It may be handy to bear in mind that the hazards of being isolated in a "island" (such as a fragment of forest), can be a major threat to species survival. That's why "connectivity" of habitat is a big issue in conservation.