DEFINATION: An adaptive landscape is a surface in multidimensional space (analogous to. a mountain range) that represents the mean fitness of a population (not the fitness of a genotype).
Many Locus affects fitness in a population. consider homozygous genotype consist of four loci. A variety of 6 possible genotypes is then possible for example AABBccdd, AAbbCCdd, aaBBccDD, and so on. According to wright, we may consider each of these genotypes to occupy an adaptive peak, which means simply a position of high fitness associated with a specific environment. As long as no other factor changes the fitness of these genotypes, each of these 6 peaks is of equal height and a population that consists entirely of any one of these genotypes would therefore achieve maximum fitness for this phenotype.
Gene combination increases astronomically by an increase in loci and alleles. A locus with 4 alleles has 10 possible diploid gene combinations and 100 loci with 4 alleles have a total of 10 raised to power 100. A potentially high adaptive peak for a population need not to coincide with a high selective peak for a genotype within the population. this discrepancy arises because selective values of genotype are based on competition with other genotypes but may not indicate their effect on the population.
consider 4 gene pairs Aa., Bb, Cc, Dd. each gene has considerably different effects. And we assign adaptive values to the effects of A and B genes on the basis of the number of capital letters genes in these two pairs, the greater the fitness. When we consider them with previous adaptive values we construct the adaptive landscape as
Note that AABBccdd has highest peak. to reach the highest valley a population need to move from inferior or non adaptive valleys. arrow in the pic indicate the movement of a population from aabbCCDD to highest peak.
Once the population reaches the adaptive peak further change will not occur unless any change in the environment occurs. and any further change will depend upon the new selective environment and the creation of a new adaptive peak. A population with one adaptive peak can no longer reach a higher peak without going through the nonadaptive valley. but the point is how the population evolves in the low valley so that it reaches the highest peak because selection wouldn’t imply on the low adaptive population how can it accept it. So wright proposed the shifting balance process that describes the movement of population from low peak to highest. he proposed to divide the population into a smaller subpopulation called demes. on them genetic drift can act and changes alleles frequency
following are the factors of process
- Random Genetic drift, allows a number of demes to change their gene frequencies and move from nonadaptive to adaptive valleys. By developing new fitness values.
- Selection pushes some of these demes up the nearest available adaptive peak by changing gene frequencies.
- Migration and mutation cause polymorphism provide further opportunity for genetic drift.
- Epistatic gene combination is also used. since in smaller populations recombination is not much likely to happen.
- Demes on the high peaks tend to exchange demes on lower peaks by increasing their size.
- Environmental change continually producing new adaptive landscapes.
- selection time and genetic accidents are needed to achieve the most optimum genotypes while genetic loads oppose optimal achievement so the highest adaptive peaks are potential and not necessarily realized.
Fisher proposed that population are large and mostly homozygous selection test each new allele independently with competition to all other alleles.so large populations primarily improves by small selective steps rather than genetic drift
Courtsey: Strickberg evolution, wikipedia