● Even though `color{Brown}"Mendel"` had talked of `color{Violet}"inheritable factors "`influencing phenotype, `color{Brown}"Darwin"` either ignored these observations or kept silence.

● In the first decade of `color{Violet}"twentieeth century"`, `color{Brown}"Hugo deVries"` based on his work on `color{Violet}"evening primrose"` brought forth the `color{Violet}"idea of mutations"` – large difference arising suddenly in a population.

● He believed that it is `color{Violet}"mutation"` which causes evolution and not the `color{Violet}"minor variations""` (heritable) that Darwin talked about.

● Mutations are `color{Violet}"random"` and `color{Violet}"directionless"` while Darwinian variations are `color{Violet}"small and directional"`.

● Evolution for Darwin was `color{Violet}"gradual"` while deVries believed `color{Violet}"mutation caused speciation"` and hence called it `color{Salation}"saltation"` (single step large mutation).

● Studies in `color{Violet}"population genetics"`, later, brought out some clarity.

● In a given population one can find out the `color{Violet}"frequency of occurrence"` of `color{Violet}"alleles of a gene"` or a locus.

● This frequency is supposed to `color{Violet}"remain fixed"` and even remain the `color{Violet}"same through generations"`.

● `color{Brown"Hardy-Weinberg principle"` stated it using `color{Violet}"algebraic equations"`.

● This principle says that `color{Violet}"allele frequencies"` in a population are `color{Violet}"stable"` and is `color{Violet}"constant"` from generation to generation.

● The `color{Brown}"gene pool"` (total genes and their alleles in a population) remains a `color{Violet}"constant"`.

● This is called `color{Brown}"genetic equilibrium"`. `color{Violet}"Sum"` total of all the `color{Violet}"allelic frequencies is 1"`.

● `color{Violet}"Individual frequencies"`, for example, can be named `color{Brown}"p, q"`, etc.

● In a `color{Violet}"diploid"`, `color{Brown}"p and q"` represent the frequency of `color{Violet}"allele A"` and `color{Violet}"allele a"`.

● The `color{Violet}"frequency of AA"` individuals in a population is simply `p^2`.

● This is simply stated in another ways, i.e., the `color{Violet}"probability"` that an `color{Violet}"allele A"` with a `color{brown}"frequency of p"` appear on `color{Violet}"both the chromosomes"` of a diploid individual is simply the `color{Violet}"product"` of the probabilities, i.e., `p^2`.

● Similarly of `color{Violet}"aa"` is `q^2`, of `color{Violet}"Aa"` 2pq.

● Hence, `p^2`+2pq+`q^2`=1.

● This is a `color{Violet}"binomial expansion"` of `(p+q)^2`.

● When frequency measured, `color{Violet}"differs from expected values"`, the difference (direction) indicates the extent of `color{Violet}"evolutionary change"`.

● Disturbance in `color{Violet}"genetic equilibrium"`, or Hardy- Weinberg equilibrium, i.e., change of `color{Violet}"frequency of alleles"` in a population would then be interpreted as `color{Violet}"resulting in evolution"`.

● `color{Violet}"Five factors"` are known to affect `color{Violet}"Hardy-Weinberg equilibrium"`.

● These are `color{Violet}"gene migration"` or `color{Violet}"gene flow"`, `color{Violet}"genetic drift"`, `color{Violet}"mutation"`, `color{Violet}"genetic recombination"` and `color{Violet}"natural selection"`.

`star` `color{Brown}"Gene Migration"` `color{Brown}"and Gene Flow"`

● When `color{Violet}"migration"` of a section of population to another place and population occurs, `color{Violet}"gene frequencies change"` in the original as well as in the new population.

● `color{Violet}"New genes/alleles"` are added to the new population and these are lost from the old population.

● There would be a `color{Violet}"gene flow"` if this gene migration, happens `color{Violet}"multiple times"`.

`star` `color{Brown}"Genetic Drift"` `color{Brown}"and Founder Effect"`

● If the same change `color{Violet}"occurs by chance"`, it is called `color{Violet}"genetic drift"`.

● Sometimes the `color{Violet}"change in allele"` frequency is so different in the `color{Violet}"new sample"` of population that they become a `color{Violet}"different species"`.

● The `color{Violet}"original drifted population"` becomes founders and the effect is called `color{Brown}"founder effect"`.

● `color{Violet}"Microbial experiments"` show that pre-existing `color{Violet}"advantageous mutations"` when selected will result in observation of `color{Violet}"new phenotypes"`.

● Over few generations, this would result in `color{Brown}"Speciation"`.

● `color{Violet}"Natural selection"` is a process in which `color{Violet}"heritable variations"` enabling `color{Violet}"better survival"` are enabled to reproduce and leave `color{Violet}"greater number of progeny"`.

● A critical analysis makes us believe that `color{Violet}"variation"` due to `color{Violet}"mutation"` or variation due to `color{Violet}"recombination"` during gametogenesis, or due to `color{Violet}"gene flow or genetic drift"` results in changed frequency of `color{Violet}"genes and alleles"` in future generation.

● Coupled to enhance `color{Violet}"reproductive success"`, natural selection makes it look like `color{Violet}"different population"`.

● Natural selection can lead to:

`star` `color{Brown}"Stabilisation"` (in which more individuals acquire `color{Violet}"mean character value"`).

`star` `color{Brown}"Directional change"` (more individuals acquire `color{Violet}"value other than"` the mean character value).

`star` `color{Brown}"Disruption"` (more individuals acquire `color{Violet}"peripheral character value"` at both ends of the distribution curve).