Variation

Continuous - Most characters are controlled by a number of genes and the differences in the character are not clear cut. A Character within a population showing gradation from one extreme to another shows Continuous Variation. An example is height. If an individual has inherited a number of alleles for tallness from the parents, that individual has the potential to grow tall.

Characters that are clear-cut easy to tell apart are controlled by a single gene. There is no intermediate types. For Example, light and dark forms in the peppered moth, the ABO blood grouping system, where the gene has more than two alleles. 

Environmental Factors also play a part played in such characteristics such as height.

Individuals that are genetically predetermined to be the same height actually grow to different heights due to variations in environmental factors such as diet.

Non-Heritable variation or Environmental influences:-

• The environment has a role in determining phenotypic variation.
• Environmental factors in humans may include diet and exercise whereas plants are affected by temperature, light and avaliable nutrients
• An organism will inherit genes, giving it a theorectical maximum size, but whether or not this is reached will depend upon nutrition during the growth period and other environmental factors.
• Thus, if organisms of identical genotype are subject to different environmental influences, they show considerable variety.
• Because these influences are varied, they are largely responsible for continuous variation in a population.

This is known as Non-Heritable Variation

Variations due to the effect of the environment have little evolutionary significance as they are not passed from one genertaion to the next.

Much more important evolution is inhertited variation that results from genetic changes. As a result of sexual reproduction, variation may be increased when the genotype of one parent is mixed with that of the other.

The sexual process has 3 inbuilt methods of creating variety:-

• The mixing of 2 different parental genotypes where cross-fertilisation occurs
• The random distribution of chromosomes during metaphase I of meiosis I
• The crossing over between homologous chromosomes during prophase I .

Although these processes may establish a new combination of alleles in one generation it is mutations that generate longlasting variation of a novel kind.

However, as previously stated, the occurrence of a useful mutation is a very rare event.

• All organisms have the reproductive potential to increase their population, although they rarely do so
• As a population increases, various environmental factors come into play to keep the numbers down
• Organisms must compete for limited resources. For example, plants compete for light, space, mineral ions; animals compete for food and shelter etc
• 

There are two types of competition:

• IntRA - Specific Competition - Competition between individuals of the sAme species. This is the basis of the origin of species by Natural Selection.
• 
• IntER - Specific Competition - Competition between individuals of diffErEnt species is illustrated by Prey-Predator Relationships.

Consider a population of rabbits. The female may produce several litters each year with a number of young in each litter. If all young survived to become adults and they, in turn, reproduced, then the rabbit population would increase rapidly.

Eventually, as the increasing numbers of rabbits eat an increasing Quantity of vegetation such as grass, food would become in short supply. Overcrowding would take place allowing disease to spread. Predators, such as foxes would increase.

These environmental factors act to reduce the rate of growth of the rabbit population. Only a small proportion of the young rabbits will develop into adults and reproduce so population growth slows.

Over a period of time the population  will oscillate about a mean level. The number of young produced is far greater than the number which will survive to become adults. Many young die before maturity and so do not reproduce .

What determines which individuals die and which survive?

• Is it a matter of luck or are some individuals born with a better chance of survival than others
• Variation within a population of organisms means that some will have characteristics which give them an advantage in the 'Struggle for Survival'

In rabbits coat colour may vary. Most rabbits have alleles which give the normal brown colour. A small number may be homozygous for the recessive allele which gives a white coat.

A white rabbit will stand out and is more likely to be killed by a predator such as a fox. As the white rabbit is unlikely to survive to become  mature adult, the chances of it reproducing and passing on its allele for white coat to the next generation are very low.

The allele for white coat will remain rare in the population. However, in the arctic winter the disadvantage becomes an advantage.

Predation by foxes is an example of a selection pressure. Selection pressure increase the chance of some alleles being passed on to the next generation, and decrease the chance of others being passed on.

The effect of such selection pressures on the frequency of alleles in a population is called natural selection.

Predation increases 'Fitness' in the prey. For example foxes kill the weakest rabbits. Selection in context of evolution, is the process by which organisms that are better adapted to thier environment surivive and breed, while those less adapted fail to do so.

These better adapted organisms are more likely to pass on their characteristics to succeding generations. The organism's environment exerts a selection pressure and this determines the spread of any alleles within the gene pool.

Gene Pool  = The total number of alleles in a population at any one time