Factors which can effect evolution:
Natural Selection as introduced by Darwin - the deciding factors are the type of environment and whether one possesses advantageous alleles that can be passed onto offspring.
Genetic Drift where chance dictates whether an allele will be passed on or not.
Definitions to help understand the topic:
Gene Pool - Complete range of alleles present in a population
Allele - different versions of a gene
Allele Frequency - how often a specific allele is present in a population
Evolution - the change in allele frequency over time
Genetic Drift - chance dictates which alleles are passed on, if the same allele is passed on more often again and again, this can lead to evolution
Gene Flow - movement of alleles (e.g intro of new ones)
Species - a group of similar organisms that can reproduce to give fertile offspring
Evolution by Natural Selection
All organisms from the same species vary; they have different alleles which give different characteristics. Variation in alleles may mean that some are advantageous whilst others may be disadvantageous. Note that alleles are only advantageous with the right selection pressures. Without selection pressures, natural selection would not take place. Selection pressures are those that create a struggle for survival. For example, a disease spreading is a selection pressure. The word selection comes in, as if an organism possesses a characteristic which makes it immune to the disease, then nature selects that organism to survive. Advantageous alleles can potentially be passed onto offspring. Therefore, overtime the frequency (allele frequency) of organisms possessing the allele will increase. Those without it will find it more difficult to survive, especially against organisms better adapted, so will eventually die out. And then we can say the species has evolved.
These are different types of selection: stabilising, disruptive and directional.
Here characteristics are within the middle range as the environment doesn't change much. Middle range characteristics make it more likely for an organism to survive and reproduce, increasing frequency. This in turn reduces the range in phenotypes. For example in a stable temperature environment such as the United Kingdom, having extreme fur lengths would make temperature regulation increasingly difficult. Therefore middle range fur length frequency increases, and the range of length reduces. Middle length fur means the organism is more likely to survive, reproduce and pass on alleles to offspring. Another example is human birthweight. A healthy baby will be between 6 and 9 pounds, anything lighter or heavier is dangerous.
Disruptive Selection - this is where selection pressures goes towards two extremes resulting in two modal values.
This is where we have characteristics of an extreme type. For example in very cold temperatures, long fur is beneficial for survival. The selection pressure moves towards one extreme moving the mode in this direction also.
Individuals show variation in their genotype. Chance dictates what alleles get pass on. By chance, there may be a more common allele being passed on which can lead to evolution. Genetic drift will have a larger affect on smaller populations as chance has greater influence. For example 8/10 or 8/100, both equivalent frequencies but over a different sized population, 8 has a larger effect on the smaller population. In larger populations, chance variations tend to even out across population. Natural selection and genetic drift work together to cause evolution.