Selection Mechanisms

Key Knowledge:

Causes of changing allele frequencies in a population’s gene pool, including environmental selection pressures and genetic drift

External factors which affect the ability of an organism to survive and reproduce are referred to as selection pressures

These pressures will reduce the variation within a population’s gene pool to create a change in allele frequency (i.e. evolution)
Selection can be driven by random processes (genetic drift), the environment (natural selection) or human intervention (artificial selection)

Genetic Drift

Genetic drift is the change in the composition of a gene pool as a result of random or chance events

It will occur faster and be more significant in smaller populations, where chance events have a bigger impact on the gene pool
Larger populations will be less affected by random events and maintain more stable allele frequencies with low genetic drift

Allele frequencies will change significantly when a large population is reduced to a small population

Two mechanisms by which this population change may occur are via population bottlenecks and the founder effect

Population Bottleneck

Population bottlenecks occur when an event reduces population size by an order of magnitude (~ >50%)

These bottlenecks may result from natural occurrences (e.g. fires, floods, etc.) or be human induced (e.g. overhunting)
The surviving population has less genetic variability than before and will be subject to a higher level of genetic drift
As the surviving members begin to repopulate, the newly developing gene pool will be divergent to the original
Example: Northern elephant seals have reduced genetic diversity compared with southern seals due to overhunting

Founder Effect

The founder effect occurs when a small group breaks away from a larger population to colonise a new territory

As this population subset does not have the same degree of diversity as a larger population, it is subject to more genetic drift
Consequently, as this new colony increases in size, its gene pool will no longer be representative of the original gene pool
The founder effect differs from population bottlenecks in that the original population remains largely intact
Example: Certain Amish communities have a higher incidence of polydactyly because of inter-marriage within the community

Natural Selection

Natural selection is a change in the composition of a gene pool as a result of the presence of selective environmental agents

Species tend to produce more offspring than the environment can support, resulting in a struggle for survival (selection pressure)
Survival pressures may include external conditions (e.g. temperature), resources (food or water access), predators and disease

Adaptations are features of organisms that aid their survival by allowing them to be better suited to their environment

Adaptations can be structural (e.g. quills), behavioural (e.g. nocturnal), physiological (e.g. homeothermy) or developmental
Organisms with beneficial adaptations will be more likely to survive long enough to reproduce and pass on their genes
This is commonly described as the 'survival of the fittest’ – whereby the 'fittest' are those most capable of surviving to reproduce

Survival of the fittest results in differential reproduction rates, meaning features that confer survival are more likely to be passed on

Over many generations, there is a change in allele frequency (evolution) favouring the presence of beneficial adaptations

Artificial Selection

Artificial selection is a change in the composition of a gene pool as a result of human intervention

This intervention can target individual organisms (via biotechnology) or entire species (via selective breeding programmes)

Selective breeding programmes allows man to intervene in the breeding of species in order to produce desired traits in offspring

By breeding members of a species with a desired trait, the trait’s frequency becomes more common in successive generations
As phenotypic extremes are being selected, targeted breeds can show significant variation in a (relatively) short period of time

Plant crops have been selectively bred to enable the generation of new types of foods from the same ancestral plant source

Plants of the genus Brassica have been bred to produce different foods by modifying plant sections through artificial selection
This includes broccoli (modified flower buds), cabbage (modified leaf buds) and kale (modified leaves)

Domestic animals have been selectively bred to fulfil a wide range of functions (e.g. dog breeds from ancestral wolves)

Racing dogs (e.g. greyhounds) were specifically bred to be sleek and fast
Herding dogs (e.g. sheep dogs) were bred for heightened intelligence in order to follow herding commands
Hunting dogs (e.g. beagles) were typically bred to be smaller in stature so as to enter fox holes
Toy dogs (e.g. chihuahuas and pugs) were selectively bred for their dimunitive size

Brent Cornell