Unit 1 KA7 Flashcards
Evolution
The gradual change in the characteristics of a population over time, as a result of natural selection working on genomic variation.
Natural selection
The non-random process that increases the frequency of advantageous sequences, and decreases the frequency of deleterious sequences.
It allows the organisms that are best suited to their environment to survive, breed and pass on the alleles that give them an advantage.
Deleterious sequences
Code for an inferior characteristic, which leave an individual poorly adapted to its environment.
These individuals fail to breed and pass on the sequence, which decreases in frequency and may disappear from the population.
Normal distribution.
A bell-shaped curve showing an even distribution about the mean.
Many characteristics are controlled by several genes (polygenic inheritance) and show continuous variation across a range, from one extreme to the other, with most organisms being ‘average’.
Stabilising selection
Intermediate phenotypes are favoured, and extremes are selected against, resulting in a decrease in genetic diversity and a narrowing of the normal distribution curve.
This tends to occur in a stable environment, where it is better to be ‘average’
eg. human birth mass
Directional selection
One extreme phenotype is favoured, which pushes the mean in one direction.
Occurs in a period of environmental change.
eg. giraffe neck length, body mass of black bears in the ice age.
Disruptive selection
Extreme phenotypes are favoured at the expense of intermediates.
Can result in the population being split into 2 distinct groups, and drives sympatric speciation.
eg. specialised beak shapes in the Galapagos finches.
Phenotype frequency
How often different phenotypes (characteristics) occur in a population.
Vertical inheritance (gene transfer)
The transfer of genetic sequences from parent to offspring as a result of sexual or asexual reproduction.
Horizontal gene transfer
Genetic material is transferred from one cell to another, between members of a population and even between different species.
Transfer occurs within the same generation, and not from parent to offspring.
It allows prokaryotes to evolve very rapidly. eg. antibiotic resistance.
Species
A group of genetically similar organisms that are able to breed to produce fertile offspring.
Speciation
The formation of new species.
Occurs following the isolation of different groups within a population, which prevents exchange of genes.
Allopatric speciation
The population is split by a geographical barrier, eg a mountain range or ocean.
The groups become isolated and are unable to exchange genes.
Random mutations in the separate groups are not shared, and natural selection acts differently on the separated populations.
Speciation has occurred if the groups can no longer breed to produce fertile offspring if reunited.
Sympatric speciation
The population is split by an ecological or behavioural barrier, eg. different food sources, courtship behaviours.
The isolated groups no longer breed and exchange genes.
Mutations in the separated populations are not shared, and natural selection acts differently on the 2 groups.
Speciation has occurred if they fail to breed and produce fertile offspring if reunited.
