Directional and Stabilising Selection Flashcards
What are selection pressures
Environmental factors that affect the chance of survival of an organism e.g. high competition for food between lions, due to low population of prey, would ‘select’ for faster, more powerful lions that are good hunters
What are the different types of selection
Stabilising and Directional
What is stabilising selection
Natural selection that keeps allele frequencies relatively constant over generations, so things stay as they are unless there is a change in the environment
Give an example of stabilising selection
In humans very-low and very-high birth weights are selected against leading to the maintenance of the intermediate birth weights. (Can be presented with bell curve)
What is directional selection
Natural selection that produces a gradual change in allele frequencies over several generations
When does directional selection occur
Usually happens when there is a change in environment/selection pressures or a new allele has appeared in the population that is advantageous
Give an example of directional selection
The presence of antibiotics is a selection pressure. Mutations are occurring in bacteria populations randomly. A mutation arises that confers antibiotic resistance - it is a beneficial allele. Bacteria with this mutation are more likely to survive and reproduce. Most bacteria without the resistance mutation die. Over generations, this leads to an increase in the frequency of beneficial allele that produces antibiotic resistance
What are adaptations
Certain alleles within a species population which produce features that make an organism better suited to its environment
Give an example of a change occurring in a species due to the rise of a favourable allele
A higher proportion of fast-twitch muscle fibres in lions legs, which is advantageous for sprinting after prey
What is the final result of natural selection
Species become better adapted to their environment
What are the three types of adaptation
anatomical, physiological and behavioural
What are anatomical adaptations
Structural/physical features
Give an example of an anatomical adaptation
The white fur of a polar bear provides camouflage in the snow so it has less chance of being detected by prey
What are physiological adaptations
Biological processes within the organism
Give an example of a physiological adaptation
Mosquitos produce chemicals that stop the animal’s blood clotting when they bite, so that they can feed more easily
What are behavioural adaptations
The way an organism behaves
Give an example of a behavioural adaptation
Cold-blooded reptiles bask in the sun to absorb heat
What do adaptations and selection pressure contribute to
The large diversity of living organisms and are major factors in the process of evolution
What is evolution
The change in adaptive features of a population over time as a result of natural selection
What will happen if an environment is static and does not change
Selection pressures will not change and evolution will not occur
How can a species form two species
If two populations of one species are isolated from each other and become so different in phenotype that they can no longer interbreed to produce fertile offspring
What is the formation of new species called
Speciation
What causes the formation of a new species from pre-existing species over time
accumulated genetic differences
How is evolution responsible for the large number of species on earth
Evolution drives speciation
