Populations and Evolution- Variation and Selection Flashcards
What is variation?
The differences that exist between individuals
What does variation within a species mean?
Individuals in a population can show a wide range of phenotypes
What causes genetic variation within a species?
Individuals have different alleles even though they have the same genes
What is the main source of genetic variation?
Mutation
How can genetic variation be brought about from meiosis?
Crossing over of chromatids and the independent segregation of chromosomes
What is the third way that genetic variation can be brought about?
Through random fertilisation of gametes during sexual reproduction
What changes in the environment can cause variation within a species?
Climate, food, lifestyle…
How is most variation within a species caused?
By a combination of genetic and environmental factors
What does most genetic variation result in?
Evolution
Why do individuals of the same species vary?
Because they have different alleles
What creates a struggle for survival?
Selection pressures
What are 3 examples of selection pressures?
Predation, disease and competition
Why are some individuals better adapted to selection pressures than others?
Because individuals vary
Which individuals are more likely to survive, reproduce and pass on their genes?
The individuals with a phenotype that increases their chance of survival
How does the frequency of beneficial alleles increase from generation to generation?
Those better adapted pass on their alleles to their offspring
What is stabilising selection?
Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce
When does stabilising selection occur?
When the environment isn’t changing, and it reduces the range of possible phenotypes
What is an example of stabilising selection?
Human birth weight, average sized babies are more likely to survive
What is directional selection?
Where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce
When does directional selection occur?
In response to an environmental change
What is an example of directional selection?
Antibiotic resistance
What is disruptive selection?
Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce
How is disruptive selection opposite to stabilising selection?
Because characteristics towards the middle of the range are lost
When does disruptive selection occur?
When the environment favours more than one phenotype