7B - Variation and selection Flashcards
What can variation be caused by?
Genes, the environment or both.
What is variation?
The differences that exist between individuals.
What does variation within species mean that individuals in a population can show?
Means that individuals in a population can show a wide range of different phenotypes.
What is variation within a species called?
Intraspecific variation.
What causes genetic variation within a species?
Although individuals of the same species have the same genes, they have different alleles (versions of genes).
What does the fact that, although individuals of the same species have the same genes, they have different alleles (versions of genes) cause?
Genetic variation within a species.
What is the main source of the genetic variation within a species?
Mutation - changes in the DNA base sequence lead to the production of new alleles.
Meiosis - through the crossing over of chromatids and the independent segregation of chromosomes).
Random fertilisation of gametes during sexual reproduction.
Other than mutations, meiosis and random fertilisation (genetic factors), what else can cause variation within a species?
Differences in the environment, e.g. climate, food, lifestyle.
What is most variation within a species caused by?
A combination of genetic and environmental factors.
What results in evolution?
Only genetic variation.
Does genetic variation result in evolution?
Yes.
Does environmental variation result in evolution?
No.
What is evolution?
A change in allele frequencies over time.
Why does evolution occur?
Natural selection, genetic variation and genetic drift.
What does natural selection cause to occur?
Evolution.
How does natural selection work?
1) Individuals of the same species vary because they have different alleles.
2) Predation, disease and competition (selection pressures) create a struggle for survival.
3) Because individuals vary, some are better adapted to the selection pressures than others.
4) This means there are differential levels of survival and reproductive success in a population. Individuals with a phenotype that increases their chance of survival are more likely to survive, reproduce and pass on their genes (including the beneficial alleles that determine their phenotype), than individuals with a different phenotype.
5) This means that a greater proportion of the next generation inherit the beneficial alleles.
6) They, in turn, are more likely to survive, reproduce and pass on their genes.
7) So the frequency of the beneficial alleles in the gene pool increases from generation to generation.
What are selection pressures?
A selection pressure is anything that affects an organism’s chance of survival and reproduction.
What do different types of natural selection lead to?
Different frequency patterns.
What are the types of natural selection that affect allele frequency in different ways?
Stabilising selection, directional selection and disruptive selection.
What are stabilising selection, directional selection and disruptive selection types of?
Natural selection that affect allele frequency in different ways.
What are the 3 types of natural selection?
Stabilising selection, directional selection and disruptive selection.
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 directional selection?
Where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce.
