Populations And Evolution Flashcards
What is Variation?
Differences that exist between individuals.
Why is there variation within a species?
Because they have different alleles causing genetic variation.
How can genetic variation come about: (4)
- random fertilisation of gametes
- mutations
- independent segregation
- crossing over of chromatids
Name one method in which evolution occurs:
Natural Selection
Explain Natural Selection:
Random mutations cause variation in a population of species
Species gain the advantage and are able to survive, reproduce and pass on genes to next generation
The next gen have the beneficial alleles and the frequency of beneficial alleles increases from gen to gen
What is Stabilising Selection?
Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.
Occurs when environment isn’t changing - 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.
This can be a response to an environmental change.
What is Disruptive Selection?
Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce.
Occurs when the environment favours more than one phenotype.
What is Speciation?
The development of a new species from an existing species.
When does Speciation occur?
When populations of the same species become reproductively isolated - changes in allele frequency means changes in phenotype so they can’t interbreed to produce fertile offspring.
When can allele frequency change?
When a physical barrier divides a population of a species causing some individuals to be seperate from the main population. (Due to a flood etc)
Known as GEOGRAPHICAL ISOLATION
What does Geographical isolation lead to?
Allopatric speciation.
Populations that are geographically separated may experience…
Different selection pressures and so different changes in allele frequencies could occur.
Different alleles will be more advantageous in different populations - directional selection will act.
The changes in allele frequency will lead to…
Differences accumulating in the gene pools of the separated populations causing changes in phenotype frequencies.
They would have adapted so much to their new environment they they can’t interbreed anymore to produce fertile offspring.
What is Sympatric Speciation?
When a population doesn’t have to become geographically isolated to become reproductively isolated.
