Selection and Evolution Flashcards
Explain how a population could increase
All organisms have the reproductive potential to increase their populations, they all over produce.
Eg rabbits produce many litters a year, if all survived to adulthood and reproduced, the rabbit popln would incr rapidly.
What prevents OVER POPULATION?
ENVIRONMENTAL FACTORS. The number of young is far greater than the number which will survive to adulthood.
What is a BIOTIC environmental factor? Give examples
Factors caused by another organism. eg predation, competition for food, infection by pathogens.
What is an ABIOTIC environmental factor? Give examples
Factors caused by non-living components of the environment. eg water supply, nutrient levels in the soil
Why is VARIATION important in selection? GIve examples
Variation within a population means some individuals will have features which give them a greater advantage in the ‘struggle for existence’.
Most mutations resulting in new allele are neutral (neither disadvantage or advantage). Some will give selective advantage and be passed on.
Eg coat colour. Most rabbits are agouti, but few are homozygous recessive for white. These will stand out distinctly from others, and more likely to be predated. Chances of a white rabbit passing on their genes is v small.
What are SELECTION PRESSURES?
Pressure that increase the chances of some alleles being passed on, and decreased the chances of others.
What is NATURAL SELECTION?
The effects of selection pressures on the frequency of alleles in a population.
Increases freq of alleles conferring an advantage, reduces frequency of alleles conferring a disadvantage.
What is a STABILISING SELECTION? Give an example
When allele frequencies stay the same. Favours the intermediate trait.
Agouti rabbits are best adapted to survive predation,so agouti allele remains most common coat colour allele. Unless something changes, natural selection will ensure this continues.
What is a DIRECTIONAL SELECTION?
The change of allele frequencies in a population to favour the variant of one extreme
What is a DIVERSIFYING SELECTION?
The change in allele freq to favour variants of opposite extremes.
How can environmental factors give rise to directional forces of natural selection? GIve an example
If the climate becomes much colder and snow covers the ground for almost all of the year, and rabbits can cope, white rabbits rabbits now have a selective advantage over agouti, as they are better camouflaged. White rabbits more likely to survive and pass on their genes. Allele for white fur incr at the expense of the allele for agouti.
What does changes in allele frequency form the basis of?
EVOLUTION
What selection pressure gave rise to melanism in the BISTON BITULARIA?
The predation of birds.
In areas of unpolluted air, tree branches are often covered in grey, brown and green lichen, speckled moths are superbly camouflaged.
However, lichens are very sensitive to pollutants (eg SO2) and don’t grow on trees near/downwind of industries releasing pollutants. Trees in the area have darker bark in which dark moths are better camouflaged.
Proportion of speckled moths expected to increase if we succeed in reducing output of certain pollutants.
What types of alleles control the black and speckled forms of the moth?
1) Speckled colouring is produced by recessive allele (c). Colouring is controlled by a single gene.
2) Black colouring produced by a dominant allele (C). The C allele has probably been present in B. betularia popln for a v long time (not produced by pollution).
Why is the freq of the sickle cell allele very high in Africa?
Part of world where sickle cell allele is common is also part where malaria is found. Malaria caused by Plasmodium parasite that multiply in RBC.
People who are heterozygous for sickle cell allele are less likely to suffer from a serious attack of malaria than a person with homozygous normal allele. Heterozygous people only have about one third the number of Plasmodium in their blood as normal homozygotes.
