Chapter 17: Selection And Evolution Flashcards
(43 cards)
What is genetic variation caused by?
■ independent assortment of chromosomes, and
therefore alleles, during meiosis
■ crossing over between chromatids of homologous
chromosomes during meiosis
■ random mating between organisms within a species
■ random fertilisation of gametes
■ mutation.
What is the difference between mutation and the other factors that cause genetic variation?
Mutation can produce completely new alleles. The other factors that cause genetic variation allow for the reshuffling of existing alleles in the population.
What produces phenotypic variation?
Offspring have combinations of alleles which differ from those of their parents and from each other. This genetic variation produces phenotypic variation.
Is variation caused by the environment passed on to the offspring?
No
What are some characteristics of discontinuous(qualitative) variation?
• A qualitative difference that has clear distinguishable
categories with no intermediates
■ different alleles at a single gene locus have large effects on the phenotype
■ different genes have quite different effects on the
phenotype.
What are some characteristics of continuous(quantitative) variation?
• A quantitative difference that has a wide range of
phenotypes
■ different alleles at a single gene locus have small effects on the phenotype
■ different genes have the same, often additive, effect on the phenotype
■ a large number of genes may have a combined effect on a particular phenotypic trait; these genes are known as polygenes.
What is fitness?
Fitness is the capacity of an organism to survive and transmit its genotype to its offspring. It is the extent to which organisms are adapted to their environment.
What do selection pressures increase?
Selection pressures increase the chances of some alleles being passed on to the next generation, and decrease the chances of others, causing changes in allele frequency.
What is natural selection and what does it do?
Natural selection is the effect of selection pressures on the frequency of alleles in a population. Natural selection raises advantageous allele frequencies and reduces the disadvantages allele frequencies within the population.
Natural selection occurs to limit exponential growth of populations and ensure survival of the fittest.
What are the two types of environmental factors that affect population growth?
o Biotic – caused by living organisms e.g. predation,
competition for food, infection etc.
o Abiotic – caused by non-living organisms e.g. water
supply, nutrient levels in soil etc.
What are the three types of selection pressures?
Stabilising, directional and disruptive selection
What is stabilising selection?
Stabilising selection: when natural selection keeps the
variety of the population the same.
o If wide variation shown, selection pressure acts
against the two extremes.
o Results in a population with a narrower range of the
characteristic
o Tends to keep the variation in a characteristic centred around the same mean value
What is directional selection?
Directional selection: when a new environmental factor or a new allele appears, causing different allele
frequencies to be produced
o Selection acts against one extreme, results in change
in a characteristic in a particular direction
What is disruptive selection?
Disruptive selection: when conditions favour both
extremes of a population (this selection maintains
different phenotypes)
Describe what the two selection pressures are on the sickle cell alleles.
■ Selection against people who are homozygous for the sickle cell allele, HbSHbS, is very strong, because they become seriously anaemic.
■ Selection against people who are homozygous
HbAHbA, is also very strong, because they are more
likely to die from malaria.
Why do heterozygotes with the sickle cell trait have a strong selective advantage?
They do not suffer from sickle cell anaemia and are much less likely to suffer badly from malaria.
What is genetic drift?
Genetic drift is a change in allele frequency due to
chance and is most noticeable in small populations
What is the founder effect?
Further genetic drift of the small population leads to
further alteration of the allele frequency, leading to
evolution and a new species – Founder’s effect
What is the Hardy-Weinberg principle used to find?
Used to calculate allele, genotype and phenotype
frequencies within a large randomly mating population
When does Hardy-Weinberg principle not apply?
o Significant selective pressure against a genotype
o Migration into or out of population
o Non-random mating
o Limited population
What is artificial selection?
When humans purposefully apply selection pressures to populations, the process is known as artificial selection.
What factors are used in selective breeding in relation to cattle?
- Progeny testing is carried out- A bull cannot
be assessed for milk production since this a sex-limited
trait. Instead, the performance of the bull’s female offspring is looked at to see whether or not to use the bull in further crosses. - Background genes- Within each organism’s genotype are all the alleles of genes that adapt it to its particular environment. These genes are called background genes. If chosen parents come from the same environment and are from varieties that have already undergone some artificial selection, it is likely that such parents share a large number of alleles of background genes, so the offspring will be adapted for the same environment.
- Increased milk trait
How is selective breeding used to improve disease resilience in plants?
Organisations are set up to screen seed collections for plants with traits such as disease resilience or climate resilience or efficient use of nitrogen fertilisers.
Why do wheat plants now have shorter stems and how?
This makes them easier to harvest and means they have higher yields (because they put more energy into making seeds rather than growing tall). The shorter stems also make the plants less susceptible to being knocked flat by heavy rains, and means they produce less straw, which has little value and costs money to dispose of. Most of the dwarf varieties of wheat carry mutant alleles of two reduced height (Rht) genes. These genes code for DELLA proteins which reduce the effect of gibberellins on growth. The mutant alleles cause dwarfism by producing more of, or more active forms of, these transcription inhibitors. A mutant allele of a different gene, called ‘Tom Thumb’, has its dwarfing effect because the plant cells do not have receptors for gibberellins and so cannot respond to the hormone.
