Unit 17: Selection & Evolution (JW) Flashcards
Explain what phenotypic variation is caused by
Phenotypic variation = genetic / environmental factors OR combo of genetic + envi factors
Explain, using examples, how the environment may affect the phenotype of individual organisms.
Phenotype results from interaction of genotype & environment
1) Environment may modify gene expression = results in continuous variation e.g. size, mass, height
due to food, nutrients.. etc.
2) Environment may switch on gene
e.g. UV light and melanin production
3) Environment may induce mutation e.g. UV light
environment effect usually greater on polygenes
Define variation
differences that exist between individuals of a species
explain what is meant by discontinuous variation
e.g. blood groups
qualitative differences in phenotypes
discrete & categoric
Feature either present / absent
no range / intermediates
does not show normal distrib curve
Explain what is meant by continuous variation
e.g. mass, height
quantitative differences in phenotypes
not discrete
range of phenotypes
Polygenic
normal distribution
explain the genetic basis of discontinuous variation
solely due to genetic factors
genotype = phenotype
Different alleles at a single gene locus have a large effect on the phenotype
e.g. different alleles at the F8 gene locus dictate whether or not normal Factor VIII is produced and whether the individual has the condition haemophilia
explain the genetic basis of continuous variation
different alleles at single gene locus have small effects on phenotype
different genes may have additive effect on phenotype
environment has effect on phenotype (interaction btwn genes + environment)
phenotype = genotype + environment
Why t-test suitable?
Compare 2 means of 2 sets of data (paired set of data)
Continuous data
Normal distribution
SD are approx the same
2 samples have < 30 values each
Describe and explain why natural selection occurs
Natural selection states organisms change overtime.
Organisms produce many offspring, more than required to maintain population. These offspring, within species, will have genetic variation due to random mutation.
When they compete for resources such as limited food, this counts as selection pressure. Some individuals may possess advantageous alleles that randomly mutated.
These fitter individuals are likely to survive, reproduce, pass on the advantageous alleles to offspring, change in allele frequency within population.
Explain how environmental factors can act as stabilising forces of natural selection
2 extremes selected against
median selected for
no change in environment
Explain how environmental factors can act as disruptive forces of natural selection
2 extremes are selected for
median is selected against
bimodal distribution
diversifying selection
maintains genetic diversity
Explain how environmental factors can act as directional forces of natural selection
1 of the extremes selected for
4 processes that affect allele frequencies
Natural selection
founder effect
genetic drift
bottleneck effect
Describe genetic drift
Gradual change in allele frequency in a population due to chance
larger effect on smaller populations
Describe founder effect
small number of individuals from a large parent population start a new population
only some of the total alleles from the parent population will be present
not all of gene pool present
small pop = more susceptible to effects of genetic drift
Describe bottleneck effect
previously large pop suffers dramatic fall in numbers
e.g. natural disasters
reduces population AND genetic diversity & variation
alleles lost -> reduction in gene pool
factors that could drive genetic changes in populations
Random mutation
artificial selection
genetic drift / bottleneck / founder effect
migration
Explain how small population of species (bottleneck effect) may result in high frequency of rare variant forms
Small population
i) reduced number of alleles, genetic variation, hybrid vigour
ii) a lot of inbreeding, may even lead to inbreeding depression
So, reduced heterozygosity, increased homozygosity, increase in frequency of recessive alleles expressed
Genetic consequences of separation of family groups
Lack of hybrid vigour/inbreeding depression
More chance that harmful recessive alleles will be expressed
decrease in hetero/increase in homo
Less genetic variation
outline how bacteria become resistant to antibiotics as an example of natural selection
Random mutation in DNA of bacteria
produces allele resistant to antibiotic
antibiotic is selection pressure
bacteria with resistant allele have selective advantage
allowing bacteria to survive and reproduce
passing on resistant allele to offspring
allele frequency increases within the population
carries on and repeats for many generations
use the Hardy–Weinberg principle to calculate allele and genotype frequencies in populations
p + q = 1
p^2 + 2pq + q^2 = 1
p = A
q = a
p^2 = AA
2pq = Aa
q^2 = aa
state the conditions when the HW principle can be applied
No mutation
no migration
no artificial selection
random mating
large population
no selective pressure against one of the alleles
sexual reproduction
equal allele frequencies in both sexes
describe the principles of selective breeding (artificial selection)
humans apply selection pressure
select parents showing desired characteristic
breed parents together (by artificial insemination)
select and breed offspring showing desired characteristic
repeat over many generations
alleles for desired characteristic passed onto offspring
increase in frequency of allele for desired characteristic
implement outbreeding to avoid inbreeding depression
Outline the use of selective breeding in the introduction of disease resistance to varieties of wheat
and rice
Humans apply selection pressure
cross parents with desired characteristics
choose & cross offspring showing desired characteristics together
desired characteristic: more grains, higher yield, fast-growing
repeat over many generations
frequency of allele giving desired characteristics increase in population
