Variation and Evolution Flashcards
define variation
differences in phenotype between individuals
what three factors cause variation?
difference in genotype - genetic factors
different epigenetic modifications - but same genotype
differences in environments - environmental factors
why is variation important?
it is important to the survival of an organism because it means that they are more likely to adapt and survive changes in the environment
dsecribe the difference between discontinuous and continuous variation
discontinuous - a particular phenotype can be controlled by one gene
continuous - controlled by more than one gene
describe the concept of non-heritable variation (environmental)
the environment can affect the way an organism’s genes are expressed, i.e. phenotypic variation
this variation cannot be passed to offspring unless an epigenetic change occurs
if the variation is not passed on to the offspring, then the variation is non-heritable
explain how the environment could lead to an epigenetic change
an environemtnal factor e.g. exercise and diet
alters DNA methylation or histone modification, leading to changes in the expression of genes
what is genetic variation also known as?
heritable variation
genetic variation is increased as a result of…?
sexual reproduction
how is genetic variation increased as a result of sexual reproduction?
crossing over between homologous chromosomes during prophase I in meiosis
independent assortment of chromosomes during metaphase I in meiosis
independent assortment of chromatids during metaphase II in meiosis
mixing of two different parental genotypes at fertilisation
what is the difference between sexual reproduction and mutations in terms of alleles?
sexual reproduction establishes new combinations of already present alleles
mutations produce new alleles which often have a more significant impact on natural selection
what is the effect of competition on variation?
inter- and intra-specific competition can have an effect on breeding success and survival
competition can place selective pressures on the survival of different phenotypes and therefore breeding success
how does an organism become suitable to its environment?
variation means some individuals have beneficial characteristics that help them access resources more easily and so reproduce more
the beneficial genes are passed onto the offspring
over generations the genes become more common, allowing the species to adapt to environmental change
define selection pressure
an environmental factor that can alter the allele frequencies of the alleles present at a particular gene locus in a population
define selective agencies
exert selection pressure, for example, Climate, Human impact, Supply of food, Breeding sites
what is the effect of organisms with well-adapted phenotypes?
those with well-adapted phenotypes have alleles which are selected for and give them a selective advantage
they are more likely to survive than those whose phenotypes are less well adapted (those whose alleles mean they are selected against)
the phenotypes which increase the chance of organisms surviving are also likely to give a higher breeding success
what selection pressure led to the increase in dark form moths during the industrial revolution?
soot from factories meant darker tree trunks
lighter moths were more visible and darker moths were camouflaged
darker moths are more likely to survive and reproduce
if a dominant allele produces a phenotype which gives a selective disadvantage, what will happen to the frequency of this dominant allele in the gene pool? explain your answer
frequency of the dominant allele decreases until it’s lost from the gene pool because any individual with the allele has the disadvantageous phenotype so it is less likely to breed and pass on the allele
will the effect be the same for a recessive allele that produces a selective disadvantage?
no
heterozygous individuals have the allele but not the disadvantageous phenotype
so it will survive and breed and pass the allele on
this means the recessive allele will remain at a low frequency in the population
define gene pool
the total of all alleles for all of the genes in a population
what ways can an allele frequency be expressed?
either as a proportion or a percentage of the total number of copies of all alleles for that gene
what letters are the frequency of the dominant and recessive alleles usually represented as?
p & q
in a population of 65 organisms, 25 are homozygous dominant and 30 are heterozygous.
what is the frequency of the dominant allele and the recessive allele in the gene pool?
total number of alleles = 65 x 2 = 130
no. of dominant alleles = 25 x 2 + 30 = 80
p (frequency of dominant allele) = 80 / 130 = 0.62
q (frequency of recessive alleles) = 1 - p = 0.380
what does the Hardy Weinberg principle state?
states that the frequencies of dominant and recessive alleles and genotypes will remain constant from one generation to the next, if certain conditions remain true
what are the 5 Hardy Weinberg conditions required so that genes remain constant?
a large population (100+ individuals)
no selection for or against any phenotypes
random mating throughout the population
no mutations
the population is isolated, i.e. no immigration or emigration
what can the Hardy Weinberg principles be used for?
can be used to estimate the frequencies of dominant or recessive alleles or of different genotypes of a characteristic in a population using the following equation
what is the equation for the Hardy Weinberg principle?
p² + 2pq + q² = 1
what does each letter in the p² + 2pq + q² = 1 equation represent?
p = frequency of the dominant allele (A)
q = frequency of the recessive allele (a)
p + q = 1.0
the three terms of this binomial expansion indicate the frequencies of the three genotypes:
p² = frequency of AA
2pq = frequency of Aa
q² = frequency of aa
The frequency of Tay-Sachs disease is 1 in 360000 births in the USA. however, the frequency is 1 in 40000 in certain populations, which isolate themselves culturally
one such population is the Old Order Amish of the Kishacoquillas Valley, Pennsylvania, USA. recent estimates put the size of the population at 40000
use the hardy weinberg equations, given below, to calculate the percentage of heterozygous individuals in the Amish community that carry the Tay-Sachs allele without suffering the disease
work out the number of people who are carriers of the Tay-Sachs allele
frequency of homozygous recessive (q²) = 1/40000 = 0.000025
q = 0.005
p + q = 1
p = 1- 0.005
p = 0.995
frequency of heterozygotes = 2pq
2pq = 2 x (0.995 x 0.005)
2pq = 0.00995
x 100 = 0.995%
0.995% of 40000 = 398
explain why the frequency of Tay-Sachs is higher in isolated populations
and predict, with a reason, what is likely to happen to the frequency of the Tay-Sachs allele in the general population
population is smaller/smaller gene pool/no migration OR higher probability
decreases because recessive alleles will be lost from one gene pool when sufferers die in childhood/selective abortion/selected against/selective disadvantages/less likely to reproduce
