Variation and Evolution Flashcards
page 17 contains an application question
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
define evolution
the change in the average phenotype of a population over time
explain the theory of natural selection
it is the theory that explain how existing species have arisen through modification of ancestral species
it encourages the transmission of favourable alleles and hinders the transmission of unfavourable alleles
describe the process of natural selection
in any population there is a variation due to mutations
population numbers remain roughly the same, despite overproduction of offspring
competition (intraspecific) means there’s a struggle for survival
the fittest have a selective advantage due to selection pressures
they survive, interbreed and pass on alleles that give their offspring a selective advantage
the process repeats over many generations and increases the frequency of the advantageous allele in the population’s gene pool
eventually the organism may genetically change so much they become a different species
what effect does natural selection have on allele frequency?
NS can drive change in allele frequency or it can maintain allele frequency
describe two types of natural selection (links to allele frequency)
examples on page 9 in booklet - important
directional selection - if the environment changes then natural selection may favour one extreme of the phenotypes resulting in a directional change in the allele frequency in a population
stabilising selection - if the environment is stable then extreme phenotypes tend to be eliminated as they do not confer any selective advantage
this prevents change and stabilises a population
what process leads to the use of a Student’s t-test?
polygenic (many genes) characteristics often show continuous variation, which can be demonstrated by plotting a frequency histogram, producing an approximately normal curve
counts or measurements of samples are made and if the distributions are approximately normal, their means may be compared using Student’s t-test
explain the difference between a student’s t-test and chi squared
stt: to test if there is a significant difference between the means of two samples of continuous data, when data is normally distributed
cs: to test if there is a significant difference between the observed and expected data in set categories/discontinuous data
define species
a group of (phenotypically) similar organisms that can interbreed to produce fertile offspring
define demes
a local population that actively interbreeds and shares a distinct gene pool
(breeding subunits)
describe and explain speciation
when populations of a species become isolated, new species can form
if demes become isolated from each other due to a barrier to reproduction, the gene pool is divided and there is no flow of genes between separate demes
if reunited after many generations, the demes may be incapable of breeding successfully (cannot interbreed to produce fertile offspring).
two new species are formed each with their own gene pools
what three things can cause speciation to occur? and define two of them
genetic drift - changing allele frequencies by chance
the founder effect (e.g. of genetic drift) - disproportionate allele frequencies in small populations
natural selection
what are the two main types of speciation? describe them briefly
allopatric speciation - geographical isolation
sympatric speciation - reproductive isolation
if 1% of a population have a particular allele, 10000 individuals and 10 individuals would have the allele in a population of 1000000 and in a population of 1000
in which of the above populations is the allele at a greater risk of being lost by chance?
the smaller population
if the 10 organisms died without breeding and passing on the allele, it would disappear
in a species of birds, blue feathers are dominant to purple feathers
in a population of 120 birds, 15 have purple feathers
a disease kills 45 of the birds in the population, including all those with purple feathers
explain why this is an example of genetic drift rather than natural selection?
birds with purple feathers continued to be born even after the events above. explain why?
allele frequency in gene pool altered due to chance
some of the blue-feathered birds were heterozygous.
two heterozygous parents could have offspring with purple feathers
describe the founder effect
when a new area is colonised by individuals from a population, those individuals may have a gene pool which, due to chance, has different allele frequencies than the original gene pool
through chance changes in allele frequencies (genetic drift), the founder population could become even more different from the original population
this effect is more significant in small populations where a chance variation in allele frequency can lead to a significant change in phenotype compared to the original population
madagascar is an island off the coast of Africa
many of the species there are unique
suggest how the founder effect may have led to these unique species arising
a small group of organisms from mainland Africa colonised Madagascar
by chance the allele frequencies in their gene pool differed from the mainland gene pool
different selection pressures on the island
over the time these allele frequencies became so different, new unique species are formed
describe allopatric speciation
occurs when there is a physical barrier isolating individuals, such as a mountain range or river that splits a population into two separate demes, preventing interbreeding and the flow of genes
the example given is of the Kaibab squirrel found on the North rim of the Grand Canyon and Abert’s squirrel found on the south rim
suggest how allopatric speciation occurred at this location, creating new gene pools
large population with common gene pool
population is separated into 2 demes by a physical barrier, preventing flow of genes
mutations and different selection pressures on each deme alter gene pool
if the barrier is removed the gene pools will be so different that interbreeding will not be successful
describe sympatric speciation
occurs when there is a reproductive barrier isolating individuals, such as being unable to attract a mate from another deme
what are four different types of isolation (sympatric speciation)
behavioural
morphological
gametic
seasonal
describe behavioural isolation
in animals with elaborate courtship behaviour, the steps in the ‘display’ of one group of organisms may fail to attract the necessary response in a potential partner from another group of organisms
describe morphological isolation
the body parts of organisms may not be compatible enough for them to mate
this is seen in insects where the rigid exoskeletons mean that the genitalia of male and females must be complementary
describe gametic isolation
there are barriers preventing gametes of different species fusing
provide two examples of gametic isolation
stigma will only produce a sugary secretion for pollen germination if the pollen is compatible (same species)
spermatozoa can often only survive in an oviduct if from the same species
describe seasonal isolation
if reproductive organs of different groups mature at different times of year the groups are unable to interbreed
this can occur due to differences in mating seasons or differences in flowering times e.g. the toad Bufo americanus mates in early summer whilst B. fowleri mates in late summer, meaning the two species remain isolated
what two things can arise as the demes become new species?
hybrid inviability
hybrid sterility
describe hybrid inviability
fertilisation may occur but incompatibility between genes of the parents prevent the development of an embryo
hybrid embryos formed from sheep and goats die in the early stages of development
describe hybrid sterility
in some cases, an embryo can survive when individuals of different species breed (e.g. in wheat plants and mules)
hybrid sterility:
a horse can breed with a donkey to produce a hybrid known as a mule
if the horse has 64 chromosomes and the donkey has 62 chromosomes, the offspring have 63 chromosomes
the hybrid offspring are unable to produce gametes and are sterile
explain why?
cannot form bivalents/homologous pairs during Prophase I
meiosis cannot take place
gametes aren’t produced
