Variation and Evolution

Question 1

define variation

Answer 1

differences in phenotype between individuals

Question 2

what three factors cause variation?

Answer 2

difference in genotype - genetic factors

different epigenetic modifications - but same genotype

differences in environments - environmental factors

Question 3

why is variation important?

Answer 3

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

Question 4

dsecribe the difference between discontinuous and continuous variation

Answer 4

discontinuous - a particular phenotype can be controlled by one gene
continuous - controlled by more than one gene

Question 5

describe the concept of non-heritable variation (environmental)

Answer 5

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

Question 6

explain how the environment could lead to an epigenetic change

Answer 6

an environemtnal factor e.g. exercise and diet

alters DNA methylation or histone modification, leading to changes in the expression of genes

Question 7

what is genetic variation also known as?

Answer 7

heritable variation

Question 8

genetic variation is increased as a result of…?

Answer 8

sexual reproduction

Question 9

how is genetic variation increased as a result of sexual reproduction?

Answer 9

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

Question 10

what is the difference between sexual reproduction and mutations in terms of alleles?

Answer 10

sexual reproduction establishes new combinations of already present alleles

mutations produce new alleles which often have a more significant impact on natural selection

Question 11

what is the effect of competition on variation?

Answer 11

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

Question 12

how does an organism become suitable to its environment?

Answer 12

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

Question 13

define selection pressure

Answer 13

an environmental factor that can alter the allele frequencies of the alleles present at a particular gene locus in a population

Question 14

define selective agencies

Answer 14

exert selection pressure, for example, Climate, Human impact, Supply of food, Breeding sites

Question 15

what is the effect of organisms with well-adapted phenotypes?

Answer 15

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

Question 16

what selection pressure led to the increase in dark form moths during the industrial revolution?

Answer 16

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

Question 17

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

Answer 17

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

Question 18

will the effect be the same for a recessive allele that produces a selective disadvantage?

Answer 18

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

Question 19

define gene pool

Answer 19

the total of all alleles for all of the genes in a population

Question 20

what ways can an allele frequency be expressed?

Answer 20

either as a proportion or a percentage of the total number of copies of all alleles for that gene

Question 21

what letters are the frequency of the dominant and recessive alleles usually represented as?

Answer 21

p & q

Question 22

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?

Answer 22

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

Question 23

what does the Hardy Weinberg principle state?

Answer 23

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

Question 24

what are the 5 Hardy Weinberg conditions required so that genes remain constant?

Answer 24

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

Question 25

what can the Hardy Weinberg principles be used for?

Answer 25

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

Question 26

what is the equation for the Hardy Weinberg principle?

Answer 26

p² + 2pq + q² = 1

Question 27

what does each letter in the p² + 2pq + q² = 1 equation represent?

Answer 27

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

Question 28

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

Answer 28

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

Question 29

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

Answer 29

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

Question 30

define evolution

Answer 30

the change in the average phenotype of a population over time

Question 31

explain the theory of natural selection

Answer 31

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

Question 32

describe the process of natural selection

Answer 32

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

Question 33

what effect does natural selection have on allele frequency?

Answer 33

NS can drive change in allele frequency or it can maintain allele frequency

Question 34

describe two types of natural selection (links to allele frequency) examples on page 9 in booklet - important

Answer 34

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

Question 35

what process leads to the use of a Student's t-test?

Answer 35

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

Question 36

explain the difference between a student's t-test and chi squared

Answer 36

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

Question 37

define species

Answer 37

a group of (phenotypically) similar organisms that can interbreed to produce fertile offspring

Question 38

define demes

Answer 38

a local population that actively interbreeds and shares a distinct gene pool (breeding subunits)

Question 39

describe and explain speciation

Answer 39

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

Question 40

what three things can cause speciation to occur? and define two of them

Answer 40

genetic drift - changing allele frequencies by chance the founder effect (e.g. of genetic drift) - disproportionate allele frequencies in small populations natural selection

Question 41

what are the two main types of speciation? describe them briefly

Answer 41

allopatric speciation - geographical isolation sympatric speciation - reproductive isolation

Question 42

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?

Answer 42

the smaller population if the 10 organisms died without breeding and passing on the allele, it would disappear

Question 43

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?

Answer 43

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

Question 44

describe the founder effect

Answer 44

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

Question 45

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

Answer 45

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

Question 46

describe allopatric speciation

Answer 46

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

Question 47

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

Answer 47

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

Question 48

describe sympatric speciation

Answer 48

occurs when there is a reproductive barrier isolating individuals, such as being unable to attract a mate from another deme

Question 49

what are four different types of isolation (sympatric speciation)

Answer 49

behavioural morphological gametic seasonal

Question 50

describe behavioural isolation

Answer 50

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

Question 51

describe morphological isolation

Answer 51

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

Question 52

describe gametic isolation

Answer 52

there are barriers preventing gametes of different species fusing

Question 53

provide two examples of gametic isolation

Answer 53

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

Question 54

describe seasonal isolation

Answer 54

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

Question 55

what two things can arise as the demes become new species?

Answer 55

hybrid inviability hybrid sterility

Question 56

describe hybrid inviability

Answer 56

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

Question 57

describe hybrid sterility

Answer 57

in some cases, an embryo can survive when individuals of different species breed (e.g. in wheat plants and mules)

Question 58

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?

Answer 58

cannot form bivalents/homologous pairs during Prophase I meiosis cannot take place gametes aren't produced