4.4 variation and evolution Flashcards
1
Q
why do organisms show variation in their phenotypes?
A
due to having different genotypes, the same genotype but different epigenetic modifications, or being exposed to different environments
2
Q
is variation heritable?
A
yes
3
Q
what does variation arise from?
A
- gene (point) mutations
- crossing over during prophase I of meiosis
- independent assortment during metaphase I and II of meiosis
- random mating i.e that any organism can mate with another
- random fusion of gametes i.e the fertilisation of any male gamete with any female gamete
- environmental factors leading to epigenetic modifications
- environmental factors can also lead to non-heritable variation within a population e.g diet
4
Q
what are the two types of variation?
A
- continuous
- discontinuous
5
Q
what are some examples of continuous variation?
A
- height
- weight
- heart rate
6
Q
continuous variation:
A
- range of phenotypes seen
- variation that cannot be categorised
- controlled by many genes (polygenic)
- follows a ‘normal’ distribution
- produces a continuous range in which a characteristic can take any value
- environmental factors have a major influence e.g diet on weight
7
Q
what is an example of discontinuous variation?
A
blood groups
8
Q
discontinuous variation:
A
- characteristics fit into distinct groups / can be categorised
- there are no intermediates
- usually controlled by one gene with two or more alleles (monogenic)
- environmental factors have little influence e.g diet has no effect on blood group
9
Q
is continuous variation usually controlled by one or many genes?
A
many genes
10
Q
what is the word for continuous variation being controlled by many genes?
A
polygenic
11
Q
does continuous variation follow a normal distribution?
A
yes
12
Q
what is it called when discontinuous variation is controlled by one gene?
A
monogenic
13
Q
what is the mean?
A
- a measure of central tendency
- sum of the values divided by the number of values
14
Q
what is the mode?
A
the most common value
15
Q
what is the standard deviation?
A
- a measure of the variation in the data either side of the mean
16
Q
if the data is normally distributed, then _____% of the data will lie within 2 standard deviations either side of the mean
A
95.4%
17
Q
statistically, if two data sets are significantly different, then no data will overlap within mean + 2SD
A
18
Q
how can you compare the means of data values of two populations?
A
using the t-test
19
Q
what is the t-test formula?
A
t = (x̄1 - x̄2) / √ ( (s1)^2 / n1) + ( (s2)^2 / n2))
where:
x̄ = mean of observations
n = no. of observations (sample size)
s = standard deviation
20
Q
for the t-test to work, must the two sample have the same number of observations?
A
yes
21
Q
how to work out the t-test?
A
- work out the means of each sample
- then subtract one from the other
- then work out the SD for each sample by subtracting the observed value from the mean for EACH observation
- then square EACH value and THEN add them up
- divide this value by the number of observations minus 1 and square root your answer
√ (Σ(x-x̄)^2 / n-1) - work out the standard deviation for the other sample
- square your deviation and divide by the number of observations in that sample, do the same for the other sample, add together and square root your answer
- finally, divide the differences in the means by
√ ( ((s1)^2 / n1) + ((s2)^2 / n2)
where s1= standard deviation for sample 1
22
Q
your calculated t-value must (exceed/be less than) the critical value in the table for 0.05% (5% probability) for the degrees of freedom (total number of observations in that-2) for you to be sure the differences seen were not due to chance
A
exceed
23
Q
what is the degrees of freedom for the t-test?
A
total number of observations - 2
24
Q
can environmental influences affect the way a genotype is expressed?
A
yes
- and result in different phenotypes
25
what is an example of the way that environmental influences have affected the way a genotype is expressed and result in different phenotypes?
- industrial melanism in the peppered moth
- there are two forms of the peppered moth: speckled and dark (melanic)
- in polluted habitats where trees are covered with soot, the dark form prevails
- but in unpolluted habitats were lichens are found, the speckled form is more common
- in each instant the moth’s colour camouflages it against the environment conferring a selective advantage, so it’s more likely to survive and reproduce, transmitting advantageous alleles to the next generation and so the numbers increase within the population
26
what are the two types of competition?
- intraspecific
- interspecific
27
what is intaspecific competition?
- a type of competition that takes place between members of the SAME species
e.g food, light, nutrients, availability of nesting sites
28
what is interspecific competition?
a type of competition that takes place between members of DIFFERENT species
e.g different plant species competing for water
29
selection pressure definition
an environmental factor that can alter the frequency of alleles in a population, when it is limiting
30
- the different resources being competed for act as a selection pressure
- therefore individuals with an advantage meaning they are more successful in gaining food and shelter are more likely to survive
- and pass those advantageous alleles onto the next generation
- this increased chance of survival and reproduction of organisms with phenotypes suited to their environment is called natural selection
31
what can natural selection lead to?
evolution
32
what is the gene pool of a population?
all if the alleles present in a population at any one time
33
population genetics considers the relative proportions of the different alleles, or allele frequencies within the gene pool
34
if the environment is stable then allele frequencies ___
also remain stable
35
however, environments do change and bring different selection pressures, which favour some alleles over others, so their frequency increases
36
allele frequency definition
the proportion/fraction/percentage of all the alleles of that gene in a gene pool
37
what is genetic drift?
variations in allele frequencies in (small) populations due to chance (rather than as a result of selection pressures)
38
what is genetic drift due to?
- random sampling (which alleles are inherited)
- chance (that an individual may survive and breed)
39
what does genetic drift lead to?
- changes in allele frequencies over time
40
where is genetic drift most significant?
- in small or isolated populations where a change will constitute a much larger proportion of the population because the population is small
- and so it may be an important evolutionary process
41
what is the founder effect?
- the loss of genetic variation in a new population established by a very small number of individuals from a larger population
(-when a small number of individuals become isolated, forming a new population with a limited gene pool. allele frequencies are not reflective of the original population)
42
- when a small number of individuals become isolated and start a new population, say by colonising a new island, the founder members of the new population are a small sample of the population from which they originated (the founder effect) and may be subject to genetic drift
- where was this seen in real life?
- in the adaptive radiation seen in Darwin’s finches on the Galapagos Islands
43
what does the hardy-weinberg principle state?
- that in ideal conditions the alleles and genotype frequencies in a population are constant
44
what are ideal conditions in the hardy-weindberg principle?
- organisms are diploid
- they have equal allele frequencies in both sexes
- reproduce sexually
- mating is random
- generations don’t overlap
- the population size is very large
- there is no migration, mutation or selection
45
what are the 2 hardy-weinberg equations?
p^2 + 2pq + q^2 = 1
p + q = 1
- where p represents the frequency of the dominant allele e.g A
- and q = frequency of the recessive allele e.g a
46
p^2 =
2pq =
q^2 =
- p^2 = frequency of homozygous dominant e.g AA
- 2pq = frequency of heterozygous e.g Aa
- q^2 = frequency of homozygous recessive e.g aa
47
it is therefore possible to use the equations to calculate allele and genotype frequencies from the instances of a disease or other phenotypic character
48
e.g
- cystic fibrosis is a recessive condition affecting around 1 in 2500 babies
- calculate the frequency of the recessive allele and the proportion of carriers in the population
- q^2 = 1/2500 =0.0004
- q = √0.0004 =0.02 so allele frequency (q) = 2%
- and p + q = 1 so p=0.98
- frequency of heterozygotes (Aa) i.e 2pq
- so frequency = 2 x 0.98 x 0.02 = 0.0392 = 0.04
- 0.04 x 100 = 4% or 1 in 25 are carriers
49
why do organisms overproduce offspring?
- so that there is a large variation of phenotypes in the population
- changes to environmental conditions bring new selection pressures through competition/predation/disease which results in a change in the allele frequency
50
what is speciation?
the evolution of new species from existing ones
51
species definition
a group of individuals with similar characteristics that can interbreed to produce fertile offspring
52
what are the two types of speciation?
- allopatric speciation
- sympatric speciation
53
what is allopatric speciation?
- it involves geographical isolation which reproductively isolate two sub-groups (demes) within a population of the same species, preventing gene flow between them
- this is followed by exposure to different environmental conditions that favour different individuals within each deme
- after thousands of generations exposed to the differing conditions, allele frequencies within the demes change as a result of different mutations
- if the barrier is removed, the two populations have changed so much that they can no longer interbreed
54
what is sympatric speciation?
- involves populations living together becoming reproductively isolated by means other than a geographical barrier
- e.g. behavioural isolation occurs in animals with elaborate courtship behaviours where members of a sub-species fail to attract the necessary response e.g stickleback
- other mechanisms include:
• seasonal (temporal) isolation - where organisms are isolated due to reproductive cycles not coinciding and so are fertile at different times of the year (this is seen in frogs where each of four types has a different breeding season)
• mechanical isolation - as a result of incompatible genitalia
• gametic isolation - from the failure of pollen grains to germinate on stigma or sperm fail to survive in oviduct e.g fruit flies
• hybrid inviability - embryo development may not occur
55
reproductive isolation can be pre-zygotic (prevents gametes from fusing to form a zygote), or post-zygotic (zygote is formed but developing organism is sterile)
56
when does hybrid sterility occur?
- when two closely related species interbreed but due to differences in chromosome structure or number, chromosomes fail to pair during prophase I of meiosis and so gametes do not form
- the resulting offspring are sterile
57
what is evolution?
- the process by which new species are formed from pre-existing ones over a long period of time
- the change in allele frequencies in a gene pool of a population over time
- occurs due to natural selection
58
- Darwin’s observations of variation within a population led to the development of the theory of natural selection and Darwinian evolution in the 19th century
59
natural selection and Darwinian evolution:
- organisms overproduce offspring, so that there is a large variation of genotypes in population
- changes to environmental conditions bring new selection pressures through competition/predation/disease
- only those individuals with beneficial alleles have a selective advantage
- these individuals then reproduce more successfully than those without the beneficial alleles
- offspring are likely to inherit the beneficial alleles
- therefore the beneficial allele frequency increases within the gene pool
60
when talking about evolution, are you talking about genes or alleles?
alleles
61
what factors provide variation between individuals?
- both genetic and environmental factors produce variation
62
name the different types of variation:
- continuous and discontinuous
- heritable and non-heritable
63
compare heritable and non-heritable variation:
- heritable = the genetic differences between individuals
- non-heritable = acquired differences in the phenotypes of individuals that cannot be inherited
64
how does natural selection cause a change in allele frequencies over generations?
- organisms with advantageous characteristics are more likely to survive and pass their favourable alleles to offspring
- frequency of unfavourable alleles decreases
65
what are selection pressures?
- environmental factors that drive evolution by natural selection and limit population sizes
- they can change the frequency of alleles in a population
66
give examples of selection pressures:
- predation
- disease
- competition (for food, habitats, mates)
- environmental conditions e.g temperature
67
how can allele frequency be expressed?
- as a percentage or proportion of the total number of all alleles for that gene
68
what is the hardy-weinberg principle?
- a model that allows the estimation of the frequency of alleles in a population, as well as whether allele frequency is changing over time
69
state the assumptions made by the Hardy-Weinberg principle:
- no mutations occur to create new alleles
- no migration in or out of the population
- no selection, alleles are all equally passed on to the next generation
- random mating
- large population
70
explain the hardy-weinberg equation for calculating allele frequency:
- the frequency for each allele for a characteristic must add up to 1
- p + q = 1
- p = frequency of dominant allele
- q = frequency of recessive allele
71
explain the hardy-weinberg equation for calculating genotype frequency:
- the frequency if each genotype for a characteristic must add up to 1
- p^2 + 2pq + q^2 = 1
72
why may speciation occur?
- genetic drift in isolated population
- founder effect
- natural selection
73
what is the term for speciation that occurs when two populations become geographically isolated?
allopatric speciation
74
what is the term for speciation that occurs when two populations within the same area become reproductively isolated?
sympatric speciation
75
outline geographical isolation:
- a physical barrier (such as a river or mountain) separated two populations of the same species
76
name the potential isolation mechanisms in sympatric speciation:
- morphological isolation
- seasonal isolation
- behavioural isolation
- gametic isolation
- hybrid sterility
- hybrid inviability
77
what is morphological isolation?
- the reproductive isolation of two populations due to the incompatibility of their reproductive systems
78
what is behavioural isolation?
- the reproductive isolation of two populations due to differences in their behaviour (such as different mating rituals)
79
what is seasonal isolation?
- the reproductive isolation of two populations due to differences in their breeding seasons
80
describe hybrid inviability:
- post-zygotic barrier
- successful fertilisation but embryo cannot develop into a living organism
81
what is hybrid sterility?
- the formation of sterile hybrid offspring from the reproduction of individuals of different species
82
why may the reproduction of individuals of didferent species produce sterile offspring?
- the chromosome sets from each parent differ so are unable to pair up during meiosis
83
give an example of a sterile hybrid
- mule
84
what is gametic isolation?
- pre-zygotic barrier
- successful fertilisation does not occur
85
when comparing data showing different types of variation, discontinuous data is compared using what??
- numbers of percentages in each group
- the ratio of the numbers in one group compared to another
(the grouping makes this type if data nominal data, as they can be put into categories)
86
when conparing data showing continuous data, what do you compare?
- the mean of each group
- the variance of the mean
(when dealing with graphs of continuous data you need to be able to recognise the mean, median and mode)
87
continuous data where the mean, median and mode have the same value are said to have a ______
normal distribution
88
characteristics which show discontinuous variation are examples of ____ data
nominal
89
characteristics which are discontinuous are often under the control of ____ gene?
one
90
data for a discontinuous characteristic can be displayed as a _____
bar graph
91
characteristics showing continuous variation are usually under the control of ____
several genes and environmental factors
92
what are some things plants compete for?
- light
- space
- mineral ions
93
what are some things animals compete for?
- food
- shelter
94
what is inter-specific competition illustrated by?
predator-prey relationships
95
- the gene pool remains stable if the environment is stable
- in large populations, assuming there is no selection, the proportion of dominant and recessive alleles of a gene remains constant. it is not altered by interbreeding
96
if the environment changes, some phenotypes will be advantageous and will be _____, whilst others will be at a disadvantage and will be _____
- selected for
- selected against
97
while the founder population remains small it may undergo genetic drift and become even more different from the large parental population. what is this due to?
- different mutations occurring in the small and large populations
- different selection pressures
- different alleles may give a competitive advantage in the isolated population compared to the original population
- the founder population is isolated from the main population; they cannot interbreed and share genes with the larger gene pool
98
disasters (earthquakes, floods and fires) can have a similar effect. they may reduce the size of the population drastically. consequently, the genetic makeup of the surviving population is unlikely to be representative of the makeup of the original population as:
- certain alleles will be overrepresented among survivors
- other alleles will be underrepresented
- some alleles will be eliminated completely
99
what is the bottleneck effect?
- alleles for some genes are likely to be lost from the gene pool, the overall genetic variability in the population is usually reduced
- this is called the bottleneck effect, where only the alleles of the surviving population remains
100
within a population of one species there are groups of interbreeding individuals. each of these breeding sub-units is called a _____
deme
101
geographical isolation:
- variation due to different, random mutations, arises within a population
- a barrier separates one part of the population from the other
- the allele frequencies in the isolated populations may be different
• in a small population the allele frequencies may be very different to the frequencies in the gene pool of the original population (founder effect)
- different selection pressures cause different changes to the gene pools in the separated populations; they evolve along separate lines
- if the barrier is removed there may be sufficient differences to prevent the formation of viable offspring. new species have evolved
102
the hardy-weinberg principle also shows that, if the conditions remain constant, then no evolution will take place
- there will be no change in the frequencies of dominant and recessive alleles and there are no selection pressures to favour one phenotype over another
103
- a recessive allele confers resistance to an insecticide in a particular insect species
- calculate the frequency of the dominant and recessive alleles for this characteristic if 36% of the insect population is resistant
- aa = 36%
- q^2 = 0.36
- q = 0.6
- p+q = 1
- p + 0.6 = 1
- p = 0.4
104
- in a sample of 1279 people in England, 363 were found to have the blood group M (genotype MM), 634 MN (genotype MN) and 282 NN
- calculate the frequencies of alleles M and N in the population of England
- the use of capital letters shows that these are co-dominant alleles, so either can be used as p or q
- alleles are in pairs, so total no. of alleles = 1279 x 2 = 2558
- total no. of M alleles = (363 x 2) + 634 = 1360 (MM + M from the MN)
- total no. of N alleles = (282 x 2) + 634 =1,198
- frequency of M = 1360/2558
- frequency of N = 1198/2558
(check add to 1)
- frequency M = p = 0.53
- frequency of N = q = 0.47
105
- a recessive allele confers resistance to an insecticide in a particular insect species
a) calculate the frequency of the dominant and recessive alleles for this characteristic if 36% of the insect population is resistant
b) calculate the proportion of the population which are homozygous dominant and heterozygotes
c) calculate the number of carriers in this population if there are 8409 insects in the total population
a)
- q^2 = 0.36
- q = 0.6
- p = 0.4
b)
- homozygous dominant = p^2 = 0.16
- heterozygotes = 2pq = 0.48
c)
- no. of carriers = 0.48 x 8400 = 4032
106
- some people are unable to produce the skin pigment, melanin
- these people are called albinos and are homozygous for a recessive allele
- each year, about 1 in 20,000 births are albino
- calculate the proportion of the population who are heterozygous for this condition and hence the number of people who are carriers of the albino allele per 1000 of the population
- allele for albinism = a, allele for normal = A
- q^2 = 1/20000 =0.00005
- q = 0.007
- p = 0.993
- heterozygotes = 2pq = 0.014
- no. of carriers per 1000 = 1000 x 0.014 = 14 people
107
polygenic characters often show _____ variation which can be demonstrated by plotting a ______, producing an approximately normal curve
- continuous
- frequency histogram
108
counts or measurements of samples are made (from continuous variation) and if their distribution is approximately normal, their means may be compared using student’s t-test
109
what is a t-test used for?
- used to compare the mean results of two groups (often controlled by vs experimental group)
- and determine if there is a significant difference between the means
- e.g testing the effectiveness of a vaccine in terms of antibody levels, or weight loss in a group of people on a diet compared to not dieting
110
when can the t-test be used?
- for sample sizes between 15-30
- for normally distributed data
- on characteristics which display continuous variation
111
in biology, what probability is used as the mark of significant difference?
0.05 / 5% probability
(under some circumstances like drug testing, might use a lower probability of 0.1% or even lower, especially when assessing the safety or determining the lowest dose)
112
t-test vs chi squared test:
T-TEST:
- used to compare two groups means
- can only be used for two groups (not multiple)
- assume the data to have a normal distribution (bell-shaped curve)
- similar standard deviations
- data must be measured values
CHI SQUARED TEST:
- can be used only on raw counts (no measurements)
- sample sizes must be more than 20
- only used to compare an experimental result w/a theoretical outcome
- it aims to test the null hypothesis of NO DIFFERENCE between data sets
113
what is the null hypothesis for a student’s t-test?
- there is no significant difference between the means of the two populations being studied
114
how to calculate a value of t?
- formulate a null hypothesis
- measure the characteristic you are comparing (minimum of 15 from each population)
- calculate the SD
- calculate the test statistic, using the sd and mean
- calculate the number of degrees of freedom
- find the critical value
- if your calculated value for t is greater than or equal to the critical value, reject the null hypothesis
- if your calculated value for t is less than the critical value, accept the null hypothesis
115
if your calculated value for t is greater than or equal to the critical value, (accept/reject) the null hypothesis?
reject
116
if your calculated value for t is less than the critical value, (accept/reject) the null hypothesis?
accept
