Populations and evolution Flashcards
1
Q
What is a population?
A
a population is a group of organisms of the same species that occupies a particular area at a particular time and that can potentially interbreed.
2
Q
what is a species?
A
a species is defined as a group of similar organisms that reproduce to produce fertile offspring. species can exist as one or more populations
3
Q
What is a gene pool?
A
all the alleles of all the genes of all individuals in a population at a given time
4
Q
what is the allele frequency?
A
how often an allele occurs in a population/gene pool)
5
Q
example: cystic fibrosis - recessive condition
Dominant allele = F
recessive allele = f
How many alleles of this does a person have?
A
Any human individual has two of these allele in every single of their cells, one in each pair of homologous chromosomes
6
Q
so when considering a gene pool how do we, how do we calculate the allele number and why?
A
as these alleles are the same in every cell of a single person, we only count one pair of alleles per gene per individual when considering a gene pool
7
Q
if there are 10,000 people in a population, how many alleles would be in the gene pool of this gene?
A
20,000
8
Q
if in the 10,000 people population had the genotype FF, what would the probability of anyone being FF and ff?
A
FF = 1.0 and ff = 0.0
frequency of FF = 100%
frequency of ff = 0%
9
Q
if in the 10,000 people population had the genotype Ff, what would the probability of anyone being Ff or fF?
A
the probability of anyone being Ff= 1.0 and the frequency of the F allele would be 50% and the frequency of the f allele would be 50%
10
Q
The Hardy-Weinberg Principle
A
11
Q
what is the Hardy-Weinberg Principle?
A
a mathematical model/ equation that is used to calculate/predict the frequency of the alleles of a particular gene in a population
12
Q
the principle assumes that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next this can be the case provided that 5 conditions are met:
A
- no mutations arise
- the population is isolated (there is no flow of alleles in or out of the population)
- there is no selection (all alleles are equally likely to be passed onto the next generation)
- the population is large
- mating within the population is random
13
Q
what is the total frequency for all the possible alleles for a characteristic in a certain population?
A
1.0 = must add up to 1
14
Q
give the equation which tells us the allele frequency? include a key.
A
p + q = 1
p = the frequency of the dominant allele
q = the frequency of the recessive allele
15
Q
what is the total frequency of all the individuals genotypes?
A
1.0 = must add up to 1
16
Q
Give the equation to calculate the genotype frequency?
A
p^2 + 2pq + q^2 = 1
p^2 = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype
q^2 = frequency of homozygous recessive genotype
17
Q
what does the Hardy-Weinberg theory predict?
A
predicts that the frequencies of alleles in a population won’t change from one generation to the next
18
Q
as long as…?
A
there is no immigration, emigration, mutations, natural selection and mating is totally random
19
Q
do practice exam questions
A
20
Q
Variation in Phenotype
A
21
Q
What is Variation?
A
the differences that exist between individuals
22
Q
what is most variation in phenotype due to?
A
a combination of genetic and environmental factors
23
Q
which is the only factor that affects evolution?
A
genetic variation
24
Q
Individuals of the same species may have the same genes but different..?
A
alleles (version of genes) -this causes genetic variation within a species
25
what are other sources of genetic variation?
Meiosis - this special form of nuclear division produces new combination of alleles before they are passed into gametes.
Random fertilisation of gametes - in sexual reproduction this produces new combination of alleles and the offspring are therefore different from parents. which gametes fuse with which at fertilisation is a random process further adding to the variety of offspring two parents can produce
independent segregation of chromosomes
26
what is the primary source of genetic variations?
Mutations - sudden changes to genes and chromosomes (may or may not be passed onto next generation)
27
give an example of genetic variation?
blood groups
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how many genes is this controlled by?
one gene
29
Variation due to environmental factors
30
explain how environment affects variation?
environments exerts an influence on all organisms. these influences affect the way the organism's genes are expressed
31
give an example - buttercups
in buttercups, one lant may be determined to grow much taller than the other plants. If, however the seed germinated in poor light or low soil nitrate, the plant may not grow properly and it will be short.
32
give examples of environmental influences?
- climatic conditions (e.g temperature, rainfall and sunlight)
- soil conditions
- pH
- food availability
33
what is meant by variation that is controlled by polygenes?
variation is not controlled by a single gene, but multiple genes
34
give an example?
height and mass
35
why are individuals who are predetermined to be the same height, do not grow to same heights when they are older?
due to variations in environmental factors such as diet.
36
what is this variation due to?
this variation is due to polygenes and the environment
37
if we measure the height of the population, what curve would we typically get?
a bell-shaped curve
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Natural Selection
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every organism is subjected to selection based on it's..?
suitability for survival under the conditions that exist at the same time
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what are selection pressures?
the environmental factors that limit the population of a species
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what are examples of selection pressures?
predation, disease and competition
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when do selection pressures vary?
from time to time and from place to place
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what does the selection pressures determine?
the frequency of alleles in the gene pool
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How does selection pressures differ from the population?
because members of the same species have different alleles, there is variation between individuals, meaning that some are better adapted to the selection pressures than others. So there are different levels of survival and reproductive success in a population.
45
What is Evolution?
the frequency of an allele in a population changes over time.
46
the process of evolution by natural selection, depends on what factors?
- organisms produce more offspring than can be supported by the available supply of food, light space etc
- there is genetic variety within the populations of all species
- a variety of phenotypes that selection operates against
47
the role of over-production of offspring in natura selection
Title
48
By which two processes can Evolution occur by?
Genetic Drift and natural selection
49
how can a species population increase, why is this way rare? Who came up with this?
can increase exponentially. Darwin said that is this ever happened, even the most slow-breeding species death rate must be extremely high
50
what has evolved to ensure that a large population survives to breed and produce the next generation
high reproductive rates have evolved
51
this prevents high death rates from?
predation, competition for food and water, extremes of temperature, and natural disasters.
52
some species have evolved lower reproductive rates along with a high degree of parental care. Explain the advantage of this?
the lower death rates that result help to maintain their population size
53
what is the link between over-production and natural selection?
there are too many offspring for the available resources, there is competition amongst individuals (intraspecific) for the limited resources available
54
the greater the population, the greater the..?
competition and more individuals die to survive
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who survives?
individuals who are best suited for prevailing conditions and will pass on successful alleles.
56
the population that survives will have a different?
combination of alleles to the previous population (unsuccessful)
57
why will sometimes an organism produce an offspring identical to itself?
because the offspring will be able to survive in the changing environment, but will be less suited for environment currently in.
58
population who show little genetic variation are more vulnerable to..?
new diseases and climate change.
59
what is it important that a species should be able to adapt to?
important that a species is capable of adapting to changes resulting from the evolution of other species
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the larger the populaton, the more..?
genetically varied the individuals within it
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so what is the advantage of variation?
provides the potential for a population to evolve and adapt to new circumstances
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Effects of Different forms of natural selection on evolution
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the effect of natural selection on allele frequencies depend on..?
the selection pressures acting on the population
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what are the three types of natural selection?
Stabilising, directional and disruptive
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Stabilising Selection
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what is Stabilising selection?
preserves the average phenotype of a population favouring average individuals, in other words selection against the extreme phenotypes
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what does stabilising selection tend to eliminate?
tends to eliminate the extremes of the phenotypes range within a population and with it the capacity for evolutionary change
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when does it tend to occur?
its tends to occur when the environmental conditions are constant over long periods of time
69
Give an example?
for example fur length in particular mammalian species. in years where temp is hotter, short fur individuals at an advantage as can loose heat more rapidly. whereas for colder opposite is true.
70
will both species be able to survive and when?
if the environment fluctuates from year to year, both extremes will survive because each will have some years when it can thrive at the expense of the other.
71
if environmental conditions do not favour the extremes they will be at a ...?
disadvantage, and will be selected against in favour of those with average fur length
72
Directional Selection
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what is directional selection?
changes the phenotype of a population by favouring phenotypes that vary in one direction from the mean of the population. in other words, selection for one extreme phenotype
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what curve is formed?
what does the mean represent?
a normal distribution curve. this curve has a mean that represents the optimum value for the phenotypic character under the existing conditions
75
why could directional selection occur?
could occur due to environmental change
76
explain how directional selection could occur due to environmental change?
if environmental conditions change, so will the optimum value for survival. some individuals will either fit to the right or the left of the mean, so will possess a combination of alleles with the new optimum for the phenotypic character. As a result there will be selection pressure favouring the the combination of alleles that favour the mean.
77
give an example of directional selection?
antibiotic resistance in bacteria
78
using fur length as an example, describe how directional selection could occur?
1. when average environmental temp is 10°, optimum length is 1.5cm, (mean for population)
2. a few individuals in the population already have a fur length of 2.0cm. if the average temp falls to 5°, these individuals are better insulated and so more likely to survive, breed and pass on successful alleles. there is selection pressure favouring individuals with long fur length.
4. over generations fur length will increase until it reaches 2.0cm - which is new optimum for 5°
79
Disruptive selection?
80
what is disruptive selection?
disruptive selection is the opposite of stabilising selection. it favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population
81
what is the least common type of selection?
disruptive
82
what is the most important selection in bringing about evolutionary change?
83
when does disruptive selection occur?
when an environmental factor such as temperature, takes two or more distinct forms
84
give an example of disruptive selection?
Coho salmon - where large males and small males have a selective advantage over intermediate-sized males in passing their alleles to the next generation. the small males are able to sneak up on the females in the spawning grounds. this leaves intermediate at a disadvantage
85
Using fur length explain disruptive selection?
1. when there is a wide range of temperatures throughout the year, there is continuous variation in fur length around the mean of 1.5cm.
2. where the summer temperature is static around 15° and the winter temperature is static around 5°, individuals with two fur lengths predominate .
4. after many generations two distinct subpopulations are formed
86
What is polymorphism?
some species of organisms have two or more distinct forms. these different forms are genetically distinct but exist within the same interbreeding population.
87
Give an example?
Peppered moths - existed as a light form until the middle of the 19th century. around this time a melanic/black variety rose as a result of mutation.
88
use the peppered moths as an example of directional selection?
these mutant moths were completely visible to predators and eaten. However due to industrial work (in Manchester), soot covered buildings etc and sulfur dioxide killed lichen, so the black moths were more camouflaged than white moths, so light form eaten more, and black moths made up most of population (98%) - this illustrates directional selection of different types in different populations, the melanic form is selected in industrial area while natural form selected in rural areas.
89
how is this an example of selective predation?
because the birds favoured individuals that lie at one extreme or the other of a range of different colour types.
90
Speciation and genetic drift
91
environmental factors do not affect the probability of a articular mutant arising but affects...?
the frequency of the mutant allele that is already present in the gene pool
92
What is speciation?
speciation is the evolution of new species from existing species.
93
what is a species?
a species is a group of individuals that have a common ancestry and so share the same genes but different alleles and are capable of breeding with one another to produce fertile offsprings.
94
when does speciation occur?
when populations of the same species become reproductively isolated/separated from other species. - changes in allele frequency causes change in phenotype, which means they can no longer interbreed to produce fertile offspring.
95
when does reproductive isolation/separation begin to occur?
What speciation does it lead to?
can occur when a physical barrier e.g flood or an earthquake divides population of a species, causing some individuals to become separated from the main population. there is no gene flow between the two populations which can lead to allopatric speciation.
96
what is this know as?
geographical isolation
97
how else can speciation occur?
when a population becomes reproductively isolated without any physical barrier - sympatric speciation
98
what is adaptive radiation?
when a population is separated in some way and undergoes mutations, it will ecome genetically different to other populations. each population will experience different selection pressures because the environment of each will be slightly different. Natural selection will change the allelic frequency of each population. the different phenotypes each combination of alleles produce will be adapted to the selection pressures of it's local environment. - this is adaptive radiation
99
what does adaptive radiation result in?
results in changes to alle frequency (evolution) of each population
100
And because of these genetic differences, the populations are now no longer..?
to interbreed together successfully. each population will be a different species each with their own gene pool
101
Summarise the whole process of speciation?
ISOLATION: 2 populations are separated (geographically or reproductively) No gene flow
GENETIC VARIATION: each population contains a variety of alleles due to different mutations and a range of phenotypes
NATURAL SELECTION: each population has different selection pressures so different advantageous alleles are passed on as individuals survive and reproduce
TIME: over many generations, populations become so different that successful interbreeding is no longer possible
102
Genetic Drift
103
What can genetic drift only take place within small populations?
this is because the relatively few members of a small population possess a smaller variety of alleles than the members of a large population. so less genetic diversity
104
what is genetic drift?
A change in allele frequency due to chance rather than selection or environmental factors.
105
why is genetic drift also referred to as random drift?
instead of environmental factors affecting which individuals survive, breed and pass on their alleles, chance dictates which alleles are passed on.
106
explain how genetic drift works?
as there are only a small number of alleles in the population there is not an equal chance of them being passed on. those that are passed on will quickly affect the whole population as their frequency is high. Any mutations to one of the alleles that is selectively favoured will also more quickly affect the population because it's frequency will be high. the effects of genetic drift will be greater and the population will change relatively rapidly, making it more likely to develop into a separate species.
107
why in a large population will the effect a mutant allele be quite insignificant?
the effect of a mutant alle will be diluted because it's frequency is far less in the much larger gene pool. the effects of genetic drift are likely to be less and development into new species will be slower
108
what are the two forms of speciation?
allopatric and sympatric speciation
109
what is allopatric speciation?
occurs when a geographical barrier physically isolates two populations of an ancestral species
110
how may geographical separation occur?
may occur due to a physical barrier e.g rivers, oceans mountains rangers and deserts that prevents them from interbreeding
111
which animal is harder to geographically separate and why?
birds because they can cross over e.g the pacific ocean
112
what type of animals are easier to separately isolate?
e.g snails as a tiny strema may act as a barrier
113
if environmental conditions of either side vary, what will be different n either side?
natural selection will influence the two populations separately and each will evolve according to their own local conditions
114
how long will these changes take place for?
hundreds and thousands of generations
115
eventually will have?
Eventually the two populations reach a degree of genetic divergence whereby they can no longer interbreed (speciation)
116
explain how allopatric speciation may lead to two different species? - using a separated forest as an example.
- species X occupy a forest. individuals within the forest form a single population with a single gene pool and freely interbreed
- climatic changes to drier conditions reduce the size of the forest to two separate regions. the distance of two regions is far too great for species X to meet each other.
- further climatic changes results in one region (Forest A) becoming colder and wetter. Natural selection acts on population X which becomes better adapted to new conditions, physiological and anatomical changes occur in this group
- continued adaption leads to evolution of a new form, population Y in forest A
- a return to the original climatic conditions result in regrowth of forest. Forest A and B are merged populations X and Y. the two populations are no longer capable of breeding as they are now two separate species
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Sympatric Speciation
118
what is sympatric speciation?
divergence of species within the same geographical location.
119
when can sympatric speciation occur?
can occur when random mutations within a population prevent individuals that carry the mutation from breeding with other members of the population that don't carry the mutation.
120
give an example of symatric speciation?
apple maggot fly - originally this insect laid eggs inside fruit of hawthorns. when apple trees were introduces, the fly started to lay eggs in apples also. females tended to lay eggs in the type of fruit they developed in and males tended to look for mates in the type of fruit they developed in. so flies raised in hawthorns mated with flies from hawthorns and flies from apples mated with flies from apples. while two types are flies are not different species, mutations in each population have led to evolution of genetic differences. in time this could result in the being incapable of breeding successfully, and therefore separate species.
121
what are reproductive isolation mechanisms?
occurs when barriers prevent two populations from interbreeding – keeping their gene pools separate
122
what are two main categories of reproductive isolation barriers?
Prezygotic isolation – occurs before fertilisation can occur (no offspring are produced)
Postzygotic isolation – occurs after fertilisation (offspring are either not viable or infertile)
123
give examples of pre-zygotic isolation?
geographical - populations are isolated y physical barriers e.g oceans
ecological - population inhabit different habitats within the same area so individuals rarely meet
temporal - the breeding seasons of each population do not coincide and so do not interbreed.
behavioural - mating is often preceeded by courtship, which is stimulated by the colour or marking of the opposite sex, the call or particular actions of the male. . Any mutations which cause variationin these patterns may prevent mating.
124
what are examples of post-zygotic isolation?
Gametic - the gametes may be prevented from meeting due to genetic or biochemical incompatibility. For instance, some pollen or grains fail to germinate or grow when they land on a stigma of different genetic make-up.
Hybrid sterility - hybrids form the fusion of gametes from different species are often sterile because they cannot produce viable gametes e.g horse - 64 chromosomes and donkey = 62 chromosomes
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