Populations And Evolution (18) Flashcards
Define gene pool
All the alleles of all the genes of all the individuals in a population at a given time
Define allele frequency
The number of times an allele occurs within the gene pool
Can every gene only have two different alleles ?
XXXX
Jeans can I have two or more possible animals. Individual humans have two alleles so two versions of each gene because we have this we are known as diploid organisms. The greater the number of potential alleles the more diversity in a given heritable trait
Cystic fibrosis alleles
Recessive f allele leads to production of thicker mucus
What do you mean by combinations of alleles ?
F - dominant
f - recessive
Heterozygous dominant Ff
Homozygous dominant FF
Homozygous recessive ff
heterozygous Ff
Bb
bb
What is the frequency of the bore allele in the population?
1/4
0.25
What is the hardy Weinberg principal?
Provides mathematical equation that can be used to calculate fréquence of the alleles of a particular gene in a population
What does the hardy Weinberg principal assume?
The proportion of dominant and recessive alleles of any gene in a population remains the same
from one generation to the next
The hardy Weinberg principal assumes that the proportion of dominant and recessive alleles of any Gene in a population will remain the same across generations.
What conditions must be met for this to remain true?
- no mutations
- population is isolated so no flow of alleles in or out of the population
- no selection (all alleles equally likely to be passed on)
- population is large
- mating within population is random
Bb
bb
Explain allele frequencies using this
Frequency of b = 3/4 = 0.75 = 75%
Frequency of B = 1/4 = 0.25 = 25%
75% of genes have the b allele
25% of the genes have the B allele
p + q = 100%
p + q = 1.0
Frequency of dominant + frequency of recessive = 1
p
Frequency of dominant allele
q
Frequency of recessive allele
How many arrangements can you get with two alleles?
Eg. Alleles A and a
Eg. AA, Aa, aA, aa
FOUR
Where:
A = dominant (frequency of A denoted as p)
a = recessive (frequency of a denoted as q)
If there are only 4 possible arrangements of the A and a alleles, what can we state?
AA
Aa
aA
aa
All the frequencies of those combinations add to 1.0
AA + Aa + aA + aa = 1.0
pp + pq + qp + qq
p^2 + 2pq + q^2 = 1.0
A particular characteristic is as a result of a recessive allele a.
We know 1 person in 25 000 display the characteristic.
How do you determine the probability of the a allele in the population? And then the probability of heterozygous individuals and ty throw the probability of genotypes and phenotypes.
Character observed if individual has aa
Probability of aa is 1/25 000 = 0.00004
Probability of aa is q^2
q^2 = 0.00004 then square root of q = 0.0063
If p + q = 1, p + 0.0063 = 1 so rearrange to find p = 0.9937
So probability of allele A is 0.9937
——
Then can find probability of heterozygous individuals: 2pq so we do (2 x 0.9937 x 0.0063) = 0.0125.
Three things genetic variations arise as a result of
1) Mutations
2) meiosis - produces new combinations of alleles before passed into gametes
3) random fertilisation of gametes - produces new combinations of alleles. Which gamete fuses with which is a random process at fertilisation.
Intraspecific variation
Variation within a species
Interspecific variation
Variation across different species
How are genetics and environment intertwined?
Genes set the limits
Environment determines where along the limits an organisms lies
Buttercups have a gene for them to grow very tall but they are actually short, why is this?
The seeds germinated in an environment of poor light or low nitrates in the soil so the plant could not grow properly
Examples of environmental influences on genetics
Temperature
Rainfall
Sunlight
Soil conditions
PH
Food availability
Polygenic
Characteristics determined by multiple genes
Example of characteristics of organisms that great into each other forming a continuum
Height and mass
Human heights as a graph is usually
Normal distribution curve
Why are the causes of variation difficult to measure?
Genetics and environment intertwined
Define sexual reproduction
Fusion of female and male gametes during fertilisation to produce a combination of alleles from both parents
Define asexual reproduction
Does not involve fusion of gametes. A clone of the parent is produced.
Advantages of sexual reproduction (3)
Produces genetic variation in the offspring
A disease is less likely to affect all the individuals in a population because they are more varied
Because there’s more variation The species can adapt to new environments which gives them a survival advantage
Disadvantages of sexual reproduction (2)
Time and energy needed to find a mate
Not possible for an isolated individual to reproduce
Advantages of asexual reproduction (4)
Population can increase
Rapidly in favourable conditions
Only one parents needed
More time and energy efficient as you don’t need to mate
Faster than sexual reproduction
Disadvantages of asexual reproduction
Disease may affect all the individuals in a population because there is less variation
It does not lead to genetic variation in a population (unless by mutation)
Define selection pressures
External factors which affect an organism is ability to survive in a given environment
Examples of selection pressures
PFPCMDC
Predators - advantageous allés to escape
Food - advantageous alleles allow access to certain food
Prey - advantages alleles to catch prey better etc
Climate - advantages alleles to survive in climate better
Mates - some might have advantage alleles that attract makes more
Disease - advantageous alleles allow survival
Competition - advantageous alleles outcompete
Positive selection pressures
Increases the frequency of a trait
Negative selection pressures
Decreases the occurrence of a trait
What do selection pressures determine?
The allele frequency within a gene pool
Name three factors that the process of evolution by natural selection depends upon?
1) organisms producing more offspring than can be supported by the environment (why is this important)?
2) there is genetic variety within POPULATIONS of all species
3) a variety of phenotypes that selection can operate against
Explain the role of over production of offspring in natural selection.
Too many offspring results in intraspecific competition over limited resources available
Larger population more competition and more individuals die trying to survive
Death NOT random. Those less well adapted to prevailing conditions die and those who ARE adapted survive.
Those with advantageous alleles survive and reproduce and pass the alleles to the next generation so this new generation has a different allele frequency compared to the one before.
Population has evolved a combination of alleles better adapted to the prevailing conditions.
What does natural selection as a result of overproduction of offspring rely on?
Genetically different individuals in the population.
(Then some will be better at feeding for example and some will die out etc otherwise nobody clearly wins during competition for resources etc)
How does allele frequency change across generations of populations?
Generation 1 has range of alleles and some are selected for so these in individuals survive and reproduce to have offspring with more combinations of these advantageous alleles
Generation 2 now has more combinations of these advantageous alleles
If an organism can survive In the conditions which it’s lives in, why does it not produce identical offspring, as they will be equally likely to be able to survive in these conditions as well.
Conditions change over time and having a wide range of genetically different individuals in a population means that some will have a combination of alleles needed to survive in almost any new circumstance
Why is little genetic diversity within a population BAD?
Populations with little individual genetic variation a more vulnerable to disease and climate change.
Why is variation among individuals important in a population?
Provides the potential for a population to evolve and adapt to new circumstances
How does a larger population lead to more genetic variation?
XXX
The larger population the more genetically varied it is.
So the larger combination of alleles produced when individuals in the population mate.
So larger chance of at least some of these combinations being advantageous
So selected for and individuals can survive and reproduce
(Ig population hasn’t been that large then perhaps correct combination of alleles to produce advantageous allele would have been made so none can survive and reproduce so those individuals die out ??)
What actually is genetic variation within a species?
All of the individuals in the population have the same genes her that they have different alleles
How does meiosis cause genetic variation?
Crossing over of chromatids
Independent segregation of chromosomes
Random fertilisation of gametes during sexual reproduction
What causes evolution
Genetic variation
What is evolution in terms of genetics
Changes in allele frequencies overtime
Describe stabilising selection
Individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.
When does stabilising selection occur?
When the environment isn’t changing
What does stabilising selection do?
Reduces the range of possible phenotypes
Give an example of stabilising selection?
In a mammal population there is a range of fur lengths
In a stable climate having for at extremes of the range reduces the chances of surviving because it is hard to maintain body temperature.
Animals with alleles for average for length are more likely to survive and reproduce, passing on these advantageous alleles to the next generation.
So these alleles for medium length for increase in frequency.
Proportion of the population with average for lunch increases and the range of fur lengths decrease.
Describe directional selection.
Where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce
When might directional selection occur?
In response to an environmental change
Give an example of directional selection?
The speed of cheaters developing through directional selection.
Individuals with alleles for increase speed are more likely to catch prey and slow individuals.
These individuals were more likely to survive reproduce and pass on the advantageous alleles.
Over time frequency of alleles for high speed increases and the population becomes faster
What is disruptive selection?
Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce
What is disruptive selection the opposite of and how?
Stabilising selection.
In disruptive selection characteristics towards the middle of the range are lost where is in stabilising selection characteristics towards the middle of the range are selected for
When does disruptive selection occur?
When the environment favours more than one phenotype
Give an example of disruptive selection.
Bird populations have a range of beak sizes.
But with large beaks can eat large seeds, but with small beaks can eat small beads.
In an environment where majority of seats are large or small but none are medium-sized, birds with medium sized beaks have a reduced chance of survival because they can’t eat the big or small seeds.
Birds with large or small beaks are more likely to survive reproduce and pass on advantageous alleles of having small or large beaks.
Alleles for large beaks and small beaks increase in frequency whilst alleles for medium-sized beaks decrease in frequency.
Overtime proportion of the population that have either small or large beaks increases
What is another example of stabilising selection
Body mass of human children at birth
Baby is born with a body mass greater or less than the 3.2 kg average have a higher risk of dying in the first few months after birth
Directional selection is a
Selection for one extreme phenotype
Stabilising selection is a
Selection against the extreme phenotypes
Disruptive selection is a
Selection against phenotypes around the mean of the population
Give another example of directional selection
Antibiotic resistance in bacteria.
Selecting for an extreme phenotype - selects for bacteria with mutation to survive antibiotic so these survive and reproduce and pass on advantageous alleles
Give another example of disruptive selection
Co-ho salmon.
Large Mills and small nails have been selected for loss intermediate the size males are selected against.
Small males able to sneak up on females in spawning grounds was large males are fierce competitors and so both can survive and reproduce
Disruptional selection is when the environment favours more than one phenotype.
When might this occur ?
When an environmental factor for example temperature takes two or more distinct forms.
For example if the temperature alternates between very low in winter (favour long fur length) and very high in summer (favouring short fur length)
Define allele frequency
The number of times and allele occurs within the gene pool
What do you environmental factors not do
They do not affect the probability of a particular mutant allele arising, they affect the frequency of the mutant allele which is already in the gene pool
What is speciation?
Evolution of new species from existing ones
Define species
A group of individuals that Have the same jeans but different alleles and are capable of breathing to produce fertile offspring
Members of species are what from other species?
Reproductively separated
When does speciation occur?
When populations of the same species become reproductively isolated
What does it mean to become reproductively isolated
Changes and are you frequency cause a change in phenotype which means individuals of the same species can no longer interbreed to produce fertile offspring
Define allopatric speciation
Mode of speciation that occurs when biological populations become geographically isolated from each other to an extent that prevents or interferes with gene flow
When might allopatric speciation occur?
When there is a physical barrier for example flood or earthquake that divides a population of species causing some of the individuals to become separated from the main population
Sympatric speciation
XXXXX
Evolution of a new species from a surviving ancestral species Well both continue to inhabit the same geographical region
Is it basically like normal speciation we learnt in GCSE so like MVARPS but carried on until a whole new species appears????
Two ways that individuals within a population can become reproductively isolated?
Sympatric speciation - become reproductively isolated when there is no physical separation.
Allopatric speciation - Become reproductively isolated because physically separated
Explain how a new species is formed when a population becomes geographically isolated?
1) Populations of the same species are geographically separated
2) They will experience slightly different conditions for example there might be a different climate on the other side of the physical barrier
3) Populations will experience different selection pressures (due to different conditions) and so different changes in allele frequencies will occur.
4) Changes in allele frequencies lead to differences in the gene pools of the separated populations which lead to a change in phenotype frequencies.
5) Eventually individuals from the populations have changed so much that they can no longer bridge to produce fertile offspring – they are reproductively isolated.
6) The two groups have become separate species
Explain how a new species is formed when a population becomes geographically isolated?
SUMMARY (6)
1) isolation
2) different conditions
3) change in alleles frequency
4) leads to change in gene pool and phenotype frequency
5) can’t interbreed to produce fertile offspring
6) separate species
If a population has become geographically isolated, name three ways the allele frequency may change
1) Different alleles will be more advantage in the different populations because they are in different environments
2) Are you frequencies will change as mutations occur independently in each population
3) Genetic drift may also affect our little frequencies in each of the separate populations
(Different alleles arise because of different environments, random mutations in separate populations, random genetic drift in separate populations)
What does sympatric speciation not require?
Geographical isolation
When does sympatric speciation occur?
When populations of a species that share the same habitat become reproductively isolated from each other
XXX so if sympatric speciation occurs with polyploidy
Allopatric speciation occurs when geographically isolated
Can speciation occur when u just have MVARPS occurring in a population that are all in one place or does this just cause diversity amongst the species but no actual new species because no reproductive isolation ??
ASK O’DONNELL
Give an example of sympatric speciation
X
Diploid
Two sets of chromosomes (one from each parent)
Chromosome 12, two chromosome 12s because one from each parent
Haploid
Cell has a single set of chromosomes
Polyploidy
Condition where cells have more than two homologous paired sets of chromosomes
(So chromosomes come in pairs but polyploidy would be if you had like a set coming in threes but you had like 6 sets which are meant to be pairs but are instead threes)
Eg. Three chromosome 12s
Aneuploidy
XXX
???
Give an example of sympatric speciation
Polyploidy organism emerging in a diploid population.
Polyploid organism is reproductively isolated from diploid organisms.
If polyploid organisms reproduces asexually, a new species may develop
BUT Polyploidy can only lead to speciation if it doesn’t prove fatal to the organism and more polyploid organisms can be produced.
In which species is it more common for polyploidy to lead to speciation and why?
Plants than animals
Plants can self fertilise and reproduce asexually
Whereas
Animals that are polyploidy would need to find another animal of the same species but opposite sex who is also polyploidy
Reproduction isolation occurs because…
Changes in alleles and phenotypes and some individuals prevent them from breeding successfully with individuals that haven’t had these changes
What types of changes might occur that could lead to reproductive isolation (individuals of same species being unable to breed)? (3)
1) Seasonal changes – individuals from same population develop different flowering on meeting seasons or become sexually active at different times of the years
2) Mechanical changes – changes in genitalia prevent successful meeting
3) Behavioural changes – a group of individuals develop courtship rituals that are NOT attractive to the main bulk of individuals in the population so they no longer mate
Explain how genetic drift leads to speciation
XX does genetic drift only occur in isolated populations or can it occur in normal population and then lead to speciation ???
Individuals within a population show variation in the genotypes
An allele is passed on by chance to offspring more often to usual
By chance the number of individuals with this allele increases.
XXX Changes an allele frequency into isolated populations could eventually lead to reproductive isolation and speciation XXXX
When does evolution by genetic drift have a greater effect?
In smaller populations where chance has a greater influence.
When does evolution by genetic drift have a smaller effect?
In larger populations, any chanced variations in allele frequency tends to even out across the whole population
What drives evolution?
Natural selection and genetic drift
What 5 things can cause speciation?
Genetic drift Geographical isolation Polyploidy Natural selection Anything that causes reproductive isolation
What is the diversity of life on Earth today as a result of?
Speciation and evolutionary change over millions of years.
To start with there was one population of organisms which then evolved into more species which evolved into more species and this process was repeated again and again
