7.3 Evolution & Speciation Flashcards
What are the 3 reasons why genetic variation arise
- Mutations
- Meiosis
- Random fertilisation of gametes
What is the main source of variation
Mutations
What are the 2 types of mutations that result in genetic variation
- Gene point mutations
- Chromosome mutations
What is necessary for natural selection
Variation
Why is variation important
When the environment changes, having a wide range of genetically different individuals means that there is more likely to be an individual that will be able to survive in the new environment
What are the 2 types of variation
- Discontinuous
- Continuous
What are the phenotypes like in discontinuous variation
Qualilative phenotypes
What are the phenotypes like for continuous variation
Quantitative phenotypes
How many genes tend to control the discontinuous variation phenotypes
1 or 2 with multiple alleles
How many genes tend to control the continuous variation phenotypes
Many genes - polygenic
How much does the environment effect discontinuous variation
Very little effect on gene expression
How much does the environment effect continuous variation
Significant effect on gene expression
Name an example of discontinuous variation
Human ABO blood group / this is a distinct group so you cannot be a bit of both you’re one or the other
Name an example of continuous variation
Height, skin colour / graduation of characterisitics
Is genetic variation or environmental factors more likely to be discontinuous variation
Genetic variation
Are environmental variation subject to natural selection
NO
Define genetic diversity
The total number of different alleles in a population
Define population
A group of individuals of the same species that live in the same place and can interbreed
What 3 things increase genetic diversity between populations
- Natural selection
- Genetic drift
- Mutations
What 1 thing decrease the genetic diversity between populations
Gene flow
What 2 things increase the genetic diversity within populations
- Gene flow
- Mutations
What 3 things decrease the genetic diversity within populations
- Natural selection
- Genetic drift
- Inbreeding
Why does natural selection increase genetic diversity between populations
Each population will experience different environment for example sunlight, so there’s a wide variety of alleles in the gene pool
Why do mutations increase genetic diversity both between populations and within populations
Because they introduce new alleles so more alleles in gene pool therefore increases genetic diversity
Why does natural selection decrease genetic diversity within populations
Since the individuals with the favourable characteristics reproduce more therefore there is less variation
Why does inbreeding decrease the genetic diversity
No new alleles just recombining alleles so no new alleles enter the gene pool
If there’s greater genetic diversity, does this increase or decrease the likelihood that an individual will survive an environmental change, and why
Increases because there’s a wider range of alleles = wider range of characteristics = greater probability that an individual will possess a characteristic that is suited to the conditions
What is the process of natural selection
- New alleles are formed by random mutations in genetic material
- Many mutations are harmful, which may mean that the individual with the mutated allele dies out, or the mutation is corrected by the ‘proof-reading’ mechanisms of the cell. Sometimes however, an allele (or combination of alleles) gives an individual an advantage, leading to increased chance of survival and reproductive success.
- These individuals then pass on their advantageous alleles to the next generation
- Over many generations, the new alleles increases in frequency in the population
Where must the mutation occur in order for it to be passed on
In gametes
When measuring a given trait in individuals of a population, what does continuous variation cause
Normal distribution
What are the 3 types of selection
- Directional
- Stabilising
- Disruptive
Define directional selection
Selection that operates towards one extreme in a range of variation / when individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce
What is the specs example of directional selection
Antibiotic resistance in bacteria
In directional selection, what happens to the position of the mean
It shifts in the direction of the more favourable trait
In directional selection, what happens to the frequency of the mean
It’s equally common
In directional selection, what happens to the distribution around the mean
It’s the same - therefore the curve looks the same but just shifted to one side
What are the general mark scheme points for this question, Why does the frequency of this trait increase in the population
- Trait X (state the trait) has a genetic basis
- Random mutations over time = genetic variation
- Explain how the trait is of an advantage ( selection pressure) and state the type of selection
- Individuals with this allele are more likely to survive, reproduce, and pass on this allele to their offspring
-Over many generations, the frequency of this allele increase in the population and so therefore does the trait
Why does the frequency of antibiotic resistance increase in the bacteria population
- Antibiotic resistance has a genetic base
- Random mutation over time = genetic variation in resistance to the antibiotic - some individuals have alleles that give them greater antibiotic resistance
- When the population is exposed to the antibiotic ( selective pressure = surviving exposure to the antibiotic), bacteria with alleles that give them less resistance are killed
- Resistant bacteria survive, reproduce, and pass on the allele for antibiotic resistance to their offspring
- Over many generations, the frequency of this allele increases in the population and therefore so does the trait
Define stabilising selection
Selection that tends to eliminate the extremes of the phenotype range within a population. It arises when environmental conditions are constant
What is the specs example of stabilising selection
Human birth weights
In stabilising selection, what happens to the position of the mean
It remains the same
In stabilising selection, what happens to the frequency of the mean
It becomes more common
In stabilising selection, what happens to the distribution around the mean
It decreases
Explain why stabilising selection occurs for human birth weigh
- Human birth weight has a genetic basis
- Random mutations over time = genetic variation in birth weight
- Selective pressure = surviving to reproductive age. Very small babies are less likely to survive (large SA:Vol & weakened immune system) and very large babies can cause problems in child birth
- Individuals with alleles for moderate birth weights are more likely to survive, reproduce, and pass on their alleles for moderate birth weight to their offspring
- Over many generations, the frequency of this allele increases in the population and so therefore does the trait
What does natural selection result in
Species that are better adapted to their environment
How do behavioural adaptations increase the likelihood an organisms surviving
The way an organism acts that increases its chance of survival
How do physiological adaptations increase the likelihood an organism survives
Processes is inside an organism’s body that increases the chance of survival
How do anatomical adaptations increase the likelihood an organism survives
The features of an organisms body that increases its chance of survival
Define disruptive selection
Where individuals with alleles for characteristics of both extremes at either end of the range are more likely to survive and reproduce
In disruptive selection, what happens to the position of the mean
It stays the same
In disruptive selection, what happens to the frequency of the mean, and what effect does this have on the SD
It decreases, so a greater SD i.e a greater spread of values about the mean
What does stabilising selection preserve
The average phenotype
What does stabilising selection select against
The extremes
When does stabilising selection occur
In stable environments
What does directional selection favour
An extreme phenotype
When does directional selection occur
When the environment changes
What does disruptive selection favour
Either of the extreme phenotypes
What does disruptive selection link to
Speciation
When discussing natural selection, the term ‘increased reproductive success’ is used. Explain what it means
When organisms are able to survive till reproductive age and passes on alleles
What changes in allele frequency would be expected over a long period of time when natural selection is acting on a population
The advantageous allele increases in frequency
What is selection
Process by which an individual is selected because it’s better adapted to it’s environment so more likely to survive and reproduce compared to those who are not adapted
Distinguish between directional and stabilising selection
Directional selection favours one of the extremes so the characteristics of the population change whereas stabilising selection favours the average phenotype so the characteristics of the population remain unchanged
What is speciation
Evolution of new species from species that already exist
What does reproductive isolation prevent
Gene flow
What is allopatric isolation
When some kind of physical divide that causes 2 populations to separate
Name an example of allopatric isolation
When sea levels rise
Why after allopatric isolation lead to the 2 population not being able to interbreed to produce fertile offspring
SInce the populations face different selection pressures, and random mutations occur and an allele will be favoured so the 2 populations become genetically different over time
What are the 4 types of sympatric isolation
- Behaviour
- Temporal
- Ecological
- Mechanical
What is mechanical isolation (type of sympatric isolation)
Mutations occurs which create anatomical differences (e.g. size) so reproduction is not possible
What is ecological isolation (type of sympatric isolation)
When the individuals use the environment differently, so then different alleles are favoured so the mutations then change how the individuals use their habitiat
What is behavioural isolation (type of sympatric isolation)
When mutations in the DNA result in individuals not being able to do the correct mating ritual - leading to reproductive isolation - no gene flow
What are the 8 main steps in the process of speciation
- Reproductively isolated
- No gene flow between isolated groups
- Different mutations in DNA
- Different groups exposed to different environments
- Subject to different selection pressures
- Natural selection so difference in gene pools
- Differences in phenotype
- No longer interbreed between the 2 groups to produce fertile offspring
- Can be considered different species
What is genetic drift
A chance, non-selective event significantly alters the alleles in the gene pool
When there’s genetic drift, what is survival down too
Chance, since there isn’t a favoured phenotype
Does genetic drift have a larger impact on small, isolated populations or on larger population
Small isolated populations
In small, isolated populations, is the change to the allele frequencies altered more by genetic drift or by natural selection
Genetic drift
How does genetic drift alter the alleles in the gene pool
- Only a few individuals survive
- Unlikely that the alleles in the survivors are exactly the same as the alleles in the original population
- So the alleles aren’t in the same proportion
What are the 2 types of genetic drift
- Bottle neck populations
- Founder effect populations
Are bottleneck populations genetically diverse or not
Genetically diverse
What happens to the genetic diverse of a bottleneck population after genetic drift
Genetic diversity is significantly reduced
Why does decreasing the genetic diversity of a population, decrease their chance of survival to environmental changes
Lower genetic diversity decreases the likelihood that any individuals has alleles that are favourabled/ advantageous to survival
What happens in the founder effect populations of genetic drift
A few individuals break away from populations
Why does the founder effect decrease genetic diversity of a population
A few individuals break away from population and it’s unlikely that the allele frequency is in the same proportion to the original population
Suggest how 2 species of palm tree arose by sympatric speciation ( 5 marks) : the flowering times of the 2 species is different
- Occurs inn the same environment
- Mutations cause different flowering times ( in the question)
- Reproductive isolation / no gene flow
- Different alleles passed on
- Disruptive selection
- Eventually the different species cannot interbreed to produce fertile offspring
What is a gene pool (1 mark)
The total number of alleles within a population
Suggest and explain how allopatric speciation may have occured (4 marks)
- The environments have different selection pressures
- Difference due to mutations
- Reproductive isolated
- No gene flow
- Different alleles are passed on
- Eventually the different species cannot interbreed to produce fertile offspring
List all the marking points that need to be said when explaining evolution of 2 geographically isolated species
- Geographical isolation/separation due to
elevation/altitude ( or whatever it’s due to) - Allopatric speciation due to isolation/separation;
- Different selection pressures
OR
Different environment(s); - (However) some overlap in distribution
OR
(Both) plants found in same area/habitat/altitude; - So (possibly) sympatric speciation;
- (Variation due to) mutation(s);
- Reproductive isolation/separation due to different
pollinators/distributions/altitudes
OR
Separate gene pools due to different
pollinators/distributions/altitudes; - Change in allele frequency (in each population)
OR
Different allele frequency (in each population); - Different species) can no longer (interbreed to)
produce fertile offspring;
