7B - Populations and Evolution Flashcards
Define species.
A group of similar organisms that can reproduce to give fertile offspring.
Define population.
A group of organisms of the same species living in a particular area at a particular time.
Can a species exist as more than one population?
Yes
Define gene pool.
The complete range of alleles present in a population.
Define allele frequency.
How often an allele occurs in a population.
How is allele frequency given?
As a percentage or decimal of the total population.
What principle is use to predict allele frequencies throughout generations?
Hardy-Weinberg
What is the Hardy-Weinberg principle?
A mathematical model that predicts that frequencies of alleles in a population won’t change from one generation to the next, assuming certain conditions are met.
What are the assumptions of the Hardy-Weinberg principle?
1) No immigration
2) No emigration
3) No mutations
4) No natural selection
5) Random mating
(6) Large population)
What things can the Hardy-Weinberg equations be used to predict?
- Allele frequencies
- Genotype frequencies
- Phenotype frequencies
- Whether or not the Hardy-Weinberg principles apply in a certain scenario or whether other factors (e.g. natural selection) are affecting the alleles
In the Hardy-Weinberg equations, what are p and q?
p = Dominant allele frequency q = Recessive allele frequency
What are the two Hardy-Weinberg equations?
- p + q = 1
* p² + 2pq + q² = 1
In the Hardy-Weinberg equations, what does p² represent?
The frequency of the homozygous dominant genotype.
In the Hardy-Weinberg equations, what does 2pq represent?
The frequency of the heterozygous genotype.
In the Hardy-Weinberg equations, what does q² represent?
The frequency of the homozygous recessive genotype.
Remember to practice Hardy-Weinberg questions.
Pgs 176 and 177 of revision guide.
The frequency of the RR genotype is 0.34 and the Rr genotype is 0.27. If R is the allele for red flowers and r is the allele for white flowers, what is the frequency of white flowers?
- p² = 0.34
- 2pq = 0.27
- q² = 1 - 0.34 - 0.27 = 0.39
- White flowers = 0.39
The frequency of cystic fibrosis (ff) in the UK is currently approximately 1 in 2500 births. From this, estimate the percentage of the UK population that are carriers (Ff).
- q² = 1 / 2500 = 0.0004
- q = 0.02
- p = 1 - 0.02 = 0.98
- 2pq = 2 x 0.98 x 0.02 = 0.039
- So the percentage of the UK population that are carriers in 3.9%
Currently, cystic fibrosis occurs in 1 in 2500 births, so the frequency of the recessive allele f is 0.02. In 50 years time, the rate changes to 1 in 3500. Find the new frequency of the recessive allele and draw conclusions.
- q² = 1 / 3500 = 0.00029
- q = 0.017
- New frequency of f = 0.017 (compared to 0.02)
- So the frequency of the recessive allele has changed. This indicates that there must have been some change between generations e.g. immigration.
What is variation?
The differences that exist between individuals.
What does variation within a species cause?
A range of different phenotypes.
Individuals of the same species have the same genes, but different…
Alleles
Give the two main sources of variation.
- Genetic
* Environment
What are the sources of genetic variation?
- Mutations
- Meiosis -> Crossing over + Independent segregation
- Random fertilisation of gametes
What are some sources of environmental variation?
- Climate
- Food
- Lifestyle
Which type of variation results in evolution?
Only genetic.
What is evolution?
The change in the frequency of an allele over time.
Describe natural selection.
1) Individuals of the same species vary because they have different alleles.
2) Predation, disease and competition (selection pressures) create a struggle for survival.
3) Variation in phenotype within a species mean that some individuals are better adapted to the selection pressures.
4) This means there are differential levels of survival and reproductive success in a population. Individuals with a phenotype that increases their chance of survival are more likely to survive, reproduce and pass on their alleles that are beneficial for survival.
5) This means a greater proportion of the next generation inherit the beneficial alleles.
6) So over time the frequency of the beneficial alleles increases.
What are the different types of selection?
- Stabilising
- Directional
- Disruptive
What is stabilising selection?
- Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.
- It occurs when the environment isn’t changing and reduces the range of phenotypes.
When does stabilising selection occur?
When the environment is not changing.
How does stabilising selection affect the range of possible phenotypes?
It reduces it.
Remember to practice drawing out the graphs for stabilising selection.
Pg 178 of revision guide
Give an example of stabilising selection.
Fur length in mammals.
What is directional selection?
- Where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce.
- It occurs when the environment changes to favour an extreme phenotype.
When does directional selection occur?
When the environment changes to favour an extreme phenotype.
How does directional selection affect the range of possible phenotypes?
Usually doesn’t change it, just shifts everything.
Remember to practice drawing out the graphs for directional selection.
Pg 179 of revision guide
Give an example of directional selection.
Cheetahs becoming faster over time.
What is disruptive selection?
- Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce. Characteristics towards the middle of the range are lost.
- It occurs when the environment favours more than one phenotype.
When does disruptive selection occur?
When the environment changes to favour BOTH extreme phenotypes.
How does disruptive selection affect the range of possible phenotypes?
It increases it.
Remember to practice drawing out the graphs for disruptive selection.
Pg 179 of revision guide
Give an example of disruptive selection.
Bird populations that favour both very small and very large peaks.
What is the type of mating in disruptive selection?
Assortative (non-random) -> The extreme phenotypes are more likely to breed amongst themselves than between each other.
Remember to revise all the examples of selection.
Pgs 178 and 179 of revision guide
What is speciation?
The development of a new species from an existing species.
What are the two types of speciation?
- Allopatric
* Sympatric
What is allopatric speciation?
Speciation that occurs as a result of geographical isolation.
What is sympatric speciation?
Speciation that doesn’t involve geographical isolation.
Describe allopatric speciation.
1) Populations become geographically separated due to a physical barrier, so interbreeding stops
2) The different populations will experience different selection pressures
3) Different alleles will be more advantageous in each population and so selection will cause allele frequencies to become different in each population
4) Allele frequencies will also change independently due to mutations and possibly genetic drift
5) The changes in allele frequency cause changes in phenotype frequencies
6) Eventually, the two populations become so different that they can no longer interbreed to produce fertile offspring, and are therefore separate species
Give some reasons why sympatric speciation may occur.
- Mutations
- Seasonal flowering or mating patterns
- Mechanical differences
- Behavioural differences
Give an example of mutations causing sympatric speciation.
- Mutation occurs that increases the number of chromosomes (polyploidy)
- This individual cannot reproduce sexually with the other individuals, leading to reproductive isolation
- If the polyploid organism reproduces asexually, this could lead to a new species
What is polyploidy?
Having too many chromosomes.
When does polyploidy mostly occur?
It is most common in plants.
When does polyploidy lead to speciation?
When it isn’t fatal to the organism.
Give an example of behavioural differences causing sympatric speciation.
- A group of individuals develop courtship rituals that aren’t attractive to the main population
- This leads to reproductive isolation
- This leads to speciation
Give an example of seasonal patterns causing sympatric speciation.
- A group of individuals from a population develop different flowering or mating patterns, or become sexually active at different times of the year
- This leads to reproductive isolation
- This leads to speciation
Give an example of mechanical differences causing sympatric speciation.
- A group of individuals have changes in genitalia that prevent successful mating
- This leads to reproductive isolation
- This leads to speciation
What is the term for groups of individuals within a population not being able to interbreed?
Reproductive isolation
What is genetic drift?
- When chance determines which individuals in a population survive and reproduce, passing on their alleles.
- In a small population, this can change allele frequencies and lead to evolution.
Describe genetic drift.
1) Individuals in a population show variation in their genotypes
2) By chance, the allele for one genotype is passed on to the offspring more than others
3) So the allele frequency increases in the next generation
4) Eventually this could lead to reproductive isolation and speciation
What are the two causes of evolution?
- Natural selection
* Genetic drift
Which process drives most evolution in small and large populations?
- Small = Genetic drift
* Large = Natural selection
What has caused there to be such great diversity of organisms?
- At first there was just one population of organisms
- The population was divided and the new populations evolved into separate species
- This process repeated multiple times to give huge diversity in organisms
