7B Populations and Evolution Flashcards
What do members of a population share?
A gene pool
What is a species defined as?
A group of similar organisms that can reproduce to give fertile offspring
What is a population?
A group of organisms of the same species living in a particular area at a particular time
So they have the potential to interbreed
Species can exist as one or more populations - give an example of this
E.g. there are populations of the American black bear (Ursus americanus) in parts of America and in parts of Canada
What is a gene pool?
The complete range of alleles present in a population
What is the allele frequency?
How often an allele occurs in a population
It’s usually given as a percentage of the total population e.g. 35% or 0.35
What does the Hardy-Weinberg principle predict?
That allele frequencies won’t change
What is the Hardy-Weinberg principle?
It is a mathematical model
It predicts that the frequencies of alleles in a population won’t change from one generation to the next
What are the conditions required for the Hardy-Weinberg principle?
- Large population
- No immigration
- No emigration
- No mutations
- No natural selection
- Random mating
What can the Hardy-Weinberg equations be used for?
To calculate the frequency of particular alleles, genotypes and phenotypes within populations
Also used to test whether or not the Hardy-Weinberg applies to particular alleles in particular populations
What can the Hardy-Weinberg equations be used to predict?
- Allele frequency
- Genotype and phenotype frequency
- The % of a population that has a certain genotype
- Show if external factors are affecting allele frequency
What is the Hardy-Weinberg equation that is used to predict allele frequency?
p + q = 1
p = the frequency of one allele, usually the dominant one q = the frequency of the other allele, usually the recessive one
When can the p + q = 1 Hardy-Weinberg principle be used?
To predict allele frequency
When a gene has two alleles, need to know the frequency of one allele
A species of plant has either red or white flowers, allele R (red) is dominant and allele r (white) is recessive.
If the frequency of R is 0.4 what is the frequency of r?
0.6
p + q = 1
1 - 0.4 = 0.6
Why is the equation to calculate the allele frequency p + q = 1?
The total frequency of all possible alleles for a characteristic in a certain pop. is 1.0
The frequencies of the individual alleles must add up to 1.0
What is the Hardy-Weinberg equation that is used to predict genotype and phenotype frequency?
p^2 + 2pq + q^2 = 1
p^2 = the frequency of the homozygous dominant gene 2pq = the frequency of the heterozygous genotype q^2 = the frequency of the homozygous recessive genotype
Why is the equation to calculate the genotype and phenotype frequency p^2 + 2pq + q^2 =1?
The total frequency of all possible genotypes for one characteristic in a certain population is 1.0
SO the frequencies of the individual genotypes must add up to 1.0
If there are two alleles for flower colour (R and r), there are 3 possible genotypes - RR, Rr and rr.
If the frequency of genotype RR (p^2) is 0.34 and the frequency of Rr (2pq) is 0.27 what is the frequency of genotype rr (q^2)?
1 - 0.34 - 0.27 = 0.39
What are the Hardy-Weinberg equations that are used to predict the % of a pop. that has a certain genotype?
p + q = 1 and p^2 + 2pq + q^2 = 1
The freq. of cystic fibrosis (genotype ff) in the UK is 1 in 2500. From this info estimate the % of people in the UK that are cystic fibrosis carriers (Ff)
p + q = 1
Find q: q^2 = 1/2500 = 0.0004
square root 0.0004 = 0.02
Calculate p: p+q=1
1-0.02=0.98
The calculate 2pq: 2pq = 2 x 0.02 x 0.98
2pq = 0.039
What can variation be caused by?
Genes, the environment or both
What is variation?
The difference that exists between individuals
What does variation within a species mean?
It means that individuals in a population can show a wide range of different phenotypes
What causes genetic variation in a species?
Individual of the same species have the same genes but different alleles
Can also be caused by the environment
What is the main sources of genetic variation?
- Mutations
- Meiosis
- Random fertilisation
How can mutations cause genetic variation?
e.g. When changes in the DNA base sequence lead to the production of new alleles
How can meiosis cause genetic variation?
- Crossing over of chromatids
- Independent segregation of chromosomes
Give an example of how genetic variation within a species can be caused by the environment?
e.g. Climate, food and lifestyle
How does evolution occur?
Only through genetic variation
What is evolution?
A change in allele frequencies over time
What are two ways in which evolution can occur?
Through natural selection and genetic drift
What is the process of natural selection?
1 - individuals of same species vary due to different alleles
2 - Predation, disease & competition create struggle for survial
3 - Individuals may vary, some are better adapted to selection pressures than others
4 - This means there are diff. levels of survival & reproductive sucess in a pop. (ones with better alleles survive)
5 - This means a greater proportion of the next gen. inherit the beneficial alleles
6 - They more likely to survive, reproduce & pass on their genes
7 - So frequency of beneficial alleles in the gene pool increases from generation to generation
What are selection pressures?
Things that create struggle for survival
e.g. Predation, disease & competition
What are the 3 different types of selection?
- Stabilising selection
- Directional selection
- Distruptive selection
What happens in stabilising selection?
Where individuals with alleles for characteristics towards the middle of the range are more likely to survive & reproduce
Occurs when the environment isn’t changing & reduces the range of possible phenotypes
How does the graph change in stabilising selection?
The graph narrows and moves towards the centre
Give an example of how stabilising selection occurs
- Range of fur lengths in a population
- In a stable climate, having fur at the extremes reduced chances of survival & harder to maintain body temp
- Animals w avg. fur length are most likely to survive, reproduce & pass on alleles
- These alleles increase in freq.
- Proportion of pop. with avg. fur length increases & range of fur lengths decreases
What is directional selection?
Where individuals with alleles for a single extreme phenotype are likely to survive & reproduce
The could be in response to an environmental change
How does the graph change in directional selection?
The mean will shift to one extreme on the graph (not both extremes)
Give an example of directional selection occurring
- Cheetahs are the fastest animals on land
- Likely this characteristic was developed through directional selection as individuals with more alleles for increased speed are more likely to catch prey
- So they’re more likely to survive, reproduce & pass on alleles
- Over time freq. of alleles for high speed increases & pop. become faster
What is disruptive selection?
Individuals with alleles for extreme phenotypes at either end of range are more likely to survive & reproduce
Opposite of stabilising selection as characteristics towards the middle of the range are lost
Occurs when environment favours more than one phenotype
How does the graph change in disruptive selection?
The means will shift to either extreme in the graph (opposite to stabilising)
Give an example of disruptive selection occurring
- Bird pop. has range of beak sizes
- Birds with large beaks specialised to eat large seeds & small beaks specialised for small seeds
- In an environment where majority of seeds are large/small - very few medium, medium beak birds
- Medium beak birds unable to eat large or small beaks seeds effectively - small & large beak birds survive & reproduce more easily
- Alleles for medium beaks decrease - proportion of small/large beaks increases
What is speciation?
Development of a new species from an existing species
How does speciation occur?
When populations of the same species become reproductively isolated (changes in phenotype means the can no longer interbreed)
What are the 2 types of speciation?
- Allopatric speciation
- Sympatric speciation
What is allopatric speciation?
When a physical barrier causes individuals to become separated (geographical isolation)
What is sympatric speciation?
When a population becomes reproductively isolated without a physical barrier
How does allopatric speciation take place?
1) Physical barrier means each population experiences different conditions (selection pressures)
2) The alleles more advantageous will survive
3) Lead to differences in gene pools and changes in phenotype frequency
4) Eventually they will have changed so much they can’t breed (become separate species)
How can sympatric speciation take place?
- Eukaryotic organisms are diploid so have 2 sets of homologous chromosomes
- Mutations can occur that increase amount of chromosomes
- Individuals with different number of chromosomes can’t produce fertile offspring
What are the 3 ways reproductive isolation occur?
- Seasonal
- Mechanical
- Behavioural
How do seasons affect reproductive isolation?
Different flowering or mating seasons or become sexually active at different points in the year
How can mechanical changes cause reproductive isolation?
Changes in genitalia prevent successful mating
How can behavioural changes cause reproductive isolation?
A group of individuals develop courtship rituals that aren’t attractive
How does genetic drift lead to speciation?
- This is when chance dictates which individuals suri]vive and pass on the alleles (eventually lead to reproductive isolation)
How has evolutionary change resulted in a genetic diversity in organisms?
To start there was one population, which divided and new populations evolved into separate species
