Section 7 - Populations and evolution Flashcards
What does gene pool mean?
All of the alleles of all of the genes present in a population
What is allele frequency?
The number of times an allele appears within the gene pool
Has a value between 0 and 1
What is the Hardy Weinberg principle used to calculate?
Used to calculate the frequencies of the different alleles of a gene in a population
What assumptions are made in the Hardy Weinberg principle?
No mutations- no new alleles
No migration- no new alleles introduced/ lost
No selection- no alleles are favoured/ eliminated
Mating is random- alleles mixed randomly
Population is large- no genetic bottlenecks
What is the equation for working out alleles?
Only 2 alleles so: P + Q = 1
Probability of dominant allele = P
Probability of recessive allele = Q
What is the equation for working out genotype?
P^2 + 2PQ + Q^2 = 1
Dominant homozygous = P^2
Recessive homozygous = Q^2
Heterozygous = 2PQ
When is P^2 + 2PQ + Q^2 = 1 used?
When given information about phenotypes/ genotypes
When is P + Q = 1 used?
When given information about allele frequency
What does the Hardy Weinberg principle assume about allele frequency?
Assumes that frequencies of the alleles in a population remain constant between generations
What is variation?
The differences that exist between individual organisms
What is interspecific variation?
Variation between species
What is intraspecific variation?
Variation between members of the same species
What is continuous variation and how would it be displayed?
Consists of a range of values between 2 extremes
Most close to mean values
Usually polygenic and affected by the environment
Use tally chart and plot results in a histogram
What is discontinuous variation and how would it be displayed?
2 or more distinct categories with no intermediate values
Monogenic
Environment has little / no effect (blood groups)
*Using bar chart
What is variation through natural selection?
Mutations produce variation
Some individuals are more likely to survive
They reproduce and pass on favourable alleles to offspring = DIFFERENTIAL REPRODUCTIVE SUCCESS
Advantageous alleles become more frequent in population
Population evolves
What is differential reproductive success?
When individuals reproduce, they pass on favourable alleles to offspring
What are selection pressures?
Environmental factors that confer an advantage to some members of a population over others
(Predation, disease, competition)
What are the 3 types of natural selection?
- Directional
- Stabilising
- Disruptive
What is directional selection?
Acts against one extreme in a range of phenotypes
One phenotype becomes rare and alternative phenotype becomes more common
What is stabilising selection?
Acts against both extremes in a range of phenotypes
What is disruptive selection?
Selection that removes individuals from centre of phenotypic disruption
Causes distribution to become bimodal
2 extremes become more common and intermediate states will be lost
Leads to 2 new species
What is speciation?
The development of a new species from an existing one
What is speciation caused by?
By reproductive isolation
Change in allele frequency which means they can no longer breed to produce fertile offspring
Why does speciation occur?
Due to geographical barriers (allopatric speciation) or without a barrier (sympatric speciation)
What is allopatric speciation?
Speciation due to geographical barrier
What is sympatric speciation?
Speciation occurring without a barrier
What 3 reasons does reproductive isolation occur for?
Timing - individuals develop different mating times
Physical changes- change in structure of gametes
Behavioural changes- courtship rituals (bird songs)
What is genetic drift?
Change determines if alleles are passed on
What is evolution by natural selection?
When selection pressures change allele frequencies over time
What does genetic drift do?
One genotype happens to get passed on to more offspring
Evolution occurs as allele becomes more common
Biggest effect in a small population
What is a recessive allele?
Only expressed in phenotype when 2 alleles are present
What is genetic drift?
Process whereby an allele becomes more common, by chance, in a population
What is a population bottleneck?
A sharp reduction in the size of a population due to environmental events
What is the founder effect?
Loss of genetic variation that occurs when a new population is established by a very small number of individuals
What does the Hardy-Weinberg principle predict? [3]
Frequency of alleles
Will stay constant from one generation to the next
Providing no mutation
What are the consequences of widely separated species?
Genetically isolated populations
Allopatric speciation
Genetic drift
Loss of alleles
