7B: Populations & evolution Flashcards
What is the definition of a species?
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
Define evolution
Change in allele frequency over time
Define gene pool
The complete range of alleles for all genes in all the organisms in a population
Define allele frequency
The proportion of a particular type of allele in a population
What are the 5 conditions that must be met for the Hardy-Weinberg principle?
- No mutations
- No movement in/out of the population
- No selection
- Random mating
- Large population
What does the Hardy-Weinberg principle predict?
It predicts that the frequencies of alleles in a population won’t change from one generation to the next (this is only true under certain conditions)
What is the Hardy-Weinberg equation?
p + q = 1
Where:
p = frequency of dominant allele
q = frequency of recessive allele
In Hardy-Weinberg what is 2pq?
2pq = frequency of heterozygous genotype
In Hardy-Weinberg what is p²?
p² = frequency of homozygous dominant genotype
In Hardy-Weinberg what is q²?
q² = frequency of homozygous recessive genotype
Brown fur coloring is dominant to skewbald fur coloring in horses. If you have 168 brown horses in a population of 200 horses. What is the predicted frequency of homozygous dominants?
36%
Brown fur coloring is dominant to skewbald fur coloring in horses. If you have 168 brown horses in a population of 200 horses. What is the predicted frequency of heterozygotes?
48%
Brown fur coloring is dominant to skewbald fur coloring in horses. If you have 168 brown horses in a population of 200 horses. What is the predicted frequency of homozygous recessive?
16%
The frequency of cystic fibrosis (ff) in the UK is currently around 1 birth in every 2500. From this information estimate the percentage of people in the UK that are cystic fibrosis carriers.
To do this you need to find 2pq.
- ff = q² = 1 / 2500 = 0.0004
so, q = 0.02 - p = 1 - q
p = 1 - 0.02 = 0.98 - 2pq = 2 x 0.02 x 0.98 = 0.039
The frequency of Ff is 0.039, so the % in the UK population is 3.9%
What is variation?
The differences that exist between individuals
What does variation within a species result in?
A wide range of different phenotypes
What is the main source of genetic variation?
Mutations (when changes in the DNA base sequence lead to the production of new alleles)
What are 2 other ways that genetic variation can happen?
- During meiosis (through crossing over and independent segregation)
- Random fertilisation
What is the definition of evolution?
Changes in the allele frequency of a population over time
Name 3 selection pressures that create a struggle for survival
Predation, disease and competition
Why does a population show a wide range of phenotypes?
Due to genetic and environmental variation
Describe natural selection
- Selection pressures create a struggle for survival.
- Organisms with phenotypes providing selective advantages are likely to produce more offspring and pass on their favourable alleles to the next generation.
- The effect of this differential reproductive success on the allele frequencies within a gene pool
What are 3 types of natural selection?
Stabilising, directional and disruptive (they affect allele frequency in different ways)
Describe stabilising selection
Where individuals with alleles for average phenotypes are more likely to survive and reproduce
- Occurs when environment is NOT changing
- Reduces the range of possible characteristics
- Results in a narrow bell curve
Describe directional selection
Where individuals with alleles for one extreme phenotype are more likely to survive and reproduce
- Could be a response to an environmental change
- Results in the bell curve shifting in the direction of selection
Describe disruptive selection
Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce
- Occurs when the environment favours more than ones phenotype
- Is the opposite of stabilising selection because characteristics towards the middle of the range are lost
What is speciation?
The development of a new species from an existing species
When does speciation occur?
It occurs when populations of the same species become reproductively isolated
What is reproductive isolation and how does it occur?
When members of the same species can no longer interbreed to produce fertile offspring. It occurs due to changes in allele frequency causing changes in phenotype.
What is geographical isolation?
When a physical barrier, e.g a flood or an earthquake, divides a population of a species, causing some individuals to become separated from the main population
Describe sympatric speciation
- Does NOT require geographical isolation
- Random mutations can occur in a population causing some to be reproductively isolated
Describe allopatric speciation
- Requires geographical isolation, so populations will experience different selection pressures + conditions
- This leads to different changes in allele frequencies, which will cause differences to accumulate in the gene pools and will eventually cause changes in phenotypic frequencies
= reproductive isolation
Name and describe 3 changes that can cause reproductive isolation
Seasonal - individuals of the same pop. develop different flowering/mating seasons or become sexually active at different times of the year
Mechanical - changes in genitalia prevent successful mating
Behavioural - a group of individuals develop courtship rituals that aren’t attractive to the main population
What is evolution by genetic drift and in what type of population does it normally occur?
When chance, rather than environmental factors, dictates which individuals survive, breed and pass on their alleles. In smaller populations
Describe how evolution by genetic drift occurs
- Individuals within a pop. show variation in their genotypes (e.g. A and B)
- By chance, the allele for one genotype (B) is passed on to the offspring more often than others
- So the number of individuals with the allele increases
- Changes in allele frequency in two isolated populations could eventually lead to reproductive isolation and speciation
Evolution by genetic drift usually has a _____ effect in _____ populations where _____ has a greater influence.
Evolution by genetic drift usually has a GREATER effect in SMALLER populations where CHANCE has a greater influence.
What is the diversity of life on earth a result of?
Speciation and evolutionary change over millions of years
