Gene Pools Flashcards
What is a population?
A group of individuals of the same species occupying a particular habitat and a particular niche within that habitat
What is the gene pool?
The sum total of all the alleles in a population at a given time
What does evolution involve?
A change in the allele frequencies within a population
When does the frequency at which different alleles occur change?
As the environment changes
What does the allele frequency describe and how is it usually expressed?
It describes what proportion of individuals carry a certain allele and is usually expressed as a decimal fraction of 1
What is the general formula that represents the frequency with which the forms of an allele occur in the gene pool of a population?
p + q = 1
P - dominant allel
q = recessive allele
What indicates whether a population is stable and unchanging or evolving?
The amount of change that takes place in the frequency of alleles in a population
What does the Hardy - Weinberg theory state?
That in a population that is not evolving the allele frequencies in the population will remain stable from one generation to the next in the abscence ot other evolutionary influences. If the population is evolving the allele frequencies will change from generation to generation and so the population is not in equilibrium
What is the algebraic equation developed by Hardy and Weinberg and what does each bit represent?
p^2 + 2pq + q^2 = 1
p^2 = frequency of homozygous dominant genotype in population 2pq = frequency of heterozygous genotype in populatuon q^2 = frequency of homozygous recessive genotype in population
How do you find out p from q^2?
Square root to get p and then substitute into p + q = 1
What are the conditions of the Hardy - Weinberg equilibrium?
- there are no mutations
- there is random mating
- the population is large
- the population is isolated
- there is no selection pressure
What mutations in animals and what mutations in plants can be passed to the offspring?
- In animals only mutatuons in the germ line cells will affect the animals of the next generation
- in plants the germ line cells are not fixed in the embryo. A mutation that tskes place in a single stem as it grows can therefore become part of the gametes of a flower that forms on that stem
What does random mating mean?
That the likelihood of any two individuals in a population will mate is completely independent of their genetic mating
When does non-random mating occur?
when some feature of the phenotype affects the probability of two organisms mating, e.g. peacocks
Why is the Hardy Weinberg equation only valid if it is applied to a large population?
- the maintenence of the genetic equilibrium depends on a random assortment of alleles
- large populations containing many individuals usually have large gene pools - the chance of losing an allele by random events is reduced in a large population
For a population to exist in isolation (as stated in the Hardy Weinberg equilibrium) what cannot happen?
• migration of genetic material
E.g. insects and wind carrying pollen
What does gene flow describe?
The migration of either whole organisms or genetic material into or out of a population and into another population tending to make different populations more alike but changing the allele frequencies within each individuals population all the time
What is a population bottleneck?
Is the effect of an event or series of events that dramatically reduces the size of a population and causes a severe decrease in the gene pool of the population, resulting in large changes in allele frequencies and a reduction in genetic diversity
What are events that could cause a population bottleneck?
An environmental disaster, a new disease, hunting by humand or other efficient predators of habitat destruction
After a catastrophic even why as the population recovers may it become so different from the original population genetically that it becomes a new species?
Because the remaining small population is vulnerable to the complete loss of some alleles and a single mutatuon or new individual can have a bigger effect than usual as a result
What is the founder effect?
The loss of genetic variation that occurs when a small number of individuals become isolated, forming a new population with allele frequencies not representative of the origunal population
Why is the founder effect referred to as the ‘voluntary population bottleneck?’
Because the alleles carried by the individuals who leave the main population are unlikely to include all the alleles or at the same frequencies as the original population. Any usual genes in the founder members of the new population mag become amplified as the population grows
What is stabilising selection?
The natural selection acting to conserve what is already present in a population (often genotypes that are already proving to be succesful), reducing variation in a population so the frequency of some alleles is very high but other alleles is greatly reduced
What is directional selection?
The ‘classic’ natural selection that occurs whenever environmental pressure is applied to a population, showing a change from one phenotypic property to a new one more advantageous in the circumstances
What is disruptive/ diversifying selection?
It gives an increase in the diversity of a population rather than the trend in one particular direction. It is common when conditions are diverse and small subpopulations evolve different phenotypes suited to their surroundings and often results in the evolution of a new species
What is genetic drift?
It describes the random changes in the gene pool of a population that occur by chance (usually due to sexual reproduction) and not because they confer any advantage or disadvantage to the offspring
Why does genetic drift have a major effect in small populations?
Because if an allele only occurs a few times and those individuals are lost that variant is lost from the population foe good
