20.5 Evolution Flashcards
Define gene pool
The sum total of all genes in a population at any given time is known as the gene pool
Define allele frequency
The relative frequency of a particular allele in a population
What does the p + q = 1 formula mean
In a diploid breeding population with two potential alleles, the frequency of the dominant allele plus the frequency of the recessive allele will always equal 1
What does the Hardy-Weinberg principle state?
In a stable population with no disturbing factors (no mutations, no selection pressures) , the allele frequencies will remain constant (random mating) from one generation to the next and there will be no evolution
- a stable population is not common the real world, but this principles is useful as it allows us to measure and study evolutionary changes when they occur
What is the Hardy Weinberg principle formula
p^2 + 2pq + q^2 =1
Where p^2 = frequency of homozygous dominant genotype in the population
2pq = frequency of heterozygous genotype in the population
And q^2 = frequency of homozygous recessive in the population
Remember p + q = 1
Factors affecting evolution
Factors that lead to changes in the frequency of alleles within a population and so they affect the rate of evolution:
- mutation: necessary for the existence of different alleles in the first place, results in genetic variation
- sexual selection: leads to an increase in frequency of alleles which code for characteristics that improve mating success
- gene flow: the movement of alleles between population (e.g. immigration and emigration - results in changes of allele frequency within a population)
- genetic drift: occurs in small populations, change in allele frequency due to random nature of mutation. Appearance of a new allele will have a greater impact on smaller population than a larger one where there is a greater number of alleles present in the gene pool
- natural selection: leads to an increase in the number of individuals that have characteristics that improve their chances of survival
What is a limiting factor?
What are the two types?
- factors which limit or decrease the size of a population
2 types:
- density-dependent factors are dependent on population size and include competition, predation, parasitism, and communicable disease
- density-independent factors affect populations of all sizes in the same way including - climate change, natural disasters, seasonal change, and human activities (e.g. deforestation)
What are population bottlenecks?
Large reductions in population size which last for at least one generation. Caused by a catastrophic event (e.g. natural disaster)
The gene pool, along with genetic diversity, is greatly reduced and the effects will be seen in future generation.
It takes thousands of years for genetic diversity to develop through the slow accumulation of mutations.
A positive aspect of a genetic bottleneck is that a beneficial mutation will have a greater impact and lead to the quicker development of a new species
Describe and explain the founder effect
Small populations can arise due to the establishment of new colonies by a few isolated individuals leading to the founder effect
These small populations have much smaller gene pools than the original population, and display less genetic variation
E.g. of the founder effect are the Amish people of America
Difference between stabilising and directional selection
stabilising:
E.g. natural selection
The norm or the average are selected for positive selection
Therefore stabilising selection results in reduction in the frequency of alleles at the extremes of the bell curve, and an increase of average alleles
Directional:
Occurs when here is change in the environment and the normal/ most common phenotype is no longer the most advantageous
Organism which are less common and have more extreme phenotypes are positively selected
Allele frequency shifts towards extreme phenotypes and evolution occurs
E.g. peppered moths
Disruptive selection
The extremes are selected for and the norm selected against
Opposite to stabilising selection
