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Pack 21 - Population and Evolution
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Population and Evolution
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biology
population
evolution
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Jacob Shepherd
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Jacob Shepherd
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Resource summary
Pack 21 - Population and Evolution
Population
A population is is a group of organisms of the same species occupying a particular space at a particular time that can interbreed
A gene pool is all the individuals in a population at a given time
Allele frequency = number of times an allele occurs within the gene pool
The Hardy-Weinberg Principle
The proportion of dominant and recessive alleles of a gene will remain constant from one generation to the next
5 assumptions:
No mutations occur
Isolated population
No selection
Large population
Random mating
p + q = 1
p = probability of dominant allele
q = probability of recessive allele
p^2 + 2pq + q^2 = 1
p^2 = frequency of AA
2pq = frequency of Aa
q^2 = frequency of aa
Evolution
Variation
Individuals within a population show a wide range of variation in phenotype
Genetic and environmental factors cause variation
Primary source is mutations
Also caused by random fertilisation
Types of variation:
Discontinuous:
Organisms fit into few distinct forms with no intermediate types
Continuous:
Organisms with many intermediate types
Natural Selection
Caused by competition, predation and disease
A biotic factor is any living component that affects the population of another organism or the environment
Competition:
Intraspecific: Competition between organisms of the same species
Interspecific: Competition between organisms of different species
Types of Selection:
Stabilising Selection:
This occurs when the environment remains constant for a long period of time
It selects against the extremes of the phenotypes
Directional Selection:
Occurs when the environment changes and so the selection pressure will be to select for individuals either to the right or the left of the mean
Disruptive Selection:
This occurs when the environment has two or more distinct forms and so the selection pressure will be to select for individuals away from the mean
It selects for both extremes
A change in allele frequency of a population
Main stages:
1) Mutation
2) Variation
3) Struggle for existence (selection pressure)
4) Survival of the fittest (advantageous allele survives)
5) Reproduction of advantageous allele
6) Natural selection (frequency of allele increases)
Isolation and Speciation
If a population becomes isolated, there will be no gene flow
So there is an accumulation of genetic differences
These genetic differences may eventually lead to different species
The isolated populations cannot interbreed with each other to produce fertile offspring
Speciation is the evolution of a new species from existing ones
Types of Speciation
Allopatric
Speciation that occurs when biological populations of the same species become isolated from each other preventing interbreeding
e.g. a river
Sympatric
Speciation that occurs without any geographical seperation
Genetic Drift
Genetic drift is a change in the allele frequency in a small population due to chance
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