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18.1, 18.2, 18.3, 18.4, 18.5 populations and evolution (Isolation and…
18.1, 18.2, 18.3, 18.4, 18.5 populations and evolution
Hardy Weinberg Principle
Assumptions:
- no mutations arise
- the population is isolated (no flow of alleles into or out of the population)
- no selection (all alleles are equally likely to be passed on to the next generation)
- large population
- mating is random
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Variation in Phenotype
Genetic Factors
mutations random changes to genes and chromosomes, may or may not be passed on to next generation. Main source of variation
meiosis nuclear division that produces new combination of alleles before they are passed into the gametes (all different) - including crossing over
random fertilisation of gametes sexual reproduction produces new combinations of alleles - offspring are different from parents. Which gamete fuses to which at fertilisation is a random process - more variety
Environmental factors
eg. climatic conditions (temp, rainfall, sunlight etc), soil conditions, pH, food availability.....
Some characteristics of organisms create a continuum - influence each other. eg. height and mass. These characterises are controlled by many genes (polygenes)
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In most cases, variation is a combination of genetic and environmental factors.
NATURAL SELECTION
Selection pressures = the environmental factors that limit the population of a species
include: predation, disease, and competition
Selection pressures determine the frequency of all alleles within the gene pool.
Gene Pool = the total number of all the alleles of all the genes of all the individuals within a particular population at a given time
Natural selection depends on:
- organisms producing more offspring than can be supported by the environment (eg. supply of food, light, space)
- genetic variety within the populations of all species
- variety of phenotypes
Over production causes natural selection because if there are too many offspring for the available resources, there is more instraspecific competition
Those with advantageous alleles and a better suited phenotype and more likely to survive and breed. So pass on their favourable allele combinations to the next generation. Changes the allele frequency.
TYPES OF SELECTION
Stabilising selection - moves towards the mean phenotype. Favours average individuals. Selection against extreme phenotypes
Occurs when environmental conditions are constant over long periods of time.
Directional selection favours one side of a phenotype. Favours one extreme.
If environmental conditions change, the optimum phenotype shifts to one side. Therefore selection pressures favour one side. One extreme is selected.
Disruptive selection favours extreme phenotypes on either side. Selection against average phenotypes.
Opposite of stabilising selection.
When an environmental factor takes two distinct forms. Eg. very hot in summer and very cold in winter.
Could ultimately lead to two different species being formed.
Another example, salmon and breeding. Large males are fierce competitors. Small males can sneak in and fertilise eggs. Middle sized males are disadvantaged.
PEPPERED MOTHS
- have two distinct forms
- genetically different
but same interbreeding population
- polymorphism
Isolation and speciation
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Species - group of individuals with common ancestry, share same genes but different alleles, capable of breeding to produce fertile offspring. Reproductively separated from other species
most new species are formed through reproductive separation followed by genetic change due to natural selection.
- population becomes separated from other populations
- under goes different mutations
- different selection pressures due to different environment
- natural selection leads to changes in allelic frequencies of each population
- population becomes adapted to its environment - adaptive radiation
- unable to interbreed to produce fertile offspring
Genetic drift - takes place in small populations - smaller variety of alleles, lower genetic diversity. As there is only small variety of alleles, there is not equal chance of alleles being passed on during breeding. Alleles passed on will quickly affect the whole population. Any advantageous allele mutation will be quickly passed on and affect the population. Effect of genetic drift will be high and population will change rapidly making it more likely to develop into a new species. This effect is less in larger populations as the mutation will be diluted because its frequency is less in a larger gene pool.
Allopatric speciation - when two populations are geographically isolated. Physical barrier between populations that prevents them interbreeding. If conditions on either side of the barrier are different, then natural selection will influence the populations differently and each will evolve to adapt and may form different species.
eg. finches on the galapogos islands. Different beak sizes for different sizes of seeds.
Sympatric speciation - form of speciation that results within a population in the same area becoming reproductively isolated eg. they have a different mating call, different courtship behaviours, different breeding season etc
isolating mechanisms
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Ecological - different habitats in the same area, but populations rarely meet.
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Behavioural - different courtship behaviours eg colour marking, mating call.
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Hybrid sterility - hybrids formed by the fusion of gametes from different species are often sterile because they cant produce viable gametes.