7B:Populations and Evolution

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  • Created by: DBaruch
  • Created on: 24-01-17 15:58

Gene pools and other terms

  • A species is defined as a group of similar organisms that can reproduce to give fertile offspring.
  • A population is a group of organisms of the same species living in a particular area at a particular time.
  • The gene pool is the complete range of alleles present in a population. How often an allele occurs in a population is called the allele frequency. Its usually given as a percentage of the total population
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Hardy-Weinberg principle

  • The Hardy-Weinberg principle is a mathmatical model that predicts the frequencies of alleles in a population won't change from 1 generation to the next. But this prediction is only true under certain conditions: it has to be a large population where there's no immigration, emigration, mutations or natural selection, there needs to be random mating- all possible genotypes can breed with all others. 
  • It can be used to estimate the frequency of particular alleles, genotypes and phenotypes within populations
  • The equations can also be used to test whether or not the Hardy-Weinberg equations applies to particular alleles in particular populations for example to test whether selection or any other facotrs are influencing allele frequencies
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Hardy-Weinberg equations

  • There are 2 equations, one is used for working out allele frequency and the other is used to workout genotype and phenotype frequencies. Both are to be used in situatations where a gene has 2 alleles.
  • Allele frequency- the total frequency of all possible allles for a characteristic in a certain population is 100%. p+q=1, p= the frequency of 1 allele (dominant), q= the frequency of the other allele (recessive).
  • Genotype frequency- all the genotypes must add up to 1 but there are 3 genotypes
  • Image result for genotype frequency hardy weinberg (http://sites.sinauer.com/ecology3e/webext/WE-06-01-02.jpg)
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Variation

  • Variation is the difference that exist between individuals. Variation within a species means that individuals in a population can show a wide range of different phenotypes. Variation can be caused by genetic and/or environmental factors. Although individuals of the same species have the same genes they have different allele, this causes genetic variation within a species. Genetic variation occurs through mutation and during meiosis (crossing over of chromatids and independent segregation of chromosomes) and because of random fertilisation of gametes,
  • Variation within a species can also be caused by differences in the enviroment, like food, climate or lifestyle. Most variation within a species is caused by a combination of genetic and environmental facotrs, but only genetic variations results in evolution
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Evolution

  • The frequency of an allele in a population changes over time- this is evolution, it can occur by genetic drift or by natural selection
  • Natural selection- organisms face different pressures that affect there chances of survival such as predation, disease and competition. These are called selection pressures and create a struggle for survival, because members of the same species have different alleles, there is variation between individuals meaning some are better adapted to selection pressures than others. This means there are different levels of survival and repoductive success.
  • Individuals with a phenotype that increases their chance of survival are more likely to survive and pass on their genes. So the frequency of the beneficial alleles in the gene pool increases from generation to generation
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Types of natural selection

  • Stabilising selection- this is where individuals with alleles for characteristics towards the middle are more likely to surivive as the environment isn't changing and reduces the range of possible phenotypes
  • Directional selection- this where individuals with a more extreme phenotype are more likely to survive and this could be in response to an environmental change
  • Disruptive selection- this is where individuals with alleles for extreme phenotypes are more likely to survive. Its the opposite to stabilising selection because characteristics towards the middle of the range are lost and occurs when the environment favours multiple phenotypes
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Speciation

  • Speciation is the development of a new species from an existing species. It occurs when populations of the same species become reproductively isolated- changes in allele frequency causes changes in phenotype, which means they can no longer interbreed.
  • Reproductive isolation can occur when there is a physical barrier. There is no gene flow between 2 populations, which can lead to allopatric speciation
  • Speciation can occur when a population becomes reproductively isolated without any physical separation, this is known as sympatric speciation.
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Allopatric speciation

  • When there is a geographical barrier, the separated populations will experience different selection pressures so changes in allele frequencies may occur
  • Different alleles will be more advantageous in different populations so natural selection occurs.
  • Allele frequencies will also change as mutations will occur in each population
  • Genetic drift may affect the allele frequencies in one or both populations
  • Over time, this can lead to speciation. The changes in allele frequencies will lead to differences accumulating in the gene pools of the separated populations, causing changes in phenotype frequencies. Eventually the original species would have changed so much they can no long breed to produce fertile offspring.
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Sympatric speciation

  • It can only occur when random mutations within a population prevent individuals that carry the mutation from breeding with other members of the population that don't carry the mutation. It doesn't involve geographical isolation.
  • It is fairly rare as its difficult for a population to become completely reproductively isolated without being geographically isolated at the same time
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Mechanisms of reproductive isolation

  • Reproductive isolation occurs because of changes in alleles, genotypes and phenotypes prevent individuals with these changes from breeding with individuals without them.
  • Seasonal changes- individuals develop different flowering or mating seasons, or become sexually active at different times of the year.
  • Mechanical changes- changes in size, shape or function of genitalia can prevent successful mating, preventing individuals from breeding
  • Behaviour changes- a group of individuals may create new courtship routines that are attractive for the rest of the population. This prevents individuals from breeding with each other even though they could
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Evolution via genetic drift

  • Evolution occurs due to genetic drift- this means that instead of environmental factors affecting which individuals surive, breed and pass on their alleles, chance dictates which alleles are passed on. Individuals within a population show variation in their genotypes. By chance, the allele for one genotype is passed on to more offspring than others. So the number of individuals with the allele increase. If by chance the same allele is passed on more often again and again it can lead to evolution as the allele becomes more common in the population.
  • Genetic drift can lead to differences in allele frequency between 2 isolated populations. If enough differences in allele frequency build up over time this could lead to reproductive isolation and speciation
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Genetic drift and pop size

  • Natural selection and genetic drift work alongside each other to drive evolution, but 1 process can drive evolution more than the other depending on the population size. Evolution by genetic drift usually has a greater affect in populations where chance has a greater influence.
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Speciation and diversity

  • To start with there was 1 population of organisms. The population was divided and new populations evolved into separate species. The new species were then divided again and the new populations evolved into more separate species. This process has been repeated over a long period of time to create millions of new species
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