how do organisms show variation in there phenotypes
- having different genotypes
- having the same genotype but different epigenetic modifications
- exposed to different environments
how is variation heritable and how does it arise
■ Gene (point) mutations.
■ Crossing over during prophase I of meiosis.
■ Independent assortment during metaphase I and II of meiosis.
■ Random mating, i.e. that any organism can mate with another.
■ Random fusion of gametes, i.e. the fertilisation of any male gamete with any female gamete.
■ Environmental factors leading to epigenetic modi cations.
■ Environmental factors can also lead to non-heritable variation within a population, e.g. diet.
what are the two types of variation
- continuous
- discontinuous
what is countinuous variation give an examples
- height
- range of phenotypes seen
- controlled by many genes - polygenic
- follows normal distribution
- environmental factors have a major influence eg diet on weight
what is discontinuosu variation and give an example
- characteristics fit into distinct groups
- no intermediates
- usually controlled by one gene with two or morre allels (monogenic)
- environmental factors have little influence eg diet has no effect on blood group
what is the mean
measure of central tendency
what is standard deviation
measure of variation in the data either side of the mean
when do you see that difference are not due to chance
- t value exceed the critical value
- 0.05 probabiity and degrees of freedom (total number of observations - 2)
selection pressure
an environmental factor that can alter the frequency of allels in a popu.ation when it is limiting
natural selectio
the increased chance of survival and reproduction of organisms with phenotypes suited to their environment,
enhancing the transfer of favourable alleles from one generation to the next.
give an example of how environmental influences affect the way the genotypes is expressed
- result in different phenotypes
- e.g. industrial melanism in the peppered moth Biston betularia.
- There are two forms of the peppered moth: speckled and dark (melanic). In polluted habitats where trees are covered with soot the dark form prevails,
- but in unpolluted habitats where lichens are found the speckled form is more common.
- In each instance the moth’s colour
camouflages it against the environment conferring a selective advantage, so it
is more likely to survive, and reproduce transmitting advantageous alleles to
the next generation and so the numbers increase within the population.
what are the two types of competition
intraspecific competition
interspecific comepetition
what is intraspecific competition
- where members of the same species vie for the same resource in an ecosystem
- eg food , light , nutrients , availability of nesting sites
what is interspecific competition
- where individuals of different species via for the ssamee resouce in an ecosystem
- different plant species competiting for water
what is a selection pressure and competition lead to evolution
- different resouces being competed for act as a selection pressure therefore individuals with an advantage meaning
- they are more successful in gaining food and shelter are more likely to survive and pass those advantageous alleles onto the next generation
- increased chance of survival and reproduction of organisms with phenotypes suited to their environment is called natural selection and can lead to evolution
gene pool
all alleles present in a population at a given time
allele frequency
the frequency of an allele is its proportion fraction or percentage of all the alleles of that gene in a gene pool
genetic drift
chance of variations in allele frequencies in a population
what is population genetics and how is it influenced by environmental change
- population genetics considers the relative proportions of the different alleles of allele frequenciess within a gene pool
- if environment is stable then allele frequencis also remain stable
- however if enviornments change they bring different slection pressures which favour some alleles over others so their frequency increase
what is genetic drift and how does it come about
- genetic drift is the change variations in the relative frequency of alleles in a population
- due to random sample (which alleles are inherites) and chance (than an individual may survive and breed)
what does genetic drift lead to
- change in allele frequencies over time and is most significant in small or isolated populations where a change with consititute a much larger proportion of the population because the population is small and so it may be an important evolutionary process
what happens to small number of individuals being isolated
- when a small number of individuals become isolated and start a new population say by colonising a new island the fouder members of a new population are a small sample of the population from which they orginated (the founder effect)and may be subjected to genetic drift
- seen in adaptive radiation seen in darwin’s finches on the galapagos islands
founder effect
loss of genetic variation in a new population established by a very small number of individuals from a larger population
what is the hardy wiengberg principle
states the idea conditions in which allele and genotype frquencies in a population are constant
