variation and evolution

• allele frequency 


  the number of times an allele appears at a particular locus in a population, expressed as a proportion or a percentage
• allopatric speciation 


  a form of speciation that occurs when two populations become geographically isolated
• behavioural isolation


  the reproductive isolation of two populations due to differences in their behaviour, e.g. different mating rituals
• continuous variation 


  a type of variation that cannot be categorised, e.g. skin colour, height. it produces a continuous range in which a characteristic can take any value. multiple genes influence continuous variation and it’s often significantly affected by environmental factors
• discontinuous variation 


  a type of variation that can be categorised, e .g blood group. a characteristic can only appear in discrete values. one or two genes influence discontinuous variation and environmental factors have little effect
• evolution 


  the gradual change in the allele frequencies within a population over time. occurs due to natural selection
• founder effect 


  a type of genetic drift in which a few individuals of a species break off from the population and form a new colony. this results in a smaller gene pools and an increases frequency of rare alleles
• gene pool


  all of the different versions of genes (alleles) in the individuals that make up a population
• genetic drift 


  variation in allele frequencies in small populations due to chance
• geographical isolation 


  a physical barrier (such as river or mountain) separates two populations of the same species
• hardy weinberg principle 


  a model that predicts that the ratio of dominant and recessive alleles in a population will remain constant between generations if the following five conditions are met: no new met mutations, no natural selection, no migrations, large population and random mating. it provides a formula for calculating the frequencies of alleles: $p^2 +$$2pq +$$q^2 =$$1$

  where p is the frequency of the dominant allele and q is the frequency the recessive allele
• heritable variation 


  genetic differences between individuals
• hybrid fertility 


  the formation of fertile hydrid offspring (e.g. wheat) due to hybridisation combined with polyploidy which doubles the chromosome number, enabling meiosis
• hybrid sterility 


  the formation of sterile hybrid offspring (e.g. mule) from the reproduction of individuals of different species; the chromosome sets from each parent differ so are unable to pair up during meiosis
• interspecific competition


  type of completion that takes place between members of different species
• intraspecific competition


  a type of competition that takes place between members of the same species
• morphological isolation 


  the reproductive isolation of two populations due to the incompatibility of their reproductive systems
• natural selection 


  the process by which the frequency of beneficial alleles grandly increases in a populations gene pool over time. this theory was developed by Charles Darwin
• non heritable variation 


  acquired differences in the phenotypes of individuals that cannot be inherited
• reproductive isolation 


  the inability of two populations of the same species to interbreed due to behavioural, morphological or seasonal barriers
• seasonal isolation


  the reproductive isolation of two populations due to differences in their breeding seasons
• selection pressures


  environmental factors that drive evolution by natural selection and limit population sizes, e.g. competition, predation and disease. they can change the frequency of alleles in a population.
• speciation 


  the formation of new species due to evolution and two reproductively separates populations. two forms: allopatric and sympatric speciation
• species


  a group of similar organisms that are able to breed with one another to produce living, fertile offspring
• t test


  a statistical test used to determine whether there is a statistically difference between the means of two data sets that show normal distribution
• sympatric speciation


  a form of speciation that occurs when two populations within the same area become reproductively isolated
• variation 


  the differences between individuals due to genes, the environment or a combination of both