variation and evolution Flashcards
what three factors cause variation?
- differences in genotypes - genetic factors producing variation between individuals
- the same genotype but have different epigenetic modifications
- differences in environment - environmental factors producing variation between individuals
why is variation important to the survival of an organism?
more likely to survive changes in the environment
describe non-heritable variation
- environmental
- the environment can affect the way an organism’s genes are expressed (i.e. phenotypic variation)
- this variation cannot be passed to offspring unless an epigenetic change occurs
- if the variation is not passed onto the offspring then the variation is non-heritable
explain how the environment could lead to an epigenetic change
an environmental factor (e.g diet) alters DNA methylation or histone modificiation.
this then leads to changes in gene expression
how might genetic variation be increased as a result of sexual reproduction?
- crossing over between homologous chromosomes during meiosis at prophase I
- independent assortment of chromosomes during meiosis at metaphase I
- independent assortment of chromatids during meiosis at metaphase II
- mixing of two different parental genotypes at fertilisation
what is heritable variation also known as?
genetic variation
define selection pressure
an environmental factor which can alter the allele frequency of the alleles present at a particular gene locus in a population
what are selective agencies?
they exert selection pressure
name 4 selective agencies
- climate
- human impact
- supply of food
- breeding sites
outline the correlation between phenotypes and survival & breeding success
- organisms with well adapted phenotypes (those whose alleles give them a selective advantage) are more likely to survive than those who phenotypes are less well adapted (those whose alleles mean they are selected against)
- the phenotypes which increase the chance of organisms surviving are also likely to give a higher breeding success
what can selection pressures change?
allele frequencies of the alleles present at a particular gene locus in a population
define gene pool
total of all alleles for all of the genes in a population
what selection pressure led to the increase in dark form moths during the industrial revolution
- soot from factories led to tree bark darkening
- dark form moths were camouflaged
- reduced predation
if a dominant allele produces a phenotype which is a selective disadvantage, what will happen to the frequency of this dominant allele in the gene pool? explain your answer.
- frequency of allele will decrease and it may disappear from gene pool
- any organism with it will have the disadvantageous phenotype so is less likely to survive, breed and pass it onto the offspring
what is the effect of a recessive allele that produces a selective disadvantage? explain your answer
- organisms which are heterozygous will not have the disadvantageous phenotype so will survive and pass along the allele
- this means that the recessive allele will remain at low frequency in the population
equation linking frequency of dominant allele and frequency of recessive allele?
frequency of dominant allele + frequency of recessive allele = 1
define genetic drift
variation in allele frequencies due to chance
define Hard-Weinburg
a principle stating that the frequencies of dominant and recessive alleles and genotypes will remain constant from one generation to the next, if certain conditions remain true
what are the conditions for the Hardy-Weinburg principle?
- a large population (100+ individuals)
- no selection for or against any phenotype
- random mating throughout any population
- no mutations
- the population is isolated, i.e. no immigration or emigration
what do the three terms of the hardy weinburg binomial expansion indicate?
- p^2 : frequency of AA (homozygous dominant)
- 2pq : frequency Aa (heterozygous)
- q^2 : frequency of aa (homozygous recessive)
