7. evolution Flashcards
what is evolution
the changes in organisms over generations as a result of genomic variations
what is natural selection
natural selection is the non-random increase in frequency of DNA sequences that increase survival and the non-random reduction in the frequency of deleterious sequences
what is stabilising selection
an average phenotype is selected for and extremes of the phenotype are selected against
what is directional selection
one extreme of the phenotype range is selected for
what is disruptive selection
two or more phenotypes are selected for
describe vertical gene transfer (natural selection in prokaryotes)
where genes are transferred from parent to offspring as a result of sexual or asexual reproduction
describe horizontal gene transfer (natural selection in prokaryotes)
where genes are transferred between individuals in the same generation
more rapid than vertical gene transfer which means that prokaryotes have faster evolutionary change
what is a species
a group of organisms capable of interbreeding and producing fertile offspring, and which does not normally breed with other groups
what is speciation
the generation of new biological species by evolution as a result of isolation, mutation and selection.
why are isolation barriers important
they prevent gene flow between sub-populations during speciation
the type of isolation barrier determines the type of speciation which occurs
which isolation barriers lead to Allopatric Speciation
geographical barriers (eg. river, sea, mountain range, desert)
which isolation barriers lead to Sympatric Speciation
behavioural and ecological barriers
which type of speciation is less common
Sympatric speciation is far less common than Allopatric Speciation because it would require a very strong, localised selective pressure to completely interrupt gene flow (breeding) between the two populations
describe the 4 stages of natural selection
-more offspring are produced than the environment can handle
-inherited variation is expressed in the offsprings phenotype due to random mutations
-selection pressures cause individuals with advantageous DNA sequences to be selected for and those with deleterious DNA sequences to be selected against
-this process continues for many generations, increasing the number of individuals displaying the advantageous characteristics (phenotypes) for that environment. It also means there is a decrease in the frequency of deleterious DNA sequences.
describe the 5 stages of speciation
-initial large interbreeding population of one species, sharing genes
-an isolation barrier splits the original population into sub-populations and prevents gene flow
-different mutations occur in each sub-population
-different selection pressures exist on either side of the barrier (mutants with advantageous sequences will be selected for by the selection pressures)
-after a very long time, the 2 sub-populations become so genetically different that they cannot interbreed to produce fertile offspring.
