evolution may lead to speciation Flashcards
why might individuals within a popualtion of a species show a wide range of variation in phenotype
genetic factors
- mutation : change in base sequence of DNA so new allele
- meiosis: crossing over and independent segregation
- random fertilisation: of gametes during sexual reproduction
environmental factors
- eg climate food lifestyle
what is evolution
change in allele frequencies in a population over time
how does evolution occur
natural selection
1. variation within a popualtion due to mutations
2. selection pressure eg. predation, disease, competition -> strugle to survive
3. some organisms have phenotypes providing selective advantages due to favourable alleles
4. they are more likely to survive and reproduce passing on favourable alleles
5. allele frequencies in gene pool change over many generations
what is directional selection
selection pressures/environment favours individuals with favourable combination of alleles in one direction from the mean
mean shifts in direction of favourable allele
eg. pepper moths -> polution means white to black moths
draw a directional selection graph
bell shifted right or left
what is stabilising selection
selection pressures act against two extremes of characteristics
mean stays the same
standard deviation smaller over time
eg. clutch size in birds -> too many egges means lack of food, too little eggs means may all die
draw a stabilising selection graph
bell gets skinner and taller
what is disruptive selection
selection against the mean
population becomes phenotypically divided - favours both extremes
most important for evolutionary change as could result in two sperate species
draw a disruptive selection garph
two bell shaped curved joined in the middle
what is speciation
formation of a new species from existing species
reproductive sepeation of two populations can result in the accumulation of differences in gene pools
new species when inability of member of the population to interbreed and produce fertile offspring
what are the two types of speciation
allopatric and sympatric
what is allopatric speciation
geographical isolation
seperate gene pools, no interbreeding
mutations cause genetic variation in each population
different selection pressures act on each population
leading to natural selection of different favourable alleles
different survival and reproductive success
leads to change of allele frequencies over a long time
memebers of different populations cant interbreed to produce fertile offspring so new species arise
what is sympatric speciation
popualtion in the same area
genetic variation due to mutations
resulting in a mechanism that makes individuals reproductively issolated
- gamete incompatibility
- temporal - different breeding seasons
- behavioural - different courtship behaviour
- mechanical - incompatible genitalia
different slectional pressures
leads to xhnage in allele freq within gene pools/ divergence of gene pools
members of diffrent popualtions cant interbreed to produce fertile offspring = new species = speciation
what is genetic drift
mechanism of evolution in which allele freq of a population change over time
when is gentic drift strongest
in small poulations as chance has greater influence
