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Flashcards in evolution and populations in ecosystems Deck (26):

what genetic factors cause variation within a species


random segregation of alleles in metaphase I meiosis

crossing over between chromatids of homologous chromosomes

random mating of organisms within a species

random fertilisation


what is mutation

mutation is a change to the DNA

mutations can either be a gene mutation or a chromosome mutation


what environmental factors could cause variation



explain how natural selection causes a change in allele frequency

organisms produce more offspring than are needed to replace the parents. most population numbers remain the same

this causes competition for existence

individuals in a species show variation.

those with advantageous alleles will survive. (survival of the fittest)

these individuals reproduce, passing on their advantageous allele

this will shift the allele frequency within the gene pool


what are the three types of natutral selection

Stabilising selection
directional selection
disruptive selection


explain stabilising selection

stabilising selection is the elimination of extreme variations in a population
e.g bettle colour- light and dark beetles may be easy to see but all those in th middle may camouflage well


define natural selection

the process by which heritable traits become either more or less common in a population due to pressures from the environment


directional selection

if an environment changes one extreme form of a trait may be favoured
e.g peppered moth frequency in industrial revolution. darker moths became harder to see so were selected for.


disruptive selection

both extremes in a trait are selected for but the middle is disadvantageous so is selected against

opposite of stabilising

e.g salmon male size
large fish can fight off opponents to fertilise eggs, small ones are fast enough to sneak in and fertilise the eggs, medium fish are selected against



is the shift of allele frequency in a gene pool caused by selection pressure


what is speciation

the formation of new species


what are the 2 forms of speciation



describe allopatric speciation

the developement of new species due to a physical seperation of a species

population seperated by geographical isolation e.g sea, mountain range etc

isolated populations subjected to different selection pressures.

natural selection causes a shift in the gene pool, resulting in the two populations being reproductively isolation and therefore a new species has evolved


describe sympatric speciation

new species formed when there are no physical barriers- reproductive isolation due to:
-behaviour- mates attracted by calls and dances etc
-structure- sex organs incompatible
-gamete mortality
-hybrid inviability- may result in a zygote that cant divide
-hybrid sterility- two species mate to form a hybrid but this cant reptroduce
-polyploidy- failure of separation of chromosomes in meiosis


genetic drift

changing of allele frequency due to chance.
e.g forest fire wipes out large proportion of population but at random- not due to an advantage that they have

this has a larger affect on a smaller population


what is the founder effect

organisms that start a new population may not be representative of original population


what is a community

all living things in a particular habitat


what makes up an ecosystem

the community (biotic) and the habitat (abiotic)


what is the carrying capacity

the maximum size of a population an ecosystem can support


what factors can affect the population size

Abiotic factors- climate factors, lack of shelter, pollution

biotic factors:
-competition within speciesfor resources e.g light, water, food etc
-competition between species for resources e.g light, water, food etc


How would you estimate a population size

two different ways
Quadrats- for slow moving or non motile organisms.
divide the area into quadrats, pick a sample of quadrats,
count how many organisms are in the sampled quadrats and multiply up for the total area

capture a population and mark them then release them.
recapture a sample from the number of marked organisms in the 2nd sample an estimation of the total population can be made

using the equation:

P-population size
M-number of animals marked captured and released
S-animals captured in second sample
R-number of marked animals recaptured


what is succession

succession is the change in structure and species composition of a community over time


what is primary succession

newly formed habitats that have not previously supported a community


what is the pioneer species

the first organism to colonise a new habitat
e.g lichen and algae on bare rock


what is secondary succession

the repopulation at sites that have previously supported a community, usually after a major environmental disturbance e.g forest fire


what is the climax community

when the community reaches and equilibrium with its environment and no further changes occur