7. genetics, pops, ecosystems

ADD HARDY WEINBERG

give 2 biotic factors which may affect pop size:

• competition

• predation

give and define the 2 types of competition:

• interspecific competition - competition for resources between different species

• intraspecific competition - competition for resources within the same species

give a key difference between interspecific and intraspecific competition:

intraspecific competition also involves competition for a mate

what effect does increasing population density have on competition?

• increase in pop density → increase in competition intensity

• so lower birth rate

what is outcompetition?

when one species/organism is more well-adapted to the environment than another

what is predation?

when an organism (predator) kills and eats another (prey)

describe a typical predator-prey graph:

• size of predator and prey pop both fluctuate

• there will always be more prey than predators

• the size of the prey population will always change before the predator population - lag time 

explain the shape of the predator-prey graph:

• increase in prey pop → more food for predators → increase in predator pop

• → more predation on prey → decrease in prey pop

• → increases competition among predators for food → decrease in predator pop

• → increase in prey pop as more successful reproduction

what is an abiotic factor?

non living conditions of an ecosystem

what is a biotic factor?

impact of the interactions between organisms

give 3 abiotic factors which affect pop size:

• pH

• availability of water

• temp

explain how pH affects pop size:

• a change in pH past the optimum temp may denature the enzymes/transport proteins

• this causes the movement of substances to decrease 

explain how availability of water affects pop size in plants:

• low availability of water would decrease the rate of transpiration and photosynthesis as less photolysis occurs

• so less glucose produced, decreasing growth

explain how availability of water affects pop size in animals:

explain how temperature may affect pop size:

• if temperature increases past the optimum, enzymes/transport proteins may denature

• decreasing movement of proteins

give 2 biotic factors that may affect pop size:

• competition - interspecific and intraspecific competition

• predation

what is the carrying capacity of a pop?

the maximum pop size it can sustainably support

what is an ecosystem?

a community and the non living components of its environment

what is a population?

a group of individuals of the same species in a habitat

what is a habitat?

the physical location where an organism lives

what is a community?

populations of diff species in a habitat

what is an ecosystem?

a community and the abiotic conditions in its habitat

what does it mean for an ecosystem to be a dynamic system?

constantly changing due to continuous interaction between biotic and abiotic factors

what is a niche?

• the exact resources/conditions an organism needs to survive

• governed by adaptation to both abiotic and biotic conditions

state the competitive exclusion principle:

2 species cannot occupy exactly the same niche

what is biodiversity?

the range/no. of diff species in a habitat

what is the carrying capacity?

the maximum stable pop size of a species an ecosystem can support

describe and explain the shape of a pop growth curve:

• initially, pop increases slowly, as birth rate exceeds the death rate

• pop then expands exponentially when there are no sig restrictions on growth

• plateaus - carrying capacity is reached due to biotic and abiotic factors

give 2 factors which each contribute +vely/-vely to a population:

+vely:

• births

• immigration

-vely:

• emigration

• deaths

give the eqn to calculate the no. of cells on agar at a certain time:

cells at time t = initial cells x 2^{(t/generation time)}

give an advantage of using log scales:

• to compare values with a large range of values

• allows more accurate reading off a graph

how do you convert from a log value to the normal value?

10^{whatever no you put in the brackets of log}

how do you read a log scale?

• one axis increases in a linear way

• the other axis has unevenly distributed lines - each represent a power of 10 (e.g. 1 ×10¹, 1× 10² etc.)

• (each marked increment on scale overlaps)   

describe and explain how succession occurs:

• colonisation by pioneer species

• pioneer species change environment, making it less hostile/more suitable for other species

• this causes an increase in biodiversity

• allowing the climax community to be reached

what is succession? name the two types of succession:

the process by which ecosystems change over time due to changes in the environment:

• primary succession

• secondary succession

what is primary succession?

succession occurring on newly formed or exposed land without soil or organic material

what is secondary succession?

succession in areas where an existing community has been removed, leaving soil intact but with no plant or animal species

how can we use succession for conservation?

• preventing succession by preserving an ecosystem in its current stage of succession

• e.g. mowing the lawn to prevent the growth of shrubs and trees

why is conservation important?

• maintains biodiversity

• e.g. moorland is preserved through grazing to prevent the loss of the plants and animals living there

• this therefore keeps the ecosystem intact through preventing succession

give 2 features of a climax community:

• same species present/stable community/pop over a long period of time

• abiotic factors constant over time

what is genetic drift?

a mechanism of evolution in which the allele freq of a pop changes over generations due to chance (i.e. surviving pop not better adapted)

which pops does genetic drift occur in?

can occur in all pops of infinite size, but effects strongest in small pops

what may genetic drift result in?

• may result in the loss of some alleles

• and the fixation (rise to 100%) of other alleles

what are the 2 types of genetic drift?

• bottleneck effect

• founder effect

what is the bottleneck effect?

• size of pop is severly reduced by events like natural disasters i.e. change in allele freq not because of a selection pressure but instead a natural catastrophe

• often most ind killed and small, random assortment of survivors remain

• allele freq diff from those of pop prior and some alleles may be missing entirely

• smaller pop will be more susceptible to the effects of genetic drift for generations to come

what is the founder effect?

• small group of ind breaks off from a larger pop to establish a colony

• new colony is isolated from the original pop and founding ind may not represent full genetic diversity of original pop

• alleles in founding pop may be present at diff freq than original pop and some alleles may be missing altogether

why may individuals w/in a pop show wide range of variation in phenotype?

• genetic factors

• environmental factors

give the 5 sources of genetic variation:

• mutation

• crossing over

• independent segregation

• random fertilisation

• random mating

give and describe the 2 the types of genetic variation - state which type the environment has a large effect on:

variation due to multiple genes (environment has large effect):

• may be due to multiple genes (polygenes)

• e.g. human height

• freq in pop represented by freq graph

variation due to one gene (environment has little effect):

• due to one gene

• e.g. human blood group

• freq in pop represented by bar chart

what is evolution?

change in allele freq in a pop

describe the process of evolution by natural selection in terms of allele freq:

• there is genetic variation w/in the gene pool due to mutation

• selection pressures - variation means that some ind may have an advantageous alleles whereas others do not

• those w/ the advantageous allele are more likely to survive and produce, passing on the advantageous allele to their offspring

• over time the freq of the the advantageous allele that leads to reproductive advantage in the gene pool increases

what are the 3 types of selection?

• disruptive selection

• stabilising selection

• directional selection

what is disruptive selection? draw the graph:

• selects for 2 extremes

• (mode stays the same, range increases, SD increases)

what is stabilising selection? draw the graph:

• the average phenotype is selected for

• (mode stays the same, range decreases, SD decreases)

what is directional selection? draw the graph:

• one extreme is selected for

• (mode, range and SD all remain the same)

what is speciation?

the evolution of new species from existing ones

give 4 reasons why 2 groups may become reproductively isolated from one another:

• 2 pops become geographically separated (leading to allopatric speciation)

• diff breeding seasons

• some active at night, some active in the day

• some active in the summer, some active in the winter

• sexual selection i.e. preference based on looks/courtship behaviour

describe sympatric speciation:

• not geographically isolated/in the same habitat

• variation due to mutation

• reproductive isolation/gene pools kept separate

• selection/survival linked to (named) traits - adapted organisms breed

• change in genetic constitution of pops/gene pools/allele freq → disruptive selection

• eventually diff species cannot interbreed to produce fertile offspring - separate gene pools

describe allopatric speciation:

• geographical isolation of pops

• variation present in pops due to mutation

• diff environmental conditions/diff selection pressures/diff phenotypes selected

• selection/survival linked to (named) traits - adapted organisms breed

• change in genetic constitution of pops/gene pools/allele freq → disruptive selection

• eventually diff species cannot interbreed to produce fertile offspring - separate gene pools

what is a gene pool?

all the alleles in a pop

how can scientists use DNA to determine whether or not individuals are of the same species?

• compare DNA base sequence

• diff in diff species

describe and explain the changes which occur in the lemming and stoat pops (6):

• 4 year cycles

• predator/stoat peaks after prey/lemming

• lemmings increase due to low numbers of stoats/available food

• more food for stoats so nos increase

• increased predation reduces no. of lemmings

• no. of stoats decreases due to lack of food/starvation

lemmings often live in isolated populations. from time to time some lemmings move and join other populations - explain how this movement is important in maintaining genetic variability in lemming pops which have large fluctuations in size (2)

• small pops have fewer diff alleles/smaller gene pool/lower genetic variability

• migrants bring in new alleles/increase gene pool

many species of cichlids are similar in size and apart from their colour, appearance - suggest how the variety of colour patterns displayed by these cichlids may help to maintain the fish as separate species (2)

• selection of mate dependent on colour pattern

• prevents interbreeding/keeps gene pools separate

what is codominance?

2 different alleles are =lly expressed in an organism’s phenotype

for codominant alleles, when the genotype is heterozygous, what phenotype is expressed?

• mixture of both alleles

• (e.g. C^R = red, C^W = white, C^RC^W = pink)

what is the notation used in codominance?

• uppercase letter for gene (e.g. C for colour)

• superscript uppercase letters for alleles (e.g. C^R = red, C^W = white)

what does it mean for two genes to be linked?

genes on the same autosome

give the steps needed to determine if genes are linked or not:

perform a test cross: AaBb x aabb

• assume no linkage - gametes = AB, Ab, aB, ab, ab

  • expected ratio = 1:1:1:1 (AaBb, Aabb, aaBb, aabb)

• assume linkage and no crossing over - gametes = AB, ab

  • expected ratio = 1:1 (AaBb, aabb)

for a linked test cross, why may the actual genotype ratios differ from the expected ratios?

• crossing over during meiosis may produce new allele combinations - recombinant offspring - and break the linkage

• i.e. mainly AaBb and aabb but fewer aaBb and Aabb

what determines how likely linked genes will be separated during crossing over?

• how close together they are on a chromosome

• the closer the genes, the more likely they are to be inherited together

what statistical test could you use to compare the goodness of fit of observed phenotypic ratios w/ expected ratios and why?

Chi-squared test: use of categorical data

suggest 2 reasons why observed phenotypic ratios obtained in offspring may not be the same as expected ratios:

• autosomal linkage

• small sample size

• epistasis

• lethal genotypes

what is epistasis?

when one gene prevents the expression of another gene

what is the epistatic gene?

the gene responsible for suppressing another gene - the hypostatic gene

what is recessive epistasis?

epistasis where the epistatic gene must be homozygous recessive to prevent the expression of the other gene

what is dominant epistasis?

epistasis which requires at least one dominant allele to prevent the expression of the other gene

give the expected phenotypic ratio for a double heterozygous cross for recessive epistasis:

9:3:4

explain how epistasis could be applied to biochemical pathways

• if any one of enzyme genes is not expressed - step in pathway is disrupted

• so prevents production of intermediate substrate needed for the next enzyme in the sequence

• ∴ early blocked step masks the activity of genes further down in the pathway

give the expected ratio for a double heterozygous cross for dominant epistasis:

12:3:1

what is the genotype?

the genetic constitution of an organism

what is the phenotype?

the expression of the genotype and its interaction w/ the environment

what is dihybrid inheritance?

inheritance involving 2 diff genes

what is the predicted ratio for a double heterozygous dihybrid cross?

9:3:3:1

what are autosomes?

all chromosomes that do not determine sex and instead regulate somatic characteristics of the body

what are sex chromosomes?

chromosomes which determine an organism’s sex by regulating sex linked traits

why are many sex-linked genetic disorders more likely to affect males than females?

• sex linked alleles carried on X chromosome and are usually recessive

• this means that males only need one allele for the trait to be expressed (as men have XY chromosomes)

• females need 2 recessive alleles for the trait to be expressed (females have XX chromosomes)

give the 2 main ways of investigating populations:

• using quadrats - random/transects

• mark-release-recapture

summarise the process of mark-release-recapature to estimate a total pop:

• capture/collect sample, mark and release

• ensure marking is non harmful/does not affect survival (i.e. is non toxic)

• allow sufficient time for organisms to randomly/evenly distribute before collecting a second sample

• pop = no. in first sample x no. in second sample / no. of marked fish in second sample

when should the mark-release-recapture method be used?

when sampling motile organisms

give 5 assumptions that the mark-release-recapture method relies on?

• proportion of marked : unmarked individuals in 2nd sample = proportion of marked : unmarked individuals in whole pop

• marked individuals distribute evenly among rest of pop

• pop remains constant w/o significant immigration/emigration/births/deaths 

• marks are non-toxic, permanent and don’t increase predation rates or change behaviour 

• pop has a definite boundary (i.e. confined to a set area)

give the formula for the mark-release-recapture method and state what each of the symbols mean:

M₁ / N = M₂ / N₂ :

• M₁ = no, of ind marked on first day

• N = total ind in pop

• M₂ = no. of marked ind found on second day

• N₂ = total no. of ind. found on second day (marked + unmarked)

why might mark-release-recapture produce unreliable results in large areas?

• less chance of recapturing organisms

• organisms less likely to distribute randomly/evenly

describe a method to determine the effect of a named environmental factor (sun/shade) on the distribution of a given species:

• use a grid/divide sunny area into squares/coordinates

• use a random number generator to generate random coordinates

• count the number/frequency in a quadrat

• take a large sample

• calculate the mean no. per quadrat

• multiply mean no. of plants per m² by area of field

• repeat for shady area

describe a method to determine the mean % cover of a given species: 

• use a grid/divide area into squares/coordinates

• use a random number generator to generate random coordinates

• count the number/frequency in a quadrat

• take a large sample

• divide total % by no. of quadrats

how can we determine a large enough sample size?

• use a running mean - when the running mean becomes constant, the sample is sufficiently large

• (if asked to estimate a number, 20+)

when should you use a quadrat?

• for plants

• for organisms that don’t move around very much

what is a transect? when should it be used?

• systematic (non-random) sampling technique - different areas in a habitat are sampled at regular intervals (e.g. every n^th interval

• allows you to determine how the distribution of a species changes across different areas w/in a habitat e.g. from a woodland to a lake

what statistical test should be used when using quadrats and why?

t test - measures diff. between means 

what is the typical p value used? what does this mean?

• p = 0.05

• there is a 5% probability that the results of a study are due to random chance