Topic 7: Genetics,Populations,Evolution Flashcards
define genotype and phenotype
genotype: genetic consitution of an organism.
phenotype: the expression of this genetic constitution and its interaction with its environment.
What are alleles and how do they arise?
- variations of a particular gene (same locus)
- arise by mutation ( changes in DNA base sequence)
how many alleles of a gene can be found in diploid organisms?
- 2 as diploid organisms have 2 sets of chromosomes ( found in homologous pairs)
- but there be many ( more than 2) alleles of a single gene in a population
Describe the different types of alleles
- dominant: always expressed (shown in the phenotype)
- recessive allele: only expressed when 2 copies preseent
- codominant allelle: both alleles expressed/contribute to phenotype ( if inherited together).
What is meant by the terms homozygous and heterozygous?
- homozygous: alleles at a specific locus ( on each homologius chromosome are the same)
- heterozygous: alleles at a specific locus ( on each homologus chromosomes) are different)
What do monohybrid and dihybrid corsses show?
- monohybrid: inheritance of one phenotypic characteristic coded for by a single gene.
- dihybrid cross: inheritance of two phenotypic characteristics coded for by different genes.
monohybrid cross practice
uplearn
dihybrid cross
uplearn
what is a sex-linked gene?
- a gene with a locus on a sex chromosome (normally X)
Explain why males are more likely to express a recessive X linked allele
- females (XX) have 2 allelles: only express recessive allele if homozygous recessive / can be carriers.
- Males (XY) have 1 allele (inherited from mother): recessive allele always expressed.
Explain how autosmal linkage affects inheritance of alleles
- two genes located on same autosome
- no alleles on same chromosome inherited together
- stay together during independent segregation of homologus chromosomes during meiosis.
- but crossing over between homologous chromosomes can create new combinations of alleles if the genes are closer together on an autosoe, they are less likely to be split by crossing over.
what is epistasis?
- interaction of non-linked genes where one masks/supresses the expression of the other
Describe when a chi-squared (X2) test can be used
- when determining if observed results are significantly different from expected results (frequencies)
- e.g. comparing the goodness fit of observed phenotypic rations with expected ratios
- data is categorical (Can be divided into groups e.g. phenotypes)
Suggest why in genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.
- fusion/fertilisation of gametes is random
- autosmal linkage/epistasis/sex-linkage
- small sample size - not representative of whole population
- some genotypes may be lethal (cause death)
Describe how a chi-squared value can be calculated
Describe how a chi-squared value can be analysed
What is a population?
- a group of organisms of the same species in one area at one time that can interbreed.
What is a gene pool?
- all the alleles of all the genes in a population at any one time.
what is allele frequency?
- proportion of an allele of a gene in a gene pool (decimal or percentage)
What does the Hardy-weinberg principle state and what are the conditions under which the principle applies?
- allele frequencies will not change from generation to generation , given:
- population is large
- no immigration/emigration ( to introduce/remove alleles)
- no mutations ( to create new alleles)
- no selection for/against particular alleles
- mating is random
Give the Hardy-Weinberg equation
- p²+2pq+q²=1
- p+q=1
- p= frequency of one dominant allelle
- q=frequency of recessive allele of the gene
- p²= frequency of homozygous genotype
- 2pq: frequency of heterozygous genotype
- q²= frequency of homozygous usually recessive genotype.
Explain why individuals within a population may show a wide range of variation in phenotype
genetic factors:
- mutations: primary source of variation
- crossing over
- independent segregation
- random fertilisation of gametes during sexual reproduction.
- environmental factors
what is evolution?
- change in the allele frequency over time/many generations in a population
- occuring through the process of natural selection
What is evolution?
- change in allele frequency over time/many generations in a population
- occuring through the process of natural selection
Describe factors that may drive natural selection
- predation,disease and competition for the means of survival.
- these result in differential survival and reproduction, i.e. natural selection
Explain the principles of natural selection in the evolution of populations
- random gene mutations can result in new alleles of a gene
- due to a selection pressure , the new allele might benefit its possessor: organism has a selective advantage
- possessors are more likely to survive and have increased reproductive success
- advantageous allele is inherited by members of the next generation (offspring)
- over many generations, allee increases in frequency in the gene pool
Explain the effects of stabilising selection
- organisms with alleles coding for average/modal variations of a trait have a selective advantage. (e.g. babies with an average weight)
- so frequency of alleles coding for average variations of a trait increase and those coding for extreme variations of a trait decrease.
- so range /standard deviation is reduced
Explain the effects of directional selection
- organiss with alleles coding for one extreme variation of a trait have a selective advantage
- so frequency of alleles coding for both extreme variations of the trait increase and those coding for the average variation of the trait decrease
- this can lead to speciation.
Describe speciation
- reprodutive separation of two populations ( of the same species)
- this can result in accumulation of differences in their gene pools
new species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring.
Describe allopatric speciation
- population is split due to geographical isolation (e.g. new river formed)
- this leads to reproductive isolation , separating gene pools by preveting interbreeding/gene flow between populations.
- random mutations cause genetic variation within each population.
- so different advantageous alleles are selected for/passed on in each population
- so allele frequencies within each gene pool change over many generations
- eventually different popultions cannot interbreed to produce fertile offspring.
Describe sympatric speciation
- population is not geographically isolated
- mutations lead to reproductive isolation, separating gene pools by preventing interbreeding/gene flow within one population e.g: different breeding seasons, different courtship behaviour mating.
- different selection pressures act on each population
- so different advantageous alleles are selected for/passed on in each population
- so allele frequencies within each gene pool change over many generations
- eventually different populations cannot interbreed to produce fertile offspring
Explain genetic drift and its importance in small populations
- GD: a mechanism of evolution in which allele frequencies in a population change over generations due to chance.
- so strongest effects in small population as gene pool is small and change has a greater influence.
- E.g. when a population is sharply reduced in size ; bottleneck effect
- E.g. when a small, new colony forms from a main population ; founder effect.
- This can reduce genetic diversity - some alleles can become fixed or lost entirely
What is a community?
- all the populations of different species living in the same place (habitat) at the same time.
What is an ecosystem?
- a community and the non-living (abiotic) components of its environment.
What is a niche?
- the specific role of a species within its habitat, e.g. what it eats, where and when it feeds
- goverened by its adaptation to both abiotic and biotic conditions
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Explain the advantage of species occupying different niches
- less competition for food/resources
- if two species tried to occupt the same niche, one would outcompete the other
What is carrying capacity?
- the maximum population size of a species that an ecosystem can support.
List the factors that influence carrying capacity
- abiotic factors: light intensity, soil ph, and minral content, humidity.
- interactions between organisms:
- interspecific competition: between organisms of different species
- intraspecific competition: between organisms of the same species
- predation: predators kill and eat other animals, called prey.
Explain how abiotic factors may affect population size/carrying capacity
- if conditions favourable, organisms mroe likely to survive and reproduce, increasing carrying capacity
- e.g. increasing light intensity increases rate of photosynthesis in plants.
- this increases CC of a vairety of plant species
- increases the number and variety of habitats, niches, and food sources for animals.
- so increasing CC of a variety of animal species.
Explain how interspecific competition may affect population size
- reduces resources available to both species limiting their chances of survival and reproduction so reduces population size of both species
- if one species is better adapted, it will outcompete the other so population size of less well adapted species declines , potentially leading to extinction .
Explain how intraspecific competition may affect population size
- as population size increases resource availability per organism decreases , so competition decreases
- as population size decreases, resource availability per organismm increases, so competition decreases so chances of survival and reproduction increase
Explain the changes which occur in populations of predators and prey
- Prey population increases so predators have more food so more predators survive and reproduce
- Predator population increases so more prey killed & eaten so less prey survive & reproduce
- Prey population decreases so predators have less food so less predators survive and reproduce.
- Predator population decreases so less prey killed & eaten so more prey survive and reproduce ( cycle repeats).
Describe how the size of a population of slow moving or non-mobile organisms can be estimated
- Divide area into a grid/squares.
- Generate a pair of coordinates using a random number generator (e.g. on a calculator)
- Place a quadrat here and count number/frequency of species
- Repeat a large number of times and calculate a mean per quadrat
- Population size= total area of habitat/quadrat area) x mean per quadrat
Describe how the mark-release-recapture method can be used to estimate the size of a population of motile organisms
- capture sample of species, mark and release
- Ensure marking is not harmful/does not affect survival
- Allow time for organisms to randomly distribute before collecting second sample
- Population size = (number in sample x number in sample 2)/ number marked in sample 2
Explain how the mark-release recapture equation can be derived
number in marked sample 1/ total population size = number in marked sample 2 / total number in sample 2
What assumptions does the mark-release-capture method make?
- Sufficient time for marked individuals to mix/distribute evenly within the population.
- Marking not removed and doesn’t affect chances of survival/predation.
- Limited/ no immigration / emigration
- No / few births/ deaths/ breeding/ change in population size
Suggest why the mark-release-recapture-method can produce unreliable results in very large areas
- unlikely that organisms will distribute randomly/evenly
- less chance of recapturing organisms ( that were marked initially)
define succession
- succession: change in a community over time due to change in abiotic factors/species.
Describe and explain how primary succession occurs
- Colonisation by pioneer species ( first to colonise)
- Pionner species change abiotic conditions
- They die and decompose , forming soil which retains water ( humus/organic matter)
- So environment becomes less hostile/more suitable for other species with different adaptations and less suitable for previous species, so better adapted species outcompete previous species
- As succession goes on , biodiversity increases
- Climax community reached - final stable community ( no further succession)
Describe features of a climax community
- same species present/stable community over a long time
- abiotic factors ( fairly) constant over time
- Populations stable around carrying capacity
Succession occurs in natural ecosystems. Describe and explain how succession occurs (5)
1. (Colonisation by) pioneer (species);
2. Change in environment / example of change caused by organisms present;
3. Enables other species to colonise / survive;
4. Change in diversity / biodiversity;
5. Stability increases / less hostile environment;
6. Climax community;
Explain how conservation of habitats involves management of succession
- further succession can be prevented to stop a climax community forming
- by removing or preventing growth of species associated with later stages e.g. by allowing grazing
- this preserves an ecosystem at a certain point/in its current stage of succession (plagioclimax)
- So early species are not outcompeted by later species and habitats / niches are not lost.
Describe the conflict between human needs and conservation as well as the importance of managing this
- human demand for natural resources e.g. timber is leading to habitat destruction . biodiversity loss
- conservation is needed to protect habitats / niches/ species biodiversity
- management of this conflict maintains the sustainability of natural resources
- meeting current needs w/o compromising the ability of future generations to meet theirs.
Describe how you could investigate the ffect of an environmental factor on the distribution of a species in a habitat ( random sampling in two areas)
RP12
- Divide two areas into grids / squares eg. place 2 tape measures at right angles
- Generate a pair of coordinates using a random number generator (eg. on a calculator)
- Place a quadrat here and count number / frequency of [named species]
- Standardise this eg. only count it if it is more than half in the quadrat
- Repeat a large number of times (10 or more) and calculate a mean per quadrat for both areas
- Measure environmental factor in each area eg. take soil moisture readings with a soil moisture meter
Suggest why percentage cover may be used rather than frequency.
- individual organisms are too small to count
Explain why random sampling is used and the importance of a large sample size
- to avoid sampling bias
- minimises the effect of anomialies + ensures sample is representatiive of the population
Describe how to decide the number of quadrats that should be usied in order to colllect representative data.
- calculate a running mean
- when enough qudrats , this shows little change
- enough to carry out a statistical test.
Describe how you could investigate the effect of a factor on the distribution of a species in a habitat
systematic sampling
- Place a transect line across an area with a environmental gradient
- Place quadrats at regular intervals e.g. 1m and record the number of organisms of species and environmental factor
- Repeat in other parallel areas and calculate mean number of plants at each point along the transect.
Explain the limitations of using systematic sampling to estimate the population of a species in a field
- not apporpriate unless there is an environmental gradient
- transects run in one direction, but to cover the entire field they would need placing in multiple directions.
Which statistical test should be used to determine the relationship between abundance and an environmental factor
- correlation coefficient e.g. Spearman’s rank
