genetics, populations, evolution and ecosystems

diploid

a cell which has 2 pairs of each chromosome (homologous chromosomes)

2 alleles of each gene

gene

a section of DNA which codes for a functional polypeptide

allele

a version of a gene

arise by mutation

2 alleles found in diploid organisms as they have 2 sets of chromosomes

genotype

the combination of alleles (genetic makeup) an organism has

phenotype

the physical expression of the genotype and its interaction with the environment

dominant allele

allele that is always expressed

recessive allele

allele which is only expressed when 2 copies of it are present (homozygous)

codominant

both alleles are present in phenotype → phenotype ends up being an intermediate between the two

locus

the position of a gene on a chromosomes

alleles have the same gene loci

monohybrid inheritance

inheritance of a characteristic controlled by a single gene

dihybrid inheritance

inheritance of two characteristics that are controlled by separate genes

epistasis

an interaction between genes: when the expression of one gene suppresses the expression of another gene

i.e. when a phenotype is controlled by more than one gene

epistasis dihybrid cross

deviates from normal phenotypic ratio

recessive epistatic allele 9:3:4

dominant epistatic allele 12:3:1

what is an autosome

a non-sex chromosome (44 autosomes)

autosomal linkage

• genes which are on the same autosome are linked

  • therefore will stay together during independent segregation

• therefore they will be inherited together

• however, new combinations of alleles can be formed during crossing over of homologous pairs

  • if genes are closer together on an autosome, they are less likely to be split by crossing over

sex linkage

the expression of alleles locates on sex chromosomes

depends on the sex of the individual

XX vs XY

• X is much longer then Y

• therefore the Y chromosome is missing many of the genes which are present on X

• females have to be homozygous recessive in order to inherit a disorder

• males only have one X chromosome → it is has a faulty allele then they are likely to suffer (only require one recessive allele)

phenotypic crosses

monohybrid:

• F1 → 100% heterozygous (if both parents are heterozygous)

• F2 → 3:1

dihybrid:

• F1 → 100% heterozygous (if both parents are heterozygous)

• F2 → 9:3:3:1

codominant:

• F1 → 100% heterozygous (if both parents are homozygous dominant for each allele)

• F2 → 1:2:1

exceptions to normal phenotypic ratios

epistasis

• 100% heterozygous (if one parent is homozygous recessive and one is homozygous recessive

• recessive epistatic allele 9:3:4

• dominant epistatic allele = 12:3:1

sex linkage

• heterozygous female x dominant male

• 3:1 (1 = male sufferer)

• heterozygous female x recessive male

• 1:1

autosomal linkage

• genes on same autosome will stay together during independent segregation

• unless they are separated in crossing over

what is speciation

the process of forming a new species

for this to happen has to be a change in allele frequency (evolution) causing differential reproductive success

Hardy-Weinberg principle

predicts the frequency of alleles, genotypes and phenotypes in a population

predicts: the frequency of an allele of a gene will stay constant over a generation

assumptions made by HWP

• no mutations (aka no new alleles created)

• no selection (no advantages provided by alleles)

• random mating (assumes females do not choose mates based on characteristics e.g. strength)

• large population (no genetic drift)

• population is genetically isolated

• no migration

Hardy-Weinberg equations

p + q = 1

• p = all dominant alleles

• q = all recessive alleles

p2 + 2pq + q2 = 1

• p2 = homozygous dominant

• 2pq = heterozygotes

• q2 = homozygous recessive

when to use X² test

• when determining if observed results are significantly different from expected results

• data is categorical

analysis of X²

if X² is greater than critical value at p = 0.05

• difference is significant so accept null hypothesis

• there is less than a 5% probability that difference is due to chance

niches + their advantages

• specific role of a species within its habitat, governed by its adaptation to both abiotic and biotic conditions

• causes less competition for food/resources. if 2 species tried to occupy the same niche, one would outcompete the other

abiotic factors affecting carrying capacity

• light intensity

• temperature

• soil pH and mineral content

• humidity

how do abiotic factors affect population size/carrying capacity

if conditions are favourable, organisms are more likely to survive and reproduce → increases carrying capacity

intraspecific variation

• between individuals of the same species

• have the same genes but different alleles → variation in their phenotype

how can intraspecific competition affect population size

1. as population size increases, resource availability per organism decreases, so competition increases

   • chances of survival and reproduction decrease → population size decreases

2. as population size decreases, resource availability per organism decreases, so competition decreases

   • chances of survival and reproduction increase → population size increases

interspecific variation

• individuals of different species

• have different genes and live in different environments → variation in their phenotypes

how can interspecific competition affect population size

• reduces [named resource] available to both species, limiting their chances of survival and reproduction

  • reduced population of both species

• if one species is better adapted, it will outcompete the other

  • population size of less well adapted species declines, potentially leading to extinction

predator prey relationship

• prey population increases → more food for predators, predators more likely to survive and reproduce

• predator population increases → more prey eaten, prey less likely to survive and reproduce

• prey population decreases → less food for predators, predators less likely to survive and reproduce

• predator population decreases → less prey eaten, prey more likely to survive and reproduce

• cycle repeats

mark-release-recapture

• 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 a second sample

• population = (no in sample 1 x no in sample 2)/number marked in sample 2

assumptions of MRR

• marked organisms have randomly redistributed within the population

• marking was not removed and did not affect chances of survival/reproduction

• no immigration/emigration

• no deaths (or birth and death rate are equal)

why is MRR unreliable in very large areas

• unlikely that organisms will distribute evenly

• less chance of recapturing organisms which were previously marked

primary succession

• colonisation by pioneer species

• pioneer species change abiotic conditions

  • e.g. they die and decompose, forming soil which retains water

• so environment becomes less hostile for other species with different adaptations and less suitable for previous species

• better adapted species outcompetes previous species

• as succession goes on, biodiversity increases

• climax community is reached, no further succession

features of a climax community

• same species present over a long time

• abiotic factors remain constant over time

• populations remain fairly stable (around carrying capacity)

conservation of habitats + management of succession

• further succession can be prevented to stop a CC forming

  • by removing or preventing growth of species associated with later stages e.g. by allowing grazing

• this preserves the ecosystem in its current stage of succession

• therefore early species are not outcompeted by later species and habitats/niches are not lost

conflict between human needs vs conservation

• human demands for natural resources → habitat destruction

• conservation is needed to protect habitats/niches/species/biodiversity

• management of this conflict maintains the sustainability of natural resources

  • meeting current needs without compromising the ability of future generations to meet theirs

sources of genetic variation

• mutation

• meiosis

• random fertilisation

continuous variation

variation in which organisms do not fall into distinct categories but show graduations from one extreme to another

discontinuous variation

variation shown when the characteristics of organisms fall into distinct categories e.g. blood groups in humans

carrying capacity

the maximum (stable) population size of a species that an ecosystem can support

causes of phenotypic variation

genetic factors

• mutations

• crossing over

• independent segregation

• random fertilisation

environmental factors

evolution

change in allele frequency over time

occurs due to natural selection

factors which drive natural selection

• predation, disease and competition

• result in differential survival and reproduction

process of natural selection

• mutation arises, causing the formation of a new allele

• new allele provides a selective advantage to its possessor

• individual with allele more likely to survive + reproduce (outcompetes)

• advantageous allele is inherited by the offspring

• over many generations, frequency of allele in the gene pool increases

stabilising selection

• selective advantage for average variation of a trait

• frequency of allele coding for average trait increase over many generations, opposite happens to alleles coding for extreme variations of a trait

• range/standard deviation reduced

directional selection

• selective advantage for organisms with one extreme allele have a selective advantage

• allele frequency which code for the extreme version of trait increase over many generations, those coding for the other extreme gene decrease

disruptive selection

• organisms with alleles coding for either extreme variation of a trait have a selective advantage

• frequency of alleles coding for either extreme trait increase over many generations

• average variation of the trait decreases

• this can lead to speciation

causes + effects of speciation

new species arises from existing species

1. reproductive separation of two populations of the same species

2. can result in an accumulation of differences in their gene pools

3. new species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring

allopatric speciation

1. population split due to geographical isolation

2. leads to reproductive isolation, separating gene pools by preventing interbreeding between populations

3. random mutations increase genetic variation between the 2 populations

4. different selection pressures/environments acting on each population

5. so different alleles are selected for/passed on in each population

6. allele frequencies within each gene pool change over many generations

7. eventually different populations cannot interbreed to produce fertile offspring

sympatric speciations

1. populations are not geographically isolated

2. mutations lead to reproductive isolation e.g.

   • gamete incompatibility

   • different breeding seasons

   • different courtship behaviour preventing mating

   • body shape/size changes preventing mating

3. different selection pressures act on each population

4. therefore different alleles are selected for/passed on to next generation

5. allele frequencies in within each gene pool change over many generations

6. eventually different populations cannot interbreed to produce fertile offspring

genetic drift

a mechanism of evolution in which allele frequencies in a population change over generations due to chance

• some alleles are passed onto offspring more/less often by chance, regardless of selection pressures and whether alleles give a selective advantage

genetic drift in small populations

• gene pool in smaller populations is small and chance has a greater influence

  • e.g. bottleneck effect (when population is sharply reduced in size)

  • e.g. founder effect (when a new, small colony forms from a main population)

• can reduce genetic diversity → some alleles can become fixed or lost entirely

RP12: why % cover > frequency

too difficult to count individual organisms

RP12: why is random sampling used

avoid sampling bias

RP12: importance of a large sample size

• minimises affect of anomalies

• ensures sample is representative

RP12: how to decide number of quadrats

• calculate a running mean

• when enough quadrats, there is little change

• enough to carry out a stats test

RP12: limitations of systematic sampling to investigate population size in a field

• not appropriate unless there’s an environmental gradient

• transects run in one direction, but to cover entire field would need to be placed in multiple directions

RP12: which stats test

correlation coefficient e.g. spearman’s rank