Topic 7: Genetics, populations, evolution, and ecosystems

genotype

genetic constitution of an organism; combination of alleles an organism has

phenotype

expression of genetic constitution (genotype) and its interaction with the environment; outward expression of genes

homozygous

(two) copies of the same allele

eg. BB or bb

heterozygous

(two) different copies of the allele

eg. Bb

dominant allele

allele whose characteristics always expressed in the phenotype (even if there is only 1)

recessive allele

allele whose characteristics only expressed in the phenotype if both alleles are recessive; no dominant alleles inherited

codominant allele

alleles that are both expressed in the phenotype; neither are recessive

carrier

a person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring

eg. Rr allele is a carrier of r

sex-linkage

female = XX, male = XY

Y chromosome much smaller so some alleles only appear on X chromosome, only one allele required to express gene: recessive phenotypes showed more frequently in men

autosome

any chromosome is not a sex chromosome

ie. autosomal genes located on autosomes

autosomal linkage

two alleles that are very close together are usually inherited together

so, during genetic variation of crossing over etc, they ‘stick together’

usually causing changes to the phenotype ratio

epistasis

alleles can mask the expression of another allele

eg. widow’s peak allele is masked by baldness: you cannot expressed that you have a widow’s peak if you are bald

many different genes can control the same characteristic (polygenic), interacting to form the phenotype

chi-squared to compare the goodness of fit of observed phenotypic ratios with expected ratios

comparing expected ratio (Punnett square value) vs. observed ratio (table of phenotypes ratios)

to prove that the difference between ratios is significant you must use statistical test: Chi-squared

difference in ratios are usually due to linkage

parental genotypes into gamete genotypes

eg. Ff Rr (one parent genotype)

F = first eg. FR

O = outside eg. Fr

I = inside eg. fR

L = last eg. fr

now a dihybrid cross can be made

population

group of organisms of the **same species** occupying a particular space at a particular time that can potentially interbreed

gene pool

complete range of alleles present in a population

Hardy-Weinberg equation (allele frequency)

p + q = 1

p = frequency of dominant allele

q = frequency of recessive allele

frequency always in **decimal**

Hardy-Weinberg equation (-zygous frequency)

p^2 + 2pq + q^2 = 1

p^2 = frequency of homozygous dominant genotype

2pq = frequency of heterozygous genotype

q^2 = frequency of homozygous recessive genotype

frequency always in **decimal**

conditions for Hardy-Weinberg equation

1. large population where there is no immigration, emigration, mutations, or natural selections

2. random mating; all possible genotypes can be breed with each other

genetic variation due to…

mutations

meiosis: crossing over and independent segregation

random fertilisation of gametes during sexual reproduction

selection pressure

anything that affects an organism’s chance of survival and reproduction

ie. predation, disease, competition, environmental

disruptive selection

selection pressure favours more than one phenotype; at two extremes/either end of the range

opposite as stabilising selection as the middle range is lost

eg. beak size: small and large favoured

small for small seeds, large for large seeds

evolution

change in the allele frequencies in a population (over time)

speciation

development of a new species from an existing species; the new species cannot interbreed/prevent gene flow with the other population and will not produce fertile offspring

the two populations of the same species become reproductively isolated (no alleles mix between populations)

allopatric speciation

physical barrier/geographical isolation

this prevents interbreeding between populations

different environments lead to different alleles being advantageous in different populations (mutations occur independently)

differences accumulate in gene pool, difference in phenotype frequency, leading to the development of new species

sympatric speciation

no geographical isolation

eg. non-random mating (changes in genitalia) (mechanical)

change in courtship behaviour

polyploidy (extra chromosome) (ie. no longer diploid, cannot breed),

different flowering times/mating seasons (seasonal)

asexual reproduction

genetic drift

difference are due to random chance; changing frequency due to random chance

could lead to reproductive isolation and speciation

greater effect on smaller populations, where chance has a greater influence

community

multiple populations of different species in a habitat

habitat

place where an organism lives

ecosystem

community and all the non-living/abiotic conditions

either small or large

biotic conditions

living features of the ecosystem

eg. predator or food

abiotic conditions

non-living features of the ecosystem

eg. temperature or availability of water

niche

role of a species within a habitat

eg. what it eats, where it feeds, when it feeds

carrying capacity

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

can vary due to: abiotic factors

interspecific/intraspecific competition

predation

in**ter**specific competition

competition between **different** species

in**tra**specific competition

competition **within** a species

randomly placed quadrats

to investigate **non-motile** organisms

count frequency of species or percentage cover of quadrat

record all abiotic conditions eg. soil pH, light intensity, temperature

quadrats along a belt transect

investigate **non-motile** organisms from a tree/hedge outwards in a straight line

count frequency of species or percentage cover of quadrat

record all abiotic conditions along the transect eg. soil pH

quadrats can be placed at intervals for longer distances = **interrupted belt transect**

estimating total population size of non-motile organisms (mark scheme)

1. measure out set area and divide into m^2

2. use random number generator to gain co-ordinates to place quadrat

3. count number of organisms/species in each quadrat

4. use large sample size and repeat until a running mean is formed

5. calculate total number of species:

mean number of organism x number of quadrats in area

RP12: which part of the quadrat is placed at the co-ordinate

bottom left corner

does not have to be specific, just consistent throughout whole practical

RP12: percentage cover

this is calculated by counting how many of the “25” squares of the quadrat contain the species

species should only be counted in a square if it covers over half of the square

number of squares covered x 100 / number of squares of quadrats = % of cover in that area

RP12: abiotic factors measurements

light intensity = photometer

pH = pH probe

temperature = thermometer

soil moisture meter (can get 3 in 1 light, pH, moisture)

humidity = hygrometers

RP12: graph

y-axis = percentage cover

x-axis = abiotic factor (eg. how temperature changes)

then statistically test data

mark-release-recapture

investigate motile organisms

capture sample of species using an appropriate technique

**mark** them in a harmless way

**release** them back into habitat

wait a week/long period of time, **capture** a second sample from same population

count how many of the second sample are marked

use **total population size equation**

appropriate technique for capturing

fly = sweep nets

aquatic = net

ground = pitfall trap

total population size equation

total population = number of 1st sample x number of 2nd sample / number of marked in 2nd sample

assumptions with mark-release-recapture method

marked sample has had enough time and opportunity to mix back in with the population

marking has not effected the individuals chance of survival

marking itself is still visible

no change in population size over time period; due to deaths, births, migration

ecosystems are dynamic

constantly changing, leading to succession

primary succession

happens on land that has been newly formed or exposed

there is no soil or organic material to start with

eg. bare rock

secondary succession

occurs when land has been cleared of all plants, but where soil remains

eg. forest fire or trees cut down by humans

much faster as skips pioneer/lichen stage

stages of succession

1. pioneer species

first species to colonise the area

abiotic conditions are extremely hostile (no water or soil)

pioneer species die and breakdown

2. basic soil forms

soil retains water; change in abiotic conditions; different species better adapted to surviving

3. more competition

larger shrubs outcompete other species

greater depth of soil

dominant species survive best

4. climax community

ecosystem support the largest and most complex community of plants

no more competition eg. trees

stable community/steady state

managing succession: seed banks

stores seeds from most plant species

grow new plants in case of extinction

managing succession: captive breeding

prevents extinction

increases numbers

eventually introduced to wild

managing succession: protected area

protected habitat eg. national parks

prevents urban expansion

managing succession: fishing quota

limits number of certain fish allowed to be caught