A Level CIE Biology: 17 Selection & Evolution

phenotype

observable characteristics of an organism

phenotypic variation

difference in phenotypes between organisms of the same species

how can phenotypic variation be explained

• genetic factors e.g. 4 diff bloo groups bc 3 possible alleles

• environmental factors e.g. diff heights of plant in diff conditions

• combination of both e.g. recessive allele for sickle cell has high freq where malaria is prevalent bc resistance if heterozygous

complete phenotype of organism equation

genotype + environment

genetic variation

• organisms of same species have v similar genotypes but 2 individuals (even twins) will have differences between their DNA base sequences

• considering size of genomes, these diffs are small betw individuals of same species

• small diff in DNA base sequences between individual organisms within a species population = genetic variation

genetic variation is…

transferred from one generation to the next and it generates phenotypic variation within a species population

what processes cause genetic variation (result in new combo of alleles in gamete/individual)

• independent assortment of homologous chromosomes during M1

• crossing over of non-sister chromatids during P1

• random fusion of gametes during fertilisation

mutation results in…

generation of new alleles

• new allele may be advantageous, disadvantageous or have no apparent effect on phenotype (because genetic code is degenerate)

• new alleles are not always seen in the individual that they first occur in

• they can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation

genes have varying effect on organisms phenotype

• may be affected by a single gene or by several

• impact that gene has on phenotype may be large, small, and/or additive

mechanism and consequence of independent assortment of homologous chromosomes during M1

random alignment of chromosomes results in different combinations of chromosomes and different allele combinations in each gamete resulting in genetic variation between gametes

mechanism and consequence of crossing over of non-sister chromatids during P1

exchange of genetic material between non-sister chromatids leads to new combinations of alleles on chromosomes. can also break the linkage between genes resulting in genetic variation between gametes

mechanism and consequence of random fusion of gametes during fertilisation

any male gamete can fuse with any female gamete (random mating in a species population) resulting in genetic variation between zygotes and resulting individuals

mechanism and consequence of mumtation

random change in the DNA base sequence results in the generation of a new allele. mutation must exist within gametes for it to be passed on to future generations resulting in genetic variation between individuals within a species population

how can the environment an organism lives in impact phenotype

different conditions affect how organisms grow and develop

• length of sunlight hours (seasonal)

• supply of nutrients (food)

• availability of water

• temperature range

• oxygen levels

can environmental pressure caused variation be inherited

no, only alterations to genetic component of gametes will ever be inherited

variation

differences that exist between at individuals of a species (intraspecific variation)

why is there variation in phenotypes

due to qualitative or quantitative differences

qualitative differences in phenotypes of individuals of population =

discontinuous variation

qualitative differences fall into ______ and _______ categories with __ ____________

discrete and distinguishable categories, usually w no intermediates (feature cant fall in between categories e.g. 4 possible ABO groups and person only has one)

is it hard to identify discontinuous variation

no, it is easy when its present in a table or graph due to the distinct categories that exist when data is plotted for particular characteristics

continuous variation

occurs when there are quantitative differences in the phenotypes of individuals within a population for particular characteristics

do quantitative differences fall into discrete categories

no, a range of values exists between 2 extremes within which the phenotype will fall (e.g. mass/height of human). the lack of categories and presence of range of values can be used to identify continuous variation when presented in table/graph

discontinuous variation refers to

the differences between individuals of a species where the differences are qualitative (categoric)

continuous variation refers to

the differences between individuals of a species where the differences are quantitative (measurable)

each type of variation can be explained by…

genetic and/or environmental factors

genetic basis of discontinuous variation

• solely due to genetic facgtors

• environment has no direct effect

• at the genetic level diff genes have diff effects on phenotype and diff alleles at single gene locus have large effect on phenotype (diploid orgs will inherit 2 alleles of each gene and can be same or diff alleles)

• e.g. f8 gene coding for blood clotting protein factor viii - diff alleles at f8 gene locus dictate whether or not normal factor viii is produced and whether haemophilia

genetic basis of continuous variation

• caused by interaction between genetics and environment

• phenotype = genotype + environment

• at genetic level diff alleles at single locus have small effect on phenotype and diff genes can have same effect on phenotype and these have an additive effect. if large no. genes have combined effect on phenotype = polygenes

t-test

statistical test used to compare means of 2 sets of data and determine whether significantly diff or not

for t test what must sets of data be

• follow roughly normal distribution

• continuous

• standard deviations should be approx equal

what must be calculated for each data set before the t test can be carried out

standard deviation

null hypothesis

this is a statement of what we would expect if there is no significant difference between 2 means and that any differences seen are due to chance

when is the null hypothesis rejected

if there is a statistically significant difference between the means of two sets of data, then the observation is not down to chance and nh rejected

steps to calculate t-test

• calc mean for each data set

• calc standard deviation for each set of data

• square the standard deviation and divide by n (no. observations) in each sample for both samples

• add values from step 3 together and take square root

• divide diff between 2 means w value calc in step 4 to get the value

• calc degrees of freedom for whole data set (v = (n1-1) + (n2-1)

• look at table that relates t values to probability that diff between data sets is due to chance to find where t value for degrees of freedom v calculated lies

• the greater the t value calc (for any dof) the lower the probability of chance causing any significant difference between the two sample means.

if t value is greater than the critical value (0.05)

any diff between the means of the two data sets is said to be statistically significant = there’s less than 5% probability that any difference is due to chance, null hypothesis rejected

if t value is less than critical value (0.05)

then no sig diff between mean of 2 data sets. probability that any diff is due to chance is higher than 5% so null hypothesis is accepted

when would a population experience exponential growth

if the offspring for every individual survived to adulthood and reproduced then the pop = exp growth

• only happens when no environmental factors or pop checks are acting on pop (e.g. lots of resources and no disease)

• one well-known but rare example of expo growth = introduction of european rabbits to aussie

• rabbits had an abundance of resources little comp and no predators so increased rapidly = pest

in reality, several env factors prevent every individual in a pop from making it to adulthood and reproducing

how do env factors limit pop sizes

by reducing rate of pop growth whenever a pop reaches a certain size

env types of factors

• biotic

• abiotic

biotic

involve other living organisms e.g. predation, competition for resources, disease

abiotic

non living parts of environment e.g. light availabiltiy, soil pH, water supply

no offspring produced is much ____ than…

higher than no. individuals that make it to adulthood

environmental factors that limit pop growth rate (african lions in wild)

1. comp for food - limited supply of prey. others also hunt. if no food = starve

2. comp for mate - more females so males compete. they fight = death/injury. loss = no mate so no offspring

3. supply of water - can be v arid during dry season. far water sources. die dehydration if dries up.

4. temp - extreme heat = overheat and die. stops from hunting so no food.

= decrease in pop growth

variation

exists within a species pop meaning some individuals within pop posses diff phenotypes bc of genetic variation in alleles they possess. members of same species have same genes

selection pressure

increases chance of individual w specific phenotype surviving and reproducing others and env factors can act as this.

fitness

ability to survive and pass on alleles to offspring. orgs w higher fitness = favoured/possess adaptations that make them better suited to environment

natural selection

process by which individuals w fitter phenotype are more likely to survive and pass on their alleles to their offspring so that the advantageous alleles increase in freq. over time and generations. when selection pressures act over several generation of species they effect frequency of alleles.

3 types of selection

• stabilising

• directional

• disruptive

stabilising selection

natural selection that keeps allele frequencies constant over generations meaning that allele frequencies stay constant unless there’s a change in environment e.g. human birth weights

directional selection

nautral selection that produces a gradual change in allele frequencies over several generations usually when there’s a change in environment or new selection pressures leading to certain alleles becoming advantageous e.g. fish size (smaller in hotter temps)

disruptive selection

natural selection that maintains high frequencies of two diff sets of alleles e.g. individuals with intermediate phenotypes/alleles are selected against e.g. galapagos birds

what does disruptive selection maintain

polymorphism: the continued existence of 2 or mroe distinct phenotypes in species (variation)

genetic drift

when allele frequencies change bc of chance

other processes that can cause allele freq changes due to chance:

• founder effect

• bottleneck effect

natural selection

• when new allele arises in pop or change in env occurs directional selec can happen producing gradual change in allele freq over sev generations

  • always phenotypic variation

  • selection pressure

  • some indi may have phenotype that aids survival in presence of selec pressure produced by particular alleles

• indi w favored phenotype fitter so reproduce and pass on to offspring

• those who dont have less likely to suurvive

• overtime and generations, freq of advantageous increases and other decreases

genetic drift

• when pop v small, chance can affect which alleles get passed on to next gen

  • meiosis = haploid gametes so fert only passes on half alleles of individual

  • half that gets passed is result of random fertilisation and other half may not make it to next gen

• overtime some alleles lost or passed purely by chance

• effects smaller pops e.g. coin toss 10 times = heads wont come but if 100 less likely etc

founder effect

• occurs when small no. individuals from large parent pop start new pop and can come about as result of chance e.g. storm separates small grp of indi from main pop

• as new pop made up of only few indi from og pop, only some of total alleles from parent pop present e.g. not all gene pool present in smaller pop

• bc pop results from founder effect is very small, more susceptible to effects of gen drift

bottleneck effect

• similar to founder and occurs when prev large pops suffers dramatic fall in numbers

• major env event can greatly reduce no. indi in pop = reduced gen diversity in pop as alleles lost

• surviving indis end up breeding and reproducing w close relatives

4 processes affecting allele freq and result

• natural selec - selec pressures produce gradual change in allele freq over sev gen

• founder effect - changes in allele freq occur in diff direction for newly isolated small pop in comparison to larger parent pop due to chance

• gen drift - gradual change in allele freq in small pop due to chance not natural selec

• bottleneck effect - reduction in gene pool of pop due to dramatic decrease in pop size

antibiotics

chemical substances that inhibit or kill bacterial cells w little or no harm to human tissue. derived from naturally occuring substances that are harmful to prok cells but dont affect euk cells (aids bodys immune system in fighting bac infection)

bactericidal vs bacteriostatic

cidal = kill

static = inhibit growth process

how do antibiotics work

target prok features but can affect both pathogenic and mutualistic bac living on body

bacterial resistance

all bac have genetic diversity and indi bac may have alleles that confer resistance to effects of antibiotic. alleles gen thru random mutation, not by anitbiotic use but use exerts selec pressure that can = increase in freq

bacteria dna

single loop of dna w only one copy of each gene so when new allele arises its immediately displayed in phenotype

when antiobitic present:

• individuals w allele for antibiotic res have massive selec adv so more likley to survive, reprod and pass genome including resistance alleles on

• those without alleles less likely to survive and reprod

• over sev gens, entire pop of bac may be resistant

staphylococcus

• have resistant strains

• due to rapid reprod rate of bacterial (gens of 20-30 mins for some species), single res bacterium can prod 10,000 mil resistant descendants in a day

can strains of bac be resistant to multiple antibiotics

yes, v difficult to treat and hard to eradicate.

hardy weinberg principle

used to predit allele freq in pop only under certain conditions in pops where:

• no natural selection happening

• no migration in or out of pop

• mating random

• pop large

• no mutations

if phenotype of trait in pop is determined by single gene w only 2 alleles then pop will consist of indi w 3 possible genotypes:

• homozygous dom (BB)

• heterozyg (Bb)

• homozygous rec (bb)

hardy weinberg equation freq of alleles

• frequencies represented as proportions of pop (no. out of 1)

• freq of alleles can be rep; this is prop of all of alleles in pop that are of particular form

  • p = dominant

  • q = recessive

  • only 2 alleles at single gene locus for a phenotypic trait in pop

• p+q = 1

hardy weinberg equation freq of genotypes

• proportion of all individuals w particular genome

• freq of homozygous dominant individuals p²

• freq of heterozygous individuals 2pq

• freq of homozygous recessive = q²

• equation: p² + q² + 2pq = 1

artificial selection

process by which humans choose organisms w desirable traits and selective breed them together to enhance the expression of these desirable traits over time and many generations also known as selective breeding e.g. increased milk yield from cattle, faster racehorses, disease-resistant crops

limits to how extreme trait can become

why do breeders accidentally enhance other traits genetically linked to desirable

• individuals are selected based on phenotypes. can negatively affect orgs health

6 principles of selective breeding

1. pop shows phenotypic variation - individuals w diff phenotypes/traits

2. breeder selects indi w desired phenotype

3. another individual w desired phenotype is selected. two selected individuals shldnt be closely related

4. two selec indi bred tg

5. offspring produced reach maturity and are then tested for desirable trait. those display desired phenotype to greatest degree selected for further breeding

6. process continues for many gens: best indi from offspring chosen for breeding until all offspring display desirable trait

selective breeding in horseracing industry. 3 phenotypes: +

• good at sprinting short dist

• good endurance over long dist

• all-rouder

how do horsebreeders breed horse for sprinting

• select fastest sprinting female and male horse

• breed two and allow offspring to grow and test sprinting speeds to find fastest

• breeder could use for racing or continue breeding w other fast sprinters

• = all fast offspring

why is most selective breeding done

to increase yield of saleable prod, not done w orgs survival or health in mind so can = unfit orgs and other traits can be accidentally enhanced e.g. disease resistance in wheat and rice varieties, hybridisation in maize, milk yield cattle

disease resistance in wheat and rice

• wheat can be badly affected by fungal diseases e.g. fusarium that causes head blight in wheat plants

• fungal diseases highly problematic for farmers as destroy wheat plant and reduce crop yield

• by using selective breeding to intorduce a fungus-resistant allele from another species of wheat, hybrid wheat plants are not susceptible to infection, and so yield increases

  • introducing allele into crop pop can take many gens and collab w researchers and plant breeders

• rice is another, e.g. bacterial blight and rice blast from magnaporthe fungus reducing yield

• scientists working to create varieties of rice plants resistant to several bac and fungal diseases

inbreeding and hybridisation in maize

• maize/corn was heavily inbred in past resulting in small and weaker plants w less vigour

• inbreeding depression which:

  • increases chance of harmfulr ecessive alleles combining in indi and being expressed in phenotype

  • increases homozygosity in indi paired alleles at loci are identical

  • leads to decreased growth and survivability

• farmer can prevent by outbreeding (breeding indi not closely related). will result in taller and heatlhier maize plants. decreases chance of harmful recessive alleles combining in indi being expressed in phenotype. increased heterozygosity and = hybrid vigour and increased growht and survivability therefore yield

• unifromity is important when growing crop as if outbreeding carried too randomly = variation so farmer needs plants to ripen and be similar hypes. this is done by buying sets of homozygous seeds from specialised companies and cross to produce f1 gen. constant for desirable traits.

improving milk yield in cattle

• female cows that have highest milk yield get crossed w male bulls related to high yield females = greater milks yields and econ bens for farmers

• doesnt take into account orgs survival as focuses on extreme characteristics

• little though given to other traits, selectively bred indi more prone to ailments e.g. mastitis (inflam of udder), milk fever, and lameness

species

group of organisms that are able to interbreed and produce fertile offspring. members of one species are repdocutively isolated rfom members of another species.

individuals of the same species have

similar behavioural, morphological (structural) and physiological (metabolic) features

gene pool

collection of genes within an interbreeding population and sum of all alleles at all of the loci within the genes of pop of a single species or a population

3 reasons gene pool changes over time:

• natural selection

• genetic drift

• founder effect

evolution

takes long but can be quicker in bacteria if short gen time

when gene pool within species pop changes sufficiently over time, the characteristics of species will also change. change so great that new species forms.

for a population to evolve into separate species it must be…

genetically and reproductively isolated from pre existing species conditions

reproductive isolation

can occur due to mutations that lead to incompatibility of gametes or sex organs, or diff in breeding behavior

when two pops are reproductively isolated, they are also…

genetically isolated so they dont exchange genes w each other in prod of offspring

speciation

changes in allele freq of isolated pop not shared so they evolvev independently of each other = 2 groups that cannot interbreed and are separate species

how can evolutionary relationships between species be shown

dna found in nucleus, mitoch and chloroplasts of cells can be sequenced

differences between nucleotide sequences/dna of diff provide what info: 2

• more similar sequence = more closely related species

• two groups of orgs w/ v similar dna will have separated into separate species more recently than 2 groups w less similarity in their dna sequences

dna sequence analysis and comparison can do what

used to create family trees/phylogenetic trees that show evolutionary relationships between species

how is dna analysis done

• dna extracted from nuclei of cells taken from org (can be from blood or skin or fossils)

• extracted dna processed, analysed, base sequence obtained

• base sequence is compared to that of other orgs determine evolutionary relationships (more similarities in dna = more closely related diff species)

when analysing dna must remember: 2

• zygote only contains mitoch of egg and none from sperm so only maternal mitochondrial dna is present in zygote

• no crossing over that occurs in mtDNA so base sequence can only change by mutation

what has allowed scientists to research origins of species, genetic drift, and migration eventsq

lack of crossing over in mtDNA.

this is how estimated when humans first lived (200,000 years ago in Africa)

• estimation of date relies on molecular clock theory which assumes constant rate of mutation over time

• greater no. diff between nucleotide sequences, longer ago common ancestor existed

• molecular clock calibrated w fossils and carbon dating

• fossil of known species is carbon-dated to estimate how long ago org lived

• mtDNA of species then used as baseline for comparison w mtDNA of other species

can only maternal mitoch dna be passed on or inherited by zygote

(yes for exma but curently research showing paternal mDNA)

evolution causes…

speciation: the formation of new species from pre-existing species over time as a result of changes to gene pools from gen to gen

2 different situations when speciation can take place:

• 2 grps within species separated by geographic barrier

• 2 grps of species are reproductively isolated but still living in same area (experiencing similar env selec pressure)

allopatric speciation

• result of geographical isolation

• most common

• species pop splits into one or more groups which become separated from each other by geographical barriers e.g. mountain, body of water, motorway

• separation = 2 pops of same species isolated so no genetic exchange occuring

• if sufficient selection pressure/genetic drift acting to change the gene pools within both populations then eventually these population will diverge and form separate species

  • changes in alleles/genes of each pop affect phenotypes present in both pops

  • 2 pops may begin to differ physiologically, behaviorally, and morphologically

example in pic

sympatric speciation

• takes place w no geographical barrier

• group of same species living in same place but in order for speciation to take place there must exist 2 pops within that group and no gene flow occurs between

• something has to split/separate population

  • ecological separation: populations are separated bc they live in diff environments within same area e.g. soil pH can differ greatly in diff areas so effects flowering and growth

  • behavioural separation: pops separated bc diff behaviors e.g. feeding, comms, social behavior

example in pic