Selection And Evolution Flashcards
What is variation?
*Variation= presence of different characteristics (phenotypes)
*Phenotype results from interaction of genotype and environment
Phenotypic variation= Genetic variation + Environmental variation
Vp=Vg+Ve
Two types of variation:
1. Discontinuous variation
2. Continuous variation
What is the difference in the type of distribution in discontinuous and continuous variation?
In discontinuous variation we have discontinuous distribution.
In continuous variation we have normal distribution.
What is the difference in the number of genes controlling phenotype in continuous vs discontinuous variation?
In discontinuous there is one/ few genes (monogenic).
In continuous variation there are many genes (polygenic)
What is the difference in the effect of different alleles at single gene locus in discontinuous and continuous variation?
In discontinuous variation it is large and different genes have different effects.
In continuous variation it is small and genes have an additive effect.
What is the difference in the type of data in discontinuous and continuous variation?
Discontinuous variation is qualitative
Continuous variation is quantitative
Describe the difference in the number of categories/ intermediates in discontinuous and continuous variation
In discontinuous variation there is distinct categories and no intermediate.
In continuous variation there is a range of phenotypes and many intermediates.
Describe the effect of environment on the phenotype on discontinuous and continuous variation
In discontinuous variation there is little or none.
In continuous variation the environment has effect and helps smooth the curve.
Give examples of discontinuous and continuous variation
Discontinuous variation: albinism, sickle cell anemia, haemophilia, Huntingdons disease
Continuous variation: height, mass
Describe genetic variation and its main sources.
Genetic variation is when the phenotype results from interaction of genotype and environment.
Main source of genetic variation:
1) meiosis and fertilisation
*Crossing over at prophase I
*Independent assortment at metaphase I
*Random fertilisation/mating
2(Mutations!
*Primary source of variations
*Results in new alleles
How does the environment influence phenotype.
Phenotype results from interaction of genotype and environment.
Environmental factors that can influence are:
Nutrients/diet
Water availability
Light intensity
Disease/parasites
Temperature
Chemicals/mutagens
Lifestyle and culture etc
Environment effect usually greater on polygenes
➡️ polygenes= many genes controlling one trait
➡️ phenotype affected by environment shows continuous variation
How does the environment influence phenotype?
The environment may…
1. Limit/ modify gene expression
*Size/ mass/ height
2. Trigger/ switch on gene
*Examples:
a) low temp and charge in animal colour
b) High temperature and gender in croc/ curly wing in drosophila
c) UV light and melanin production
d) wavelength of light and plant growth
3. Induce mutation which affect phenotype
Describe how the environment of low temperature can cause a change in animal colour.
*Dark pigmentation in Himalayan rabbits
➡️Controlled by both genotype and environment
At low temp:
*Allele for dark pigment expressed
*Forming dark tips at ears, paws, paws, nose and tail
➡️Coldest part of rabbit
Describe how the enviroment of high temperature can determine gender in crocs/curly wing in drosophila
Gender of crocodiles depend on temperature of eggs!
*Temperature of 32-34 degrees celcius=males
*below 32/above 34=females
In fruit flies eith the curly wing mutation….
*Temperature of 19 degrees celcius=straight wings
*Temperature of 25 degrees=curly wings
Describe how the enviroment of UV light and melanin production
After a few hours of exposure to UV radiation:
*Melanocytes produce melanin in skin
➡️Causing skin to tan/form dark spots/freckles
➡️Protecting cells from DNA damage
Describe how the enviroment of wavelength of light affects plant growth
*Red and blue light are most effective for plant growth
*Blue light=helps with seed germination
*Red light=helps flower bloom, but leaves will have stretched and elongated appearance.
What is a population?
Population=group of individuals of the same species living in the same area that can interbreed.
What is an allele?
Different forms of a gene
What is allele frequency?
The number of occurrences of an allele in a population
What is a gene pool?
All genes/alleles present in a population
What is the theory of evolution?
Theory of evolution=idea that organisms change over time
Forms new varieties and species of organisms over time from pre-existing species
Organisms are descended from a common ancestor
The more closely related the species is, the more recent the common ancestor.
Due to change in allele frequency
Natural selection is one mechanism by which evolution occurs
Genetic drift and artificial selection can change allele frequencies too.
Describe natural selection
Observation 1: overproduction
*Organisms produce many offspring
➡️organisms have great reproductive potential
➡️reproduce more than is necessary to maintain population
➡️Reproduce more than can be supported by food supply/ space
BUT population size is constant over time
Deduction 1: “struggle for existence”
*Individuals/members of the same species compete for survival (intraspecific competition)
*Many die due to environmental factors and therefore cannot reproduce
*Selection pressure occurs=environmental factor causes difference in survival between individuals with different traits
Observation 2:Variation
*Within a species their is variation in phenotype
➡️Due to genetic variation
➡️Primarily caused by mutations that introduces new alleles
Deduction 2: “survival of the fittest”
*Some individuals have advantages alleles
➡️Better adapted to survive and reproduce more (I.e. have more offspring)
➡️Pass on advantageous alleles to the next generation
Advantageous alleles are selected for
Disadvantageous alleles are selected against
Over time… Over many generations…
*This causes changes in allele frequency/gene pool
➡️Frequency of advantageous alleles increase over time
➡️Result in adaptation=structure/behaviour/physiological trait that is a result of natural selection over time, in a species of a population.
➡️Can lead to speciation=formation of a new species, if it is reproductively isolated from other populations.
What is the importance of variation?
Variation means the presence of different characteristics
Resulting in different survival rates
Leads to either reproductive success or failure
Allow some individuals within a population to adapt to the changing environment
*Enables a population to survive despite a changing environment
Low genetic diversity=susceptible to disease/enviromental changes
What are the types of natural selection!
When there is selection pressure, natural selection occurs
The favourable allele will always be selected for
Less favourable allele will be selected against
Leads to changes in allele grey
3 types of natural selection:
Stabilising selection:
Intermediate phenotypes are selected for
Extremes of the phenotype are selected against
Directional selection
One extreme characteristic selected for
The other extreme and intermediate phenotype are selected against
Disruptive selection
Both extreme traits are selected for
Intermediate traits are selected against
Give examples of natural selection
a) antibiotic resistance in bacteria
b) industrial melanism in peppered moth
c) sickle cell anaemia
Describe antibiotic resistance
*When antibiotics are no longer effective against bacteria
*Antibiotic variation are no longer effective against bacteria
E.g.
penicillin inhibits cross link formation between peptidoglycans in bacteria cell wall
Many bacteria, have penicilinase enzymes which can break down penicillin
Become resistant to penicillin
Caused by:
1. Spontaneous/random mutation in some bacteria
*Mutation cause change in protein/ production of new protein that can be targeted by antibiotics
Natural selection enables resistance genes to spread
- Antibiotic is the selection pressure
*Bacteria with allele coding for penicillinase has selective advantage
*Antibiotics only kill bacteria that are non-resistant - Resistant bacteria more likely to survive and reproduce
*More likely to pass down allele to offspring via binary fission - Overtime, resistance allele frequency increases
Describe industrial melanism
Peppered moth
Spends the day resting on tree barks/branches/trunks
Camouflage to protect itself against its predators (birds)
Before 1849 moths with a speckled appearance was most common
No. of moths with a melanin/black appearance increased in industrial areas
BUT other places speckled form still more common
➡️Natural selection caused change in allele frequencies
- Variation present
*Black moths and speckled moths - Selection pressure= predation by birds
*Industrial cities tree barks has a darker shade due to high pollutants
*Black moths have a selective adaptation as they were more camouflaged
*White moths are selected against - Black moths more likely to survive and reproduce
*More likely to pass down allele to offspring - Overtime , allele for black colour increased in frequency in industrial cities
Describe why the frequency of sickle cell anaemia is highest in areas where malaria is common
Selection pressure= SCA and malaria
3 genotypes:
- HsHs: homozygous for sickle cell alleles
*RBC cannot carry oxygen very well, may die from SCA
*SCA selects against HsHs - HNHN: Homozygous for normal Hb allele
*No SCA
*Plasmodium parasite affects RBC
*Malaria is lethal, so more likely to die from malaria
*Malaria selects against HNHN - HNHS: Heterozygous for sickle cell anaemia and malaria
*Have sickle cell trait
*Do not die from SCA
*Less likely to suffer severe effects of malaria
*Contains less plasmodium in their blood
*Has selective advantage
*More likely to survive and reproduce
*More likely to pass on HN and Hs
*Sickle cell alleles is maintained within population because of sickle cell trait individuals
What is the hardy Weinberg principle used for and what are its requirements?
Used to calculate allele, genotype and phenotype frequencies in populations.
5 requirements of the principle:
1. The population is large
2. There is random mating within a population
3. No immigration and no emigration
4. No mutations
5. No selection pressure against one phenotype
If frequencies in real life are not as expected, this m bay mean that one of the assumptions above does not apply
What are the Hardy-Weinberg equations?
Equation 1-For alleles
p+q=1
P=frequency of dominant allele (A)
q=Frequency of recessive allele (a)
1= all alleles in a gene pool
Equation 2- for genotypes
p^2 + 2pq + q^2=1
P^2= Homozygous dominant (AA)
2pq= Heterozygous (Aa)
q^2=heterozygous recessive (aa)
What processes can change allele frequency?
1.Natural selection
2. Genetic drift
*Founder effect
*Bottleneck effect
3.Artificial selection
Describe genetic drift
*Random process due to chance, unlike natural selection
→Changes in allele frequencies fluctuate due to random events
→Cannot be predicted
*Affects small populations more than large populations
→Higher chance that allele will be lost from population
*Random events usually cause small effects only
*Unless….there is:
1. Migration →Founder effect
2. Natural disaster →Bottleneck effect
Describe and explain the founder effect of genetic drift.
*Due to migration
*Few individuals move to a new region
→Become geographically isolated from the larger population
*New population is established by a small number of individuals
→Only carry a fraction of the alleles of the original population
→Gene pool may not be representative of gene pool of original population
→Lower genetic diversity than original population
*Over time, population may become genetically distinct from the original population
→May develop into separate species=speciation
Describe and explain the bottleneck effect of genetic drift.
*Large decrease in genetic diversity
→Due to large decrease in population numbers
→Common when natural disasters occur
→But can occur due to overhunting/human activities too!
*Small group of survivors
→Gene pool will not be representative of gene pool of original population
→Limited gene pool compared to previous population
p/s: Conservation effort may increase population number BUT does not increase genetic diversity
Low genetic diversity=suscpetible to disease/enviromental changes
Describe artificial selection
*AKA selective breeding
*Induviduals with desired features are selected to breed
→By humans
*Selection pressure=humans
- Humans choose parents with desirable features
- Parents with desired features are crossed
- Select offspring with desirable features
- Repeat for many generations
→Increase in allele frequency for ideal charcteristics
→Decrease in frequency of undesired characteristics
What are the disadvanatages of artificial selection?
*Artificial selection can result in inbreeding!!
→Increase in homozygosity
→Harmful recessive alleles may be expressed
→Inbreeding depression/loss of hybrid vigour
→Limited gene pool/decrease in genetic variation
*Offspring may show desired traits, but may not be well-adapted to its enviroment
Describe the differences between artificial selection and natural selection.
AF: Humans are the selection pressure
NS: Environmental selection pressure
AF: Selected feature for human benefit
NS:Selected feature of organisms benefit
AF: Not for survival/evolution
NS:Promotes survival/evolution
AF: Inbreeding common
NS: Outbreeding common
AF: Genetic diversity lowered
NS: Genetic diversity remains high
AF: Results in inbreeding depression
NS: Increases hybrid vigour
AF: Increased homozygosity/decreased heterozygosity
NS: Increased heterozygosity/decreased homozygosity
AF: No isolation mechanism operating
NS: Isolation mechanism operating
AF: Usually faster
NS:Usually slower
Give examples of artificial selection
a)Improving the milk yield od dairy cattle
b)Disease resistance in varieties of wheat and rice
c)The incorporation of mutant alleles for gibberellin synthesis into dwarf species
d)Inbreeding and hybridisation of maize
Describe the role of artificial selection in improving the milk yield of dairy cattle
Desired traits:
*Increased docility/calm temperament
*Fast growth rates (for meat production)
*High milk yield
*Fat-rich milk
*High fertility
*Disease resistance
*Breed individuals by collecting sperm from bull
→Can freeze and use later
→Artificial inseminating defrosted semen into cow during its fertile period
→Can avoid inbreeding by referring to pedigree records
Describe the role of artificial selection in disease resistance in varieties of wheat and rice
Wheat, triticum aestivum
*Resistance to fungal infections
*E.g. wheat rust
Rice, oryza sativa
*resistance to bacterial and fungal diseases
*E.g. bacteria blight, “spots”, “smuts” and rice blast
Other desired traits in crop improvement:
*High yield…so bigger ears/more grains per ear/bigger grains
*Fast grow rates
*Tolerance to high temperature
*Pest-resistance
*Gluten-rich grain for bread flour
Describe the role of artificial selection in the incorporation of mutant alleles for gibberellin synthesis into dwarf varieties
Desired trait: Shorter stem, dwarf varieties in wheat
→Greater porportion of energy put into grain instead of height=higher yield
→Less susceptible to being knocked over by weather
→less straw produced
*Giberellin (GA) stimulate stem elongation
→Incorporate mutant alleles for gibberellin synthesis into wheat by crossing shorter plants
→Mutant alleles are of the Rht (reduced height) gene
→Mutant alleles code for faulty enzymes in GA synthesis pathway
→Inhibit gibberellin synthesis
→Della protein nor broken down and continues to bind to transcription factor PIF
→Inhibit transcription of growth genes
→Dwarf variety
Describe the role of artificial selection in inbreeding and hybridisation of maize.
Maize, Zea mays
*Problem:inbreeding leads to uniformity
*Homozygous plants are less vigorous than heterozygous ones
*Inbreeding caused plants of each gen to progressively becomes smaller and weaker (inbreeding depression)
*Outbreeding produces heterozygous plants that are healthier, grow taller and have high yields
*BUT problem! there will be no uniformity-hard to harvest and sell
Solution: Inbreeding and hybridisation!
1. Companies use inbreeding to produce homozygous maize plants of desired traits for many generations
→E.g. more kernels/big kernels/ high yield etc.
- Cross-pollinate two inbred lines=hybridisation
→Produce F1 seeds - farmers buy seeds and plant them
→F1 plants are all hybrids/heterozygotes
*All have the same genotype
*Uniform!
*Has hybrid vigour
What is the molecular evidence for the theory of evolution
Molecular evidence for evolution is found in:
1. Amino acid sequences of protein
*Compare amino acid sequence of 2 species
*The more similar the amino
2. Mitochondrial DNA (mtDNA)
Describe and explain the molecular evidence of amino acid sequence of protein for the theory of evolution
*Compare amino acid sequence of 2 species
*The more similar the amino acid sequence, the more closely related the species
*More similar amino acid sequence=less time has elapsed since the most recent common ancestor.
For example:
Cytochrome c
*Electron carrier in the ETC
*Important proteins like these are highly conserved \
*Length and amino acid sequence for the protein are identical in rat and in mouse
*But human cytochrome c has 9 amino acids that are diff
→Mouse and rat share a more recent common ancestor
→Human is more distantly related
Describe and explain the molecular evidence of mitochondrial DNA (mtDNA) for the theory of evolution
*Compare nucleotide sequence in mtDNA of 2 species
*There are less differences in nucleotide sequence, the more closely related the species
*Few mutations=less time elapsed since the most common ancestor
Characteristics of mtDNA:
1. mtDNA is circular, does not undergo crossing over
2. Mutations occur at a constant rate
→Can act as a “molecular clock”
3. mtDNA mutates faster than nuclear DNA
→Changes only arise by mutation and are not repaired
→Not associated with histones
→Oxidative phosphorylation can produces reactive oxygen species that can acts as mutagens
4. Smaller, has fewer genes
→mtRNA analysis is quicker
5. mtDNA is passed from mother to offspring
→All descendants of one female have identical mtDNA
Define species
Species: Group of similar organisms with the same
*Morphological,
*Physiological
*Behavioural
*Biochemical features
*Which can interbreed to produce fertile offspring
*i.e. NOT reproductively isolated from each other
*members of one species are reproductively isolated from another species
Define speciation
*Formation of new species
*Speciation involves a reproductive barrier between populations.
Two types of speciation:
1. Allopatric speciation
*Caused by geographical isolation
2. Sympatric speciation
*Same location
Define allopatric speciation
*Speciation that occurs as a result of geographical barrier of same species
- Geographical barrier
E.g. river, mountain, sea
*2 populations of same species physically separated
*No breeding between populations= gene flow
*Barrier prevents interbreeding between populations = populations reproductively isolated - Different selection pressures act on separated populations
*Isolated population subjected to different environmental conditions
*Individuals with beneficial alleles are selected for
➡️ More likely to survive and reproduce more
➡️ Pass on beneficial alleles to offspring
➡️ Change in allele frequency/ gene pool
*Different mutations occur giving rise to new alleles
*Genetic drift occurs randomly in separate populations
- Over a long time/ many generations…
*Population have sufficient differences
*Populations are unable to interbreed to produce fertile offspring = reproductively isolated
➡️New species
Define sympatric speciation
*Same location, no physical isolation
- Different features/behaviour (or other isolating mechanisms)
➡️Cause a population to not interbreed
➡️This is a barrier to gene flow - Over time…
*Change in allele frequency due to genetic drift
➡️Cause populations to become reproductively isolated
➡️Cannot breed as they are now different species
Give an example of sympathetic speciation
E.g. Polyploidy in plants
*Polyploid plants= has more than two sets of chromosomes
➡️Result of complete non-disjunction in meiosis
➡️Chromosomes fail to segregate to opposite poles
➡️Result in gametes with extra/ missing chromosome
*Non disjunction= chromosomes fail to segregate to opposite poles
➡️Can cause an individual to have extra/missing chromosomes
➡️Cause diseases like Down syndrome, Triple X chromosome etc
Complete non-disjunction= all identical chromosomes fail to segregate to opposite poles
➡️Result in diploid gametes
➡️ If fertilised (2n+n), can result in trisomy (3n)
*In general, fusion of two mutated gametes result in
➡️New individuals with different chromosome number as parents
➡️Cannot interbreed with original parent species
➡️New species
Describe and explain extinction
*In past, mass extinction events were all natural
*E.g. due to severe changes to climate and physical conditions
*There were 5 previous mass extinction events
*Humans may cause a 6th mass extinction event!
*The international union for conservation of nature (IUCN) is the world’s largest global environment organisation
*The IUCN red list of threatened species valuates the conservation status of plants and animals species.
Some endangered species are high profile, others less photogenic or not enough publicity.
Extinction is largely due to:
*increased competition from a better adapted species
*Habitat loss
(Draining of wetlands, cutting down rainforests, pollution of air, water and soil)
*Hunting/killing/poaching for sport or food or medicine
*Climate change
Which species are more likely to be affected by extinction?
- Smaller population are less resilient than larger populations
*Populations with reduced genetic diversity face increased risk of extinction
