Variation Flashcards
Natural selection
the process by which individuals with a particular set of alleles are more likely to survive and reproduce that those with other alleles
reduces allele frequency conferring a disadvantage
define Population
group of individuals of the same species living in the same
area that can interbreed
Selection pressure
is an environmental factor that affects the chance of survival of an organism
it increases chances of some alleles being passed on to the next generation and decreases chances of others
Stabilizing selection
When natural selection keeps things as they are. A common phenotype remains
equally distributed mean
Directional selection
If a new environmental factor or a new allele appears then natural selection may cause allele frequency to change over successive generation
one extreme phenotype
Disruptive selection
Maintains different phenotypes (polymorphism) in a population
favors 2 extremes
variation
presence of different characteristics
phenotypes results from interaction of genotype and environment
Phenotypic variation = Genetic variation + Environmental variation
VP = VG + VE
continuous variation
normal distribution and many genes control the phenotype.
Genes have an additive effect of different alleles at a single gene locus
Type of data Quantitative
Range of phenotypes, many
intermediates
Environment has effect
Helps smooth the curve
Examples Height, mass
Geographical isolation
Separation by a geographical barrier (Stretch of water or a mountain range)
Reproduction isolation
The inability of 2 groups of organisms to breed with one another
ecological separation
The separation between 2 populations because they live in different environments of the same and CANNOT breed together
Behavioral separation
The separation of 2 populations because they have different behaviors which prevent them from breeding together
Allopatric speciation
The development of new species following geographical isolation
(most common)
sympatric speciation
the development of new species without any geographical separation
biological species
A group of organisms with similar morphology and physiology which can breed together to produce fertile offspring
Morphological species
A group of organisms that’s share many physical physical feature that distinguish them from other species
ecological species
A population of individuals of the same species living in the same area at the same time
discontinuous variation
Discontinuous distribution
controlling phenotype One / few genes (Monogenic) Many genes (Polygenic)
Effect of diff alleles at
single gene locus
Large
Diff genes have diff effects
Discrete categories, no
intermediates
Effect of environment
on phenotype Little or none
Examples Albinism, sickle cell anemia,
hemophilia, Huntington’s disease
main source of genetic variation
1) Meiosis and fertilization
* Crossing over @ Prophase I
* Independent assortment @ Metaphase I
* Random fertilization / mating
2) Mutations!
* Primary source of variations
* Results in new alleles
How the environment influences phenotype
Phenotype results from interaction of genotype and environment
Environmental factors that can influence phenotypes:
* 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
→ Phenotypes affected by environment often show continuous variation
Bottle neck effect
- 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
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
Migration → Founder effect
Natural disaster → Bottleneck effect
founder effect
→ 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 original
population
→ May develop into separate species = speciation
Antibiotic Resistance
When antibiotics are no longer effective against bacteria
* Antibiotic resistance can be spread from bacteria to bacteria
E.g.
* Penicillin inhibits cross-link formation btwn peptidoglycans in
bacteria’s cell wall
* Many bacteria have penicillinase enzymes can break down penicillin
* Become resistant to penicillin
Antibiotic Resistance is caused by
- Spontaneous/random mutation in some bacteria
* Mutation cause change in protein/production of new protein that cannot
be targeted by antibiotics
a) Antibiotic Resistance Chap 10 Recap
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 - Over time, resistance allele frequency increases
Industrial Melanism
(key words)
→ 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 selective advantage 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 - Over time, allele for black colour increased in frequency in industrial cities
Sickle Cell Anaemia & Malaria allele
- 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 HN HN - HNHS : Heterozygous for sickle cell allele
* 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 both HN and HS
- Sickle cell allele is maintained within population
because of sickle cell trait individuals
The Hardy-Weinberg Principle and its requirments
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 or emigration
4. No mutations
5. No selection pressure against one genotype
The Hardy-Weinberg Equation for alleles
p+q=1
p- Dominant allele
q- recessive allele
The Hardy-Weinberg Equation for genotypes
p² + 2pq + q² = 1
Examples for evolution by natural selection
a) Antibiotic resistance in bacteria
b) Industrial melanism in peppered moth
c) Sickle cell anaemia
Examples of Artificial Selection
a) Improving the milk yield of dairy cattle
b) Disease resistance in varieties of wheat and rice
c) The incorporation of mutant alleles for gibberellin synthesis into dwarf varieties
d) Inbreeding and hybridization of maize
The incorporation of mutant alleles for
gibberellin synthesis into dwarf varieties
→ Incorporate mutant alleles for gibberellin synthesis into wheat by crossing shorter plants
→ Mutant alleles are of the reduced height gene
→ Mutant alleles code for faulty enzymes in GA synthesis pathway
→ Inhibit gibberellin synthesis
→ DELLA protein not broken down and continues to bind to transcription factor PIF
→ Inhibit transcription of growth genes
→ Dwarf variety
