7) Genetics, Populations, Evolution, Ecosystems Flashcards
Gene definition
A sequence of bases on a DNA molecule that codes for a protein which results in a characteristic
Allele definition
A different version of a gene
Genotype
The genetic constitution of an organism
Phenotype definition
The expression of the genetic constitution (the genotype) and its interaction with the environment
Locus definition
The fixed position of a gene on a chromosome
Alleles of a gene are found at the same locus on each chromosome in a pair
Diploid definition
Cells that have 2 sets of chromosomes, so have 2 alleles for each gene
Dominant definition
An allele whose characteristic appears in the phenotype even when there is only 1 copy
Recessive definition
An allele whose characteristic only appears in the phenotype if 2 copies are present
When to use a dihybrid cross:
When looking at how 2 different genes are inherited at the same time
Why do you sometimes not get the expected phenotypic ratio?
Due to:
- Sex linkage
- Autosomal linkage
- Epistasis
Codominant definition
Alleles are both expressed in the phenotype
Neither one is recessive
How to write alleles which are co-dominant
Cannot use upper and lower case letters for the alleles- this would imply that 1 was dominant to the other
Use different letters and put them as superscripts above a letter that represents the gene
Sex linked gene definition
A gene that is carried on either the X or Y chromosome
Sex linkage:
Females: XX
Males: XY
X chromosome is much longer than Y chromosome which means for most of the length of the X chromosome, there is no equivalent homologous portion of the y chromosome
Because males only have 1 X chromosome, they express the characteristic of this allele even if it’s recessive
Autosome definition
A chromosome that isn’t a sex chromosome
Autosomal linkage definition
Genes on the same chromosome are linked
How does autosomal linkage change the expected phenotypic ratio?
- Genes on same autosome- so will stay together during independent segregation of chromosomes in meiosis 1
- Alleles passed to offspring together
- They do not segregate in accordance with Mendel’s Law of Independent Assortment
- Only reason this won’t happen is if crossing over splits them up first
Epistasis definition
Arises when the allele of 1 gene affects or masks the expression of another in the phenotype
When to use the chi-squared test + what is it:
Used to see whether the results of investigations are significantly different to expected outcomes or whether deviations could be due to chance alone
Use if: have categoric or discrete data, large sample size, raw data (not %)
How to work out degrees of freedom:
Number of classes - 1
In the chi-squared test, when do you accept the null hypothesis?
If probability deviation if greater than the critical value
Means any difference is due to chance and not significant
In the chi-squared test, when do you reject the null hypothesis?
If probably deviation is less than critical value
Any difference is significant
Population definition
A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed
Gene pool definition
The complete range of alleles in a population
Allele frequency definition
How often an allele occurs in a population
What is the Hardy-Weinberg principle?
A mathematical model which predicts that allele frequencies will not change from generation to generation
The Hardy-Weinberg principle is only true under what conditions?
Has to be:
- Large population
- An isolated population- no immigration or emigration
- Random mating
- No selection- all alleles are equally likely to be passed to the next generation
- No mutations arise
What calculations are used in the Hardy-Weinberg equation?
p + q = 1
p^2 +2pq + q^2
p = frequency of dominant allele
q = frequency of recessive allele
q^2 = aa
If the Hardy-Weinberg principle does no longer apply to a population, why does this suggest?
The frequency of the allele has changed between generations
There must have been factors affecting allele frequency (immigration, emigration, mutations, natural selection)
Intraspecific variation definition
Individuals within a population of the same species show variation in phenotype due to genetic and environmental factors
What causes genetic variation?
- Primary source = mutations
- Random fertilisation of gametes during sexual reproduction + meiosis
Evolution definition
A change in allele frequencies in a population
Selection pressure definition
Anything that affects an organism’s chance of survival and reproduction
How does natural selection cause evolution?
1) Variation within a population of a species as individuals have different alleles
2) Selection pressures (eg predation, disease, competition) create a struggle for survival
3) Organisms in a species with phenotypes providing selective advantages are likely to survive
4) So more likely to reproduce + pass on their favorable alleles to the next generation
5) This means a greater proportion of the next generation inherit the beneficial alleles- more likely to survive + reproduce
6) Frequency of beneficial alleles in gene pool increases from generation to generation
3 types of natural selection:
- Stabilising
- Directional
- Disruptive
Stabilising selection:
- Individuals with alleles for characteristics towards the middle of the range are more likely to survive + reproduce
- Occurs when environment is not changing
- Reduces range of possible phenotypes
Directional selection:
- Individuals with alleles for a single extreme phenotype are more likely to survive + reproduce
- Could be a response to an environmental change
Disruptive selection
- Individuals with alleles for extreme phenotypes at either end of the range are more likely to survive + reproduce
- Opposite of stabilising selection as characteristics towards the middle of the range are loss
- Occurs when the environment favours more than 1 phenotype
Speciation definition:
The process where new species arise due to genetic differences leading to an inability of members of the population to interbreed + produce fertile offspring
2 types of speciation:
Geographical
Sympatric
Allopatric speciation process
(Geographical speciation)
1) Populations of same species become geographically separated
2) Populations experience different selection pressures + so different changes in allele frequencies occur
3) Allele frequencies also change as mutations occur independently in each population
4) Changes in allele frequency lead to differences accumulating in the gene pools of the separated populations, causing changes in phenotype frequencies
5) Eventually, 2 groups change so much- unable to breed together to produce fertile offspring- reproductively isolated
Sympatric speciation
1) Mutation occurs
2) Leads to isolating mechanism (eg temporal differences- different breeding seasons, different courtship displays
3) Different selection pressures operate
2 ways evolution can occur:
- Speciation
- Genetic Drift
Genetic drift definition
When chance dictates which individuals survive, breed and pass on their alleles
Why is genetic drift only important in small populations?
- In larger populations, any chance variations in allele frequency tend to even out across the whole population
- Evolution by genetic drift usually has a greater effect in smaller populations where chance has a greater influence
Community definition
Populations of all different species in an ecosystem at one time
Habitat definition
A physical place where an organism lives
Ecosystem definition
A community, plus all the non-living (abiotic) conditions in the area in which an organism lives
Adaptation definition
A feature that members of a species have that increases their chance of survival + reproduction
Abiotic conditions definition
Non-living features of an ecosystem
eg temperature, availability of water
Biotic conditions definition
The living features of the ecosystem (eg food)
Niche definition
The role of a species within its habitat
Every species has its own unique niche
A niche can only be occupied by 1 species
Carrying capacity definition
The size of certain populations an ecosystem can support
The population size can vary as a result of:
- The effect of abiotic factors
- Interaction between organisms (interspecific + intraspecific competition, predation)
Interspecific competition
- When organisms of different species compete with each other for the same resources
- Can result in resources available to both organisms being reduced- both populations become smaller
- If 1 species is better adapted to its surroundings than the other, the less well adapted species is likely to be out-competed
Intraspecific competition
- When organisms of the same species compete with each other for the same resources
- Population of a species increases when resources are plentiful
- Eventually resources become limiting- population declines
- Smaller population = less competition- population grows again
(Produces an alternating curve)
Describe how the population sizes of predators and prey are interlinked (predation)
1) Predators eat their prey- prey population reduced
2) Fewer prey available- predators are in greater competition with each other for the prey that is left
3) Predator population decreases
4) Fewer predator means fewer prey eaten- more prey survive + reproduce and their population increases
5) More prey available as food so predator population increases
Importance of random sampling:
Avoids bias which ensures that the data obtained is reliable
Using quadrats to estimate the size of populations:
- Used when investigating slow-moving or non-motile organisms
- Used to obtain measures of abundance (the number of individuals of a species within a given area)
1) Lay out 2 long tape measures at right angles along 2 sides of the study area
2) Obtain a series of coordinates using a random number generator
3) Place a quadrat at the intersection of each pair of coordinates and record the species within it
OR- this other method can be used when investigating a gradual change in communities of organisms (eg stages of succession)
1) Stretch tape measure across the ground in a straight line
2) A quadrat is laid down alongside the line and the species within it recorded
3) It is then moved its own length along the line
Benefit + negative of using percentage cover when estimating the size of populations
- Quick way to investigate populations- don’t have to count all individual organisms
- Less useful where organisms occur in several overlapping areas
3 factors to consider when using quadrats:
- Size of quadrat to use- depends on size of organism + how distributed in an area
- Number of quadrats used- more used, the more reliable the results will be
- Position of each quadrat within the study area- random sampling must be used to produce statistically significant results
Mark-release-recapture method to estimate the size of populations:
Used to investigate the distribution of motile organisms
1) Capture a sample of species using appropriate technique
2) Mark them in a harmless way
3) Release them back into their habitat
4) Wait a period of time, then take a second sample from the same population
5) Count how many of the second sample are marked
6) Use equation to find population size
Mark-release-recapture equation to find population size:
Total population size = number caught in 1st sample X number caught in 2nd sample / number marked in 2nd sample
Assumptions when using the mark-release recapture method to estimate the size of populations:
- Marked sample has had enough time + opportunity to mix back in with the population
- Marking has not affected the individual’s chance of survival
- There are no changes in population size due to births, deaths and migration during period of study
Succession definition
The process by which an ecosystem changes over time
Primary succession definition
Succession which occurs on land that has been newly formed or exposed
eg where a volcano has erupted- only bare rock to start off with
Process of primary succession:
1) Pioneer species colonise rocks (eg lichens grow, break down rocks, release minerals + change abiotic conditions)
2) Change in environment (eg lichens die + form soil)
3) Enable other species to colonise
4) Change in biodiversity
5) Stability increases- less hostile environment
6) Climax community
Conservation definition:
The protection and management of ecosystems in a sustainable way
Conservation of ecosystems techniques:
- Sometimes involves preventing succession in order to preserve an ecosystem in its current stage of succession
- Fishing quotas, protected areas
- Allowing animals to graze on land, managed fires
