Genetics populations evolution and ecosystems Flashcards
Why do we use the chi squared test
Used to compare the ‘goodness of fit’ of observed phenotypic ratios with expected ratios
In other words, it is used to determine if there is a statistical difference between the expected and observed ratios from the results of a genetic cross
Define the term multiple alleles
- This where there are more than two alleles of a particular gene. However, only two alleles can be present in a genotype
- E.g. There are 3 different alleles for the blood group gene - I^A , I^B and I^O
Define the term ‘sex linked ‘ genes
Where the gene is carried on only one type of sex chromosome usually the X chromosome
Define the term phenotype
The expression of this genetic constitution and its interaction with the environment
Describe what conditions the hard-weinberg principle depends on
- Population is large and isolated
- Mating within the population is random
- No mutations occur
- There is no selection i.e. all alleles are likely to be passed to next generation
Describe what hardy-weinberg principle is
The Hardy-Weinberg principle provides a mathematical model, which predicts thatallele frequencies in a population will not change from generation to generation
Describe what is meant by allele frequency
Number of times an allele of a particular gene occurs within the gene pool
Describe what is meant by gene pool
All the alleles of all the genes of all the individuals in a population at any one time
Describe what is meant by population
A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed
Describe genetic drift
- By chance, the allele of a particular gene is passed on to the offspring more often than other alleles of the same gene.
- The frequency of this allele increases in successive generations.
- Genetic drift has a much greater effect in small populations where there is likely to be a smaller variety of alleles in the gene pool, i.e. genetic diversity is less
- This can lead to speciation occurring more rapidly than in a large population where the frequency of the mutant allele is likely to be much less and thus has less of an effect
Describe sympatric speciation
- Random mutations within a population could lead to a group of organisms not reproducing with organisms of the same species in the same habitat e.g. due to different flowering seasons or mating seasons
- Although living in the same habitat, the populations are reproductively isolated with no gene flow between the gene pools of the populations
- Allelic frequencies will also change as mutations may occur independently in each population.
- Over a long period of time the allelic frequencies in the separated gene pools become so different that different species develop i.e. organisms from the different gene pools cannot interbreed to produce fertile offspring
Describe allopatric speciation
- Population with variation is already present due to mutations and meiosis
- Population is split into two or more separate groups by geographical isolation - this may be result of any physical barrier which separates a population and prevents breeding between the two populations so they are reproductively isolated
- The populations are isolated with no gene flow between the gene pools of the separated populations
- Different selection pressures will operate in the different environments resulting in selection for different phenotypes
- Organisms possessing the selected phenotypes survive and breed, passing on their advantageous alleles to successive generations
- The frequency of the different selected phenotypes and thus alleles in each isolated population increases
Describe stabilising selection
Stabilising selection - this occurs when the environment is not changing i.e. is stable, natural selection will favour those organisms with alleles for characteristics towards the middle of the range e.g. fur length
Describe directional selection
Directional selection - This occurs when the environment is changing. The change in the environment selects for those organisms with alleles for a phenotype towards the extreme of a range e.g. a decrease in temperature may select for individuals of a species of mammal that possesses longer fur than the mean fur length
Describe disruptive selection
Disruptive selection - Involves selection of the phenotypes at the two extremes at the expense of the intermediate phenotypes e.g. , the environment may change to produce much lower winter temperatures and much higher summer temperatures. A lower temperature during the winter would select for animals in the population with a longer fur length, and a higher temperature during the summer would select for animals with a shorter fur length
Give 3 examples of selective pressures present in an environment
Predation
Disease
Competition e.g. for food and/or shelter
Describe the sources of generic variation
- Gene mutations - changes in the base sequence of DNA often results in the production of different polypeptides/enzymes
- Crossing over - during meiosis, the exchange of alleles of the same genes (between the chromosomes of a homologous pair) results in new combinations of alleles
- Independent segregation of chromosomes - during the first meiotic division separation of the two members of a homologous pair of chromosomes occurs independently of the separation of other homologous pairs
- Random fertilisation of gametes occurs during sexual reproduction
Describe variation in populations
- Individuals within a population of a species may show a wide range of variation in phenotype
- Similarities and differences between individuals within a species may be the result of genetic factors, differences in environmental factors or a combination of both
Describe primary succesion
- The gradual change in plant and animal communities from colonisation by pioneer species to the climax community
- Pioneer species are the first organisms to colonise an area
At each stage in succession, certain species change the abiotic environment in some way (e.g. by causing the development of soil) - These changes to the habitat make it more suitable for new species with different adaptations to colonise the habitat
- The new species may change the environment in such a way that it becomes less suitable for the previous species - they are often better competitors and so the earlier species may become reduced in abundance
- The changes in the abiotic environment result in a less hostile environment and changing species diversity (usually an increase in diversity)
- Succession usually proceeds to form a climax community (the final community). The stability and diversity of the ecosystem increases as food webs become more complex.
Describe why a population size may vary
- As a result of the effect of abiotic factors e.g. temperature, light intensity etc
- As a result of interactions between organisms, interspecific and intraspecific competition and predation
Describe the mark-release-recapture method
- Capture a representative sample of organisms from the population, record the number captured and mark the individual organisms.
- The marked organisms are released back into habitat and left for a suitable period of time to enable the organisms to mix randomly within the population
- After a suitable period a second sample of organisms is captured and the number of marked individuals is counted.
Describe how you could use the belt transect technique
Tape is run along the ground
At regular intervals, a frame quadrat is laid down along the tape
The percentage cover or the frequency of the species can be recorded
Describe why we use a large number of quadrats
So that the results are representative of the population
Describe the importance of random sampling
Ensures there is no bias in the collection of results
Describe what is meant by an ecosystem’s ‘carrying capacity’
The size of population of a species an ecosystem can support
Describe what is meant by a species’ niche
- An organisms abiotic requirements in the habitat and its position in the food web.
- It is a description of how the organism fits into the ecosystem.
Describe what is meant by a ecosystem
Formed by a community and the non-living components of its environment
Describe what is meant by a community
Populations of different species
