Variation and Natural Selection
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What is meant by variation when looking at members of a population?
- Variation are differences between organisms
- We lok for phenotypic variation that is of interest in a population
List and briefly describe the 4 main groups of variation
- structural variations are the variations that are seen physically
- physiological variations are differences in how an individual's body functions
- biochemical variations are the differering abilities to produced biochemical molecules (enzymes)
- Behavioural variations are the different behavious (right handed, left handed, mating calls)
What are the two major causes of variation?
- Genetic influences, there is a range of alleles for most genes, alleles inherited give a certain genetic potential (eg - having the pigments for tan skin)
- Environmental influences can affect genetic inheritance (eg - living in a sunny environment for your skin to tan)
- differences in genes are the basis of variation, but their expression is altered by changing internal and external environmental influences
What is the difference between genotypic variation and phenotypic variation?
- genetoypic variation is variation in the types of alleles inherited.
- phenotypic variation is variation in the traits displayed by an individual. Internal functioning and biochemistry are also aspects of phenotype
Do individuals with the same genotype have the same phenotype?
No, individuals with the same genotype do not always have the same phenotype as the environment can affect the expression of the genes inherited
Do individuals with the same phenotype have the same genotype?
No, this is because individuals who show a dominant phenotype can be heterozygous or homozygous for the alleles responsible
Explain monogenic and discontinuous variation
- Characteristics determined by one gene are monogenic
- As the phenotypes fall into discrete groups, the variation is described as being discontinuous
- eg - ABO blood types
Explain polygenic and continuous variation
- Characteristics determined by a number of genes, each at different locus of different chromosomes
- As the phenotypes do not fall into discrete groups, it is said to be continuous
- Can be represented by a bell curve
- Eg - weight of the population
List the sources of variation (6)
- Gene mutation - introduces new alleles
- Chromosomal mutations - change in size and number
- Crossing over and recombination - shuffles the paternal and maternal alleles within a homologous pair of chromosomes
- Independant assortment of chromosomes - allows different combinations of alleles to be formed in the resulting gametes
- Mate selection - different mates provide different alleles
- gamete selection - many are produced but few are selected for fertilization
Which is a more important source of variation - Mutation or recombination?
- Mutation is a more important source because it gives rise to new alleles
- Recombination of alleles is also important because it shuffles the alleles into new combinations
- ALthough there is no additional variation, the new combination of alleles could lead to a better set of characterisitic in offspring
How are evolution and natural selection related?
- Evolution occurs as the result of natural selection
- evolution refers to the changes in species over time
- natural selection is the process that aids the change
What must exist in a population for natural selection to occur?
- phenotypic variation must exist in the popution
- if all individuals have the same phenotype, selection for the better suited phenotype wouldn't occur
Is selection at a phenotypic or genotypic level?
- Natural selection occurs at the phenotypic levelindividuals with the better suited phenotype will have a selective advantage in their environment
- less well suited individuals tend not to survive and reproduce
How does natural selection affect allele frequency?
As phenotype is largely determined by genotype, alleles that code for the better suited phenotypes will become more common
Does the biological fitness of an individual vary? If so, what determines as individual's fitness?
- Determined by the varying environemnts
- eg - Polar bear is biologically fitter in the Artic circle, not in the Simpson desert
Which individuals are more likely to survive in a particular environment?
Individuals who have the phenotypes that better suit the environment
Which individuals are more likely to reproduce in a particular envirnoemtn?
- Individuals that are well adapted to their environment
- poorly suited individuals are less likely to survive and reproduce
- If they do, they would produce less offspring
How long must an individual live in order to pass on their alleles (genetic information) to the next generation?
Live long enough to reproduce
Gene pool and allele frequency
- The gene flow of a population is the total availble alleles present in the population.
- over a numebr of generations, the alleles that give individuals an advantage will increase in frequency in the gene pool
Difference between natural and artificial selection
- the selecting agent
- natural: environemnt
- artificial: humans
- both result in evolutionary change
What is the most important test to distinguish different species?
Test to see whether individuals can mate and produce viable and fertile offsping
Features of static gene pools
- random mating
- equal fitness of allelles
- no natural selection
- no gene flow
- large population size
Define allele frequency
- The relative number of an allele in a population
- doesn't depend on whether its expression in phenotype is dominant or recessive
What causes shifts in allele frequencies?
- recombinatino and mating by change
- different slections of phenotypes in a population
Movement out of the population
movement into the population
Define genetic drift
The change in the frequencies of alleles in a population due to change
- Trait controllled by a polygene
- polygene are a group of genes at more than one locus, that cumulatively affect the overall expression of one phenotype, causing a continuous variation in that charateristic
Factors provided by the environment that act to favour one phenotype over another
List the kinds of selecting agent/pressures
- may be an aspect of the physical environment - climate, chemical composition, moisture levels
- may be the influence of another species - predation or parasitism
- may be population pressure and the competition from members of the same species
- a process occuring in a particular environment that selects individuals with better suited phenotypes to survive and reproduce and those without it to decline in number
- can be articial or natural
- The selection of organisms with desirable phnotypes for slective bredding to produce individuals with speficic charaterists
- when humans increase the frequency of alleles that code for the desirable traits over generations
- Record books that show genetic relationships betwween individuals considere as potential breeding stock
- used in commercial artificial selection and breeding programs for endangered species
The movement of alleles in between populations due to migration of individuals
A group of organisms of the same species living at a point of time in a particular area
A group of organisms that can mate and reproduce viable and fertile offspring
- The process that leads to a new species
- allopatric speciation
- variation exist within a population
- Gergraphical barrier seperates the initial population so there is no gene flowDifferent selection pressures = different natural selection which selecs for different alleles
- OR genetic drift in small populations creates diffeernt combinatinos of alleles in the 2 populations
- over generations the populations become so different
- OR a mutation arises in one population which means that polutaions can no longer mate to produce viable fertile offspring
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