7 Populations Flashcards
gene pool
all the alleles of all genes of all the individuals in a population at a given time
allelic frequency
the number of times an allele occurs within the gene pool
population
a group of organisms of the same species that occupies a particular space at a particular time and that can potentially interbreed
the hardy-weinberg principle
provides a mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population
makes the assumption that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next this can be the case provided that five conditions are met
species
exist as one or more populations
a group of individuals that have a common ancestry and so share the same genes by different alleles and are capable of breeding with one another to produce fertile offspring
hady-weinberg principle conditions
- no mutations arise
- the population is isolated, that is, there is no flow of alleles into or out of the population
- there is no selection, that is, all alleles are equally likely to be passed to the next generation
- the population is large
- mating within the population is random
hardy-weinberg equations
p + q = 1.0
probability of dominant allele= p
probability of recessive allele= q
p^2 + 2pq + q^2 = 1.0
what do genetic variations arise as a result of?
mutations
meiosis
random fertilisation of gametes
mutations
these sudden changes to genes and chromosomes may, or may not, be passed on to the next generation
mutations are a main source of variation
meisosis
this special form of nuclear division produces new combinations of alleles and the offspring are therefore different from parents
which gamete fuses with which at fertilisation is a random process further adding to the variety of offspring two parents can produce
variation due to environmental influences
environment exerts influence on all organisms
these influences affect the way the organism’s genes are expressed
An example is two plants that possess the same alleles for the flower colour. However an environmental factor of one plant growing in a soil that lacks a certain mineral may mean the pigment in one will not develop so will have a slightly different flower colour. If a large enough sample is taken out of this trait then a normal distribution will be shown
selection pressures
environmental factors hat limit the population of a species
predation, disease and competition
determine the frequency of all alleles within the gene pool
process of evolution by means of natural selection depends upon what factors
- organisms produce more offspring than can be supported by the available supply of food, light, space, etc
- there is genetic variety within the populations of all species
- a variety of phenotypes that selection operates against
role of over-production of offspring in natural selection
where there are too many offspring for the available resources, there is competition amongst individuals (intraspecific competition) for the limited resources available
those individuals in a population best suited to prevailing conditions will be more likely to survive
these individuals will be more likely to breed and so pass on their more favourable allele combinations to the next generation, which will therefore have a diff allele freq from the previous one
role of variation in natural selection
the larger a population is, and the more genetically varied the individuals within it, the greater the chance that one or more individuals will have the combination of alleles that lead to a phenotype which is advantageous in the struggle for survival
these individuals are therefore more likely to breed and pass their allele combinations on
variations therefore provides the potential for a population to evolve and adapt to new circumstances
types of selection
stabilising
directional
disruptive
stabilising selection
preserves the average phenotype of a population by favouring average individuals
selection against extreme phenotypes
occurs where the environmental conditions are constant over long periods of time
directional selection
changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population
selection for one extreme phenotype
occurs when the environmental conditions change and the phenotypes best suited to the new conditions are more likely to survive.
disruptive selection
favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population
occurs when an environmental factor such as temp takes two or more distinct forms
speciation
the evolution of new species from existing ones
how are new species formed
reproductive separation followed by genetic change due to natural selection
within a species thee are one or more populations
although individuals tend to breed only with others in the same population, they are nevertheless capable of breeding with individuals in other populations
if a population becomes separated in some way from other populations and undergoes different mutations- it will become genetically different from the other populations,
each of the populations will experience diff selection pressures as the environment of each will be slightly diff
natural selection will lead to changes in the allelic frequencies of each population
diff phenotypes each combination of alleles produces will be subject to selection pressure that will lead to each population becoming adapted to its local environment. adaptive radiation. results in changes to the allele freq of each pop
if pops were no longer physically separated, they would be unable to interbreed successfully
each pop would now be a diff species each with it sown gene pool
genetic drift
something that can take place in small populations.
this is because the relatively few members of a small population possess a smaller variety of alleles than the members of a large population.
their genetic diversity is less
as these few individuals breed, the genetic diversity of the population is restricted to those few alleles in the original population.
as there are only a small number of diff alleles, there is not an equal chance of each being passed on.
those that are passed on will quickly affect the whole population as their freq is high.
any mutation to one of these alleles that is selectively favoured will also more quickly affect the whole population because its freq will be high.
the effects of genetic drift will be greater and the pop will change relatively rapidly, making it more likely to develop into a separate species.
in large pops the effect of a mutant allele will be diluted as its freq is far less in the larger gene pool.
the effects of genetic drift are likely to be less, and development into a new species is likely to be slower
two forms of speciation
allopatric
sympatric
allopatric speciation
allopatric means diff countries and describes the form of speciation where two populations become geographically separated
geographical separation may br the result of any physical barrier between two pops which prevents them from interbreeding
these barriers include oceans, rivers, mountain ranges and deserts
if environmental conditions either side of the barrier vary, then natural selection will influence the two populations differently and each will evolve leading to adaptations to their local conditions
these changes may take many hundreds or thousands of generations, but ultimately lead to reproductive separation and the formation of separate species
