Mr G bio 4 Gene pools, selection and speciation Flashcards
what is allele frequency?
how often an allele occurs in a population- normally given as a percentage of the whole pop
what is a gene pool?
all the alleles in a population
what conditions is Hardy weinberg true under?
-large population
-no immigration or emigration
-no mutations
-no natural selection
-random mating
what does the H-W principle predict?
the frequency of alleles will stay constant from one generation to the next providing no mutation/ selection etc
what are the H-W equations used to estimate?
the frequency of particular alleles, genotypes and phenotypes within a population
what are the H-W equations?
p + q = 1
p^2 + 2pq + q^2 = 1
what is variation and what is it caused by?
the differences between individuals caused by genetic +/or environmental factors
what is the main source of genetic variation?
mutation or crossing over of chromatids, independent assortment and random fertilisation of gametes
what is evolution?
where the frequency of an allele in a population changes over time which occurs by genetic drift or natural selection
what is natural selection?
organisms face selection pressures- affect their chance at surviving and members of same species have diff alleles so some better adapted to survive and and reproduce and pass on the favourable alleles so frequency of beneficial alleles increases
what are the 3 types of natural selection?
-stabilising
-directional
-disruptive
what is stabilising selection?
-individuals with alleles for charactersistics towards he middle of the range are more likely to survive and reproduce
-selective pressure at both ends of the distribution
-reduces the range of possible phenotypes by eliminating extremes
-occurs when environment is changing
what is directional selection?
-individuals with alleles for a single extreme phenotype are more likely to survive and reproduce
-could be in response to an environmental change
what is disruptive selection?
-individuals with alleles from extreme phenotypes more likely to survive and reproduce
-opposite to stabilising as chracteristics towards middle of range are lost
-occurs when the environment favours more than one phenotype
what is speciation?
-development of a new species from an existing species
-occurs when populations of the same species become reproductively isolated
what does reproductively isolated mean?
changes in allele frequency causes changes in phenotype, which means they can no longer interbreed to produce fertile offspring
what is allopatric speciation?
-new species is created when populations are isolated to geographical barriers
-diff selection pressures
-mutations lead to variation which leads to changes in allele frequencies
-no gene flow so populations become reproductively isolated from each other
-cannot reproduce and produce fertile offspring
how does a change in allele frequency occur during allopatric speciation?
-variation occurs due to mutations producing different alleles that will be more advantageous in diff populations so there is differential reproductive success
-genetic drift
why do changes in allele frequency lead to speciation?
differences in gene pools so changes in phenotype frequencies and eventually phenotypes will be so diff that will be reproductively isolated so new species have been formed
in what 3 ways does reproductive isolation prevent breeding?
-seasonal changes
-mechanical changes
-behavioural changes
how does seasonal/ temporal isolation prevent breeding?
individuals develop diff flowering or mating seasons or become sexually active at diff times of the year, orgnaisms dont breed as arent reproductively active as same time
how does mechanical isolation prevent breeding?
changes in the size or function of genitalia can prevent successful mating, preventing individuals from producing fertile offspring
how does behavioural isolation prevent breeding?
a group of individuals may, for example, develop diff courtship rituals that arent attractive to the rest of species
what is sympatric speciation?
-occurs when random mutation within a population prevents individuals that carry mutations from reproducing with individuals that dont carry mutation
-occurs in same population (no geographical isolation)
what is evolution by genetic drift?
instead of environmental factors affecting which individuals survive, reproduce and pass on alleles, chance dictates which alleles are passed on
what is the process of genetic drift?
-individuals within a population show variation in their genotypes
-by chance, the allele for one genotype is passed on to more offspring than the others- so no. of individuals with the allele increases
-if by chance allele passed on more often again + again then can lead to evolution
how does genetic drift lead to speciation?
can lead to differences in allele frequency between 2 isolated populations and if enough differences build up over time then could lead to reproductive isolation and speciation
does genetic drift tend to affect smaller or larger populations more? and why?
usually greater effect in smaller populations where chance has greater influence as in larger pops chance tends to even out
what impact does genetic drift have on genetic diversity?
causes it to decrease
what are the 2 types of genetic drift?
-genetic bottlenecks
-the founder effect
what is a genetic bottleneck?
-an event causes a big reduction in a population
-reduces no. of diff alleles in a gene pool and so reduces allele frequency and genetic diversity
-survivors reproduce and larger pop created from a few individuals
what is the founder effect?
few organsims from a population start a new colony and only a small no. of diff alleles in initial gene pool
-allele frequency in new pop may be very diff to original pop
-sometimes leads to higher incidence of genetic disease
