population genetics, selection and speciation Flashcards
how does genetic variation occur
through mutations that generate new alleles and give rise to different combinations of alleles via meiosis and random fertilisation
what are the two forces that change genetic variation
genetic drift
natural selection
what is genetic drift
causes changes in allele frequency in small populations - random changes in allele frequency due to chance
what is natural selection
when alleles that enhance fitness of the individuals that carry them rise in frequency - better adapted to environment so reproduce more, increase in allele frequency, provides selective advantage,
due to mutation
what are alleles
different forms of genes an organism has to determine the characteristics of an individual
what is a gene pool
all the alleles in the population at one time
what is discontinuous variation
characteristics fall into distinct categories, there is a genetic cause, usually inherited by only a few genes e.g. human blood groups
what is continuous variation
variation spread over a range of measurements e.g. height
data shows normal distribution curve ( bell shaped ) with most individuals at the mean
due to genetics and environment
how does natural selection occur
1) variation within the population due to mutations in meiosis and random fertilisation ( crossing over etc )
2) selection pressure acting on the population
3) differential survival and reproduction, those with advantageous alleles survive and reproduce
4) offspring inherit their favourable characteristics
5) over many generations, allele frequency increases in the populations gene pool
what is stabilising selection
- occurs in all populations where the environment is stable ( no environmental change )
- selection pressure at both ends of the distribution
- favours the average
- eliminates extremes
- reduces variability
- reduces opportunity for evolutionary change
- mode stays the same
what is directional selection
- when there is environmental change that produces a new selection pressure that favours an extreme characteristic
- mean in population represents optimum for existing conditions
- conditions change and so does optimum for survival
- some organisms will possess new optimum
- over time these will predominate and the mean will shift
what is a genetic bottle neck
occurs when a population is greatly reduced in size, limiting genetic diversity
this can happen due to :
- forest fires
- floods
- human activity
- disease
- drought
what is disruptive selection
- occurs in a population where extreme phenotypes are favoured
- selection pressure against the mean
- favours the extremes
what is a definition of species
a group of individuals that can interbreed to produce fertile offspring
what is the founder effect
a few individuals from a population start a new population with a different allele frequency than the original population
describe allopatric speciation
any physical barrier ( GEOGRAPHICAL ISOLATION ) which prevents interbreeding
1) no interbreeding between 2 gene pools
2) isolated populations may be subject to different selection pressures
3) variation due to mutation - could lead to changes in genotype or phenotype
4) those with advantageous alleles survive , reproduce and pass on allele
5) increase in advantageous allele frequency
-in small pops genetic drift also causes changes
6 ) gene pools diverge but are unable to interbreed to produce fertile offspring
7) separate species
definition of allopatric speciation
new species arising from different populations / gene pools from geographically isolated populations
describe sympatric isolation
same geographical area but have REPRODUCTIVE ISOLATION
- variation due to random mutation within the population prevents individuals that carry the mutation from breeding with others who do not have the mutation e.g. different behaviour patterns
- allele frequency in gene pool changes
- cannot interbreed to produce fertile offspring
what are the types of isolating mechanisms in sympatric isolation
- temporal = breed at different times
- ecological = different habitats within the area
- behavioural = different behaviour patterns e.g. courtship
- mechanical = anatomical differences making it impossible for gametes to come together
- gametic = incompatibility between gametes that prevent hybrid from forming
- hybrid sterility = organisms interbreed but offspring are infertile
what is the definition of sympatric speciation
a formation of new species via reproductive isolation from a population that are not geographically isolated
what is allelic frequency
the number of times an allele occurs within a gene pool
what is the equation in the hardy Weinberg principle
p + q = 1.0
where p = frequency of dominant allele
and q= frequency of recessive allele
therefore
p2 + 2pq + q2 = 1.0
where p2 = frequency of homozygous dominant
q2 = homozygous recessive
2pq = heterozygous
what are the factors that would affect the hardy Weinberg equation
- no immigration or emigration
- no mutation
- no selection for or against a particular allele
- large population needed
- mating should be random
