populations + variation Flashcards
population
group of organisms of the same species occupying the same area at the same time
Allopatric speciation
Occurs when there is geographical isolation
Means they face different selection pressures
Variation due to mutations
Different alleles are advantageous
Individuals with those alleles more likely to survive and reproduce
Leads to change in allele frequency
Happens overtime till populations can no longer breed with reproductive success
Sympatric speciation
Occurs when there is no geographical isolation
Mutations cause variation
Reproductive isolation occurs as changes in alleles and phenotypes prevent breeding
Could be due to differences in mating seasons, changes to reproductive organs
Becomes separate species when can no longer interbreed and produce fertile offspring
Genetic drift
Individuals in a population show differences in their genotypes
By CHANCE the allele for one genotype is passed on more than others
So number of individuals with this allele increases
Changes in allele frequency could lead to speciation
bigger impact in smaller populations
- in larger, chance variations even out
Natural selection
Mutations create new alleles of a gene
If this gene is beneficial, organisms survived and passes it on
Individuals with this beneficial allele more likely to survive and reproduce
Increases allele frequency of beneficial allele
what is the result of natural selection?
species that are better adapted to their environment
anatomical - structural features
eg blubber
behavioural - way it acts
eg playing dead
physiological - processes within body
eg hibernation
what is a gene pool?
all the alleles of all the genes in a population at a given time
what is allele frequency?
how often an allele occurs in a population (gene pool)
what is a population?
group of organisms of the same species occupying a particular area at a particular time
sources of genetic variation
differences in base sequences between organisms of the same species, due to:
mutations
random fertilisation
meiosis
- independent segregation
- crossing over
= new alleles (can be beneficial or not)
why do individuals show variation?
due to genetic and environmental factors
eg mutation or different selection pressures
differences caused by genetic variation passed on
- inherited in DNA
environmental not
how do new species arise from existing?
when they become reproductively separated
canโt breed to create fertile offspring
stabilising selection
individuals with characteristics towards middle of range more likely to survive and reproduce
- alleles selected for
occurs when environment isnโt changing - reduces range of phenotypes
- preserves characteristics
eg human birth weights
those with mid weights more likely to survive, range smaller
directional selection
individuals with alleles for extreme characteristics more likely to survive and reproduce
- alleles at one extreme selected for
could be in response to environmental changes
- results in change in allele frequency
eg antibiotic resistance
those with resistance allele survive, passed on, increased number with resistance allele
disruptive selection
individuals with alleles for extreme phenotypes at either end of range more like to survive and reproduce
- extreme alleles selected for
- average phenotypes selected against (decrease in frequency)
when environment favours more than one phenotype
- result in change in allele frequency
- increase in frequency of extremes
what is evolution?
a change in allele frequency overtime
organisms become better adapted
occurs by
- natural selection
- genetic drift (chance)
what has created variety of species?
evolutionary change and speciation
populations divided over many years, into many different species
what is the Hardy Weinberg principle used to calculate?
predicts frequency of
alleles, genotypes and phenotypes
hardy-weinberg equations for allele frequency
p + q = 1
p = frequency of one allele
q = frequency of other
hardy-weinberg principle for genotype and phenotype frequency
p2 + 2pq + q2 = 1
p2 = homozygous dominant (PP)
2qp = heterozygous (Pp)
q2 = homozygous ressesive (pp)