(18) population and evolution Flashcards
define species
a group of similar organisms that can reproduce to give fertile offspring
define population
a group of organisms of the same species living in a particular area at a particular time (potential to interbreed)
what is a gene pool
complete range of alleles present in a population
what is allele frequency
how often an allele occurs in a population (usually given as a % of the total population)
how can the hardy weinberg equation be used to predict allele freq (if you know the freq of one allele)
p + q = 1 where p is the freq of the dominant allele and q is the freq of the recessive one
what is the equation for predicting the freq of one genotype when you know the frequencies of the others
p^2 + 2pq + q^2 = 1
p^2 = freq of homozygous dominant
2pq = freq of heterozygous genotype
q^2 = freq of homozygous recessive genotype
what are the conditions needed for the hardy weinberg principle to apply
- large population
- no immigration / emigration
- no mutations
- no natural selection
- random mating
what causes genetic variation within a species
individuals of the same species having different alleles of the same genes. this is due to mutations, independent segregation (during meiosis) or random fertilisation
what is evolution
change in allele freq over time
how does natural selection work
variation means some are more adapted to selection pressures than others. individuals with a phenotype that increases their chances of survival are more likely to reproduce so there will be higher freq of beneficial alleles in gene pool
what is stabilising selection
individuals with alleles for characteristics towards middle of range are more likely to survive and reproduce occurs in a non changing environment and reduces possible range of phenotypes
what is directional selection
individuals with alleles for a single extreme phenotype are more likely to survive and reproduce usually in response to an environmental change
what is disruptive selection
individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce. characteristics towards middle of range are lost and occurs when environment favours more than one phenotype.
define speciation
development of a new species from an existing species
when does speciation occur
when populations of the same species become reproductively isolated with or without physical separation
what is allopatric speciation
when a physical barrier e.g. flood or earthquake divides population causing some individuals to become separated from main population. (geographical isolation)
different alleles would be more advantageous in diff populations.
what is sympatric speciation
random mutations can occur within population preventing interbreeding e.g. if a polyploid organism emerges in a diploid population.
seasonal- individuals from same population develop different flowering or mating seasons
mechanical- changes in genitalia prevent mating
behavioural- group of individuals develop courtship rituals that aren’t attractive to main population
what is genetic drift
when chance rather than environmental factors dictates who survives and reproduces. eg by chance the allele for one genotype is passed on more frequently than others. change in allele freq between 2 isolated populations could lead to reproductive isolation / speciation
why does genetic drift have a greater effect in smaller populations
because chance has a greater influence but in larger populations any chance variations in allele freq tend to even out across the whole population
