7B - Populations and Evolution Flashcards
What is the definition of ‘species’?
- group of similar organisms that can reproduce to give fertile offspring
What is a ‘population’?
- group of organisms of same species living in a particular area at a particular time
What is a ‘gene pool’?
- complete range of alleles present in a population
What is meant by ‘allele frequency’?
- how often an allele occurs in a population
- given as a percentage
What does the Hardy-Weinberg Principle predict?
- that frequencies of alleles in a population won’t change from one generation to the next
- but prediction is only true under certain conditions - large population w/no immigration, emigration, mutations or natural selection
What Hardy-Weinberg equation is used to predict allele frequency?
- p + q = 1
- p = frequency of dominant allele
- q = frequency of recessive allele
What Hardy-Weinberg equation is used to predict genotype and phenotype frequency?
- p^2 + 2pq + q^2 = 1
- p^2 = frequency of homozygous dominant genotype
- 2pq = frequency of heterozygous genotype
- q^2 = frequency of homozygous recessive genotype
What is meant by ‘variation’?
- differences that exist between individuals
What causes genetic variation?
- individuals of same species have same genes, but have different alleles
What factors are there in genetic variation?
- mutations
- meiosis (crossing over of chromatids and independent segregation of chromosomes)
- random fertilisation of gametes during sexual reproduction
What is the process of natural selection?
- predation, disease and competition create struggle for survival
- some are better adapted to selection pressures than others
- individuals w/phenotype that increases chance of survival are more likely to survive, reproduce and pass on beneficial alleles
- greater proportion of next gen inherit beneficial alleles
What is meant by stabilising selection?
- individuals w/alleles for characteristics towards middle of the range are more likely to survive and reproduce
- occurs when the environment isn’t changing, it reduces the range of possible phenotypes
What is meant by directional selection?
- individuals w/alleles for a single extreme phenotype are more likely to survive and reproduce
- could be in response to environmental change
What is meant by disruptive selection?
- individuals w/alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce
- opposite of stabilising selection as characteristics in middle of range are lost
- occurs when environment favours more than one phenotype
What is ‘speciation’?
- development of a new species from an existing species
What is reproductive isolation?
- changes in allele frequency cause changes in phenotype, which mean they can no longer interbreed to produce fertile offspring
What is geographical isolation?
- a physical barrier e.g. a flood or earthquake divides a population of species, causing some individuals to become separated from main population
What does geographical isolation lead to?
- allopatric speciation
What is sympatric speciation?
- when a population becomes reproductively isolated without any physical separation
How does allopatric speciation occur?
Geographical isolation occurs between two populations
- climate will be different on either side with different selection pressures
- means different alleles will be favourable and due to variation different alleles will survive and be passed on
- after time the two populations will become so different that they will become reproductively isolated and not be able to produce fertile offspring
What makes sympatric speciation different from allopatric speciation?
- doesn’t require geographical isolation
- instead, random mutations could occur within a population, preventing members of that population breeding with other members of the species
What are the 3 examples of reproductive isolation?
- seasonal; individuals of same pop develop diff flowering or mating seasons or more sexually active at diff times of year
- mechanical; changes in genitalia prevent successful 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 which individuals survive, breed and pass on their alleles
How does genetic drift work?
- individuals within pop show variation in genotypes
- by chance, allele for one genotype is passed on to offspring more often than others
- so no. of individuals with allele increases
- changes in allele frequency in two isolated pops could eventually lead to reproductive isolation and speciation
Why does genetic drift have greater effect on smaller populations?
- chance has greater influence
- in larger populations, any chance variations in allele frequency tend to even out across the whole population
