Topic 7 Populations and Evolution Flashcards
what is a species?
a group of similar organisms that can reproduce to give fertile offspring
what is a population?
group of organisms of the same species living in a particular area at a particular time - therefore, have potential to interbreed
what is a gene pool?
complete range of alleles present in a population
what is an allele frequency?
- how often an allele occurs in a population
- usually given as a % of the population
what is the Hardy-Weinberg principle?
- mathematical model that predicts the frequencies of alleles in a population won’t change from one generation to the next
what conditions must be present for the Hardy-Weinberg to be valid?
- has to be a large population where there’s no immigration, emigration, mutations or natural selection
- there needs to be random mating - all possible genotypes can breed with all others
what can the Hardy-Weinberg equations be used for?
- estimate the frequency of particular alleles, genotypes, and phenotypes within populations
- also used to test whether or not the Hardy Weinberg principle applies to particular alleles in particular populations
(if frequencies do change between generations in a large population, then there’s an influence of some kind)
what is the Hardy-Weinberg equation to show allele frequency?
p + q = 1
p = frequency of dominant allele
q = frequency of recessive allele
what is the Hardy-Weinberg equation to show genotype frequency?
p² + 2pq + q² = 1
p² = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype
q² = frequency of homozygous recessive gentoype
what is allele frequency?
total frequency of all possible alleles for characteristics in a certain population = 100% (1)
what is genotype frequency?
total frequency of all possible genotypes for one characteristic in a certain population = 1
what can the genotype frequencies be used to work out?
- phenotype frequencies
- if you know how the genotype relates to the phenotype
ALSO
- works when the 2 alleles are codominant, or if you don’t know which allele is recessive and dominant
(in this situation, you can decided which allele is p and which one is q)
what is variation?
- differences that exist between individuals
what is intraspecific variation?
- variation within a species
- means individuals in a population can show a wide range of diff. phenotypes
what can variation be caused by?
- genetic and environment factors
- although individuals of the same species have the same genes, they can have different alleles (versions of genes) - this causes the genetic variation within a species
- main source of variation is mutation (e.g. changes in DNA base sequence leads to production of diff. allele)
- genetic variation can also be introduced during meiosis (through crossing over of chromatids and the independent segregation of chromosomes
- random fertilisation of gametes during sexual reproduction
how can variation be caused by the environment?
- can be caused by differences in the environment (e.g. food, climate, lifestyle)
what kind of variation results in evolution?
genetic variation
what is evolution?
how the frequency of an allele in a population has changed over time
how can evolution occur?
- genetic drift
- natural selection
describe natural selection
- organisms face many pressures that affect their chances of surviving (e.g. predation, disease, and competition) = selection pressures
- selection pressures create a struggle for survival because members of the same species have diff. alleles there is a variation between individuals, meaning that some are better adapted to the selection pressures than others
- meaning that there are differential levels of survival and reproductive success in a population
- individuals with a phenotype that increases their chance of survival are more likely to survive, reproduce and pass on their genes (including good alleles that determine their phenotype), than individuals with diff. phenotype
- meaning, a greater proportion of the next generation inherit the beneficial alleles
- therefore, are more likely to survive, reproduce and pass on their genes
- so, frequency of beneficial alleles in the gene pool increase from one generation to the next
describe stabilising selection
- where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce
- occurs when the environment isn’t changing
- reduces range of possible phenotypes
(look at images from textbook pg. 406)
describe directional selection
- where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce
- could be in response to an environmental change
(look at images from textbook pg. 406)
describe disruptive selection
- where individuals with alleles for extreme phenotypes are more likely to survive and reproduce
- oppo. of stabilising selection because characteristics towards the middle of the range are lost
- occurs when the environment favours more than one phenotype
(look at images from textbook pg. 407)
what is speciation?
- development of a new species from an existing species
- occurs when populations of the same species become reproductively isolated