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
what does reproductively isolated mean?
changes in allele frequency cause changes in phenotype, meaning they can no longer interbreed to produce fertile offspring
what is geographical isolation?
- when a physical barrier (e.g. a flood or earthquake), divides a population of a species, causing some individuals to become separated from the main population
- there is no gene flow between the 2 populations which can lead to allopatric speciation
describe allopatric speciation
- populations that are geographically separated will experience slightly diff. conditions (e.g. there might be a diff. climate on each side of the physical barrier)
- the populations will experience diff. selection pressures, meaning diff. changes in allele frequencies can occur:
1. diff. alleles will be more advantageous in the diff. populations, so natural selection occurs (e.g. if geographical separation places one population in a colder climate than before, longer fur length will be beneficial . directional selection will then act on the alleles for fur length in this population, increasing the frequency of the allele for longer fur length)
2. allele frequencies will also change as mutations will occur independently in each population
3. genetic drift may also affect the allele frequencies in one or both populations - over time, this can lead to speciation
- the changes in allele frequency will lead to differences accumulating in the gene pools of the separated populations, causing changes in phenotype frequencies
- eventually, individuals from the diff. populations will have changed so much that they won’t be able to breed with one another to produce fertile offspring - they’ll have to become reproductively isolated
- 2 groups have to become separate species
describe sympatric speciation
- can occur when random mutations within a population prevent individuals that carry the mutation from breeding with other members of the population that don’t carry the mutation
- doesn’t involve geographical isolation
- generally, pretty rare as its difficult for a section of a population to become completely reproductively isolated from the rest of the population without being geographically isolated too
why does reproductive isolation occurs?
changes in alleles, genotypes, and phenotypes prevent individuals with these changes from successfully breeding with individuals without them
what changes are involved in order for reproductive isolation to occur?
- seasonal changes - individuals develop diff. flowering or mating seasons, or become sexually active at diff. times of the year - meaning that they can’t breed together, as they aren’t reproductively active at the same time
- mechanical changes - changes in size, shape or function of genitalia can prevent successful mating, preventing individuals from breeding
- behavioural changes - a group of individuals may (e.g. develop courtship rituals that aren’t attractive to the rest of the species, such as a change in a song for birds). this prevents individuals from breeding with each other, even if they could do so successfully
what is evolution by natural selection?
selection pressures can change the allele frequencies of a population over time
when does evolution occur?
- occurs due to genetic drift - just means that instead of environmental factors affecting which individuals survive, breed and pass on their alleles, chance dictates which alleles are passed on
- therefore, genetic drift is also called random drift
how does evolution via genetic drift work
- individuals within a population show variation in their genotypes
- by chance, the allele for one genotype is passed on to more offspring than the others - so, no. of individuals with the allele increases
- if by chance the same allele is passed on more often again and again, it can lead to evolution as the allele becomes more common in the population
what can genetic drift lead to?
- differences in allele frequency between 2 isolated populations
- if enough differences in allele frequency build up over time, this could eventually lead to reproductive isolation and speciation
describe the effect of evolution by genetic drift in small populations
has a greater effect in smaller populations where chance has a greater influence
describe the effect of evolution by genetic drift in large populations
any chance factors tend to even out across the whole populations
how are new species created?
- to start, there was one population of organisms
- populations was divided and the new populations evolved into seperate species
- the new species were then divided again and the new populations evolved into more separate species
- this process has been repeated over a long period of time to create millions of new species
