7.3 and 7.4 Flashcards
(29 cards)
Explain why individuals within a population of a species may show a wide range of variation in phenotype:
Genetic factors:
- mutation: primary source of genetic variation
- crossing over between homologous chromosomes during meiosis
- independent segregation of homologous chromosomes during meiosis
- random fertilisation of gametes during sexual reproduction
Environmental factors
What is evolution?
Change in allele frequency over time/many generations in a population
Occuring through the process of natural selection
Describe factors that may drive natural selection:
Predation, disease and competition for the means of survival
These result in differential survival and reproduction
Explain the principles of natural selection in the evolution of populations:
Mutations: random gene mutations can result in new alleles of a gene
Advantage: due to selection pressure, the new allele might benefit its possessor so organisms has a selective advantage
Reproduction: possessors are more likely to survive and have increased reproductive success
Inheritance: advantageous allele is inherited by members of the next generation (offspring)
Allele frequency: Over many generations, allele increases in frequency in the gene pool
Explain the effects of stabilising selection:
Organisms with alleles coding for average/modal variations of a trait have a selective advantage
So frequency of alleles coding for average variation of a trait increase and those coding for extreme variations of a trait decrease
So range/standard deviation is reduced
Explain the effects of directional selection:
Organisms with alleles coding for one extreme variation of a trait have a selective advantage
So frequency of alleles coding for this extreme variation of the trait increase, and those coding for the other extreme variation of the trait decrease
Explain the effects of disruptive selection:
Organisms with alleles coding for either extreme variation of a trait have a selective advantage
So frequencies of alleles coding for both extreme variations of the trait increase, and those coding for the average variation of the trait decrease
This can lead to speciation
Describe speciation:
Reproductive separation of two populations
This can result in accumulation of differences in their gene pools
New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring
Describe allopatric speciation:
Population is split due to geographical isolation
This leads to reproductive isolation, separating gene pools by preventing interbreeding/gene flow between populations
Random mutations cause genetic variation within each population
Different selection pressures/environments act on each population
So different advantageous alleles are selected for/passed on in each population
So allele frequencies within each gene pool change over many generations
Eventually different populations cannot interbreed to produce fertile offspring
Describe sympatric speciation:
Population is not geographically isolated
Mutations lead to reproductive isolation, separating gene pools by preventing interbreeding/gene flow within one population:
- gamete incompatibility
- different breeding seasons
- different courtship behaviour preventing mating
- body shape/size prevents mating
Different selection pressures act on each population
So different advantageous alleles are selected for/passed on in each population
So allele frequencies within each gene pool change over many generations
Eventually different populations cannot interbreed to produce fertile offspring
Explain genetic drift and its importance in small populations:
Genetic drift= a mechanism of evolution in which allele frequencies in a population change over generations, due to chance
Some alleles are passed onto offspring more/less often by chance
- regardless of selection pressures and whether alleles give a selective advantage
So strongest effects in small population with no interbreeding with other populations as gene pool is small and chance has a greater influence
- e.g genetic bottleneck
- e.g founder effect
This can reduce genetic diversity as some alleles have much higher frequencies whilst others are lost
What is a communtiy?
All the populations of different species living in the same place at the same time
What is an ecosystem?
A community and the non-living (abiotic) components of its environment
What is a niche?
The specific role of a species within its habitat (e.g what it eats, where and when it feeds)
Governed by its adaptation to both abiotic and biotic conditions
Explain the advantage of species occupying different niches:
Less competition for food/resources
If two species tried to occupy the same niche, one would outcompete the other
What is carrying capacity?
The maximum stable population size of a species that an ecosystem can support
List the factors that influence carrying capacity:
Abiotic factors
- e.g light intensity, temp, soil pH, humidity
Interactions between organisms:
- interspecific competition between organisms of different species
- intraspecific competition between organisms of the same species
- predation
Explain how abiotic factors may affect population size/carrying capacity:
If conditions favourable, organisms more likely to survive and reproduce, increasing carrying capacity
e.g increasing light intensity increases rate of photosynthesis, which increases the carrying capacity of a variety of plant species so increases the number and variety of habitats, niches and food sources for animals. Therefore increases carrying capacity for animal species
Explain how interspecific competition may affect population size:
Reduces (named resource) available to both species, limiting their chances of survival and reproduction
- so reduces population size of both species
If one species is better adapted, it will outcompete the other
- so population size of less well adapted species declines, potentially leading to extinction
Explain how intraspecific competition may affect population size:
As population size increases, resource availability per organism decreases, so competition increases
- so chances of survival and reproduction decrease, as does population size
As population size decreases, resource availability per organisms increases, so competition decreases
- so chances of survival and reproduction increase, as does population size
Explain the changes which occur in populations of predators and prey:
Prey population increases so predators have more food
- so more predators survive and reproduce
Predator population increases so more prey killed and eaten
- so less prey survive and reproduce
Prey population decreases so predators have less food
- so less predators survive and reproduc
Predator population decreases so less prey killed/eaten
- so more prey survive and reproduce
Describe how the size of a population of slow-moving or non-motile organisms can be estimated:
Divide area into grid
Generate a pair of coordinates using random number generator
Place a quadrat here and count number/frequency of species
Repeat a large number of times and calculate a mean per quadrat
Population size= (total area of habitat/quadrat area) x mean per quadrat
Describe how the mark release recapture method can be used to estimate the population size of motile organisms:
Capture sample of species, mark and release
Ensure marking is not harmful/doesn’t affect survival
Allow time for organisms to randomly distribute before collecting second sample
Population = (number in sample 1 x number in sample 2) / number marked in sample 2
What assumptions does the mark release recapture method make?
Sufficient time for marked individuals to mix/distribute evenly within the population
Marking not removed so marked organisms recaptured OR marking is not toxic so doesn’t affect survival chances OR marking not visible so doesn’t affect predation
Limited/no immigration/emigration
No/few births/deaths/breedings/change in pop. size
