Chapter 18 - Populations and evolution Flashcards
Define species
Species – a group of organisms with similar characteristics that can reproduce to produce fertile offspring (exist as one or more populations)
Define population
Population – a group of organisms of the same species, occupying a particular space at a particular time
Define allele frequency
Allele frequency – the number of times an allele occurs within a population
Define gene pool
Gene pool – all the alleles of all the genes in a population
What is the hardy Weinberg principle used to calculate
- Allele frequency
- Genotype and phenotype frequency, in a population
What assumptions are needed for the hardy Weinberg principle to work
Assumes allele frequency will not change from generation to generation
This is because:
* No mutations arise
* No selection
* Mating is random
* Population is isolated
* Population is large
What are the two equations used in the hardy Weinberg principle
p = frequency of dominant allele in a population
q = frequency of recessive allele in a population
- p + q = 1.0 (used to calculate allele frequency)
- p2 + 2pq + q2 = 1.0 (used to calculate phenotype / genotype frequency)
p2 and q2 is squared
What are the genotype frequencies represented by in the Hardy Weinberg equations
Genotype frequencies:
p2 = homozygous dominant
2pq = heterozygous
q2 = homozygous recessive
What are the phenotype frequencies represented by in the Hardy Weinberg equations
Phenotype frequencies:
p2 + 2pq = dominant phenotype
q2 = recessive phenotype
table of frequencies from Hardy Weinberg equations
What causes variation in phenotype
- Genes
- environment
- both
example of only genetics in phenotypic variation
Example: blood group
Usually controlled by a single gene
Environment has very little influence
Creates discrete groups – discontinuous variation
Shown as bar chart
Mutations are the primary source of genetic variation in all organisms.
Meiosis and random fertilisation during sexual reproduction produce further genetic variation.
example of only environmental factors in phenotypic variation
Only scars and tattoos
example of both genes and environmental factors in phenotypic variation
Example: height, weight
Usually a combination of genetic and environmental factors
Usually controlled by many genes (polygenes)
Creates a continuum of variation – continuous variation
Shown as normal distribution curve
example natural selection answer
- Random mutations can result in new alleles of a gene
- Selection pressure = (any abiotic factor such as predation, competition or disease)
- Individuals with the advantageous allele survive, reproduce and pass on the advantageous allele to the next generation
- Over many generations, the new allele increases in frequency in the population
What is directional selection
- Phenotypes falling one side of the mean are selected for.
- Increased frequency of allele to one side of the mean.
e.g Antibiotic resistance,fur legnth
diagram for directional selection
What is stabilising selection
- Phenotypes around the mean being selected for.
- Stabilising selection occurs when environmental conditions are constant over a period of time.
- A species no longer needs the range of phenotypes to survive
diagram for stabilising selection
What is disruptive selection
- Occurs when extreme environmental conditions occur.
- Extreme phenotypes are favoured.
- They survive and reproduce.
- Over time, it may lead to two sub-populations or separate species arising (speciation) when they each favour differing conditions.
diagram for disruptive selection
define speciation
Speciation - Evolution of a new species from an existing one
Define genetic drift
Genetic drift - A change in allele frequency that takes place randomly, by chance, especially in small populations
Define reproductive isolation
Reproductive isolation - Inability to reproduce to produce fertile offspring
define allopatric speciation
Allopatric speciation - Formation of a new species from different populations in different areas
define sympatric speciation
Sympatric speciation - Formation of a new species from a population living in the same area, without geographical isolation
How does speciation generally occur
- Populations become reproductively isolated
- Mutations occur in each population
- Changes in allele frequency occur in each population by natural selection due to differing selection pressures
- Genetic differences accumulate in each gene pool
- Over time, new species arise as individuals of the separated populations are now unable to interbreed and produce fertile offspring.
- This means there is restricted gene flow
How does allopatric speciation occur
- When two populations are geographically isolated by a physical barrier (examples = rivers, mountain ranges, deserts) and there is restricted gene flow
- Variation due to mutations occurs in each population
- Different environmental conditions/selection pressures for each population
- Selection for advantageous alleles leads to differential reproductive success in each population
- Over time, a change in allele frequency occurs in each population
- Leads to reproductive isolation and formation of two species (separate populations cannot interbreed to produce fertile offspring)
How does sympatric speciation occur
- Occurs in the same area (not geographically isolated)
- Mutations lead to reproductive isolation and therefore restricted gene flow between gene pools
- Different mutations occur and selection pressures operate
- Differential reproductive success leads to a change in allele frequency occurs
- Eventually they cannot breed to produce fertile offspring
Can be an example of disruptive selection
What are the 5 types of isolating mechanism
- temporal − different breeding seasons, feeding times
- ecological – different niches, habitats, feeding areas
- behavioural − different courtship displays
- mechanical − mismatch of reproductive parts
- gamete incompatibility − genetic/biochemical incompatibility e.g. sperm killed in female’s reproductive tract
What is genetic drift
- Genetic drift - a change in allele frequency that takes place randomly
- Magnified in smaller populations
- Sometimes if a random event occurs, alleles may be completely lost from a population’s gene pool
