18.1 - 18.5 Populations and evolution Flashcards
Define population
A group of organisms of the same species that occupy the same space and can interbreed
Define gene pool
All of the alleles of all genes of all the individuals in a population
Define allelic frequency
Number of times an allele occurs
Give an overview of cystic fibrosis
- Thicker than normal mucus produced in the lungs
- Recessive disease
Define the Hardy-Weinberg principle
States that the proportion of dominant and recessive alleles of a gene remains the same generation to generation.
What is the shorter Hardy-Weinberg equation?
p + q = 1
p = frequency of dominant allele
q = frequency of recessive allele
What is the longer Hardy-Weinberg equation?
p2 + 2pq + q2 = 1
p2 = frequency of homozygous dominant individuals
q2 = frequency of homozygous recessive individuals
2pq = frequency of heterozygous individuals
Causes of variation
Can be due to genetic and/or environmental factors
If it is mostly due to genetics the resulting phenotypes will fit into distinct groups i.e blood groups, inherited diseases
If the environment has an impact it will result in more continuous phenotypes i.e height, weight
Describe discontinuous variation
- Distinct categories with no overlap
- Controlled by a small number of genes
- Largely unaffected by the environment
What are the causes of genetic variation?
- Meiosis - Produces new combinations of alleles (crossing over, independent segregation)
- Mutations - Random changes to the genetic material. Most common reason for variation.
- Random fusion of gametes - Produces new combinations of alleles
Describe continuous variation
- No categories to place individuals
- Controlled by large number of genes
- Significantly affected by the environment
How might the environment impact gene expression?
- Weather (light intensity, temperature, rainfall, wind speed)
- Nutrient availability
- pH
- Food availability
Define polygenic characteristics
A characteristic which is controlled by multiple genes
More likely to be influenced by the environment
What factors does natural selection depend on?
- Organisms produce more offspring than can be supported
- Genetic variation in a population
- Selection pressures against certain phenotypes
Describe natural selection
- Within any population there will be a gene pool containing a wide variety of alleles
- Random mutation of an allele may result in a new allele of a gene
- In a certain environment this allele may give its possessor an advantage against selection pressure
- These individuals are more likely to obtain resources and survive long enough to reproduce
- Individuals that successfully reproduce will pass on their alleles
- New individuals are in turn more likely to survive and reproduce
- Over many generations allele frequency will increase
- Name type of selection
Define selection
A process in which organisms that are better adapted to the environment survive and breed while less well adapted organisms do not
Define selection pressure
Any environmental factor that alters the frequency of alleles in a population
Define stabilising selection
Eliminates the extremes of phenotype in both direction. Tends to occur when the environment remains constant over extended periods.
Define directional selection
Favours one phenotypic extreme. Tends to occur when the environment changes overtime.
Define disruptive selection
Favour the extremes of phenotypes in both directions. Tends to occur in a changing environment.
Define differential reproductive success
Those organisms best adapted to a given environment will be most likely to survive to reproductive age and have offspring of their own
Define speciation
Evolution of a new species from an existing one. Has lead to greater diversity.
Define the two types of speciation
- Allopatric - Geographically isolated populations
- Sympatric - Reproductively isolated populations
Describe speciation
- Variation in an interbreeding population. Variety of alleles due to mutations.
- Population geographically or reproductively isolated which prevents interbreeding. No gene flow between the two populations.
- Variations due to mutations in each population
- Environmental conditions faces by each population are different, different selection pressures
- Natural selection occurs in both populations. Different alleles are more beneficial for survival.
- Different changes in allele frequency in two populations
- Become genetically different from each other which causes further reproductive separation
- Formation of a new species - no longer able to interbreed to produce fertile offspring
List and describe the different mechanisms of isolation
- Geographical - Oceans, mountain ranges etc
- Ecological - Different habitats within the same area
- Temporal - Different breeding seasons
- Behavioural - Different courtship behaviours
- Mechanical - Anatomical differences that make mating impossible
- Gametic - Gametes prevented from meeting due to genetic or biochemical incompatability
- Hybrid sterility - Hybrids are often sterile because they cannot produce viable gametes
Define genetic drift
A method of changing the frequency of an allele by chance where some specific individuals reproduce more than others. This may result in loss of beneficial alleles or increase in harmful alleles as it is not linked to natural selection.
What are the conditions for the Hardy-Weinberg principle to apply?
- The population is isolated and large
- No mutations occur
- No selection pressures for any particular allele
- Mating within the population is random
