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Evolution
The change in the gene pool of a population over time.
Gene Pool
The collection of genes within a population.
Allele Frequency
The frequency of certain alleles within a population or gene pool.
What is the relationship between allele frequencies and evolutionary change?
Evolution can only occur with changes in allele frequencies.
Genetic Drift
Random changes in allele frequency due to chance
How can genetic drift change a gene pool?
It changes the allele frequency, which potentially eliminates a gene and reduces genetic variation.
Founder effect
A small group of individuals from a larger population are separated from that population and form a new gene pool.
Bottleneck effect
When a natural event causes a large portion of a population to be wiped out, leaving a small remaining population with a new gene pool.
Emigration
Leave the original area/habitat (Exit)
Immigrate
Move into a new area/habitat (Into)
Phenotypic Range
The range of phenotypes within a population
Stabilising selection
A form of natural selection where organisms with average phenotypes (not on either extreme) are selected for, compared to organisms with extreme phenotypes.
Disruptive selection
A form of natural selection where organisms of extreme phenotypes are selected for, compared to organisms with the average phenotype.
Directional selection
A form of natural selection where organisms of one extreme phenotype are selected for, compared to organisms with average phenotypes or the phenotype of the opposite extreme.
Ploidy
The number of chromosome sets (pairs) in a cell.
Polyploidy
The state of having 3 or more sets of chromosomes.
How does polyploidy occur?
Due to nondisjunction during cell division, meaning one daughter cell has more chromosomes than usual, and one daughter cell will have less chromosomes than usual.
What does polyploidy result in?
Instant speciation as the offspring becomes reproductively isolated from the original species due to different numbers of chromosome sets.
Nondisjunction
Where chromosomes fail to separate during cell division (meiosis), meaning that instead of moving towards opposite poles of the cell, a pair of chromosomes move to the same pole.
Autopolyploidy
Polyploidy involving the multiplication of identical sets of chromosomes from the same species.
Allopolyploidy
Polyploidy involving the combination of chromosomes from different species to form a (usually sterile) hybrid with chromosomes from each parent species.
Species
A group of organisms within a population that are able to interbreed to produce fertile offspring.
Why is a species hard to define?
Ring species and hybridisation
Ring species
A type of species that can interbreed with neighboring populations, but not with populations that are further away, creating a "ring" of connected populations.
Hybridisation
Where organisms produced by 2 different species can sometimes be fertile.
Speciation
The formation of a new species from an original species due to reproductive isolation.
Allopatric speciation
Speciation which is caused by a geographical barrier or geographic isolation separating members of the same species.
How does geographic isolation lead to allopatric speciation?
Species become isolated and prevented from interbreeding. Gene flow is prevented and different selection pressures favour different traits, meaning allele frequencies for certain genes change over time.
Sympatric speciation
Speciation which occurs when members of the same species occupy different niches in the same geographical location.
How does speciation occur without a geographical barrier?
Polyploidy in plants, and changes in food and habitat preferences within different niches. These factors allow different members of the same species to face different selection pressures, which over time creates barriers to successful interbreeding.
Reproductive isolating mechanism
Barriers to gene flow which prevents successful breeding between species.
Why are geographical barriers not reproductive isolating mechanisms?
They donât operate through the organisms themselves.
What is the role of reproductive isolating mechanisms in speciation?
When organisms become reproductively isolated, they are no longer able to breed with members of their original species and they become 2 separate populations.
Prezygotic RIM
Acts before fertilisation to prevent successful reproduction.
What are the 5 prezygotic RIMâs?
Ecological, temporal, behavioural, structural and gamete mortality.
Ecological RIM
Species occupy different habitats within the same geographical area.
Temporal RIM
Species have different activity patterns in relation to time, eg. diurnal, nocturnal, or seasonal breeding patterns.
Behavioural RIM
Different behaviours allow different species to recognise potential mates of the same species.
Structural RIM
Organisms need to have similar body structures, appearance and chemical make-up.
Gamete mortality RIM
For fertilisation to occur, sperm and egg need to successfully unite.
Postzygotic RIM
Acts after fertilisation to prevent successful reproduction.
What are the 3 postzygotic RIMâs?
Hybrid inviability, hybrid sterility, and hybrid breakdown.
Hybrid inviability
When the fertilised egg fails to develop properly, causing the death of the offspring, or low viability (survivability).
Hybrid sterility
When the hybrid is viable but sterile (unable to breed).
Hybrid breakdown
When the first generation is fertile, but following generations will be infertile or non-viable.
Divergent evolution
2 or more species which are formed from a common ancestor.
How does divergent evolution occur?
When organisms from the same species become reproductively isolated due to allopatric speciation. When the species becomes separated by a geographical barrier, they are exposed to different environment and selection pressures, causing them to diverge into 2 different species from the same ancestral species.
Homologous structures
Features which are similar in structure and origin, but have different functions.
Convergent evolution
2 different species form common traits, despite no common ancestor.
How does convergent evolution occur?
When different species living in the same environment are subjected to the same selection pressures, which select for similar phenotypes.
Analogous structures
Features which are similar in function, but have different structures and origin. (Bird and butterfly)
Coevolution
When evolutionary changes in one species act as a selection pressure for another, resulting in evolutionary changes of the other species.
Interspecific relationships
Relationships between members of different species.
Mutualism
When both species benefit from an interspecific relationship.
Parasitism and Predation
One species is benefited and one species is harmed by the interspecific relationship.
Competition
Both species are harmed by the interspecific relationship.
How do interspecific relationships cause coevolution to occur?
The species will evolve in response to the selection pressures inflicted by the other species.
Gradualism
Populations slowly diverge from one another by slowly accumulating adaptive characteristics in response to different selection pressures.
Punctuated equilibrium
Most of a speciesâ existance is spent in stasis (no evolutionary change), followed by short bursts of significant change.
Adaptive radiation
The rapid evolution of a number of species from a single common ancestor.
What is the rapid speciation required for adaptive radiation caused by?
A sudden availability of many niches which were previously occupied by a different species.
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