10.3 Gene Pools and Speciation Flashcards
allele frequency
indicates the proportion of the different alleles in a population
Mutation def.
spontaneous mutations can alter alleles frequencies and create new alleles. Mutations are random changes in the DNA code that may alter structures, functions and behaviour
Gene flow def.
genes can flow into or out of gene pools as individuals move from one gene pool to another e.g. immigration and emigration
Small population size
in small populations, allele frequencies can change randomly from generation to generation e.g. genetic drift, population bottleneck
Sexual reproduction
Sexual reproduction can introduce new gene combinations and alter allele frequencies if mating is assortative
Genetic drift
The change in the composition of a gene pool as a result of a chance or random event
Natural selection affect on gene pool
The change in the composition of a gene pool as a result of differentially selective environmental pressures
Reproductive isolating mechanisms
mechanisms that keep the new gene pool different from the original (prevents members of the same species from reproducing)
Prezygotic mechanisms
isolating mechanisms that occur before fertilisation
Geographical isolation
prezygotic
happens when physical barriers such as land or water prevent mating
Ecological isolation
prezygotic
different populations may occupy different habitats within the same geographical area
Behavioural isolation
prezygotic
occurs when two populations exhibit different specific courtship patterns
Temporal isolation
prezygotic
occurs when two populations differ in their periods of activity or reproductive cycles
Structural imcompatibility
prezygotic
occurs when physical structures prevent successful mating (e.g. incompatible copulatory apparatuses)
Postzygotic mechanisms
mechanisms that act after fertlisation
Hybrid inviability
postzygotic
hybrids are produced but fail to develop to reproductive maturity
Hybrid sterility
postzygotic
hybrids fail to produce functional gametes
Speciation
is the process by which a new species is formed by the splitting of an existing species, occurs when gene flow has ceased
two types:
allopatric and sympatric
Allopatric speciation
Allopatric speciation occurs when a geographical barrier physically isolates populations of an ancestral species
The two populations begin to evolve separately as a result of cumulative mutation, genetic drift and natural selection
Eventually the two populations reach a degree of genetic divergence whereby they can no longer interbreed (speciation)
Sympatric speciation
Sympatric speciation is divergence of species within the same geographical location (i.e. without a physical barrier)
Sympatric speciation may result from the reproductive isolation of two populations as a result of genetic abnormalities
Typically, a chromosomal error may arise which prevents successful reproduction with any organism lacking the same error
Speciation in the genus Allium
The genus Allium is comprised of monocotyledonous flowering plants and includes onions, garlic, chives and leeks
In many of these species polyploidy has occurred, resulting in reproductively isolated populations with distinct phenotypes
Gradualism
is the view that evolutionary changes occur slowly from one form to another over time
Punctuated equilibrium
has long periods with no change and short periods of rapid evolution
Stabilising selection
Where an intermediate phenotype is favoured at the expense of both phenotypic extremes
This results in the removal of extreme phenotypes (phenotypic distribution becomes centrally clustered to reflect homogeneity)
Operates when environmental conditions are stable and competition is low
An example of stabilising selection is human birth weights
Directional selection
Where one phenotypic extreme is selected at the cost of the other phenotypic extreme
This causes the phenotypic distribution to clearly shift in one direction (towards the beneficial extreme)
Operates in response to gradual or sustained changes in environmental conditions
Directional selection will typically be followed by stabilising selection once an optimal phenotype has been normalised
An example of directional selection is the development of antibiotic resistance in bacterial populations
Disruptive selection
Where both phenotypic extremes are favoured at the expense of the intermediate phenotypic ranges
This causes the phenotypic distribution to deviate from the centre and results in a bimodal spread
This occurs when fluctuating environmental conditions (e.g. seasons) favour the presence of two different phenotypes
Continued separation of phenotypic variants may eventually split the population into two distinct sub-populations (speciation)
An example of disruptive selection is the proliferation of black or white moths in regions of sharply contrasting colour extremes
The founder effect
The founder effect occurs when a small group breaks away from a larger population to colonise a new territory
As this population subset does not have the same degree of diversity as a larger population, it is subject to more genetic drift
Consequently, as this new colony increases in size, its gene pool will no longer be representative of the original gene pool
The founder effect differs from population bottlenecks in that the original population remains largely intact
Population bottleneck
Population bottlenecks occur when an event reduces population size by an order of magnitude (~ >50%)
These bottlenecks may result from natural occurrences (e.g. fires, floods, etc.) or be human induced (e.g. overhunting)
The surviving population has less genetic variability than before and will be subject to a higher level of genetic drift
As the surviving members begin to repopulate, the newly developing gene pool will be divergent to the original
