10.3 Speciation Flashcards
What is a gene pool?
A gene pool represents the sum total of alleles for all genes present in a sexually reproducing population
What does a large gene pool indicate?
A large gene pool indicates high amounts of genetic diversity, increasing the chances of biological fitness and survival
What does a small gene pool indicate?
A small gene pool indicates low amounts of genetic diversity, reducing biological fitness and increasing chances of extinction
What can gene pools be used to determine?
Gene pools can be used to determine allele frequency – the proportion of a particular allele within a population
What is evolution?
Evolution is the cumulative change in the heritable characteristics of a population across successive generations
What reflects evolution in gene pools?
This requires that allele frequencies change within the gene pool of the population to reflect these evolving characteristics
What 5 key processes may lead to a change in allele frequency
mutation
gene flow
sexual reproduction
genetic drift
natural selection
What is mutation?
A random change in the genetic composition of an organism due to changes in the DNA base sequence
What is gene flow?
The movement of alleles into, or out of, a population as a result of immigration or emigration
What is sexual reproduction?
Sex can introduce new gene combinations and alter allele frequencies if mating is assortative
What is genetic drift?
The change in the composition of a gene pool as a result of a chance or random event
What is natural selection?
The change in the composition of a gene pool as a result of differentially selective environmental pressures
When will genetic drift be faster and more significant?
It will occur faster and be more significant in smaller populations, where chance events have a bigger impact on the gene pool
Why is genetic drift not as significant in larger populations?
Larger populations will be less affected by random events and maintain more stable allele frequencies with low genetic drift
When do allele frequencies change significantly?
Allele frequencies will change significantly when a large population is reduced to a small population
By what two mechanisms may allele frequencies change significantly?
Two mechanisms by which this population change may occur are via population bottlenecks and the founder effect
When do population bottlenecks occur?
Population bottlenecks occur when an event reduces population size by an order of magnitude (~ >50%)
What may bottlenecks be a result of?
These bottlenecks may result from natural occurrences (e.g. fires, floods, etc.) or be human induced (e.g. overhunting)
What does the surviving population exhibit in terms of genetic drift? (bottleneck)
The surviving population has less genetic variability than before and will be subject to a higher level of genetic drift
How will the gene pool vary over time? (bottleneck)
As the surviving members begin to repopulate, the newly developing gene pool will be divergent to the original
What is an example of population bottlenecks?
Example: Northern elephant seals have reduced genetic diversity compared with southern seals due to overhunting
When does the founder effect occur?
The founder effect occurs when a small group breaks away from a larger population to colonise a new territory
What is the small group more subject to? founder
As this population subset does not have the same degree of diversity as a larger population, it is subject to more genetic drift
Consequently, what will happen to the gene pool of the small group? founder
Consequently, as this new colony increases in size, its gene pool will no longer be representative of the original gene pool
How does the founder effect differ from population bottle necks?
The founder effect differs from population bottlenecks in that the original population remains largely intact
What is an example of the founder effect?
Example: Certain Amish communities have a higher incidence of polydactyly because of inter-marriage within the community
What do allele frequencies represent?
Allele frequencies represent the prevalence of a particular allele in a population, as a proportion of all the alleles for that gene
How can allele frequencies be represented?
Consequently, allele frequencies are either represented as a percentage or as a value from 0 to 1.0
What can allele frequencies reflect (2)
Changes in allele frequency can reflect either random processes (genetic drift) or differential processes (natural selection)
What will the founder effect and population bottlenecks do in terms of genetic differences?
Population bottlenecks and the founder effect will exacerbate genetic differences between geographically isolated populations
What is natural selection?
Natural selection is the change in the composition of a gene pool in response to a differentially selective environmental pressure
How does natural selection affect the allele frequency?
The frequency of one particular phenotype in relation to another will be a product of the type of selection that is occurring
What 3 types of selection are there?
stabilising selection
directional selection
disruptive selection
What phenotype is favoured in stabilising selection?
Where an intermediate phenotype is favoured at the expense of both phenotypic extremes
What phenotype is removed in stabilising selection?
This results in the removal of extreme phenotypes (phenotypic distribution becomes centrally clustered to reflect homogeneity)
When does stabilising selection operate?
Operates when environmental conditions are stable and competition is low
What is an example of stabilising selection?
An example of stabilising selection is human birth weights (too large = birthing complications ; too small = risk of infant mortality)
What phenotype is favoured in directional selection?
Where one phenotypic extreme is selected at the cost of the other phenotypic extreme
How does the phenotypic distribution shift in directional selection?
This causes the phenotypic distribution to clearly shift in one direction (towards the beneficial extreme)
What does directional selection operate with regards to?
Operates in response to gradual or sustained changes in environmental conditions
What will directional selection typically be followed by?
Directional selection will typically be followed by stabilising selection once an optimal phenotype has been normalised
What is an example of directional selection?
An example of directional selection is the development of antibiotic resistance in bacterial populations
What phenotypes are favoured in disruptive selection?
Where both phenotypic extremes are favoured at the expense of the intermediate phenotypic ranges
How does the phenotypic distribution shift for disruptive selection?
This causes the phenotypic distribution to deviate from the centre and results in a bimodal spread
When may disruptive selection occur?
This occurs when fluctuating environmental conditions (e.g. seasons) favour the presence of two different phenotypes
What may continued separation lead to in disruptive selection?
Continued separation of phenotypic variants may eventually split the population into two distinct sub-populations (speciation)
What is an example of disruptive selection?
An example of disruptive selection is the proliferation of black or white moths in regions of sharply contrasting colour extremes
When does reproductive isolation occur?
Reproductive isolation occurs when barriers prevent two populations from interbreeding – keeping their gene pools separate
What are the two main categories of reproductive isolation barriers?
prezygotic and postzygotic
What is prezygotic isolation?
Prezygotic isolation – occurs before fertilisation can occur (no offspring are produced)
What is postzygotic isolation?
Postzygotic isolation – occurs after fertilisation (offspring are either not viable or infertile)
How can prezygotic isolation barriers differ?
Prezygotic isolation barriers can be temporal, behavioural, geographic / ecological or mechanical
How can postzygotic isolation barriers differ?
whereas postzygotic isolation barriers include the inviability, infertility or breakdown of hybrid organisms
When does temporal isolation occur?
Temporal isolation occurs when two populations differ in their periods of activity or reproductive cycles
What is an example of temporal isolation?
Leopard frogs and wood frogs reach sexual maturity at different times in the spring and hence cannot interbreed
When does behavioural isolation occur?
Behavioural isolation occurs when two populations exhibit different specific courtship patterns
What is an example of behavioural isolation?
Certain populations of crickets may be morphologically identical but only respond to specific mating songs
When does geographical isolation occur?
Geographic isolation occurs when two populations occupy different habitats or separate niches within a common region
What is an example of geographic isolation?
Lions and tigers occupy different habitats and do not interbreed (usually)
What is hybrid inviability?
hybrids are produced but fail to develop to reproductive maturity
What is hybrid infertility?
hybrid fails to produce functional gametes (sterility)
What is hybrid breakdown?
F1 hybrids are fertile but F2 generation fails to develop properly
What is speciation?
Speciation is an evolutionary process that results in the formation of a new species from a pre-existing species
When does speciation occur?
It occurs when reproductive isolating mechanisms prevent two breeding organisms from producing fertile, viable offspring
What are the two basic mechanisms by which speciation can occur?
allopatric / sympatric speciation
What is allopatric speciation?
Allopatric speciation occurs when a geographical barrier physically isolates populations of an ancestral species
How do the two populations begin to separate in allopatric speciation?
The two populations begin to evolve separately as a result of cumulative mutation, genetic drift and natural selection
AT what point does speciation occur in allopatric speciation?
Eventually the two populations reach a degree of genetic divergence whereby they can no longer interbreed (speciation)
What is sympatric speciation?
Sympatric speciation is divergence of species within the same geographical location (i.e. without a physical barrier)
What may sympatric speciation result from?
Sympatric speciation may result from the reproductive isolation of two populations as a result of genetic abnormalities
What is a typical error that causes sympatric speciation?
Typically, a chromosomal error may arise which prevents successful reproduction with any organism lacking the same error
What failure may cause sympatric speciation?
Sympatric speciation is most commonly caused as the result of a meiotic failure during gamete formation
What happens when meiotic cells fail to undergo cytokinesis (sympatric)?
If meiotic cells fail to undergo cytokinesis, chromosomal number will double in the gamete (e.g. diploid instead of haploid)
What will the offspring exhibit when cells do not undergo cytokinesis? (sympatric)
This will result in offspring that have additional sets of chromosomes (polyploidy)
When will speciation occur? (sympatric and meoisis)
Speciation will result if the polyploid offspring are viable and fertile but cannot interbreed with the original parent population
What do two fertile polyploid offspring require in terms of parents? What is an exception?
Fertile polyploid offspring will typically require two polyploid parents (unless allopolyploidy occurs)
Why do fertile polyploids need parents that are also polyploid?
This is because reproduction with the original parent population results in offspring with an uneven number of chromosome sets
Give an example of why polyploid parents are needed for fertile offspring
Example: diploid gamete + haploid gamete = infertile triploid zygote (cannot halve an uneven number when forming gametes)
In what species is polyploidy more common and why?
Consequently, polyploidy is far more common in plant species as they may lack separate sexes or can reproduce asexually
WHat is self-pollination?
Self-pollination – many plant species possess both male and female reproductive parts (monoecious) and can hence self fertilise
Why is asexual reproduction useful for polyploidy?
Asexual reproduction – infertile polyploids can still reproduce asexually via vegetative propagation
Are polyploidy crops useful?
YES
Polyploid crops may be particularly desirable to farmers
What are two reasons why polyploid crops may be desirable?
Allows for the production of seedless fruits (e.g. triploid watermelons are infertile and hence do not produce seeds)
Polyploid crops will typically grow larger and demonstrate improved longevity and disease resistance (hybrid vigour)
Therefore what may farmers do to plants?
Consequently, farmers may induce polyploidy in certain plant species by treating plants with certain drugs (e.g. colchicine)
What is the genus allium composed of?
The genus Allium is comprised of monocotyledonous flowering plants and includes onions, garlic, chives and leeks
What has occurred in the genus Allium?
In many of these species polyploidy has occurred, resulting in reproductively isolated populations with distinct phenotypes
Give 3 examples of polyploidy in the genus Allium
Diploid (2n) = ~ 16 chromosomes (e.g. Allium cepa – garden onion)
Triploid (3n) = ~ 24 chromosomes (e.g. Allium carinatum – keeled garlic)
Tetraploid (4n) = ~ 32 chromosomes (e.g. Allium tuberosum – chinese chives)
In what ways may evolution occur?
Evolution occurs both within a species (microevolution) and across the species barrier (macroevolution = speciation)
Via what two methods may evolution via speciation occur?
Evolution via speciation may occur by one of two alternative models: phyletic gradualism or punctuated equilibrium
What does phyletic graduation suggest?
According to this model, speciation generally occurs uniformly, via the steady and gradual transformation of whole lineages
What type of process does phyletic gradualism suggest speciation is?
In this view, speciation is seen as a smooth and continuous process (big changes result from many cumulative small changes)
What evidence is there for phyletic gradualism?
This view is supported by the fossil record of the horse, with many intermediate forms connecting the ancestral species to the modern equivalent
What does punctuated equilibrium suggest?
According to this model, species remain stable for long periods before undergoing abrupt and rapid change (speciation)
What type of process does punctuated equilibrium suggest speciation is?
In this view, speciation is seen as a periodic process (big changes occur suddenly, followed by long periods of no change)
What evidence is there for punctuated equilibrium?
This view is supported by the general lack of transitional fossils for most species – however such absences could also be explained by the relatively rare and irregular conditions required for fossilisation
