Genetics - 10.3 Speciation Flashcards
population definition
group of individuals of the same species living in the same area and can breed successfully
Gene populations information and definition
it is defined as: the sum total of all the genes and their alleles present in a population at any one time
each individual is a carrier of part of the total genetic complement of the population
=> by determining the allele frequency and genotypes it is possible to determine the state of the gene poll
the importance of evolutionary and allele frequency and genetic changes (with time in populations)
state of the gene pool indicates if it is STABLE or UNDERGOING change
genetic change is an important indicator of evolutionary events ==> evolution requires that allele frequencies change with time in the population
list of selection processes that can alter allele frequencies
- Mutation
- gene flow
- small population size
- natural selection
- non-random mating
Genetic change in gene pools understanding
evolution requires that allele frequencies change with time in populations
Mutation - selection processes that can alter allele frequencies in gene pools
spontaneous mutations can alter allele frequencies and create new alleles. Mutations are random changes in the DNA code that may alter structures, functions and behaviour
A random change in the genetic composition of an organism due to changes in the DNA base sequence
gene flow - selection processes that can alter allele frequencies in gene pools
genes can flow into or out of gene pools as individuals move from one gene pool to another
The movement of alleles into, or out of, a population as a result of immigration or emigration
small population size - selection processes that can alter allele frequencies in gene pools
in small populations, allele frequencies can change randomly from generation to generation
Natural selection - selection processes that can alter allele frequencies in gene pools
selection pressure may favour certain allele combinations which may increase reproductive success
The change in the composition of a gene pool as a result of differentially selective environmental pressures
non-random mating - selection processes that can alter allele frequencies in gene pools
individuals seek out particular phenotypes to make with
(Dawin called it “sexual selection”)
emigration definition
when an organism LEAVES a population
immigration definition
when an organism ENTERS a population
gene flow definition
“the flow of genes between populations” + all alleles in an interbreeding population
species formation - isolating mechanisms in regards to gene flow and divergence to form a new species
different types of isolating mechanisms may operate and different amounts of gene flow may take place as two populations diverge to form new species
A gene pool represents…
the sum total of alleles for all genes present in a sexually reproducing population
a large gene pool indicates…
high amounts of genetic diversity, increasing the chances of biological fitness and survival
A small gene pool indicates…
low amounts of genetic diversity, reducing biological fitness and increasing chances of extinction
Evolution is…
the cumulative change in the heritable characteristics of a population across successive generations
–> This requires that allele frequencies change within the gene pool of the population to reflect these evolving characteristics
Genetic drift is…
the change in the composition of a gene pool as a result of chance or random events
Things affecting a genetic drift (faster + smaller or less + larger)
It will occur FASTER and be more significant in smaller populations, where chance events have a bigger impact on the gene pool
Larger populations will be less affected by random events and maintain more stable allele frequencies with low genetic drift
Population Bottlenecks occur when an event reduces population size by an order of magnitude (~ >50%)
Things affecting them:
These bottlenecks may result from NATURAL OCCURENCES (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
Founder Effect
occurs when a small group breaks away from a larger population to colonise a new territory
Things affecting them:
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
What are allele frequencies
Allele frequencies represent the prevalence of a particular allele in a population, as a proportion of all the alleles for that gene
either represented as a percentage or as a value from 0 to 1.0
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 (too large = birthing complications ; too small = risk of infant mortality)
Stabilising Selection Operates when
environmental conditions are stable and competition is low
stabilishing selection example
An example of stabilising selection is human birth weights (too large = birthing complications ; too small = risk of infant mortality)
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
directional selection example
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
Continued separation of phenotypic variants may eventually split the population into two distinct sub-populations (speciation)
disruptive selection occours when…
This occurs when fluctuating environmental conditions (e.g. seasons) favour the presence of two different phenotypes
disruptive selection example
An example of disruptive selection is the proliferation of black or white moths in regions of sharply contrasting colour extremes
Reproductive isolation occurs when
barriers prevent two populations from interbreeding – keeping their gene pools separate
There are two main categories of reproductive isolation barriers:
Prezygotic isolation – occurs before fertilisation can occur (no offspring are produced)
Postzygotic isolation – occurs after fertilisation (offspring are either not viable or infertile)
Prezygotic isolation
occurs before fertilisation can occur (no offspring are produced)
Postzygotic isolation
occurs after fertilisation (offspring are either not viable or infertile)
Temporal Isolation - Prezygotic isolation = reproductive isolation
Temporal isolation occurs when two populations differ in their periods of activity or reproductive cycles
Example: Leopard frogs and wood frogs reach sexual maturity at different times in the spring and hence cannot interbreed
Behavioural Isolation - Prezygotic isolation = reproductive isolation
Behavioural isolation occurs when two populations exhibit different specific courtship patterns
Example: Certain populations of crickets may be morphologically identical but only respond to specific mating songs
geographic Isolation - Prezygotic isolation = reproductive isolation
Geographic isolation occurs when two populations occupy different habitats or separate niches within a common region
Example: Lions and tigers occupy different habitats and do not interbreed (usually)
Speciation is
an evolutionary process that results in the formation of a new species from a pre-existing species
It occurs when reproductive isolating mechanisms prevent two breeding organisms from producing fertile, viable offspring
There are two basic mechanisms via which speciation can occur:
- Allopatric speciation (geographical isolation)
- Sympatric speciation (reproductive isolation)
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 is most commonly caused as the result of a meiotic failure during gamete formation
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
polyploidy
offspring that have additional sets of chromosomes
causes of polyploidy
If meiotic cells fail to undergo cytokinesis, chromosomal number will double in the gamete (e.g. diploid instead of haploid)
sympatric Speciation will result if
the polyploid offspring are viable and fertile but cannot interbreed with the original parent population
polyploidy is far more common in plant species as they may lack separate sexes or can reproduce asexually
Self-pollination – many plant species possess both male and female reproductive parts (monoecious) and can hence self fertilise
Asexual reproduction – infertile polyploids can still reproduce asexually via vegetative propagation
Fertile polyploid offspring will typically require two polyploid parents (unless allopolyploidy occurs)
This is because reproduction with the original parent population results in offspring with an uneven number of chromosome sets
Example: diploid gamete + haploid gamete = infertile triploid zygote (cannot halve an uneven number when forming gametes)
Polyploid crops may be particularly desirable to farmers for a number of reasons:
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)
Punctuated Equilibrium is when
species remain stable for long periods before undergoing abrupt and rapid change (speciation)
In this view, speciation is seen as a periodic process (big changes occur suddenly, followed by long periods of no change)
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
Phyletic Gradualism is when
speciation generally occurs uniformly, via the steady and gradual transformation of whole lineages
In this view, speciation is seen as a smooth and continuous process (big changes result from many cumulative small changes)
This view is supported by the fossil record of the horse, with many intermediate forms connecting the ancestral species to the modern equivalent
Evolution via speciation may occur by one of two alternative models:
- phyletic gradualism
- punctuated equilibrium
speciation is brought about by
the development of reproductive isolating mechanisms that keep the new gene pool different from the otiginal
reproductive isolation - development
barriers may occur between gene pools which prevents members of the same species from reproducing
as time goes on more mutations accumulate and more natural selection causes the two gene pools to become more different until they can no longer reproduce successfully
Prezygotic mechanisms
- geographical isolation
Prezygotic mechanisms
- geographical isolation
- ecological islation
- behavioral isolation
- temporal isolation
- structural incompatibility
- gamete mortality
geographical isolation - Prezygotic mechanisms
physcial barriers - eg land or water that prevent mating
ecological isolation - Prezygotic mechanisms
different species may occupy different habitats within the same geographical area
behavioral isolation - Prezygotic mechanisms
species may have specific call, rituals, etc that enable them to recognise potential mates
temportal isolation - Prezygotic mechanisms
incompatable time frames that prevent reproduction
structual incompatibility - Prezygotic mechanisms
for successgul mating, species must have compatible copulator apparatuses, appearances and chemical make-up
gamete mortality - Prezygotic mechanisms
if sperm and egg fail to unite, fertilization will be unsuccessful
postzygotic mechanism
postzygotic (post-fertilization) mechanisms act after fertilization to prevent successful reproduction
hybrid inviability and sterility - postzygotic mechanisms
fertilized egg may fail to develop properly
or
hybrid of 2 species may be VIABLE but STERILE
speciation =
the process in which new species are formed by the splitting of an existing species = occours when gene flow has ceased between populations (where it previously existed)
speciation (2 forms)
- allopatric speciation
- sympatric speciation
steps in allopatric speciation
- migration
- geographical (ecological) isolation
- formation of a subspecies
- reproductive isolation
self explanatory - if full explanation required = page 42 of booklet
sympatric speciation in plants vs animals
rarer than allopatric speciation among animals but is a major cause of speciation among plant
sympatrick speciation may occour via:
- change in host preference, good preference or habitat preference
- the partitioning of an essential but limiting resource
- instant speciation as a result of polyploidy
polyploidy -
mutation that involves the multiplication of whole sets of chromosome
Sometimes in animals, often in plants
when such individuals spontaneously arise, they are instantly reproductively isolation from their parent population
allopolyploidy
polyploid individual having a chromosome set of 2+ chromosome sets derived more or less complete from DIFFERENT species
gradualism =
view that evolutionary changes occour slowly from one form to another over time
if species evolve by gradulism there should be transitional forms seen in the fossil record - many fossil records show a succession of small changes (also we do not see rapid evolution happening today so conclude that it has always happened this way
eg the environment changes slowly
punctuated equilibrium
when there appear to be no transitional formed = often due to fossil evidence had not been found yet… or that changes happened rapidly
punctuated equilibrium (definition)
has long period with no change and short periods of rapid evolution such as divergence
often rapid natural selection in response to rapid environmental change
eg volcanic eruptions, meteor impacts and ice ages
organisms that arrive to a new ecosystems may undergo rapid evolution in response to the new environment
pressures in the gradualism
constant/slow enviornment change
pressures in the punctuated equilibrium
fast enviormental change
co-evolution occurs where
each species has a reciprocal evolutionary effect/each species acts as a natural selective influence on the other
3 types of natural selection
- stabilising
- disruptive
- directional
stabilising selection
maintains the population in a stable form because it FAVOURS the AVERAGE phenotype at the expense of the extreme in either direction - acts AGAINST EXTREMES and favors intermediate phenotypes
eg birth weight
Directional selection
favours the phenotype of ONE EXTREME over the AVERAGE or the OTHER extreme - ie there is a shift of the average in the one direction
eg peppered moths
Disruptive selection
favours BOTH EXTREMES at the expense of the average
(2 peaks!)
Genetic drift …. small populations (fill in the gap!)
the random changes in the alleles frequencies of a population and is a feature of SMALL POPULATIONS
- not all individuals will be able to contribute their genes to the next generation, as a result, random changes occur in the allele frequencies in all populations
genetic drift includes
- bottleneck effect
- founder effect
The Founder effect
Occasionally, a small number of individuals become isolated from the original population
this colonizing/founder population will have a small and probably non-representative sample of alleles from the main populations’s gene pool
colonizing population may evolve in a different direction than the parent population
SMALL FOUNDER POPULATIONS ARE SUBJECT TO THE EFFECTS OF RANDOM GENETIC DRIFT
eg. islands
Population bottleneck - causes
populations may be reduced to low numbers due to:
- seasonal climatic change
- heavy predation or disease
- catastrophic events
- human influence
