Speciation and Selection Flashcards
Hardy Weinberg assumptions
No mutations No natural selection No migration (gene flow in/out)
Large population (genetic drift has little affect) Random mating
What causes variation within a population
Genetics
Environmental factors
Sources of genetic variation
Random mutations
Meiosis
Random fertilisation of gametes
Sexually reproducing organisms increase variation by all 3
When do species become extinct
Rapid change in environment where previous advantageous allele is now selected against
Mutations are not quick enough for new characteristics to be made
Population dies
More likely if variation is low/small population
What 3 things can a mutation cause
A selective advantage
Neutral
A selective disadvantage
Monogenic
Variation caused predominantly by genetic factors are discontinuous
So produce categoric data like bar chart
Usually caused by a single gene (monogenic)
Blood groups in humans (4 distinct groups linked to a particular allele)
What is a mutation
A sudden, random change in the base sequence of DNA
Introduces new alleles to a population
How does environment affect variation
Influences the ways an organisms genes are expressed in the phenotype
Genotype sets limits for characteristics but environment determines where within these limits an organism lives
Examples of environmental influences
Climate conditions (temperature, light intensity, humidity) Abiotic/biotic factors
Polygenic
Characteristics controlled by more than one gene
Often form continuous variation
Environmental factors play a big role in where the organism will lie
Normal distribution curve
Height and Mass
What is a normal distribution curve
A bell shaped curve that measures a phenotype against its frequency in a large population
What is selection
A process in which organisms that are better adapted to their environment survive and reproduce in greater numbers
Resulting in an increase of the frequency of the advantageous allele within the population
Over time
Natural selection
Variation due to a mutation
Selection pressure causes advantageous alleles to be selected for
Those with them survive and reproduce in greater numbers
To produce offspring with same alleles
Those without more likely to die
Over time frequency of advantageous allele increases in population
Stabilising selection
Occurs in population where environment is stable
Selection pressure occurs at both ends of distribution (
Favouring the average
Usually eliminating extremes
Reduces variation and standard deviation
Reduces opportunity for evolutionary changes
Example of stabilising selection
Birth mass
Very heavy and very light babies have a high neonatal mortality rate than those with medium mass
Over time selection operates to reduce number of very heavy and very light babies
Directional selection
Mean in population represents optimum phenotype for existing conditions
Environmental change may produce a selection pressure favouring an extreme phenotype due to advantageous allele
Some organisms possess new optimum phenotype and so survive and reproduce in greater numbers
Over time, this allele becomes predominant
Mean phenotype will shift
Important to note about selection and mutations
Organisms do not develop a phenotype because of a change in the environment
Phenotype will usually be there before the selection pressure and change in environment due to a random mutation
Example of directional selection
Thicker fur in mice is an advantage in a cold climate
Over time selection operates against disadvantageous allele for thin fur
Mean shifts towards the favoured extreme
Frequency of alleles causing long hair increase
Least common but most important type of selection in evolution
Disruptive selection
Disruptive selection
Environment has 2 selection pressures that favour the 2 extreme phenotypes
Optimum phenotype for existing environment selected against
Some organisms will possess the extreme new optimum phenotype and so survive and reproduce
Over time selection means that the two extremes will dominate and the mean will shift in both directions
Example of disruptive selection
Mice fur at a beach with light-coloured sand tall patches of grass
Light-coloured agouti mice blend in with the sand so favoured
Dark-coloured mice can hide in the grass so favoured
Medium-coloured mice would not blend in with either and would be more likely to be eaten by predators
How do 2 populations become genetically distinct
Must undergo a period of reproductive isolation (prevented from interbreeding)
Via an accumulation of mutations and different selection pressures
What does natural selection lead to
Change in frequency of alleles in a population
A mechanism by which new populations of a species can arise and even evolve into 2 distinct species
Genetic drift
Random increase/decrease in allelic frequency
Important in small populations where random loss of individuals with certain alleles causes significant change in frequency of alleles in population and future generations
Not a selection pressure but a random event
Gene flow
Change in allele frequency due to the migration of fertile individuals
Founder effect or genetic bottle neck
Founder effect
Example of gene flow
Migration can lead to new isolated populations with mixed or same alleles due to chance
Always draw a diagram
Explain how geographical isolation can lead to new species
A physical barrier (river, road, mountain) prevents population meeting therefore reproductive isolation and can’t interbreed
Isolated populations subject to different selection pressures
Variation in both caused by random mutations
Different alleles selected for/against in each so organisms with advantageous alleles survive and reproduce
Leads to increase in advantageous allele frequency
Over a long time, gene pools become so different that they can no longer breed together to produce fertile offspring
Allopatric speciation
Speciation associated with geographical isolation
Selection occurs on different sides of a geographical barrier that stops them interbreeding
Organisms change via such an extent that they become a different species
Can’t produce fertile offspring when bred.
Sympatric speciation
Same country and so organisms in the same area become reproductively separated
Through choice of food or season to mate or hibernation etc
Example of allopatric speciation
Darwin’s finches
Example of sympatric speciation
Apple maggot fly
Flies mate exclusively on or near fruit of their host
Difference in host preference can result in virtually complete premating isolation among species
Flies feeding on Hawthorn do not mix with flies that feed on the apples
Isolated genetically and can develop through selection to become different species
Types of isolation mechanisms
Temporal Ecological Behavioural Gametic Hybrid Mechanical
Temporal isolation
Organisms breed at different times of year
Ecological isolation
Different habitats within the same area
Behavioural isolation
Different behaviour patterns like courtship and birds songs or plumage
Mechanical
Anatomical differences making it impossible for gametes to come together
Gametic isolation
Incompatibility (genetic or physiological) between gametes prevent hybrid from forming
Sperm may be destroyed in birthing canal if its too acidic
Hybrid isolation
Organisms interbreed but offspring are infertile
Horse and donkey produce an infertile mule
Number of chromosomes of mule is odd and so bivalent can’t form in prophase 1 due to not having homologous chromosomes and meiosis can’t occur to produce gametes
