7.3 Evolution may lead to Speciation Flashcards
(17 cards)
Individuals within a population show a wide range of variation in phenotype due to?
Genetic and Environmental factors
what are the 3 sources of genetic variation
Mutations, random fertilisation of gametes, Meiosis (independent assortment + crossing over)
Which biotic factors may limit population size?
Predation, competition for resources and disease
Which abiotic factors may limit population size?
Light availability, water supply and soil pH
Changes in allele frequency and phenotype frequency can occur due to what 2 things?
- Natural selection– When selection pressures act over several generations of a species individuals with a fitter phenotype are more likely to survive and pass on their alleles to their offspring so that the advantageous alleles increase in frequency over time and generations
- Isolation– new population may experience new (i.e. different) selection pressures from the main population. Over many generations, this will cause the allele and phenotype frequencies in the new population to change
What are the 3 types of selection
Stabilising selection
Disruptive selection
Directional selection
Give an overview of stabilising selection
Stabilising selection is natural selection that keeps allele frequencies relatively constant over generations
This means things stay as they are unless there is a change in the environment
A classic example of stabilising selection can be seen in human birth weights
Very-low and very-high birth weights are selected against leading to the maintenance of the intermediate birth weights
(mean allele frequency is in the middle of bell shaped curve)
Give an overview of Directional selection.
Directional selection is natural selection that produces a gradual change in allele frequencies over several generations
This usually happens when there is a change in environment / selection pressures or a new allele has appeared in the population that is advantageous.
(mean allele frequency shifted slightly to favour one direction)
Give an overview of Disruptive selection.
Disruptive selection is natural selection that maintains high frequencies of two different sets of alleles
In other words, individuals with intermediate phenotypes or alleles are selected against
This can occur in an environment that shows variation
(mean allele frequency favours both extremes and not the intermediate– bell shape curve is disrupted in the middle)
When does Reproductive separation occur
When changes in the alleles and phenotypes of some individuals in a population prevent them from successfully breeding with other individuals in the population that don’t have these changed alleles or phenotypes
Give 3 examples of allele or phenotype changes that can lead to reproductive separation:
Seasonal changes - some individuals in a population may develop different mating or flowering seasons (becoming sexually active at different times of the year) to the rest of the population (i.e their reproductive timings no longer match up)
Mechanical changes - some individuals in a population may develop changes in their genitalia that prevent them from mating successfully with individuals of the opposite sex (i.e. their reproductive body parts no longer match up)
Behavioural changes - some individuals in a population may develop changes in their courtship behaviours, meaning they can no longer attract individuals of the opposite sex for mating (i.e. their methods of attracting a mate are no longer effective)
Outline what occurs in any type of speciation
If the two populations are no longer reproducing with each other, then they do not interchange genes with each other in the production of offspring
Changes that occur in the allele frequencies of each group are not shared, so they evolve independently of each other, which can lead to the formation of two populations that are no longer successfully able to interbreed
When the genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring, speciation has occurred
What are the two situations in which speciation can take place?
Two populations of a species are separated by a geographical barrier and become genetically isolated from each other– Allopatric
Two populations of species are living in the same area (experiencing similar environmental selection pressures) but still become genetically isolated from each other– Sympatric
Give an Overview of Allopatric speciation
Allopatric speciation occurs as a result of geographical isolation
Allopatric speciation occurs when populations of a species become separated from each other by geographical barriers– natural like a body of water/a mountain range or can be man-made, like a motorway
Two populations of the same species created who are reproductively separated from each other, and as a result, no genetic exchange can occur between them
(no gene flow)
If there are sufficient selection pressures acting to change the gene pools (and allele frequencies) within both populations then eventually these populations will diverge and form separate species
The changes in the alleles/genes of each population will affect the phenotypes present in both populations
Over time, the two populations may begin to differ physiologically, behaviourally and morphologically (structurally)
Give an overview of Sympatric speciation
Sympatric speciation takes place with no geographical barrier
Something has to happen that splits or separates the two populations:
1. Ecological separation: Populations are separated because they live in different environments within the same area
For example, soil pH can differ greatly in different areas. Soil pH has a major effect on plant growth and flowering, so a population growing in soil with a slightly different pH may flower at a different time from another population, leading to reproductive separation (and eventually genetic isolation) of the two populations
Behavioural separation: Populations are separated because they have different behaviours
For example differences in feeding, communication or social behaviours, such as courting behaviours to attract a mate
Give an overview of genetic drift
This is when chance (instead of environmental selection pressures) affects which individuals in a population survive, breed and pass on their alleles
For example, when a population is significantly small, chance can affect which alleles get passed onto the next generation
Over time, some alleles can be lost or favoured purely by chance
When there is a gradual change in allele frequencies in a small population due to chance and not natural selection, genetic drift is occurring
In large populations, genetic drift is less likely to have an effect because any chance variations in allele frequencies usually even out across the whole population
Natural selection has a much more influential role in the evolution of large populations
What are the 4 ways in which evolution can take place?
Mutation, Gene flow, Genetic drift, and Natural selection. Mutation creates new genetic variation in a gene pool. Gene flow and genetic drift alter allele frequencies in a gene pool.
