7.3 Flashcards
Give three examples of selectional pressures
Competition
Disease
Predation
What do selection pressures determine?
Allele frequency in a gene pool
What causes variation in a population?
Genetic factors
Environmental factors
What are the sources of genetic variation?
Mutations
Meiosis
Random fertilisation
How does variation as a result of genetics alone compare to variation as a result of genetics and environment statistically?
Genetics alone- discontinuous variation- bar chart
Genetics + environment- continuous variation- normal distribution curve
How do selectional pressures result in natural selection?
Variation occurs within a species due to genetics variation
Selectional pressures create struggle for existence
Individuals who have a selectional advantage due to having advantageous phenotypes will be better adapted to the environment so more likely to survive so more likely to pass on favourable alleles
If selectional pressures don’t change, offspring with favourable alleles more likley to survive and pass on
Frequency of beneficial allele will increase in gene pool over successive generations
What are the three types of natural selection?
Stabilising
Directional
Disruptive
What kind of environment does stabilising selection occur in? What does this mean for variation? How is this shown on a distribution curve?
Very stable
Extremes not selected for so variation decreases
Narrow curve, mean remains the same
What kind of environment does directional selection occur in? What does this mean for variation? How is this shown on a distribution curve?
Changing environment
One extreme selected for in favour of the other extreme so variation increases
Curve moves one way or another, mean changes as optimum survival value changes
What kind of environment does disruptive selection occur in? What does this mean for variation? How is this shown on a distribution curve?
Environment with two distinct forms
Both extremes selected at the expense of intermediate phenotypes so variation increases
Two curves that dip at the mean
Which is the least common type of natural selection?
Disruptive
Which type of natural selection results in the most evolutionary change?
Disruptive
How does a populations environment affect allele frequency? (3)
Selection pressures within environment
Favourable alleles that code for advantageous phenotypes more likely to survive
Frequency of favourable alleles increases within gene pool as they are passed on
Define speciation
Formation of a new and distinct species through the process of evolution
How do new species form?
Portion of existing species needs to change so no longer has significantly similar characteristics so can’t interbreed
What two things does speciation require?
Isolation = population separated due to change in climatic condition that produces a body of water that separates the habitat
Different selectional pressures
What is it called when speciation occurs due to geographical isolation?
Allopatric speciation
How does speciation occur due to geographical isolation?
Adaptive radiation= diversification of a group of organism into forms that fill an ecological niche within a given habitat
Leads to change in allele frequency in each population
Each population evolves
What is it called when speciation occurs due to behavioural or mechanical isolation in a population in the same area?
Sympatric isolation
How does speciation occur through a genetic niche?
Happens in small populations- small gene pool
Genetic diversity restricted as only a small number of allele types to be passed around
Any new alleles that arise through mutation and are a selectional advantage will quickly increase in frequency in subsequent generations
Effects of genetic drift will happen quickly within small population
Why doesn’t speciation due to a genetic niche occur in large populations?
Larger gene pool so any changes less pronounced so more allelic variety
Generic drift effects and new species development will be much slower
