Module 6 Section 3: Evolution Flashcards
What is evolution
This is how the frequency of alleles in a population changes over time
What is a gene pool
This is the complete range of alleles present in the population
How is the rate at which an allele occurs displayed
The allele frequency is given as a percentage of the total population
Process of natural selection
Individuals in a population vary because they have different alleles (caused by meiosis and mutations)
Predation, disease and competition (selection pressures) create a struggle for survival.
Because individuals vary, some are better adapted to the selection pressures than others.
Individuals that have an allele that increases their chance of survival (advantageous allele) are more likely to survive, reproduce and pass on the advantageous allele, than individuals with different alleles.
This means that a greater proportion of the next generation inherit the advantageous allele.
They, in turn, are more likely to survive, reproduce and pass on their genes.
So the frequency of the advantageous allele increases from generation to generatio
This process is called natural selection.
What characteristics are selected by a stable environment
When the environment isn’t changing much, individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.
This is called STABILISING SELECTION and it reduces the range of possible phenotypes.
Example of how certain characteristics are chosen by a stable environment
Stabilising selection:
In any mammal population there’s a range of fur length.
In a stable climate, having fur at the extremes of this range reduces the chances of surviving as it’s harder to maintain the right body temperature.
Animals with alleles for average fur length are the most likely to survive, reproduce and pass on their alleles.
So these alleles increase in frequency.
The proportion of the population with average fur length increases and the range of fur lengths decreases
What characteristics are selected by a changing environment
When there’s a change in the environment, individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce.
This is called DIRECTIONAL SELECTION.
Example of how certain characteristics are chosen by a changing environment
If the environment becomes very cold, individual mammals with alleles for long fur length will find it easier to maintain the right body temperature than animals with short fur length.
So they’re more likely to survive, reproduce and pass on their alleles. Over time the frequency of alleles for long fur length increases
How is genetic drift different compared to natural selection
This relies on chance instead of environmental factors to dictate which alleles are passed on
Process of how does genetic drift work
Individuals within a population show variation in their genotypes (e.g. A and B).
By chance, the allele for one genotype (B) is passed on to the offspring more often than others.
So the number of individuals with the allele increases.
If by chance the same allele is passed on more often again and again, it can lead to evolution as the allele becomes more common in the population
What processes is evolution influenced by in a natural environment
Natural selection and genetic drift work alongside each other to drive evolution
One process can drive evolution more than the other depending on the population size.
How will natural selection and genetic drift work together in a smaller population
Evolution by genetic drift usually has a greater effect in smaller populations where chance has a greater influence and each allele makes up a greater proportion of the population (due to small gene pool) making it more likely to be passed on
If the environment changes for a small population, the lack of variety in the gene pool means that they will more likely go extinct than adapt as its less likely they will have an advantageous allele
How will natural selection and genetic drift work together in a larger population
Genetic drift less prevalent in larger populations because the appearance of a new allele (from mutation) in a diverse gene pool will have less of an impact as it makes up a smaller portion of the gene pool
In larger populations any chance variations in allele frequency tend to even out across the whole population
Example of genetic drift
Different Native American tribes show different blood group frequencies.
For example, Blackfoot Indians are mainly group A, but Navajos are mainly group O.
Blood group doesn’t affect survival or reproduction, so the differences aren’t due to evolution by natural selection.
In the past, human populations were much smaller and were often found in isolated group
By chance the allele for blood group O was passed on more often in the Navajo tribe, so over time this allele and blood group became more common.
When may genetic drift have a greater effect
During a genetic bottleneck
During the founders effect
What is a genetic bottleneck
This is an event (such as a natural disaster) that causes a big reduction in a population’s size, leading to a reduction in the gene pool and genetic diversity
New population may not be representative of the original population as rarer alleles may not be present in survivors
Example of a genetic bottleneck
The mice in a large population are either black or grey.
The coat colour doesn’t affect their survival or reproduction.
A large flood hits the population and the only survivors are grey mice and one black mouse.
Grey becomes the most common colour due to genetic drift.
What is the founders effect
The founder effect describes what happens when just a few organisms from a population start a new population and there are only a small number of different alleles in the initial gene pool
How does the founders effect work
Individuals within a population show variation in their genotypes (e.g. alleles ABCD).
Some of these individuals start a new population.
By chance these individuals are mostly one particular genotype (A).
Without any further ‘gene flow’ (i.e. the introduction of new alleles from outside the population) the new population will grow with reduced genetic variation.
As the population is small, it’s more heavily influenced by genetic drift than a larger population
When can the founders effect occur
Can occur as a result of migration leading to geographical separation
Or if a new colony is separated from original population for other reasons e.g. religion
E.g. the Amish have little genetic diversity so have a high incidence of certain genetic disorders
What does the Hardy-Weinberg principle predict
Predicts that the frequencies of alleles in a population won’t change from one generation to the next.
But this prediction is only true under certain conditions
Hardy-Weinberg principle conditions
Has to be a large population where there’s no:
Immigration
Emigration
Mutations
Natural selection
There also needs to be random mating where all possible genotypes can breed with all others
What can the Hardy-Weinberg equations be used for
They can be used to estimate the frequency of particular alleles and genotypes within populations
If the allele frequencies do change between generations in a large population then immigration, emigration, natural selection or mutations have happened
How to use Hardy-Weinberg equation to predict allele frequency
p+q = 1
p: frequency of dominant allele
q: frequency of recessive allele
Total frequency of all possible alleles for a characteristic in a population is 1.0 so p and q add to 1.0
