Evolution Flashcards
What is a gene pool?
the complete range of alleles in a population
What is allele frequency?
how often an allele occurs in a population. it is usually given as a percentage of the total population or a decimal.
What happens to the frequency of an allele over time?
it changes. this is evolution.
How does evolution by natural selection occur?
1- individuals in a population vary due to diff. alleles. new alleles arise by mutations.
2- predation, disease, and competition (Selection pressures) create a struggle for survival
3- some are better adapted to selection pressures than others. this increases their chance of survival. they are more likely to survive, reproduce and pass on the advantageous allele.
4- a greater proportion of the next generation inherits the advantageous allele
5- they are more likely to survive, reproduce and pass it on. the frequency of the adv. allele increases from generation to generation.
What can’t occur without a selection pressure?
natural selection
When is an allele advantageous?
with the right selection pressure
What happens in a stable environment?
individuals with alleles for characteristics in the middle of the range are more likely to survive and reproduce- this is called stabilising selection. it reduces the range of possible phenotypes.
What happens in a changing environment?
individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce. this is called directional selection.
What causes evolution by genetic drift?
change dictates which alleles are passed on.
Compare evolution by natural selection and genetic drift.
in natural selection, characteristics become more common if they increase an organism’s likelihood of survival. by genetic drift, they become more common by chance.
Evolution via genetic drift
1- individuals within a population show variation within their genotypes
2- by chance, the allele for one genotype is passed on to the offspring more often than others. the no. of individuals with this allele increases.
3- 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.
Which 2 processes work alongside each other to drive evolution?
natural selection and genetic drift, but one drives evolution more depending on the population size.
In what type of population does evolution by genetic drift have a greater effect on evolution?
in smaller populations where chance has a greater influence
How can genetic drift be a problem?
it decreases genetic diversity. of a population. this may make a species less able to adapt to future changes in their environment, which can be a problem for smaller populations.
What is a genetic bottleneck?
an event (like a natural disaster) that causes a big reduction in a population’s size, leading to a reduction in the gene pool.
When does evolution by genetic drift have a greater effect?
when there’s a genetic bottleneck.
What is the founder effect?
when a few organisms from a population start a new population and there are only a small no. of diff. alleles in the initial gene pool.
The founder effect
1- individuals within a population show variation within their genotypes
2- some of these individuals start a new population. by chance, they mostly have one particular genotype.
3- without any further ‘gene flow’, the new population will grow with a reduced genetic variation. as the population is small, it’s more heavily influenced by genetic drift.
When can the founder effect occur?
as a result of migration leading to geographical separation or if a new colony is separated from the original population for another reason, such as religion. (the Amish people)
What is the Hardy-Weinberg principle?
a mathematical method that predicts the frequencies of alleles in a population won’t change from one generation to the next. it is only true under certain conditions.
Hardy-Weinberg conditions
- has to be a large population with no immigration. emigration, mutations, or natural selection
- needs to be random mating so all possible genotypes can breed with all others
What are the 2 Hardy Weinberg equations?
one for allele frequency and another for genetic frequency
Hardy Weinberg- allele frequency equation
p+q=1
p= frequency of the dominant allele
q= frequency of the recessive allele
Hardy Weinberg- genotype frequency equation
p^2 +2pq+q^2=1
p^2= freq. of homo dom genotype
2pq=freq. of hetero genotype
q^2= freq. of homo recessive genotype
What is artificial selection?
when humans select individuals in a population to breed together to get desirable traits e.g. modern dairy cattle and bread wheat.
How are modern dairy cows produced?
1- farmers select a female with a very high milk yield and a males whose mother had a very high milk yield and breed them
2- they select the offspring with the highest milk yield and breed them too
3- this is continued over several generations until a very high milk-yielding cow is produced
What are other characteristics selected for in dairy cows?
high milk quality, long lactation period, large udders, resistance to mastitis (udder inflammation) and other diseases, and a calm temperament
What are modern techniques used instead of artificial selection?
- artificial insemination and IVF to give farmers more control over which cows reproduce.
- animal cloning allows farmers to produce genetically identical copies of their best cows
How is bread wheat produced?
1- wheat plants with a high wheat yield are bred together
2- their offspring with the highest yields are bred together
3- this is continued over several generations to produce a plant with a very high yield
What are other characteristics selected for in bread wheat?
higher tolerance of cold, short stalks (don’t collapse under the weight of the ears), and uniform stalk heights to make harvesting easier
What techniques are useful in the artificial selection of plants?
plant cloning
Problems with artificial selection
1- reduced gene pool so can be more affected by a new disease and an increased incidence of genetic disease
2- potentially useful alleles are accidentally lost from the population when other alleles are being selected
3- can exaggerate certain traits, leading to health problems e.g. pedigree dog breed
Artificial selection problems- pedigree dog breed example
- modern PDB descended from a single wolf-like ancestor. they went through many generations of AS. —dogs like pugs and French bulldogs have been bred to -have a flat, squashed up face which has caused them breathing problems.
- dogs like dalmatians and English bull terriers have a high incidence of hereditary deafness
What is a species?
a group of similar organisms that can reproduce to give fertile offspring
What is speciation?
the development of a new species
When does speciation occur? W
when populations of the same species become reproductively isolated- changes in allele freq. cause changes in phenotype that mean they can no longer breed together to produce fertile offspring
What is allopatric speciation?
when populations become reproductively isolated through a combination of geographical isolation and natural selection
When does geographical isolation occur?
when a physical barrier e.g. a flood or earthquake, divides a population of a species, causing some individuals to be separated from the main population
What happens to those that are geographically isolated?
they experience slightly diff. conditions e.g. a diff. climate on each side of the physical barrier. since the environment is diff. for each population, diff. characteristics will become more common due to natural selection.
Allopatric speciation and natural selection
1- due to diff. characteristics being advantageous on each side, the allele freq. will change in each population
2- mutations will occur independently in each population, also changing the allele freq.
3- changes in the allele freq. will lead to changes in the phenotype freq.
Eventually, individuals from diff. populations will change so much that they cant breed to produce fertile offspring. The 2 groups will become separate species.
When does reproductive isolation occur?
changes in the alleles and phenotypes of 2 populations prevent them from successfully breeding together
How does reproductive isolation occur?
1- Seasonal changes- individuals of the same pop. develop diff. flowering or mating seasons, or become sexually active at diff. times of the year
2- Mechanical changes- changes in genitalia
3- Behavioural changes- a group of individuals develop courtship rituals that aren’t attractive to the main population
What is sympatric speciation?
Speciation without geographical isolation. Random mutations occur within a pop., resulting in the same changes as mentioned before, preventing members from breeding with other members of the species.
Why is sympatric speciation rare?
It’s difficult for a section of a pop. to become completely reproductively isolated without being geographically isolated too.
An example of sympatric speciation- polyploid organisms
Individuals with diff. no. of chromosomes can’t reproduce to give fertile offspring. So if a polyploid organism emerges in a diploid pop. it will be reproductively isolated. If the polyploid organism reproduces asexually, a new species can develop. Polyploidy can lead to speciation if it isn’t fatal to the organism and more polyploid organisms can be produced. It’s more common in plants than animals.
What is polyploidy?
when one possesses more than 2 sets of homologous chromosomes in their cells
What is a difference between sympatric and allopatric speciation?
sympatric occurs in the same place and allopatric occurs in populations that are away from each other
