Topic 7B - Populations and Evolution Flashcards
Define ‘species’.
A group of similar organisms that can reproduce to give fertile offspring
Define ‘gene pool’
The complete range of alleles present in a population
Define ‘allele frequency’
How often an allele occurs in a population (usually given as a %)
What is the ‘Hardy-Weinberg’ principle
A mathematical model that predicts that allele frequencies wont change from 1 generation to the next.
What 2 conditions must take place in order to apply the hardy-weinberg principle?
- It has to be a large population where there’s no immigration, emigration, mutations or natural selection.
- There needs to be random mating - all possible genotypes can breed with all others.
What is the Hardy Weinberg equation USED FOR?
- To calculate frequency of particular alleles, genotypes, and phenotypes within populations
- To test whether or not the Hardy-Weinberg principle applies to particular alleles in particular populations i.e to test whether selection or any other factors are influencing allele frequencies - if frequencies do change between generations in a large population then there is an influence of some kind.
Give the equation that will help you predict allele frequencies?
If a gene has 2 alleles, you can figure out the frequency of 1 of the alleles of the gene, if you know the frequency of the other allele, using this equation:
p+q = 1
p= frequency of 1 allele, usually the dominant one
q= frequency of other allele, usually the recessive
* the total frequency of all possible alleles for a characteristic in a certain population is 1 so the frequency of individual alleles must add up to 1
Give the hardy weinberg equation that will help you predict genotype and phenotype frequency?
You can figure out the frequency of 1 genotype if you the know the frequencies of the others using
p^2 + 2pq + q^2 = 1
p^2 = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype
q^2 = frequency of homozygous recessive genotype
* the total frequency of all possible alleles for a characteristic in a certain population is 1 so the frequency of individual alleles must add up to 1.
Do practice questions on the hardy-weinberg principle
page 176-177
Define variation?
The differences that exist between individuals
Does genetic variation exist between species?
Yes, even though they have the same genes. they have different alleles
What are the main 3 factors that causes genetic variation?
- Mutations
- Meiosis (through crossing over and independent segregation)
- Random fertilisation of gametes
Define evolution
The change in allele frequencies over time
How does natural selection work?
- Individuals with an allele that increases their chance of survival are more likely to survive, reproduce and pass on their genes
- This means that a greater proportion of next generation inherits beneficial allele
- They, in turn, are more likely to survive, reproduce and pass on genes
- Frequency of beneficient allele increases from generation to generation
- Over generations, this leads to evolution as advantageous alleles become more common in pop
What is a selection pressure?
Anything that affects an organisms chance of survival and reproduction e.g. competition, disease and predation
Define intraspecific variation
Variation within a species
Define stabilising selection
Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. It occurs when the environment isn’t changing, and it reduces the range of possible phenotypes
Define directional selection
Where individuals with alleles for a single extreme characteristic are more likely to survive and reproduce. It occurs when there is a change in the environment.
State the 3 types of selection
- Stabilising selection
- Directional selection
- Disruptive selection
Define disruptive selection
example on page 179
Where individuals with allele for extreme characteristics at either end of the range are more likely to survive and reproduce. It’s the opposite of stabilising selection as characteristics towards the middle of the range are lost. It occurs when the environment favours more than 1 phenotype
What is speciation?
Speciation is the development of a new species from an existing species
When does speciation occur?
When populations of the same species become reproductively isolated - changes in allele frequency causes changes in phenotype, which mean they can no longer interbreed to produce fertile offspring
- Reproductive isolation can happen due to geographical isolation: when a physical barrier e.g. flood/earthquake, separates individuals from main species OR due to sympiatric speciation (no physical barrier)
How can geographical isolation lead to allopatric speciation?
diagram
ALLOPATRIC SPECIATION
- Population of 1 species exist
- Physical barriers stop interbreeding between pops.
- Pops adapt to new environment (due to directional selection, mutations and genetic drift)
- Allele and phenotype frequency change, leading to reproductive isolation and the development of new species.
Describe how sympiatric speciation works?
- Most eukaryotic organisms are diploid, with 2 sets of homologous pairs. Sometimes, mutations occur that increase number of chromosome (polyploidy)
- Individuals with different numbers of chromosomes can’t reproduce sexually to give off fertile offspring, so polyploid organism will be reproductively isolated.
- If polyploid reproduces asexually successfully, a new species could develop.
Name the 3 categories of allele and phenotype changes that lead to reproductive isolation
- Seasonal
- Mechanical
- Behavioural
Describe the ‘seasonal’ category.
Seasonal: Individuals from same population develop different flowering or mating seasons, or become sexually active at different times of the year.
Describe the mechanical category.
Mechanical: Changes in genitalia prevent successful mating.
Describe the behavioural category.
Behavioural: A group of individuals develop courtship rituals that aren’t attractive to the main population.
What 2 factors can cause evolution
- Natural selection
2. Genetic drift
What is genetic drift?
This is when chance, rather than selection pressures, dictates which individuals survive, breed, and pass on their allele.
- Individuals within pop. show variation in genotypes.
- By chance, allele for 1 genotype is passed on to offspring more often than others
- So frequency of this allele increases, which can lead to reproductive isolation and speciation.
Does natural selection and genetic drift work together to separately to drive evolution?
They work alongside each other to drive evolution, but 1 process may drive evolution more than the other depending on pop. size.
- Evolution by genetic drift usually has greater effect in smaller pops where chance has a greater influence. In larger populations, chance variations are too insignificant to count towards evolution.
What has led to the diversity of life today?
Speciation and evolutionary change over millions of years
