Section 7 - 18 Populations and evolution Flashcards
What is a population?
A group of organisms of the same species that occupies a particular space at a particular time and that canpotentially interbreed
Any species exists as one or more populations.
What is a gene pool?
All the alleles of all the genes of all the individuals in a population at a given time.
What is the allelic frequency?
The number of times an allele occurs within the gene pool.
What does cystic fibrosis effect?
Mucus produced by affected individuals is thicker than normal.
How can we work out the allele frequency of mixed populations?
The Hardy-Weinberg principle
What does the Hardy-Weinberg principle provide?
A mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population.
What assumption is made in the Hardy-Weinberg principle?
The proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next.
What 5 conditions must be met in order to assume that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next?
- No mutations arise
- the population is isolated - no flow of alleles into or out of the population
- no selection - all alleles are equally likely to be passed to the next generation
- population is large
- mating within the population is random
EQUATION
Hardy-Weinberg principle.
The probability of allele A = p
and the probability of allele a = q
p + q = 1.0
There are only 4 possibilities for two alleles -
AA + Aa + aA + aa = 1.0
p^2 + 2pq + q^2 = 1.0
Why must p + q = 1.0?
There are only two alleles and so the probability of one plus the other must be 1.0 (100%)
What factors affect variation in species phenotypes?
genetic and environmental factors.
What is the cause of three genetic variations?
- Mainly mutations
- meiosis
- random fertilisation of gametes during sexual reproduction
How does genetic variation arise from mutations?
Sudden changes to genes and chromosomes may or may not be passed on to the next generation.
Main source of variation.
How does genetic variation arise from meiosis?
A special form of nuclear division produces new combinations of alleles before they are passed into the gametes, all of which are therefore different.
How does genetic variation arise from random fertilisation of gametes?
In sexual reproduction, this produces new combinations of alleles and the offspring are therefore different from parents.
Which gamete fuses with which at fertilisation is a random process further adding to the variety of offspring two parents can produce.
Give environmental influences that affect the phenotype
- Climatic conditions
- soil conditions
- pH
- food availability
What are the three forms of selection?
Stabilising
Directional
Disruptive
How does stabilising selection affect the characteristics of a population?
Preserves the average phenotype of a population by favouring average individuals in other words, selection against the extreme phenotypes.
How does directional selection affect the characteristics of a population?
CHanges the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population, in other words, selection for one extreme phenotype.
How does disruptive selection affect the characteristics of a population?
Favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population.
What is eliminated due to stabilising selection?
Eliminate the extremes of the phenotypes
and with it the capacity for evolutionary change.
When is it most likely for stabilising selection to occur?
Where the environmental conditions are constant over long periods of time.
What graph can be formed due to directional selection?
normal distribution curve.
A mean that represents the optimum value for the phenotypic character under the existing conditions.
What is the result of the directional selection?
one extreme of a range or variation being selected against in favour of the other extreme or even the average.
What is the opposite of stabilising selection?
Disruptive selection
What does disruptive selection favour?
extreme phenotypes at the expense of the intermediate phenotypes.
When does disruptive selection occur?
When an environmental factor, such as temperature, takes two or more distinct forms.
What is polymorphism?
Some species of organisms have two or more distinct forms.
These different forms are genetically distinct but exist within the same interbreeding population.
What is the allelic frequency?
The number of times an allele occurs within the gene pool.
Affected by selection which is due to environmental factors.
What is speciation?
The evolution of new species from existing ones.
What is a species?
A group of individuals that have a common ancestry and so share the same genes but different alleles and are capable of breeding with one another to produce fertile offspring.
Members of a species are reproductively separated from other species.
What is adaptive radiation?
The different phenotypes each combination of alleles produces will be subject to selection pressure that will lead to each population becoming adapted to its local environment.
What is the result of adaptive radiation?
CHanges to the allele frequencies of each population and so each population evolves.
Populations will no longer be able to interbreed successfully.
As each population would be a different species each with its own gene pool.
When can genetic drift happen?
In small populations
Why does genetic drift happen in small populations?
The relatively few members of small populations possess a smaller variety of alleles than the members of a large population.
Their genetic diversity is less.
What are the two forms of speciation?
allopatric speciation
sympatric speciation
What does allopatric mean?
Different countries.
What is allopatric speciation?
Where two populations become geograohically separated.
This can be the result of any physical barrier between two populations which prevents them interbreeding.
What does sympatric mean?
Same country
describing the form of speciation that results within a population in the same area leading to them becoming reproductively separated
State the 7 types of variation
- Geological
- Ecological
- Temporal
- Behavioural
- Mechanical
- Gametic
- Hybrid sterility
What is the geological variation?
Populations isolated by physical barriers such as oceans, mountain ranges, rivers
What is ecological variation?
Populations inhabit different habitats within the same areas and so individuals rarely meet.
What is temporal variation?
The breeding seasons of each populations do not coincide and so they do not interbreed.
What is behavioural variation?
Any mutations which cause variations in courtship patterns may prevent mating.
What is mechanical variation?
Anatomical differences may prevent mating occuring.
What is gametic variation?
Prevented from meeting due to genetic or biochemical incompatibility.
What is hybrid sterility variation?
Hybrids formed from the fusion of gametes from different species are often sterile because they cannot produce viable gametes.
