18. Populations and Evolution Flashcards
How does genetic variation arise in sexual organisms?
- Mutations
- Meiosis
- Random variation of gametes
How does genetic variation arise in asexual organisms?
Mutation only.
How does the environment cause genetic variation?
Factors such as climate, diet, accidents, culture and lifestyle
What are polygenes?
Variation controlled by a single gene but by many genes
What is a ‘normal distribution curve’?
The normal distribution is a continuous probability distribution that is symmetrical on both sides of the mean, so the right side of the centre is a mirror image of the left side. Forms a bell shape.
Example: height of a population.
How is genetic distribution, such as blood type, measured?
Measured using a bar chart or pie graph.
How is environmental distribution, such as plant growth, measured?
Often represented as a line graph.
Define ‘section pressure’
The environmental factors that limit the population of a species, e.g. predation, disease and competition. Selection pressure determines the numbers of alleles within a gene pool.
Which factors alter the process of evolution by natural selection?
- Organisms produce more offspring
- There’s genetic variety within the populations of all species
- A variety of phenotypes that selection operates against
What’s the role of overproduction of offspring in natural selection?
Where there are too many offspring for the limited available resources there’s competition. The greater the number of offspring, the more competition and more organisms will die in the struggle to survive. The individuals in the population more adapted to the prevailing conditions will survive. These individuals are more likely to pass on their favourable allele combinations to the next generation, which will then have a different allele frequency from the previous one.
What’s the role of variation in natural selection?
Conditions change over time so having a population widely genetically different is important. The larger and more variation in a population increases the chance that individuals will have the advantageous alleles that lead to a phenotype suitable for survival. Populations showing little variation are more vulnerable to disease and climate changes. It’s also important that a species is capable of adapting to changes resulting from evolution of other species.
Describe stabilising selection
Preserves the average phenotype of a population by favouring average individuals- selection against the extremes of the phenotypes. Tends to occur when environmental conditions are constant over a period of time. E.g. body mass of children at birth.
Describe directional selection
Changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population- selection for one extreme of a phenotype. If environmental conditions change within a normal distribution curve, so will the optimum value for survival. Some individuals, to the left or right of the mean, will possess a combination of alleles with the new optimum for the phenotypic character. There will be a selection favouring the combination of alleles that results in the mean moving left or right. E.g. antibiotic resistant bacteria.
Describe disruptive selection
Favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population. Least common form of infection but most common in bringing evolutionary change. Occurs when an environmental factor, such as temperature takes 2 or more distinct forms. E.g. Coho salmon, large males are fierce competitors, and small males are able to sneak up to females in spawning grounds. Intermediate sized males are therefore disadvantaged.
Define ‘gene pool’
All the alleles of all the genes of all the individuals in a population at a given time.
Define ‘allelic frequency’
The number of times an allele occurs within the gene pool.
How is allelic frequency affected by selection?
The allelic frequency is affected by selection, which in turn is due to environmental factors. Environmental changes therefore affect the probability of an allele being passed on in a population, and hence the allelic frequency (not variation just the number of the existing allele).
Define ‘speciation’
Speciation is the evolution of new species from existing ones.
Define ‘species’
A species is a group of individuals that have common ancestry and 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.
How does separation of populations encourage speciation?
If a population is separated from another it will undergo different mutations and therefore be genetically different from other populations. Each population will experience different selection pressures because each environment will differ. Natural selection then leads to changes in the allele frequencies of each population. The different phenotypes each combination of alleles produces will be subject to selection pressure that will lead to each population being adapted to its local environment. This is known as ‘adaptive radiation’ and results in changes to the allele frequencies (evolution) of each population. As a result of these genetic differences it may be that, even if the populations were no longer separated, they would be unable to interbreed successfully. Each population would now be a different species with its own gene pool.
How does genetic drift encourage greater speciation in small populations than larger populations?
Because members of a small population possess a smaller variety of alleles than members of a large population, so the genetic diversity is less. As these few individuals breed, the genetic diversity is restricted to those few alleles in the original population. As there are only a small number of different alleles there’s not an equal chance of each being passed on. Those that are passed on will quickly affect the whole population as their frequency is high. Any mutation to an allele that is selectively favoured will also quickly affect the whole population because its frequency will be high. The effects of genetic drift will be greater and the population will change rapidly, making it more likely to develop into separate species. In large populations the effect of a mutant allele will be diluted because its frequency is far less in a much larger gene pool. The effects of genetic drift are much less, and development into new species is likely to be slower.
Describe allopatric speciation
Allopatric means different countries and is the form of speciation where 2 populations become geographically separated. If environmental conditions either side of the barrier vary, then natural selection will influence the populations differently, each will evolve and adapt to their own conditions. These changes may eventually lead to reproductive separation and the formation of separate species.
Describe sympatric speciation
Sympatric means same country and describes the form of speciation that results within a population in the same area leading to them becoming reproductively separated.
Describe geographical isolation
Populations isolated by physical barriers, e.g. oceans, mountains, rivers.
Describe ecological isolation
Populations inhibit different habitats within the same area and so individuals rarely meet.
Describe temporal isolation
The breeding seasons of each population do not coincide and so they don’t interbreed.
Describe behavioural isolation
Mating is often preceded by courtship, which is stimulated by the colour of markings of the opposite sex, the call or particular actions of the mate. Any mutations that cause variations in these patterns may prevent mating.
Describe mechanical isolation
Anatomical differences may prevent mating from occurring.
Describe gametic isolation
The gametes may be prevented from meeting due to genetic or biochemical incompatibility.
Describe hybrid sterility
Hybrids formed from the fusion of gametes from different species are often sterile because they can’t produce viable gametes.
State four factors that lead to diffrential survival and reproduction
- Predator
- Competition for resources
- Disease
- Natural disasters
Sickle cell anaemia is a debilitating genetic disease which causes premature death but provides some resistance to the malarial parasite. Explain how selection might affect the distrubrution of the gene causing sickle cell anaemia in both malarial and non-malerial regions.
In malarial regions, the diasdvantages of having the disease will be offset by the advantages of having resistance to malaria and so there will be little if any selection against the genes and its frequency will be relatievly high. In non-malaerial regions there is no advantage to gaving resistance to malaria and so individuals with sickle cell anaemia will be at a disadvantage; they will be selected against and the frequency of the gene will be low.
What type of selection is the following?
‘A baby with a birth rate greater than 4.0kg or less than 2.5kg has an increased risk of dying.’
Stabilising
What type of selection describes the following?
‘Elephants have evolved longer trunks enabling them to reach leaves higher up trees.’
Directional
What kind of selection describes the following?
‘Small mammals can escape from predators by hiding in small spaces while large ones can reissit attack by predators.’
Disruptive
Suggest which form of the peppered moth, Biston betularia, is now most common in cities like Manchester and explain why in terms of selection pressure.
The light coloured form because pollution control means that buildings are no longer black. The melanic form is therefore more conspicuous than the light form and prefrentially eaten by predators. The light form is more likely to survive and reporduce to give more light-coloured offspring. There is a selection pressure in favour of the light form, leading to it outnumbering the melanic form.
Explain how the geographical serparation of 2 populations of a species can result in the accumulation of the differences in their gene pools.
Geographically isolated populations may experience different environmental conditions. In each population, phenotypes that are best suited to the environment are selected. The composition of the alleles in each gene pool therefore changes as they pass to subsequent generations. The composition of the gene pool becomes increasingly different over time. Being geographically isolated, individuals of each population cannot interbreed so the 2 gene pools remain separtae and different.
What does the Hardy-Weinberg principle calculate?
The frequencies of the alleles of a particular gene in a population
What conditions need to be met for the Hardy-Weinberg principle to be accurate?
- No mutations
- Isolated population (no alleles flow in/out)
- No selection pressure
- Large population
- Random mating
What are the calculations needed to find the allele frequency?
p+q=1.0
(allele A+allele a=1)
p2 + 2pq + q2 = 1.0
(AA +Aa + aA + aa= 1.0)
