Variation and Evolution Flashcards
what are the three main factors variation (differences in phenotype) between individuals is caused by?
Differences in genotype – genetic factors.
•Different epigenetic modifications - but same genotype.
•Differences in environment – environmental factors
Why is variation important to the survival of an organism?
because it means that they are more likely to adapt and survive changes in the environment
what does discontinuous variation?
A particular phenotype can be controlled by one gene
What is continuous variation?
A particular phenotype can be controlled by more than one gene
what is non-heritable variation (environmental)? (2)
- The environment can affect the way an organism’s genes are expressed, i.e. phenotypic variation.
-This variation cannot be passed to offspring unless an epigenetic change occurs.
If the variation is not passed on to the offspring, then the variation is non-heritable.
Explain how the environment could lead to an epigenetic change:
-An environmental factor (e.g. diet) alters DNA methylation or histone modification.
-This then leads to changes in the expression of the genes
what is genetic variation also known as
heritable variation
what may genetic variation be increased as a result of?
Crossing over between homologous chromosomes during meiosis.
Independent assortment of chromosomes during meiosis.
Independent assortment of chromatids during meiosis.
Mixing of two different parental genotypes at fetilisation.
what does sexual reproduction do compared with mutations?
Sexual reproduction establishes new combinations of already present alleles.
Mutations produce new alleles. These often have a more significant impact on natural selection.
what can enter an inter and intra specific competition have an effect on?
breeding success and survival.
What can competition do?
Competition (e.g. for food/nutrients) can place selective pressures on the survival of different phenotypes and therefore breeding success.
what is the definition of selective pressure?
an environmental factor that can alter the allele frequencies of the alleles present at a particular gene locus in a population
what is the definition of selective agencies?
exert selection pressure, for example, Climate, Human impact, Supply of food, Breeding sites
Who are more likely to survive?
Organisms with well-adapted phenotypes (those whose alleles are selected for and give them a selective advantage) are more likely to survive than those whose phenotypes are less well adapted (those whose alleles mean they are selected against).
what are the phenotype which increase the chance of organism surviving also likely to do?
likely to give a higher breeding success
What selection pressure led to the increase of dark moths during the industrial revolution?
Soot from the factories led to tree bark becoming darker – dark form moths were camouflaged – reduced predation.
if a dominant allele produces a phenotype which gives a selective disadvantage what will happen to the frequency of the dominant allele in the gene pool?
-The frequency of the allele will decrease and it may disappear from the gene pool.
-As any organism with it will have the disadvantageous phenotype so is less likely to breed and pass on the allele
Will the effect be the same for a recessive allele that produces a selective disadvantage? Explain your answer.
No, organisms who are heterozygous will not have the disadvantageous phenotype, so will survive and breed and pass the allele on.
This means the recessive allele will remain at a low frequency in the population.
Only homozygous recessive have a disadvantageous phenotype
what is the gene pool?
the total of all alleles for all of the genes in a population
what is allele frequency?
Number of copies of that allele with all the alleles for that gene
what is the allele frequency equation?
Frequency of dominant allele + frequency of the recessive allele = 1
In a population of 65 organisms, 25 are homozygous dominant and 30 are heterozygous.
What is the frequency of the dominant allele in the gene pool? Show your working.
Total number of allele = 65 x 2 = 130
Number of dominant alleles = 25 x 2 (homozygous ) + 30 (heterozygous)
Number of dominant alleles = 80
Dominant allele frequency = number of dominant alleles /
total number of alleles
What is the hardy Weinberg principal state?
that the frequencies of dominant and recessive alleles and genotypes will remain constant from one generation to the next, if certain conditions remain true
What do the conditions include?
•a large population (100+ individuals);
•no selection for or against any phenotype;
•random mating throughout the population;
•no mutations;
•the population is isolated, i.e. no immigration or emigration.
what can the hardy Weinberg principle be used to do?
to estimate the frequencies of dominant or recessive alleles or of different genotypes of a characteristic in a population using the following equation: p2 + 2pq + q 2 = 1
what are the three terms of the equation?
p2 = frequency of AA (homozygous dominant)
•2pq = frequency of Aa (heterozygous)
•q2 = frequency of aa (homozygous recessive)
what is the natural selection explanation?
-variation by mutation of DNA creates new alleles
-Advantageous alleles give individuals a selective advantage
(only when a selection pressure exists in the environment)
-so more likely to survive and reproduce and so pass on the advantage all over many generations
-So the allele frequency of this allele increases in the gene pool
(selection pressure)
Explain why the frequency of Tay-Sachs is higher in isolated populations and predict, with a reason, what is likely to happen to the frequency of the Tay-Sachs allele in the general population. [2]
(Hardy Weinberg principle does not apply because) population is small immigration}/higher probability of two carriers having children (1) Decrease because h alleles will be lost from gene pool when sufferers die in childhood/selective abortion/selected against/selective disadvantage/less likely to reproduce (1)
What is evolution?
the change in the average
Phenotype of a population over time
what does natural selection and what does it do?
-the theory that explains how existing species have arisen through modification of ancestral species.
-It encourages The transmission of favourable alleles and hinders the transmission of unfavourable alleles.
In any population, what is variation due to?
due to mutations.
What is the process of natural selection?
•In any population there is variation due to mutations.
•Population numbers remain roughly the same, despite overproduction of offspring.
•Competition (intraspecific) means there’s a struggle for survival.
•The fittest have a selective advantage due to selection pressures.
•They survive, interbreed and pass on alleles that give their offspring a selective advantage.
•The process repeats over many generations and increases the frequency of the advantageous allele in the population’s gene pool.
•Eventually the organisms may genetically change so much they become a different species.
What can natural selection do to all frequency?
can drive change in allele frequency, or it can maintain allele frequency
What is directional selection?
If the environment changes then natural selection may favour one extreme of the phenotypes resulting in a directional change in the allele frequency in a population
What is stabilising selection?
If the environment is stable then extreme phenotypes tend to be eliminated as they do not confer any selective advantage.
This prevents change and stabilises a population.
The elephant shark has shown the lowest rate of evolution of all vertebrates and has changed little in over 300 million years.
What type of selection is the elephant fish likely to have mainly being experiencing? Explain your answer.
Stabilising selection, stable environment, extreme phenotypes selected against, stable gene pool
Use the concept of directional selection to explain the differences between Australopithecus and modern humans.
Natural selection favoured alleles for – height / larger brain
Less sexual dimorphism / less body hair – directional selection
occurred to form modern humans over long period of time
what do polygenic characteristics often show?
continuous variation, which can be demonstrated by plotting a frequency histogram, producing an approximately normal curve.
Why would you carry out a t-test?
To test if there is a significant difference between the means of two samples of interval/continuous data, when data is normally distributed.
how would you do the T test?
STEP 2: Calculate the MEAN values for both data sets….
STEP 3: Calculate the STANDARD DEVIATION values for both data sets…
STEP 4: You should now have 4 values (mean 1, mean 2, standard deviation 1, standard deviation 2)..
STEP 5: Put them into the T test equation to calculate a T value..
STEP 6: Work out the degrees of freedom (n, + n2 - 2).. Then work out the CV at P=0.05…
STEP 7: If the T value is > CV then P < 0.05 (*remember it goes the opposite way)
STEP8: State what that P value means to you… e.g. There is “less than 5% probability that the difference between the means is due to chance”.. so the difference in means IS significant..
STEP 8: Make sure to comment on significant or no significant differences in means…
STEP8: Accept or Reject the null hypothesis..
What is the definition of a species?
a group of phenotypically similar organisms that can interbreed to produce fertile offspring
Where are demes (breeding subunits) found?
In a population of one species
What can form when populations of species are isolated?
new species can form (speciation)
what does it mean if demes become isolated from each other due to a barrier to reproduction?
the gene pool is divided and there is no flow of genes between separate demes
what will happen if the demes are reunited after many generations (2)?
-demes may be incapable of breeding successfully (cannot interbreed to produce fertile offspring).
-Two new species are formed each with their own gene pools.
what can speciation occurred due to?
-Genetic drift
–The founder effect
–Natural selection
what are the two types of speciation?
–Allopatric speciation – geographical isolation
–Sympatric speciation –reproductive isolation
what is genetic drift?
Variation in allele frequencies can occur due to chance
Why does genetic drift have a bigger impact and why?
Smaller population
Because an allele can be lost from the gene pool through a random chance
In a species of birds, blue feathers are dominant to purple feathers. In a population of 120 birds, 15 have purple feathers. A disease kills 45 of the birds in the population, including all those with purple feathers.
Birds with purple feathers continue to be born- why?
-Some of the birds with blue feathers were heterozygous.
-Two heterozygous parents could have offspring with purple feathers.
What is the founder effect (an example of genetic drift)?
When a new area is colonised by individuals from a population those individuals may have a gene pool which, due to chance, has different allele frequencies than the original gene pool.
what could happen to the founder population (2)
through chance changes in allele frequencies (genetic drift) the founder population could become even more different from the original population.
where is the founder effect more significant?
in small populations where a chance variation in allele frequency can lead to a significant change in phenotype compared to the original population.
Madagascar is an island off the coast of Africa. Many of the species there are unique (only found on the island).
Suggest how the founder effect may have led to these unique species arising.
-A small group of organisms from main land African colonised Madagascar. By chance the allele frequencies in their gene pool differed from the mainland gene pool.
-Different selection pressures on the island. Over the time these allele frequencies became so different new unique species formed.
What is allopatric speciation?
when there is a physical barrier isolating individuals, such as a mountain range or river that splits a population into two separate demes, preventing interbreeding and the flow of genes.
what is an example of Allopatric speciation?
Kaibab squirrel found on the North rim of the Grand canyon and Abert’s squirrel found on the south rim.
What is the process of speciation?
1.Large population with common gene pool.
2. Population is separated into
2 demes by a physical barrier, preventing flow of genes (breeding).
3. Mutations and different selection pressures on each deme alter gene pool.
4. If the barrier is removed the gene pools will be so different that interbreeding will not be successful.
what is sympatric speciation?
when there is a reproductive barrier isolating individuals, such as being unable to attract a mate from another deme.
what are two examples of sympatric speciation?
behavioural isolation
morphological isolation
gametic isolation
seasonal isolation
What is behavioural isolation?
In animals with elaborate courtship behaviour, the steps in the ‘display’ of one group of organisms may fail to attract the necessary response in a potential partner from another group of organisms.
What is morphological isolation?
The body parts of organisms may not be compatible enough for them to mate.
This is seen in insects where the rigid exoskeletons mean that the genitalia of male and females must be complementary.
what is gametic isolation?
There are barriers preventing gametes of different species fusing
what are examples of gametic isolation?
-Stigma will only produce a sugary secretion for pollen germination if the pollen is compatible (same species)
-Spermatozoa can often only survive in an oviduct if from the same species
what is seasonal isolation?
the reproductive organs of different groups mature at different times of year the groups are unable to interbreed.
What can seasonal isolation be occur due to?
differences in mating seasons or differences in flowering times. e.g. the toad Bufo americanus mates in early summer whilst B. fowleri mates in late summer, meaning the two snecies remain isolated.
What can arise as the demes become new species?
Hybrid inviability
Hybrid sterility
what is hybrid inviability?
-Fertilisation may occur but incompatibility between genes of the parents prevent the development of an embryo.
-Hybrid embryos formed from sheep and goats die in the early stages of development.
what is hybrid sterility?
some cases, an embryo can survive when individuals of different species breed (e.g. in wheat plants and mules).
What is an example of hybrid sterility?
A horse can breed with a donkey to produce a hybrid known as a mule. If the horse has 64 chromosomes and the donkey has 62 chromosomes, the offspring have 63 chromosomes.
why are the hybrid offspring unable to produce gametes and sterile?
hybrids are unable to form bivalents (homologous pairs) during prophase I of meiosis so cannot produce gametes (sterile)
What is a deme?
Breeding sub-unit with a population of a species
