Variation and Evolution (C2)

Question 1

What is continuous variation? shape of graph?

Answer 1

• phenotypes are controlled by a number/more than one gene e.g height, hair colour, skin colour
• environmental factors do have an effect (a plant may not grow tall despite having the alleles for tallness due to lack of sunlight)
• bell shapes curve, no gaps

Question 2

What is discontinuous variation? shape of graph?

Answer 2

• phenotypes are controlled by a single gene e.g tongue rolling, ear lobes, blood type
• environmental factors have NO effect - the phenotype will remain the same regardless of external factors
• random variable graph, gaps between/distinct categories

Question 3

What is non-heritable variation?

Answer 3

• where the environment can affect the way an organisms genes/phenotypes are expressed
• this variation cannot be passed on to offspring (unless an epigenetic change occurs - transcription of DNA/histone proteins is modified, the sequence is not but the expression is altered)
• e.g diet/nutrients, exercise, temperature, light

Question 4

What is heritable variation? Increased by?

Answer 4

• this is genetic variation - sexual reproduction establishes new combinations of already present alleles, mutations may produce new alleles (can impact natural selection) may be passed on to offspring
• variation may be increased by;
crossing over in prophase I,
independent assortment in metaphase I and II,
mixing of two parental genotypes at fertilisation

Question 5

What is the effect of inter and intra specific competition on breeding success and survival?

Answer 5

• competition places selective pressures on the survival of different phenotypes and hence breeding success (essentially driving natural selection)

Question 6

What is selective pressure? selective agency examples? affects on organisms alleles?

Answer 6

• environmental factors which can alter allele frequencies of alleles present at a particular gene locus in a population
• SELECTIVE AGENCIES exert selection pressure
e.g supply of food, climate, breeding sites, human impacts
• organisms with well adapted phenotypes have alleles giving them a SELECTIVE ADVANTAGE therefore more likely to survive (having higher breeding success) than less well adapted, whose alleles are SELECTED AGAINST

Question 7

What is the ‘gene pool’?

Answer 7

The total of all alleles for all of the genes in a population
( gene = portion of DNA that determines a certain trait, an allele is a specific form of a gene ie the gene for hair colour; the allele for brown hair)

Question 8

What is genetic drift?

Answer 8

• can cause variation in allele frequencies due to CHANCE

natural selection is selected upon whereas genetic drift is due to chance

Question 9

What is meant by allele frequency?

Answer 9

allele frequency refers to;

how common an allele is in a population

Question 10

What is the Hardy Weinberg principle used for? what does it state? conditions needed?

Answer 10

• used to estimate frequencies of dominant or recessive alleles or of different genotypes of a characteristic in a population
• it states;
that the frequencies of dominant and recessive alleles, and genotypes will remain constant from one generation to the next, IF certain conditions remain true
- 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

Question 11

What is the Hardy Weinberg equation? which letter represents what?

Answer 11

p2 + 2pq + q2 = 1

where:
p = frequency of the DOMINANT ALLELE (i.e A)
q = frequency of the RESSESIVE ALLELE (i.e a)
(p + q = 1)

p2 = frequency of AA (homozygous dominant)
2pq = frequency of Aa (heterozygous)
q2 = frequency of aa (homozygous recessive)

Question 12

What is the concept of speciation?

Answer 12

when populations of a species become isolated new species can form - this is speciation

• evolution, in terms of speciation, will not take place if the conditions, under which the Hardy-Weinberg principle applies, do not change
• speciation, formation of a new species, can occur due to:
• GENETIC DRIFT in isolated population
• the FOUNDER EFFECT of disproportionate allele frequencies in small populations
• NATURAL SELECTION

Question 13

What is the concept of isolation? types?

Answer 13

the separation of species, where demes (little populations/sub units to a species) become isolated form each other due to a barrier to reproduce
isolation can be:
- ALLOPATRIC isolation
- SYMPATRIC isolation

Question 14

What is the definition of a species? when are two species distinguished/a new one formed?

Answer 14

• a group of phenotypically similar organisms that can INTERBREED to produce FERTILE offspring

*species are not classed as the same if they can not successfully interbreed to produce fertile offspring

Question 15

What is the founder effect?

Answer 15

• when a new area is colonised by individuals from a population those individuals may have a gene pool which, due to chance, have brought different allele frequencies from the original gene pool
• through chance changes in allele frequencies (GENETIC DRIFT) or different SELECTION PRESSURES the founder population could become even more different from the original population - effect is more significant is smaller pops

Question 16

What is allopatric speciation? examples? process?

Answer 16

• occurs when a PHYSICAL barrier splits a population into two separate demes
• this prevents interbreeding and the FLOW of genes
e.g rivers, lakes, forests, mountain ranges

1. large population with common gene pool
2. pop is separated into two demes, by physical barrier
3. flow of gene and interbreeding is prevented
4. mutations and different selection pressures on each deme, change gene pool
5. if barrier is removed, gene pools became so different that interbreeding will not be successful
   THEREFORE; become classed as different species

Question 17

What is sympatric speciation? types? x6

Answer 17

the process forming two species, from one original species due to REPRODUCTIVE isolation whilst occupying the SAME geographical location
this can be;
- BEHAVIOURAL isolation (courtship behaviour)
- MORPHOLOGICAL isolation (body parts of organisms may not be compatible to mate - ridged exoskeleton insects)
- GAMETIC isolation (prevention of gametes of different species fusing)
- SEASONAL isolation (maturing at different times of year - unable to interbreed)
- HYBRID INVIABILITY (fertilisation may occur - zygote/embryo not viable)
- HYBRID STERILITY (some cases embryos survive i.e liger - hybrid offspring are sterile BAR wheat, a fertile hybrid)

Question 18

Why, when hybrid sterility occurs, is the offspring sterile, in terms of meiosis? (example of mule)

Answer 18

• mule is a hybrid offspring of a male donkey and a female horse (different species with different number of chromosomes)
• due to differing number of chromosomes, bivalents/homologous pairs cannot form during prophase I
• meiosis can therefore not take place
• gametes aren’t produced
• offspring are sterile

Question 19

What is hybrid fertility in wheat? How does it occur, in terms of meiosis?

Answer 19

wheat is a fertile hybrid, which is interbreed between species but can produce fertile offspring

• this is due to endo mitosis
• chromosome number doubles/duplicates WITHOUT cytokinesis, thus one cell (occurring in early embryo)
• therefore, there are two/full sets of EACH chromosome
• this allows pairing during meiosis so gametes can be produced
• offspring are fertile

Question 20

What is the definition of natural selection? passed on advantageous alleles?

Answer 20

a change in the average phenotype of a population overtime

• may occur due to selection pressures affecting the survival of different phenotypes in a population
  e. g. selective predation, camouflage, mimicry
• the advantageous selected alleles are passed on to offspring, if individuals survive to reproductive age
• this changes the allele frequencies over time due to the natural selected advantageous alleles increasing

Question 21

What is the basis of Darwin’s theory of evolution?

Answer 21

Darwin’s theory of evolution:

that EXISTING species have arisen through MODIFICATION of ANCESTRAL species by NATURAL SELECTION

Question 22

What is the process of natural selection? how it occurs?

Answer 22

• in any pop there is VARIATION due to MUTATIONS
• COMPETITION means there is a struggle for survival
• those with the most favourable alleles/ the ‘fittest’ have a SELECTIVE ADVANTAGE
• they survive, INTERBREED and PASS ON alleles giving the offspring a selective advantage
• process repeats over many generations
• INCREASES FREQUENCY of the advantageous allele in the populations GENE POOL
(organisms may become so genetically different they become different species)