chapter 14 - evolutionary mechanisms

Question 1

what are sources of variation

Answer 1

• random fertilisation: mother and father contribute genes, each individual produces a number of sperm / ova, infinite number os possible combinations of alleles
• random assortment: of chromosomes during meiosis, homologous pairs separate independently of every other pair, 8.4 million combinations for just one sperm / ova
• crossing over: of chromatids during meiosis (prophase 1), matching regions of homologous pairs break off and reattach to other chromosome (genetic recombination)
• nondisjunction: failure of chromosomes to separate during meiosis, causing monosomy (45) or trisomy (47), occurs during meiosis 1 or 2, called aneuploidy

Question 2

how do changes to allele frequencies in gene pools occur

Answer 2

• random genetic drift, founder effect, migration, barriers to gene flow, genetic disease

Question 3

what is random genetic drift

Answer 3

• RGD: the occurrence of characteristics in a population as a result of CHANCE rather than natural selection, occurs in all populations of non-infinite size (effects strongest in small populations), sewall right effect
• dunkers: pennsylvania, from hesse community (germany), religion = marry within group (isolated breeding pop.), allele freq. varies from hesse and US, small size = RGD = variation of alleles
• bentick islanders: island was part of mainland (high IA low IB), rising sea level = cut off, isolated + small = RGD = variation in alleles from mainland (high IB, no IA)
• > mornington islanders = travelled to mainland via small islands = similar freq. to mainland

Question 4

what is the founder effect

Answer 4

• FE: type of RGD, new pop. formed by individuals moving, expand and grow as new community, become unrepresentative of original pop. (isolation, little migration, small)
• pitcairn islanders: 1970, 9 mutineers populated pitcairn island (pacific), joined 6 men and 12 women on island, isolation (not many alleles introduced) = little variation compared to parent pop.
• tristan da cunha: islands in south atlantic (south africa / america), 7 surnames (traced = 15 british immigrants), little migration + small (250) + isolated = diff. gene pool to british pop. today
• finland: small pop. settled south west of finland, remained isolated + little migration = diff. gene pool to neighbouring european countries pop.
• pingelap: typhoon in micronesia, 20 survivors (founding pop.), were carriers for achromatopsia (TCB), 5% in pingelap 30% carriers compared to 0.00033% world wide

Question 5

what is migration

Answer 5

• M: gene flow, transfer of alleles from one population to another
• china: in past = high Rh+, 16th century = trading with european countries = introduced Rh-
• mongols (east asia): high IB, introduced this to western europe when they invaded them (had little to no IB)
• australia: aborigines prior to settlers = no contact / resistance to european diseases (chicken / small pox, measles), colonisation = drop in 90% of aborigine population = change in allele freq. = survivors (natural selection, resistant gene)

Question 6

what are barriers to gene flow

Answer 6

• B: barriers that inhibit the amount of interbreeding, isolation = separate gene pool
• geographical: australian’s (island = isolated many years), bentick islanders (isolated from mainland australia)
• sociocultural: basque people (pyrenees between france and spain = own language = isolation = developed broad foreheads, narrow jaws, distinctive patterns in blood groups; 30% Rh- compared to 16% of neighbours),

Question 7

what is genetic disease

Answer 7

• GD: changes to allele freq. in gene pool, usually allele that causes death disappears but some stay
• tay sachs: disorder of lipid metabolism, missing enzyme accumulation of fat in NS, death by 4-5, carriers = resistance to tuberculosis (survival advantage)
• -> jewish = ashkenazi = freq. of 1 in 2500 births compared to 1 in 500000 worldwide, RGD (small, isolated), introduced TSD to cajun population - southern louisiana US (isolated due to language, but migration of jewish family)
• thalassaemia: anaemic (defect in formation of haemoglobin, carries iron in blood), require transfusions / medication,
• -> lots marriages in mediterranean countries = high incidence of thalassaemia, occurs in aus. in greek / italian immigrants (migration / genetic)
• sickle-cell anaemia: substitution of valine for glutamic acid (haemoglobin gene, sickle shaped RBC = low O2), 2 SCA = fatal, 2 normal = susceptible to malaria, 1 of each = resistant to malaria + no SCA (selective / survival advantage)

Question 8

what is natural selection

Answer 8

• NS: process by which a species becomes better adopted to its environment
• darwins observations: variation (all members of species vary, variations passed on), birth rate (rate of reproduction > food supply / available resources = overcrowding), natures balance (although birth rate was high, numbers remained relatively constant)
• interpretations: struggle for existence (high birth rate = limited resources = competition), survival of fittest (variation = those with characteristics best suited to environs thrive / reproduce), favourable characteristics passed on, allele freq. change in gene pool
• principles: variation, birth rate, natures balance, struggle for existence, survival of fittest, favourable characteristics, allele freq.

Question 9

what are examples of natural selection

Answer 9

• eskimo (BS): favourable (short limbs / long bodies = low SA = heat gain, temp is cold = selective pressure = increased reproduction due to survival advantage = more of favourable allele in gene pool), unfavourable (long limbs / short bodies = high SA = heat loss, temp is cold = selective pressure = die due to cold = allele not passed on = less of allele in gene pool)
• africans (BS): favourable (long limbs / short bodies = high SA = heat loss, temp is hot = selective pressure = increased reproduction due to survival advantage = more favourable allele in gene pool), unfavourable (short limbs / long bodies = high SA = heat gain, temp is hot = selective pressure = die due to heat = less of allele in gene pool)
• sickle-cell anaemia: favourable (SCA = resistance to malaria = in areas of malaria (selective pressure) = pass on resistance (survival advantage) = more of allele in gene pool), unfavourable (normal RBC = in area of malaria (selective pressure) = die (no survival advantage) = less of allele in gene pool

Question 10

what is speciation

Answer 10

• S: the process of new species developing, caused by variation within a species, barriers to gene flow and reproductive isolation, struggle for existence and differential selection
• selection: no two environments are the same, different alleles are favourable in one environment (selective pressures) = overtime more of this phenotype of thrive / reproduce = more offspring with advantageous allele = change in gene pools
• over many generations: two populations become less alike and develop characteristics that best suit them to their environment
• speciation: sometimes species become so different that even if the barriers to reproduce were eliminated interbreeding of fertile offspring would not be possible, leads to two seperate species