variation and evolution

Question 1

what three factors cause variation?

Answer 1

• differences in genotypes - genetic factors producing variation between individuals
• the same genotype but have different epigenetic modifications
• differences in environment - environmental factors producing variation between individuals

Question 2

why is variation important to the survival of an organism?

Answer 2

more likely to survive changes in the environment

Question 3

describe non-heritable variation

Answer 3

• environmental
• 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 onto the offspring then the variation is non-heritable

Question 4

explain how the environment could lead to an epigenetic change

Answer 4

an environmental factor (e.g diet) alters DNA methylation or histone modificiation.
this then leads to changes in gene expression

Question 5

how might genetic variation be increased as a result of sexual reproduction?

Answer 5

• crossing over between homologous chromosomes during meiosis at prophase I
• independent assortment of chromosomes during meiosis at metaphase I
• independent assortment of chromatids during meiosis at metaphase II
• mixing of two different parental genotypes at fertilisation

Question 6

what is heritable variation also known as?

Answer 6

genetic variation

Question 7

define selection pressure

Answer 7

an environmental factor which can alter the allele frequency of the alleles present at a particular gene locus in a population

Question 8

what are selective agencies?

Answer 8

they exert selection pressure

Question 9

name 4 selective agencies

Answer 9

• climate
• human impact
• supply of food
• breeding sites

Question 10

outline the correlation between phenotypes and survival & breeding success

Answer 10

• organisms with well adapted phenotypes (those whose alleles give them a selective advantage) are more likely to survive than those who phenotypes are less well adapted (those whose alleles mean they are selected against)
• the phenotypes which increase the chance of organisms surviving are also likely to give a higher breeding success

Question 11

what can selection pressures change?

Answer 11

allele frequencies of the alleles present at a particular gene locus in a population

Question 12

define gene pool

Answer 12

total of all alleles for all of the genes in a population

Question 13

what selection pressure led to the increase in dark form moths during the industrial revolution

Answer 13

• soot from factories led to tree bark darkening
• dark form moths were camouflaged
• reduced predation

Question 14

if a dominant allele produces a phenotype which is a selective disadvantage, what will happen to the frequency of this dominant allele in the gene pool? explain your answer.

Answer 14

• frequency of allele will decrease and it may disappear from gene pool
• any organism with it will have the disadvantageous phenotype so is less likely to survive, breed and pass it onto the offspring

Question 15

what is the effect of a recessive allele that produces a selective disadvantage? explain your answer

Answer 15

• organisms which are heterozygous will not have the disadvantageous phenotype so will survive and pass along the allele
• this means that the recessive allele will remain at low frequency in the population

Question 16

equation linking frequency of dominant allele and frequency of recessive allele?

Answer 16

frequency of dominant allele + frequency of recessive allele = 1

Question 17

define genetic drift

Answer 17

variation in allele frequencies due to chance

Question 18

define Hard-Weinburg

Answer 18

a principle stating that the frequencies of dominant and recessive alleles and genotypes will remain constant from one generation to the next, if certain conditions remain true

Question 19

what are the conditions for the Hardy-Weinburg principle?

Answer 19

• a large population (100+ individuals)
• no selection for or against any phenotype
• random mating throughout any population
• no mutations
• the population is isolated, i.e. no immigration or emigration

Question 20

what do the three terms of the hardy weinburg binomial expansion indicate?

Answer 20

• p^2 : frequency of AA (homozygous dominant)
• 2pq : frequency Aa (heterozygous)
• q^2 : frequency of aa (homozygous recessive)

Question 21

define evolution

Answer 21

change in the average phenotype of a population over time

Question 22

define natural selection

Answer 22

• 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

Question 23

outline the process of natural selection

Answer 23

• in any population, there is variation due to mutations
• there is the potential for large numbers of offspring but the number of offspring stays roughly the same
• competition means there is 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 genetically change so much they become a different species

Question 24

what are the two types of natural selection?

Answer 24

1. directional selection
2. stabilising selection

Question 25

describe directional selection

Answer 25

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

Question 26

describe stabilising selection

Answer 26

- 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

Question 27

define polygenic

Answer 27

controlled by more than one gene

Question 28

explain why you would carry out a student's t-test

Answer 28

to test if there is a significant difference between the means of two samples of intervals/continuous data, when data is normally distributed

Question 29

explain why you would carry out a chi-squared test

Answer 29

to test if there is a significant difference between the observed and expected data in set categories or discontinuous data

Question 30

define demes

Answer 30

in a population of one species there are breeding sub units

Question 31

define species

Answer 31

a group of phenotypically similar organisms that can interbreed to produce fertile offspring

Question 32

describe speciation

Answer 32

- 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 - if reunited after many generations, the demes are incapable of breeding successfully - two new species are formed, each with their own gene pools

Question 33

what can speciation occur due to?

Answer 33

- genetic drift, in isolated population, changing allele frequency by chance - the founder effect, of disproportionate allele frequencies in small populations - natural selection

Question 34

what are the two main types of speciation?

Answer 34

- allopatric speciation - sympatric speciation

Question 35

define founder effect

Answer 35

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

Question 36

madagascar is an island off the coast of africa. many of the species there are unique. suggest how the founder effect may have led to these unique species arising

Answer 36

- a small group of organisms from mainland Africa colonised Madagascar - by chance the allele frequency in their gene pool differed from mainland gene pool - different selection pressures on the island - over time, these allele frequencies become very different - unique species formed

Question 37

define allopatric speciation

Answer 37

when a physical barrier splits a population into new separate demes, preventing interbreeding and the flow of genes

Question 38

define sympatric speciation

Answer 38

when a population is split into separate demes by a non geographical factor, e.g. behaviour, preventing interbreeding and flow of genes

Question 39

what are examples of physical barriers?

Answer 39

mountain river

Question 40

outline the process of allopatric speciation

Answer 40

1. large population with common gene pool 2. population is separated into 2 demes by a physical barrier, preventing flow of genes 3. mutations and different selection pressures on each deme alter the gene pool 4. if the barrier is removed the gene pools will be so different that interbreeding will not be successful

Question 41

what causes sympatric speciation?

Answer 41

- behavioural isolation - morphological isolation - gametic isolation - seasonal isolation - hybrid inviability - hybrid sterility

Question 42

describe behavioural isolation

Answer 42

in animals with elaborate courtship behaviour, the steps in the 'display' of one group or organisms may fail to attract the necessary response in a potential partner from another group of organisms

Question 43

describe morphological isolation

Answer 43

the body parts of organisms may not be compatible enough for them to mate - this is seen in insects where the rigid exoskeleton mean that the genitalia of male and females must be complementary

Question 44

describe gametic isolation

Answer 44

barriers preventing gametes of different species fusing

Question 45

give examples of gametic isolation

Answer 45

- stigma will only produce a sugary secretion for pollen germination if the pollen is compatible - spermatazoa can only survive in the oviduct if from the same species

Question 46

describe seasonal isolation

Answer 46

in reproductive organs of different groups mature at different times of year the groups are unable to interbreed - this can occur due to differences in mating seasons or differences in flowering times

Question 47

describe hybrid inviability

Answer 47

fertilisation may occur but incompatibility between genes of the parents prevents the development of an embryo - hybrid embryos formed from sheep and goats die in the early stages of development

Question 48

describe hybrid sterility

Answer 48

in some cases, an embryo can survive when individuals of different species breed. - the hybrid offspring are unable to produce gametes and are sterile (unfertile)

Question 49

explain why hybrid organisms are sterile

Answer 49

- cannot form bivalents/homologous pairs during prophase I - meiosis cannot take place - gametes aren't produced