6 - ICH - Variation & Selection

Question 1

Define:

• Discontinuous variation
• Continuous variation

Answer 1

Discontinuous variation = Phenotypes that have qualitative differences i.e. clear cut groups e.g. blood groups.

• Characteristics are controlled by a single gene

Continuous variation = Phenotypes that have quantitative differences i.e. a wide range of overlapping intermediates e.g. height and mass.

• Characteristics are controlled by at least 2 genes (polygenes)

Question 2

6 Sources of variation

Answer 2

• Mutations - produces new alleles
• Crossing over in prophase 1 of meiosis
• Independant assortment of homologous pairs of chromosomes in metaphase 1 of meiosis
• Independant assortment of daughter chromatids in metaphase 2 of meiosis
• Random fertilisation to produce different combinations of genomes
• Environmental influences

Question 3

State 2 processes which act on a natural population of organisms to alter allele frequencies

Give an example of each

Answer 3

Random processes e.g genetic drift

Non-random processes e.g. selection

Question 4

What is genetic drift?

Where is it likely to occur + why?

Answer 4

Genetic drift = The change in allele frequency that occurs by chance, rather than by natural selection

More likely to occur in island populations as they’re small + isolated from other populations:

• ∴ increases levels of inbreeding
• They form a small sample of the original population ∴ unlikely to be representative of the large population’s gene pool

Question 5

What is the Founder effect?

Answer 5

Founder effect - Genetic drift:

When this process occurs on a recently isolated small population

• More likely to occur in island populations as they’re small + isolated from other populations:
• ∴ increases levels of inbreeding
• They form a small sample of the original population ∴ unlikely to be representative of the large population’s gene pool

Question 6

What does natural selection cause? (3)

Answer 6

• Change in allele fequency of a gene in a population
  
  • Arises because individuals have different fertilities
  • i.e. individuals with one genotype have more offspring than those of another genotype. Genotype with highest fertility rate = fit
• Increases chance of advantageous adaptive alleles being passed onto the next generation
• Decreases the chance of non-advantageous non-adaptive alleles being passed onto next generation

Question 7

Summarise Natural selection is 6 points

Answer 7

1. Individuals in a population compete for environmental resources e.g. food, space, light…
2. There is genetic variation between individuals in a population
3. One or more individuals have a competative advantage over the others because they possess a favourable allele of a gene
4. These individuals will have more offspring than other without the advantageous allele i.e. they’re fitter
5. Fitter individuals pass on their favourable allele onto some of their offspring
6. More individuals in the next generation will possess the favourable allele - Increased allele frequency. This is natural selection

Question 8

What does an individual’s fitness within a population mean?

Answer 8

Their ability to transmit their alleles to the next generation as a result of natural selection

Question 9

Define selection pressure

Answer 9

Selection pressure = A environmental factor that gives greater chances of survival amd reproduction to some individuals

Question 10

What is stabalising selection? (5)

Illustrate this concept with an example

Answer 10

Staballising selection occurs when environmental conditions remain the same ∴ characteristics are selected over several generations

• Works against both extremes + reduces the range of possible phenotypes
• Fittest phenotype is in the middle of the range + the closer you are to the middle the more likely to survive + reproduce you are - increased fitness
• After selection, variation around modal value is less
• Allele frequences remain the same

E.g. baby mass

Question 11

What is directional selection? (6)

Illustrate this concept with an example

Answer 11

Directional selection occurs when environmental conditions change e.g. climate changes giving longer/ colder winters

• Fittest phenotypes are those at one end of the range. Works againsts one extreme
• Selection results in a new modal value
• Allele frequencies change

E.g. House mice and length of hair

• Mice living in cold stores have an advantage if they have long hair rather than short hair
• Over a period of time selection should result in mice with long hair

Question 12

What are isolating mechanisms? Their purpose?

Answer 12

Can lead to populations with different gene pools from each other. This can result in the evolution of a new species i.e. speciation - formation of a new species from an exisiting one

Question 13

State 3 forms of isolation

Answer 13

Ecological (geographical) isolation

Seasonal (temporal) isolation

Reproductive isolation

Question 14

What is allopatric speciation

Answer 14

Populations becoming isolated through a combination of geographical isolation and natural selection

Question 15

What is ecological (geographical) isolation?

What does it do? (4)

What is it an example of?

Answer 15

Ecological (geographical) isolation = 2 or more populations are seperated from each other by a geographical barrier e.g. mountain range, ocean…

• Habitat conditions differ on either side of the barrier ∴ there’s different selection pressures
• Mutations in the seperated populations are independant of each other
• Both populations evolve to suit their particular habitat by natural selection
• If the populations meet up man generations later, they may have evolved into different species

Is an example of allopatric speciation

Question 16

What is seasonal (temporal) isolation?

Answer 16

Seasonal (temporal) isolation = 2 populatiions may be unable to breed as they are active at different times of the day or year

Question 17

What is reproductive isolation?

Give 5 examples

Answer 17

Reproductive isolation = Mechanisms that prevent closely related species interbreeding

E.g.

• Different courtship behaviour
• Mechanical problemts with mating e.g. different sized/ shaped reproductive organs
• Gametes aren’t compatible & can’t fuse e.g. different chromosome number
• Zygote is not viable i.e. it dies
• Hybrid offspring are sterile

Question 18

Define gene pool

Answer 18

Gene pool = All of the genes and their different alleles in a population. Each individual has a portion of the gene pool in their genome

Question 19

Define speciation

Answer 19

Speciation = Formation of a new species from an existing one

Question 20

What does sympatric speciation mean?

Answer 20

Sympatric speciation = Speciation without geographical isolation

Question 21

What is the biological species concept? (2)

Problems with this definition? (3)

Answer 21

“A group of similar organisms that can interbreed and produce fertile offpring”

The group is ∴ reproductively isolated from others.

Problems:

• Not all organisms reproduce sexually
• Some members of a species may look different from others of the same
• Can’t be used for extinct species or fossils

Question 22

What is the phylogenetic (cladistic / evolutionary) species concept?

• Why is it a better definition than the biological species concept?
• What is the concept based upon?
• What is a clade in relation to species?

Answer 22

“A group of organisms that have similary morphology (shape), physiology (biochemistiry), embryology (stages of development), behaviour and occupy the same ecological niche”

• It’s based on the knowledge that closely related species have more similar DNA base sequences and molecular structures than species that are less closely related
• Definition can be used for all organisms, even if it’s not known whether or not 2 species can interbreed

Clade = Any group of organisms with base sequences that are more similar to each other than any other group

Question 23

What does the phylogenetic descent of an organism mean?

Answer 23

Phylogenetic descent = The common ancestor of organisms within a clade

Question 24

What is a monophyletic and paraphyletic group?

Answer 24

Monophyletic group = Single ancestry - used to describe members within a clade

Paraphyletic group = Ignores some of the clades that might have been included e.g. reptiles ignored birrds which are related to reptiles

Question 25

What is a **cladistic approach** to classification? What does this approach focus on? (4)

Answer 25

**Cladistic approach** = Molecular analysis * Evolution (phylogenetic relationships) * Molecular analysis * DNA sequencing * Inclusion of extinct species

Question 26

What is **artificial selection**?

Answer 26

**Artificial selection** = When humans select individuals in the population to breed together to get desirable traits * Characteristics of species are changd fairly rapidly as humans direct their evolution

Question 27

Why does artificial selection cause characteristics of a species to change relatively rapidly?

Answer 27

Humans are directly their evolution

Question 28

_Artificial selection:_ Modern dairy cow * What are they mainly selected for? (3) * Sequence of events in modified artificial selection for fairy cow (7)

Answer 28

*NOTE: Unconcious selection already bagan before humans intervened as farmers only kept the desirable aimals for breeding* Modern dairy cow - example of artificial selection for fertility, milk yield and quality: 1. Each cow's milk yield is measured + recorded 2. Offspring of bulls are tested to find which bulls produce daughters w/ high milk yields 3. Best bulls are selected + kept 4. Best cows are given hormones to stimulate them to produce many eggs 5. Eggs are fertilised in vitro w/ sperm from selected bulls 6. Embroys may be cloned into identical embryos 7. Embryos implanted into surrogate mothers

Question 29

Apart from fertility, milk yield and quality what other characteristics are selected for in dairy cows? (4)

Answer 29

* Long lactation period - produce milk for longer * Large udders - makes miking easier * Resistance to mastitis (inflammation of the udders) and other diseases * Calm temperament

Question 30

Name 2 modern techniques that make artificial selection for dairy cows much easier than back in the oldern days

Answer 30

IVF Artificial insemination

Question 31

_Artificial selection:_ Bread wheat and why is it an example of **polyploidy**

Answer 31

Bread wheat = plant from which flour is produced for bread-making. * It originated from a hybrid of several plants. These hybrids were sterile but chance doubling of chromosome number (**polyploidy**) restored fertility on at least 2 occasions

Question 32

_Artificial selection:_ Bread wheat * What is it selected for? * How is it achieved?

Answer 32

* High wheat yield * Disease resistance * Higher tolerence to changes in climates * Short stalks (so they don't collapse under the weight of the ears) * Uniform stalk heights (makes harvesting easier) * Wheat plants with high wheat yield (e.g. large ears) are bred together * Offspring with the highest yields are bred together * This is continued for several generations to produce a ant that has very high wheat yield

Question 33

4 similarities between natural and artificial selection

Answer 33

* They change the allele frequencies in a population * They change the phenotypic characterisics of a species * They occur over many generations * They are evolutionary processes

Question 34

5 differences between natural and artificial selection

Answer 34

Question 35

Define **population genetics**

Answer 35

**Population genetics** = The study of the behaviour and distribution of alleles within a population and explains it in amthematical terms

Question 36

What does studying allele frequencies between generations allow?

Answer 36

Can see if selection is taking palce (allele frequency change) or is the population is in equilibrium (allele frequencies stay the same)

Question 37

Name the mathematical model which predicts that the frequencies in a population won't change from one generation to the next

Answer 37

Hardy-weinberg principle

Question 38

What conditions needed in order for the **Hardy-weinberg law** to apply (6)

Answer 38

* Population is large * No selection is happening * No mutation * No immigration * No emigration * Mating is random - all possible genotypes can breed with all others

Question 39

Give the equations for the **Hardy-weinberg principle** + explain the use of each one and how the results are useful

Answer 39

_Allele frequency:_ **p + q = 1** * **p** = freq of dominant allele * **q** = freq of recessive allele * **Σ allele freq \> 1** they're not alleles for the same gene (characteristic) * **Σ allele freq \< 1** there's more than 2 alleles for that gene _Genotype frequency:_ **p² + q² + 2pq = 1** * **p²** = freq of homozygous dominant genotype * **q²** = freq of homozygous recessive genotype * **2pq** = freq of heterozygous genotype