Genes Evolution & Development L4

Question 1

What is the biological definition of a species?

Answer 1

Population of reproducing organisms that is isolated from other populations.

Question 2

Various ways/forms in which species can arise?

Answer 2

• Over time along ancestral lineage
• Across locations -> geographic isolation
• Variety of genetic effects
   Identified by comparison of populations -> population genetics.
   These processes can be observed.

Question 3

What is the controversy amongst scientists surrounding the definition of species?

Answer 3

• (Biological definition) -> Population of reproducing organisms that is isolated from other populations.
   However:
  Prokaryotes & eukaryotes -> not strictly sexual Eg. plants.
  Doesn’t consider extinct organisms
  Bacteria & archaea -> horizontal gene transfer.
• Definition is widely debated/ discussed among scientists. -> each bases on their own field of interest.

Question 4

What is a species?

Answer 4

• Group of individuals with similar characteristics & DNA sequences, sharing common ancestors, which can reproduce to produce fertile offspring.

Question 5

What is speciation? Outline the process of speciation.

Answer 5

• The evolutionary process in which new, distinct species evolve from an existing population due to reproductive isolation.
• Population divides -> populations become isolated from one another -> selection pressures, adaptations, changes in characteristics, mutations -> do not recognise when reintroduced.
   Population becomes reproductively isolated following start of speciation due to
   Selective pressures
   Genetic Drift
   Genetic mutations
  Of each individual population.
• Consequential event of specific conditions, involving by-products of subsequent selection.
• Occurs over long time periods -> selection not involved.

Question 6

Types of speciation?

Answer 6

1. Allopatric
2. Sympatric
3. Paraphatic

Question 7

Describe allopatric speciation with an example

Answer 7

1. Allopatric speciation:
    (Allos = other; patra =homeland) -> Classic, widespread.
    Geographical isolation of existing population
   Eg. Antelope Squirrels -> Grand Canyon. -> Birds on both sides show no such effects/speciation.

Question 8

Describe sympatric speciation with an example.

Answer 8

2. Sympatric speciation:
    (Sym = same ; patra = homeland) -> Theoretically debated -> animals -> few examples
   -> Commonly found in plants
    No geographical isolation of groups.
   Eg. Change in behaviour -> nocturnalism of one group
   -> natural selecton -> use of different resources
   Eg. Shortcleuch Waters –(see also allypolyploid evolution below)

Question 9

What is paraphatic speciation?

Answer 9

 Somewhat but not complete geographical isolation -> reproductive barriers when restricted gene flow.

Question 10

Average length of speciation event?

Answer 10

(Mean time: Single eukaryotic population -> Two reproductively isolated populations
> Up to 5,000,000 years.

Question 11

• Different types of selection?

Answer 11

• Directional selection -> population tends in direction of one characteristic
• Disruptive Selection -> range is separated to two extremes
• Stabilizing selection -> focused population around mean

Question 12

• What is genetic drift?

Give examples of how it could occur.

Answer 12

• What is genetic drift?
Substantial change in genetic variation due to the chance disappearance of particular genes of a population, without regarding advantageous values of characteristics involved.

Eg. 1. Bird -> alleles for two different beak sizes -> by chance offspring may inherit only one particular beak size -> one allele disappears & genetic variation reduced.
Eg. 2. Certain plant -> alleles for blue & yellow flowers -> during a fire, many yellow flowers destroyed -> blue flowers are now dominant by chance -> reproduce more blue than yellow -> eventual disappearance of yellow altogether as reproduction rate not as high.

Question 13

Name the types of genetic drift.

Answer 13

1. Bottleneck

2. Founder Effect .

Question 14

What is a bottleneck?

Answer 14

Bottleneck -> Random event -> loss of certain characteristics/alleles -> decreases range of allele frequency.

Question 15

What is the founder effect?

Answer 15

.Founder Effect -> New population started by few members of original population -> decreased genetic variation.

Question 16

Describe the effect of different sized populations.

Answer 16

• > Small population -> leads to substantial genetic differences.
• > Large randomly mating pop -> Unlikely for large fluctuations in gene pool variety.
• > Small pop -> random effects -> major changes without influence of selective process.

Question 17

Why does genetic drift occur?

Answer 17

No intention/purpose – pure chance - just because of statistics of small population.

Question 18

Two examples of speciation?

Answer 18

1. Blackcap
    Breeds -> Central Europe
    Winter -> Spain (& UK for last 30yrs)
   Assortative mating -> Strong signs of genetic & morphological differences across Spanish & UK populations
   • Varied arrival time at breeding grounds -> rapid genetic change.
2. Hooded & Carrion crows:
    Distribution: -> split down the middle of Europe (carrion->west ; hooded->east)
    Different colour patterns -> mate with others of similar pattern & different dominance behaviour.
    Appear distinct species but have extremely similar genetic sequences.
    Some fertile hybrids.
    Found 83 fixed differences in genetic sequence -> 81 occurring on a small region of single chromosome -> chromosome 16.
   Transcription factors -> pigmentation & vision.
   Likely contained in an inversion -> free genetic interchange of chromosomes except in F-K region -> cross over in this region -> inviable zygotes.
   • Inversion keeps genes together -> pigment & preference for associated pigment -> stable system.

Question 19

What is an inversion?

Answer 19

Breakage & backward insertion of part of a chromosome back into the full chromosome.

Question 20

Name the genetic factors in speciation.

Answer 20

• Genetic Drift & Bottlenecks -> long periods of isolation
• Natural selection

Question 21

Describe the process of reinforcement as a role of selection/Role of selection in divergence after selection.

Answer 21

Two populations have divided enough -> unfit hybrids
Natural selection prevents formation -> strengthens reproductive isolation & difference in species through reinforcement.
Individuals who can avoid unproductive matings (infertile offspring) -> advantageous
> If mating does result in fertile hybrids -> hybrid breakdown at/after F2.
Natural selection directly favours evolution of species a s distinct groups.

Question 22

What are hybrid zones?

List the possible outcomes of a hybrid zone.

Answer 22

• Areas in which hybrids are made as a result of sympatric speciation & reinforcement by selection can be observed.
  1. Fusion
  2. Reinforcement of isolation
  3. Stability

Question 23

Describe how fusion occurs as a result of a hybrid zone.

Answer 23

1. Inter-fertile -> Lots of gene flow between populations = fusion of two groups.

Question 24

Describe how reinforcement of isolation occurs as a result of a hybrid zone.

Answer 24

2. Some gene flow -> significant loss of fitness in hybrids = reinforcement of isolation.

Question 25

Describe how stability occurs as a result of a hybrid zone.

Answer 25

3. Limited gene flow -> few hybrids -> negligible loss of fitness = stability

Question 26

Explain the process of polypoloid speciation with an example.

Answer 26

(Allypolyploid evolution)
• Occurs in plants -> incr. genetic, physiological & developmental flexibility.
1. Plant doubles it’s number of chromosomes due error in formation of gametes. -> Can no longer reproduce with others of it’s species.
2. Therefore reproduces with itself to produce a new species.
Eg. M. guttatus -> 14 pairs chromosomes ;
M. luteus -> 30/31 pairs
-> Interbreeding produces triploid hybrids -> 3 copies of each chromosome pair. -> Sterile – Can’t produce fertile offspring.
M. peregrinus -> six copies of each chromosome pair -> evolved from duplication event in infertile offspring.
Can produce normal haploid gametes with 46 chromosomes – x4 guttatus.
Are inter-fertile & isolated from other closely related species.