Week 5

Question 1

what is the species concept?

Answer 1

Species consist of interbreeding populations that evolve independently of other populations

Question 2

morphosepcies concept

Answer 2

Species defined by morphological differences and similarities
Cryptic species (look the same species but aren’t the same) make this hard

Question 3

phylogenetic species concept

Answer 3

Species are identified by estimating the phylogeny of closely related populations and finding the smallest number of monophyletic groups

Question 4

biological species concept

Answer 4

Reproductive isolation
Organisms that only mate together, do not often hybridise and produce fertile offspring

Question 5

bacteria and archaea and problems with defining species

Answer 5

Best defined by their ecological context and what environment they can thrive in which is dependent on genes

Question 6

speciation

Answer 6

begins when gene flow is disrupted and populations become genetically isolated.

Question 7

physical isolation as a barrier to gene flow

Answer 7

Causes reproductive isolation
Allopatric speciation
Dispersal (part of a species moves to an island etc.) or vicariance (a divide like a mountain range appears)

Question 8

polyploidy and chromosome changes as a barrier to gene flow

Answer 8

Changes in chromosome number isolate populations genetically
Speciation triggered by changed in chromosome number is especially important in plants

Question 9

adaptation to different habitats as a mechanism of divergence

Answer 9

Natural selection can cause populations to diverge based on ecological differences

Question 10

assortative mating as a mechanism of divergence

Answer 10

Assortative mating is when individuals choose mating partners according to traits and this can cause reproductive isolation

Question 11

what is hybridisation?

Answer 11

when recently diverged populations interbreed
Prezygotic and postzygiotic isolation
Polyploid hybrid speciation
Homoploid hybrid speciation
Hybridisation can have a variety of outcomes depending on the fitness of the hybrids relative to parental fitness. Can cause:
Species of hybrid individuals
Formation of a stable hybrid zone
Reinforcement

Question 12

what is reinforcement (related to hybridisation)

Answer 12

occurs when hybrid offspring have low fitness and natural selection leads to assortative mating and the prezygotic isolation of populations.

Question 13

what drives diversification?

Answer 13

Ecological controls
Range size
Intrinsic properties of organisms

Question 14

allopatric speciation by dispersal: drosophila

Answer 14

Distinct species on each volcanic island
These islands were never connected
This has happened by allopatric speciation driven by dispersal
Knowledge of geology is really important

Question 15

allopatric speciation by vicariance: Panamanian snapping shrimp

Answer 15

Isthmus - land with water on both sides
Multiple species in Caribbean Sea and multiple in Gulf of Panama
Isthmus used to be submerged but water levels went down when ice age formed lots of ice at poles about 3mya
Closest relative is on the other side of the barrier (Jordon’s rule) - pair-wise speciation event
Deep water species diverge earlier than shallow water species - shallow water species have had less time in allopatry - could still cross land bridge for longer than deep water species

Question 16

sympatric speciation

Answer 16

Don’t have a physical barrier - how do these species still diverge?
Strong reproductive isolation needs to occur
Need to convincingly show that the two species have always been together with an overlap
Got to be dealing with sibling taxa, cannot be an allopatric phase
A group starts to specialise on something - maybe an area with different nutritional value
Assortative mating - female prefers mates in same area of nutritional value as her - disruptive selection which should lead to divergence

Question 17

lord Howe island sympatric speciation

Answer 17

Split between two Howea palms happened after island formed
Island is too far from other islands for allopatric speciation to occur
Peak flowering time differs between palms (temporal separation) - reinforcement by assortative mating
One species began to specialise on a type of soil (different nutrition) and this caused difference in flowering time

Question 18

genetic drift as a mechanism of speciation

Answer 18

Case study: ring species of salamander
Divided by a valley
Cross-valley pairs are separated allopatrically
Two of the subspecies have encountered again but are still genetically diverse
Collected salamanders from each contact zone between species - what percentage seemed to be hybrids? Varied between pairs - 11%-89%
Is level of hybridisation (level of genetic admixture) due to cological divergence by natural selection or random by drift?
No strong relationship between climatic and vegetation dissimilarity and degree of admixture
Strong negative correlation between levels of hybridisation and nDNA distance and this may have just built up over time because of the mutations accumulating in the separated species

Question 19

natural selection as a method of speciation

Answer 19

Red Crossbills
Feed on pinecones
North America
Populations vary in bill morphology
Medium bill depths have highest survival
Optimal bill depth for feeding efficiency is about the same as survival
Feeding efficiency is correlated with survival (optimality model)
Looked at feeding efficiency and bill depth eating four other types of conifers
Fitness peaks and fitness valleys between all the kind of coniferous cones
Different populations have evolved to become specialised on particular conifer species
Hybrids are unlikely to survive as their bill sizes do not suit any type of cone so survival decreases

Question 20

mechanisms of speciation: divergence

Answer 20

Reinforcement: natural selection increases reproductive isolation between populations ie. selection against hybrids

Prezygotic vs postzygotic

Green-eyed tree-frog
Northern and southern and isolated southern populations
Rainforest species - arid cycles force frogs into refuges
When it gets wetter again, they have secondary contact
What is selecting against hybridisation?
N male with S female —> all failed (postzygotic)
S male with N female —> survived
Differences in male calls from different areas may be prezygotic isolation
Female choice experiment: northern females were not very discriminate, southern females preferred males from the south
And isolated southern frogs never choose northern males (strong reinforcement of prezygotic selection)

Question 21

mechanisms of speciation: hybridisation

Answer 21

If it is a fit hybrid, it is selected for
Common in plants (responsible for 70% of plant diversity) - because of polyploidy
Very uncommon in animals (3% of diversity)

Question 22

monophyletic group

Answer 22

clade eg. All mammals and their common ancestor

Question 23

paraphyletic group

Answer 23

has the common ancestor but not all of the descendants

Question 24

polyphyletic group

Answer 24

includes some but not descendants and does not include the common ancestor

Question 25

polytomy

Answer 25

section of phylogeny in which the relationships cannot be fully resolved to dichotomies

Question 26

parsimony analysis

Answer 26

tree with fewest evolutionary changes

Question 27

reversals

Answer 27

a trait is lost

Question 28

what traits can we use?

Answer 28

Homologous traits, use lots of traits but now DNA is mainly used

Question 29

what DNA can we use?

Answer 29

Neutral genetic markers (non-adaptive), not under strong selection, we know the mutation rate