Lecture 21

Question 1

Anagenesis

Answer 1

• microevolution
• evolution within the lineage (within the level of species)
• i.e. drought changed average beak depth over time. Must be heritable (genetic basis), and variability

Question 2

Cladogenesis

Answer 2

• macroevolution
• splitting of lineages into two or more lineages
• as it creates new species, we can see it as evolution above the level of species
• any evidence for common descent-homologous structures, biogeography, embryology-should suffice to demonstrate cladogenesis, but some won’t accept this

Question 3

Implication of genetics in reproductive isolation

Answer 3

• reciprocal translocation (produce aneuploid gametes)
• polyploidy: autopolyploidy, allopolyploidy
• hybrid sterility in Drosophila
• genetic incompatibilities (common ancestry, phylogenetic trees and parts, relatedness)

Question 4

Reciprocal Translocation

Answer 4

• non-homologous chromosomes exchange DNA
• this exchange can fix (goes to 100% frequency) in one population, while another population does not have it
• when gametes from these two populations meet in a hybrid, this individual produces aneuploid gametes and has reduced fertility

Question 5

Aneuploidy

Answer 5

• a genome lacks or has an extra copy(ies) of a chromosome(s). usually bad because it disrupts balance that exists for gene products in a cell
• ex in humans trisomy 21

Question 6

Hybrids

Answer 6

• have plenty of opportunity for aneuploid gametes (reduced fertility)
• the hybrid of an autopolyploid and a diploid produces aneuploid gametes and is largely sterile (look at slide) can’t fertilize each other or go back and fertilize parents

Question 7

Autopolyploid

Answer 7

• formed with the union of unreduced gametes of an individual(s) from the same species
• look at slide

Question 8

Allopolyploid

Answer 8

-is formed with the union of gametes from individuals of different species

Question 9

Bottom Line

Answer 9

-either by autopolyploidy or allopolyploidy, new species can arise because hybrids will produce aneuploid gametes and will therefore have reduced fertility

Question 10

Hybrid Sterility in Drosophila

Answer 10

• took females from D. simulans and crossed with males from D. mauritiana
• took hybrid females and back crossed with males from D. Simulans
• scored hybrid male fertility
• when male had sex chromosome from other species fertility would reduce to almost 0 and the only difference is the genetic composition between species
• this experiment implicated genetics in replicative isolation in drosophila

Question 11

Allopatric Populations

Answer 11

-isolated populations that do not exchange alleles
-incompatible genetic variants can evolve in separate populations due to vicariance
-geographic barrier prevents gene flow between populations and each can reach fixation in their own trait
-if they go back to sympatry they will be reproductively isolated and will constitute 2 species (if they mate hybrid dies)
-

Question 12

Sympatric Populations

Answer 12

• populations that exist in the same location and whose individuals can meet
• genetic variants that cause reproductive isolation may not increase in frequency easily
• one trait may take over via natural selection

Question 13

Indirect Evidence for Allopatric Speciation

Answer 13

-as you increase geographic distance between any pair of species, the species are more reproductively isolated

Question 14

Direct Evidence that Reproductive Isolation Can Evolve

Answer 14

• yeast in high salinity (S) and low glucose (M)
• mate to get S/M hybrid
• measured mitotic fitness in low glucose–S/M had lowest fitness and M had the highest
• measured mitotic fitness in high salinity–S/M had lowest fitness, S had highest
• measured meiotic fitness in environment permissive to sexual reproduction (count of daughter cells after 72 hr)–S/M had lower fitness

Question 15

Selection for Prezygotic Isolation Reinforcement–Dobzhansky-Muller Model

Answer 15

• initial populations is separated in two allopatric populations (no gene flow)
• two genes L and R
• allopatric populations accumulate incompatible genetic variants (red and green)
• L gene mutates into red so 1/3 are red and R gene mutates into green so 1/3 are green
• incompatible genetic variants increase in frequency due to selection and/or drift
• populations reunited in sympatry before reaching full reproductive isolation (barrier disappears before speciation totally complete)
• 3 possible crosses in the one where the red and green alleles cross 25% of offspring die because they are bad together–individuals in this cross have lower fitness and they like and/or are willing or capable of mating with the other population
• result of these genotypes having lower fitness?–>selection against breeding with the other population–>REINFORCEMENT (because of natural selection)

Question 16

DM Model-Speciation

Answer 16

• reinforcement finishes off speciation with population in sympatry
• reinforcement selects for prezygotic isolation
• postzygotic isolation increases and now have two species
• locus under selection due to reinforcement is neither L or R loci, it is a third locus which affects willingness/capacity to breed with the other population

Question 17

Indirect Evidence for DM Model***

Answer 17

• prezygotic isolation was stronger among sympatric than among allopatric pairs of taxa
• sympatric species have more prezygotic isolation because they breed together so there is reinforcement for individuals to recognize their species and select against hybrids
• allopatric don’t interbreed so there are no hybrids to begin with so reinforcement can’t happen
• if you have sympatric with total prezygotic isolation reinforcement doesn’t happen?

Question 18

Phlox

Answer 18

• why does P. drummondii have red flowers in sympatry?
• drift–>too much gene flow
• ecological natural selection (flower color is advantageous)–experiment: allow them to grow in sympatry and if hypothesis is right red would be more fit and would grow more–>no significant differences in fitness
• reinforcement (requirement: species must be in sympatry so they can breed and hybrids will be less fit and selection can select against them)–if this is the case would expect hybrids between cuspidata and the red to be selected against because they have lower fitness (assuming reinforcement has already happened)

Question 19

How does flower color influence hybridization?

Answer 19

• info for pollinators
• 3 arrays with 3 species each all with P. cuspidata and light-blue P. drummondii but then each one with light-red, dark-red and dark-blue P. Drummondii respectively
• reduced visiting between dark and P. cuspidata for situations with dark-blue, light-blue and P. cuspidata and dark-red, light-blue, and P. cuspidata