Evolution and speciation

Question 1

Definition of evolution

Answer 1

change in inherited traits of a population through successive generations: the genetic content of a population changes over time

Question 2

what is microevolution

Answer 2

changes in allelic frequencies in a population of a species over time

Question 3

3 main mechanisms of microevolution

Answer 3

natural selection, genetic drift, gene flow

Question 4

Testing of Darwin’s Hypothesis - Rosemary and Peter Grant 1976-1978

Answer 4

parents -> deep beaks/small beaks = offspring with deep/small beaks = genetic component to determination of beak characteristics

• drought decreased seeds available + were harder and larger -> many birds died -> 1978 only 90 birds hatched and mean beak depth increased–> selection for bigger birds with deeper stronger beaks

therefore: natural selection can drive evolution

Question 5

Chernobyl Example - selective pressure

Answer 5

• brighter frogs in b/g radiation conditions
• darker frogs in high radiation conditions
  –> due to natural selection as melanin protects against ionising radiation

Question 6

All 3 of the Darwinian postulates

Answer 6

1) individuals within a species are variable
2) at least some of these variations are heritable
3) reproduction is not random: survival of the fittest

–> populations adapt genetically to the environment

Question 7

types of mutations

Answer 7

insertions, deletions (both cause frameshift), substitutions

Question 8

allele fixation

Answer 8

the probability and the expected time for a rare allele to increase to a significant frequency within a population - natural selection will fix favourable alleles in a population

Question 9

dominant advantageous allele

Answer 9

Aa/AA - rises rapidly in frequency in the population = fixation is rapid

Question 10

recessive advantageous allele

Answer 10

only visible to natural selection as homozygotes - aa –> take long to accumulate = fixation takes longer

Question 11

what is genetic drift

Answer 11

the change in allele frequency of a species population due to chance - sampling error, absence of natural selection

Question 12

deme

Answer 12

subpopulation - localized group of organisms from a single species that tend to reproduce with other members of their deme

Question 13

metapopulation

Answer 13

collection of demes

Question 14

Null effects of mutations

Answer 14

Not all genetic changes alter the phenotype
- no effect due to degenerate code
- some mutations in introns have no effect (non coding section of RNA transcript - are spliced out before translation)
- most mutations between genes have no effect

Question 15

Richard Lenski - Long term evolution experiment method (LTEE)

Answer 15

Day 1: 2 flasks of E. Coli 24th Feb 1988, Ara+ and Ara-

Day 2: 12 flasks inoculated 1/100 dilution and grown in medium with glucose + citrate

Day 3 onwards: each flask subcultured, every 75 days, mean fitness estimated and samples frozen - experiment still running

Question 16

LTEE findings/ timeline

Answer 16

• in all flasks: growth rate increased and cell size increased
• in some flasks, defects in DNA repair evolved, giving ‘mutator’ phenotypes with elevated mutation rates -> can evolve more rapidly
• in one flask, Ara-3, the ability to use citrate in aerobic conditions evolved, + also evolved a mutator phenotype - generating multiple versions of the original cit+ cells
• loss of Cit+ by genetic drift even though they had a selective advantage: sampling error - not re-evolved to date

Question 17

How did citrate use in aerobic conditions (normally only in anaerobic conditions) evolve?

Answer 17

the Cit+ strain: duplication of the gene encoding citrate transporter-> deregulated transcription -> citrate transporters expressed in aerobic growth

Cit+ strains use citrate + glucose as C and energy sources -> selective advantage

Question 18

gene flow definition and general methods

Answer 18

the movement of alleles between previously separate populations

• migration of adults and subsequent mating
• movement of gametes e.g. pollen + fertilisation

Question 18

3 main mechanisms of gene flow

Answer 18

1) genetic drift removes genetic variation within demes (sub pops) but leads to differentiation between demes -> random changes in allele frequencies

2) Gene flow intro new alleles into demes within a metapopulation -> can lead to genetic homogeneity between demes

3) selection and reproductive isolation

Question 19

what can lead to speciation?

Answer 19

combination of gene flow, genetic drift and selection promotes population divergence -> speciation

Question 20

species definition at a eukaryotic level

Answer 20

a population of organisms that can potentially/ actually interbreed, giving viable fertile offspring

Question 21

3 types of reproductive isolation

Answer 21

geographic, temporal (different mating times), behavioral