Lecture 4: Evolution and Speciation [G]

Question 1

Is molecular evidence very powerful evidence when working out lineages?

Answer 1

Yes

Question 2

Who redefined evolution in 1937?

Answer 2

Theodosius Dobzhansky redefined evolution: a change in allele frequency in a gene pool.

Question 3

Who thought that natural selection was the driving force behind evolution?

Answer 3

Charles Darwin

Question 4

Is microeveolution evolution below the species that can be inherited by the next generation?

Answer 4

Yes

Question 5

What was Darwin’s first postulate?

Answer 5

For any particulate trait, individuals within a species are variable

Question 6

What was Darwin’s second postulate?

Answer 6

Some variation is heritable and is passed from parent to offspring.

Question 7

What was Darwin’s third postulate?

Answer 7

Reproduction is not random, but is selected by Nature. individuals that reproduce the most are those with the most favourable variations (survival of the fittest). Nature favours such traits.

Question 8

Did Darwin have any experimental evidence for his theory of evolution by natural selection?

Answer 8

No

Question 9

Describe how Rosemary and Peter Grant tested Darwin’s 1st postulate (1976-1978)

Answer 9

• They noticed that the medium ground finch, Geospiza fortis, showed variation in beak depth.
• They captured every bird on the island and ringed it so that they could identify each bird.
• Every time the birds mated, they measured the size of the beaks of the offspring
• There was a normal distribution of beak size centred around 9.5 mm (beak depth). ranging from ~6 mm to 14 mm, average ~9.5 mm.
• This proved postulate 1 to be correct

Question 10

Are narrow beaks recessive?

Answer 10

Yes, because in 1978 there was a loss of small-beaked birds

Question 11

How did Rosemary and Peter Grant’s experiment prove Darwin’s second postulate to be correct?

Answer 11

• The Grants ringed all birds to track parent-offspring relationships and found that offspring beak size correlates with parental beak size.
• Showed that parents with small beaks tend to have offspring with shallow beaks, and that parents with broad beaks produced offspring with broad beaks.
• Thus proving that there is a large genetic component to determination of beak depth.

(This experiment was also repeated in 1978 despite changes in population size, confirming that beak size is a heritable trait.)

Question 12

Describe how Rosemary and Peter Grant’s experiment prove Darwin’s third postulate to be correct (nature favours organisms with favourable traits)

Answer 12

• At the start of the drought, seeds were soft and small, but by the end of the drought, the seeds got bigger and harder.
• By 1978, the population mean beak depth had increased, meaning that the change in food supply had created a selection for bigger birds with deeper stronger beaks.
• Thus, this proved Darwin’s third postulate and showed that natural selection can drive evolution.
• Conclusion: Natural selection favoured birds with larger beaks, altering allele frequencies in less than two years. (microevolution)

Question 13

How have Darwin’s postulates been proven?

Answer 13

Both experimentally and observationally (due to the help of Rosemary and peter grant as well)

Question 14

Is it true that microevolution is both a theory and a fact?

Answer 14

Yes

Question 15

Is it true that many mutations have little effect on ‘fitness’ and so they can’t be acted upon by natural selection?

Answer 15

Yes

Question 16

A dominant advantageous allele…

Answer 16

is ‘visible’ to natural selection as both heterozygote Aa and after the first generation, homozygote AA: it rapidly rises in frequency in the population.

Question 17

A recessive advantageous allele..

Answer 17

is only ‘visible’ to natural selection as homozygotes, and these take a long time to accumulate, so fixation takes longer

Question 18

What does natural selection do to favourable alleles in a population?

Answer 18

natural selection will tend to ‘fix’ favourable alleles in a population.

Question 19

On the top strand, how is dna read?

Answer 19

from left to right

Question 20

On the bottom strand, how is dna read?

Answer 20

from right to left

Question 21

Is DNA always read from 5’ to 3’?

Answer 21

Yes

Question 22

Do mutations in introns have an effect or not?

Answer 22

Mostly not, as they’re not selected for or against because they don’t have a critical function.

Question 23

Do mutations between genes have an effect?

Answer 23

Mostly not. Non-coding and not under a strong selective pressure.

Question 24

Some mutations in genes have no effect

Answer 24

Some mutations in genes have no effect

Question 25

Is it true that the frequencies of mutations can change in populations even in the absence of natural selection?

Answer 25

Yes

Question 26

Who tested genetic drifts effect on evolution experimentally?

Answer 26

Richard Lenski

Question 27

Describe Richard Lenski’s experiment

Answer 27

• Lenski’s E. coli Experiment began in 1988 and continues today.
• Lenski established 12 populations of E. coli, derived from a single ancestral strain. (Six Ara+ strains and Six Ara- strains)
• He filled 2 flasks with E coli. One had the sugar Arabanose, the other one didn’t. He grew them over night.
• He innoculated the flasks, diluted them, and grew them in a medium with glucose and citrate.
• He grew them over night, diluted them, and grew them again and again and again.
• This experiment has been running for over 35 years

Question 28

What was the timeline and findings of Richard Lenski’s experiment?

Answer 28

• In all flasks: populations evolved to grow faster and achieve greater cell density in the limited glucose environment, reflecting adaptation to their conditions. Populations evolved larger cells, an advantageous trait in this environment.
• In some flasks, defects in DNA repair evolved, giving ‘mutator’ phenotypes with elevated mutation rates. These were therefore able to evolve more rapidly
• In one flask, Ara-3, the ability to use citrate as a carbon source in aerobic conditions evolved, a trait rare in E. coli. Hence it is evident that Ara-3 evolved a mutator phenotype called Cit+.
• However, there was soon the loss of Cit+ by genetic drift, even though they had a selective advantage: sampling error. Cit+ has not re-evolved to date

(This metabolic innovation provided a significant growth advantage in the glucose-limited medium.)

Question 29

What now gives the Cit + strain of E coli a selective advantage?

Answer 29

• Cit+ strains can now utilise citrate as well as glucose as C and energy sources. (Citrate is transported into the cell using the citT transporter. It can then be oxidized via the citric acid cycle, contributing to ATP production and biosynthetic pathways). (additional carbon and energy source alongside glucose) (change from minimal medium)

Question 30

How was the Cit+ lost?

Answer 30

By genetic drift. even though they had a selective advantage: sampling error. Cit+ has not re-evolved to date

Question 31

What was the point of Richard Lenski’s experiment?

Answer 31

to explore how populations adapt over time through the mechanisms of mutation, natural selection, and genetic drift.

Question 32

What were the key findings of Leniski’s experiment?

Answer 32

• Fitness increases rapidly at first and then levels off, suggesting diminishing returns on adaptation.
• Beneficial mutations accumulate and interact with one another, often producing unpredictable outcomes.
• Genetic drift plays a significant role, even in large populations, as seen in the temporary loss of advantageous Cit+ strains.
• Evolution exhibits predictable trends (e.g., fitness increases) but also contingent events (e.g., citrate usage).
• The Cit+ E. coli in Ara-3 may represent the emergence of a new “species,” as citrate usage differentiates it from ancestral E. coli.

Question 33

What is a defining feature of E coli?

Answer 33

inability to utilise citrate in aerobic conditions (separating it diagnostically from pathogenic Salmonella species)

Question 34

What is gene flow?

Answer 34

The movement of alleles between previously separate populations, through migration, mating, or dispersal

Question 35

How can alleles move through the process of gene flow?

Answer 35

• migration of adults and subsequent mating
• movements of gametes (e.g. pollen) and subsequent fertilisation

Question 36

Genetic drift and gene flow

Answer 36

• Genetic drift removes genetic variation within demes (sub-populations) but leads to differentiation between demes, all by random changes in allele frequencies.
• ② Gene flow introduces new alleles into demes within a metapopulation and by itself can lead to genetic homogeneity between demes.
• If any of the demes becomes reproductively isolated, then it’s easy for natural selection to work upon it.
• combination of gene flow, genetic drift and selection promotes population divergence and can lead to speciation

Question 37

What is a species?

Answer 37

At eukaryotic level: a population of organisms that can potentially or actually interbreed, giving viable fertile offspring. Reproductively isolated

Question 38

Why are the tree frogs Hyla chrysocelis and H. versicolor not a good example of separate species?

Answer 38

• Because recent studies indicate occasional gene exchange

Question 39

Is the tube mosquito a good example of 2 separate species?

Answer 39

Yes

Question 40

Describe the Culex pipiens pipiens mosquito

Answer 40

• a bird-biting mosquito
• Females need a blood meal before laying eggs
• Breed seasonally
• Swarm
• Lay eggs in open spaces
• Inactive over winter

Question 41

Describe the Culex molestus mosquito

Answer 41

• a mammal-biting mosquito
• Females can lay eggs without a blood meal
• Breed continuously
• Do not swarm
• Lay eggs in enclosed spaces
• Active during the winter.

Question 42

What is an example of macroevolution?

Answer 42

The Big bird lineage

Question 43

Is it true that a dominant advantageous allele is ‘visible’ to natural selection as both heterozygote Aa and after the first generation, homozygote AA: it rapidly rises in frequency in the population?

Answer 43

Yes

Question 44

Is it true that a recessive advantageous allele is only ‘visible’ to natural selection as homozygotes, and these take a long time to accumulate, so fixation takes longer ?

Answer 44

Yes

Question 45

What is speciation?

Answer 45

The formation of new species through reproductive isolation.

Question 46

Through which mechanisms can speciation occur?

Answer 46

Reproductive Barriers

Gene Flow Reduction

Question 48

Describe real time speciation on daphne major

Answer 48

A male finch from the nearby Española Island flew to Daphne Major, a small, isolated island, and mated with a local female of a different species.

This marked a significant gene flow event, introducing new genetic material into the local population.

The first-generation offspring (F1) were 50% immigrant genes. These hybrids were viable and fertile, meaning they could reproduce with others.

Female finches chose mates based on two key factors: beak, and song. This selective mating behavior reduced the likelihood of hybrids breeding with other local finches, creating a reproductive barrier.

Over several generations, the hybrid population became reproductively isolated from other finches on the island.

This isolation led to the emergence of a new species, colloquially called the “big birds.” The new species displayed distinct traits (larger size and beak shape) and did not interbreed with other local species.

Question 49

Define macroevolution

Answer 49

Large-scale evolutionary changes, such as speciation events or the emergence of major new traits.

Question 50

What is an example of macroevolution?

Answer 50

Hybrid finches on Daphne Major demonstrate macroevolution over just a few years.

Question 51

Are the london tube mosquitoes an example of speciation?

Answer 51

Yes