Introduction to Evolution I

Question 1

What is evolution?

Answer 1

It is the accumulated, heritable changes within a population, over generations, giving rise to new species.

Question 2

Describe the rapid evolution of influenza virus by reassortment.

Answer 2

Different flu strains infecting the same cell can swap parts of their genetic material.
This is why the 2009 (and 2010) H1N1 strain contains alleles from human, avian and swine flu.

Question 3

How can we observe evolution?

Answer 3

1) Obtain 12 colonies of E. Coli from a single clone
2) Subculture it every day, for 27 years
3) Freeze the culture samples every 500 generations (75 days)
4) Over the completed experiment, 63,500 generations would have been observed
5) We then test the cultures for the ability to grow in citrate in the presence of oxygen

The ability to use citrate evolved after about 30,000 generations in one group in particular. Earlier generations from this same ancestral E. Coli were repeatedly able to evolve the ability to utilise citrate at about the same time.

Question 4

How does a bacteria culture evolve the ability to utilise citrate?

Answer 4

The ability of bacteria to utilise citrate is controlled by its citrate gene, which is controlled by the citrate promoter. The gene is normally turned off in the presence of oxygen. It happens to be situated next to the RNK promoter, which controls the RNK gene. This is turned on in the presence of oxygen.

It was found that, in cultures that were able to utilise citrate, the citrate gene and RNK promoter were duplicated right after each other. This didn’t affect the existing citrate gene, but the duplicated citrate now has an RNK promoter before it. This meant that the citrate gene would now work in the presence of oxygen.

Question 5

How do changes in the genotype arise?

Answer 5

• mistakes can occur during DNA replication
• natural selection: organisms better adapted to the environment survive to produce more offspring
• genetic drift: variation in relative frequency of different genotypes in small populations, due to chance
• gene flow: transfer of alleles (genes) from one population to another
• applied selection: plant/animal breeding, antibiotic/drug/pesticide resistance, etc.

Question 6

Sickle Cell Anaemia is autosomal recessive and causes early death. According to the theory of natural selection, why is SCA common in tropical regions?

Answer 6

The places where SCA is prevalent correlate with the regions of endemic malaria. Heterozygotes (carriers) of SCA have improved malaria survival rates due to lower parasitaemia and fewer severe complications.
So, even though it’s a harmful mutation, it confers heterozygote advantage.

Question 7

What is taxonomy?

Answer 7

It is the naming of groups of organisms (taxa).

Question 8

What is classification?

Answer 8

It is the arrangement of taxa into an ordered, hierarchical system.

Question 9

What is phylogeny, and what are phylogenetic trees?

Answer 9

Phylogeny is the determination of ancestral relationships or organisms and their evolutionary history.

Phylogenetic relationships can be represented using phylogenetic trees.
The Phylogenetic Tree of Life illustrates that all Bacteria, Archaea and Eucaryota are derived from a distant, common ancestor called LUCA.

A phylogenetic tree can also be based on morphological criteria.

Question 10

Describe the fossil record.

Answer 10

Rock and organic material can be dates by measuring the decay of naturally occurring radioactive isotopes.
Fossils usually form in sedimentary rock. Fossil dates can be bracketed by the age of surrounding igneous rock.

Question 11

Describe how fossils form.

Answer 11

1) There are surface erosions that occur, creating sediment.
2) When an organism dies, it settles onto the sediment, which then becomes sedimentary rock.
3) Thus, a fossil forms of that organism. The soft sediment preserves the fine details in the bones, teeth, and leaves of plants.

Question 12

How does the fossil record provide evidence for evolution?

Answer 12

The fossil record allows for the order and age of organisms to be determined.
When we do that, we find that only simpler life forms are present in older rock. We also find fossilised organised, that no longer exist.

This proves that evolution takes place, for more complex organisms to have evolved from the simpler organisms, and for the extinct organisms to have died out due to their inability to survive in the environment.

Question 13

What is radiometric dating?

Answer 13

It is measuring the ratio of parent/daughter nuclides.

Radioisotopes can be used to date rocks and fossils.

Question 14

What are some requirements for radiometric dating?

Answer 14

• the rate of radioactive decay is constant: not affected by temperature, pressure, chemicals, electrical or magnetic fields
• the parent and daughter nuclides cannot leave or enter material after it is formed
• there must be no intrusion of other, later rock
• there must be no contamination

Question 15

What is the Cambrian Explosion?

Answer 15

It is the rapid appearance (and disappearance) of complex organisms in the fossil record (about 525 million years ago).

Question 16

What do we mean by the term homologous?

Answer 16

We say two structures are homologous when they are derived from a common ancestral version.
Because these structures are derived from a common ancestor, they form by a common mechanism.
An example would be tetrapod limbs.

Question 17

What is meant by the term analogous?

Answer 17

We say two structures are analogous when they have no common ancestor - the similar structure are produced by selection to meet similar function (convergent evolution).
Although they have different developmental mechanisms, some molecular components may be the same.
An example would be insect wings and bird wings.

Question 18

Describe tetrapod limbs.

Answer 18

Tetrapod limbs are homologues, but with great diversification.
Some common bones between them are the humerus, radius and ulna.
All modern tetrapod limbs have evolved from later remodelling of the elements (eg. human, bird, bat, horse, whale).

Question 19

What is molecular analogy?

Answer 19

It’s also known as molecular convergence.
It’s when proteins have the same function with no structural identity.
An example would be crystallins (lens proteins)

There are many different transparent proteins in different species. They all have other functions (in addition to being stable, transparent and refractive).

Question 20

What is molecular homology?

Answer 20

Molecular homology is determined from amino acid or nucleic acid sequence.

• two genes from the same organism (by duplication) are called PARALOGS
• two genes from different organisms (common ancestor) are called ORTHOLOGS

Question 21

How is gene duplication a key driver in evolution?

Answer 21

When a gene is duplicated, the original gene can go back to executing it’s original function. The duplicated gene, however, is free to be modified.
It may perform functions complementary to the original gene’s function (acting as a backup). It may also, over time, evolve non-overlapping functions.

Question 22

What are Hox genes?

Answer 22

Hox genes are developmentally important transcription factors.
They bind DNA in a sequence-specific fashion and regulate the expression of adjacent/nearby genes.
The arrangement of Hox genes along the chromosome matches their expression patterns in the embryo. They’re ordered along the Anterior-Posterior (cranio-caudal) axis.