Chapter 17 - Evidence for Evolution

Question 1

What have scientists conclude?

Answer 1

All living things use the same DNA code, adding weight to the hypothesis that all living things are related to each other and all have evolved from a common ancestor.

Question 2

Despite the common ancestor, the code in the DNA will be different for different species. Explain why.

Answer 2

• Although all species of organism have DNA, the sequence of bases in the DNA will vary.
• New genes are gained by mutation; others are lost by natural selection, genetic drift or some other process.

Question 3

What is speciation?

Answer 3

It is an ancestral species giving rise to two or more new species.

Question 4

What happens when Speciation occurs?

Answer 4

• The new species would have very similar DNA.
• However, as the new species gradually change through the processes mention in chapter 16, they accumulate more and more differences in their DNA.
• Species that are more distantly related have more differences in their DNA, whereas species that are more closely related share a greater portion of their DNA.

Question 5

What is a Genome?

Answer 5

It is the complete set of DNA in each cell of an organism.

Question 6

Through the examination of the genome which is the closest living relatives to the human species?

Answer 6

The chimpanzees, shows that they share more than 98% of our DNA.

Question 7

Interestingly, humans have 23 pairs of chromosomes while chimpanzees have 24. How do scientists explain this?

Answer 7

Scientists believe this can be explained by two small chromosomes found in chimpanzees having fused to form one of the human chromosomes at some time in the past.

Question 8

What do comparisons of junk DNA provide?

Answer 8

• It provides similar results as those for other parts of the genome - more closely related species have more junk sequences in common.
• This observation only makes sense if related species have evolved from a common ancestor.

Question 9

What is a good example of stretches of apparently non-functional DNA?

Answer 9

Endogenous Retroviruses (ERVs)

Question 10

What is an ERV?

Answer 10

Endogenous Retroviruses; is a viral sequence that has become part of an organism’s genome.

Question 11

Describe Retroviruses.

Answer 11

• Retroviruses store their genetic information as RNA, not DNA.
• Upon entering a cell, a retrovirus copies its RNA genome into DNA - a process known as reverse transcription.
• The DNA then becomes inserted into one of the host cell’s chromosomes.

Question 12

How does a Retrovirus become endogenous?

Answer 12

• A retrovirus only becomes endogenous if it inserts into a cell whose chromosomes will be inherited by the next generation, that is, an ovum or sperm cell.
• The offspring of the infected individual will then have a copy of the ERV in the same place, in the same chromosome, in every single one of their cells.

Question 13

What have scientists found out about the location of ERVs?

Answer 13

ERVs make up 8% of the human genome, and that other primates also possess some of the same ERVs in exactly the same locations in their genomes.

Question 14

What is a compelling evidence that humans and chimpanzees share a common ancestor?

Answer 14

• When comparing the chromosomes of humans and chimpanzees, scientists have found 16 instances of human ERVs matching exactly with chimpanzee ERVs.
• This is because any retrovirus that became inserted into the genome of a common ancestor would be inherited by both chimpanzees and humans at exactly the same location in the chromosome.

Question 15

What is the strong indication that all “these” animals shared a common ancestor, diverging at differing periods?

Answer 15

The experimentation with the DNA strands from different species suggests an increasing genetic distance between humans and the other primate groups as one progresses from chimpanzees, to gorillas, to orang-utans, to gibbons and to Old World monkeys.

Question 16

What do mitochondrial DNA contain?

Answer 16

• 37 genes
• 13 with instructions for making some of the enzymes necessary for the reactions of cellular respiration
• 24 with the code fork making the transfer RNA molecules that are involved in protein synthesis

Question 17

Why is it much easier to find and extract mtDNA from mitochondrion than DNA from the nucleus?

Answer 17

This is because most cells contain large numbers of mitochondria and therefore usually have between 500 and 1000 copies of the mtDNA molecule.

Question 18

What makes mtDNA particularly interesting?

Answer 18

• Unlike nuclear DNA that is inherited equally from both parents, mtDNA is inherited only from the mother.
• Thus your mitochondrial DNA is a copy of that which was in your mother’s egg cell.
• This occurs because when a sperm penetrates an egg at fertilisation, the mitochondria in the sperm are rapidly destroyed.

Question 19

Why has human mtDNA been slowly diverging from the mtDNA of our original female ancestor?

Answer 19

Due to the higher rate of mutation of DNA in mitochondria;

the amount of mutation is roughly proportional to the amount of time that has passed.

Question 20

How are scientists able to use the similarity between the mtDNA of any two individuals to provide an estimate of the closeness of their relationship through their maternal ancestors?

Answer 20

• If their mtDNA is identical they will be closely related, perhaps even siblings.
• On the other hand, if the mtDNA is very different, their last common maternal ancestor lived long aong.

Question 21

When has the use of mtDNA been found to be most valuable?

Answer 21

• When comparing individuals within a species and for species that are closely related.
• In this way it has allowed scientists to track the ancestry of many species back hundreds of generations.

Question 22

How are Protein Sequences formed?

Answer 22

1. Proteins consist of long chains of amino acids - some proteins may contain as few as a hundred amino acid units, while others contain thousands.
2. Linking together particular amino acids in a precise sequence determined by the DNA creates these proteins.
3. There are tens of thousands of types of proteins in living things and are all fabricated from 20 kinds of amino acids.

Question 23

How does modern biochemical techniques enable the sequence of amino acids in a protein to be determined?

Answer 23

By comparing the type and sequence of amino acids in similar proteins from different species, the degree of similarity can be established.

Question 24

Compare Protein Sequences to DNA analysis.

Answer 24

• Just like DNA analysis, the degree of difference between proteins enables an estimate of the amount of evolution that has taken place since two species developed from a common ancestor.
• The longer the period of time involved, the greater the number of amino acids that are different.
• Animals of the same species have identical amino acid sequences in their proteins, and those from different species have different amino acids or they are arranged in a different order.

Question 25

What is the system of coding scientists have adopt to make a comparison of amino acid chains easier?

Answer 25

• Usually amino acids are represented by a three-letter code, frequently the first three letters of their name.
  1. This system involves using one letter to represent on particular amino acid.
  2. By listing the amino acids for a particular protein in sequence, a comparison can be made with other specie.

Question 26

Define Ubiquitous Proteins.

Answer 26

One of a group of proteins that appears to be in all species, from bacteria to humans; the small protein called ubiquitin was so-named because it is present in all types of cells.

• Such proteins perform very basic but essential tasks that all organisms require for life.
• Ubiquitous proteins are found in all organisms from bacteria to humans and are completely independent of an organism’s specific function or the environment in which it lives.
• Such proteins carry out the same functions no matter where they are found.

Question 27

Define Cytochrome c?

Answer 27

It is a well-researched example of an ubiquitous protein that shows how protein sequences can provide evidence for evolution.

• This protein performs an essential step in the production of cellular energy.

Question 28

What strongly suggests that cytochrome c proteins have descended from an ancestral cytochrome c molecule found in a primitive microbe that existed more than 2000 million years ago?

Answer 28

1. It appears to have changed very little over millions of years of evolution.
2. Human cytochrome c contains 104 amino acids.
3. Regardless of the species tested, 37 of these have been found at the same positions in every sequenced cytochrome c molecule.

Question 29

How to compare cytochrome c sequences?

Answer 29

1. They need to be aligned so that the maximum number of positions containing the same amino acids can be determined.
2. The more similarity there is between two molecules, the more recently they have evolved from a common ancestor.
3. By doing such comparisons, scientists have determined that the cytochrome c of chimpanzees and gorillas is the same as that for humans, and for rhesus monkeys it only differs by one amino acid when compared with that of humans.

Question 30

What are other protein sequences that have been examined and yielded similar results?

Answer 30

1. The alpha and beta chains of the blood protein, haemoglobin, are identical in humans and chimpanzees, but the same protein sequences in gorillas differ by one amino acid.
2. When the same chains are examined in gibbons, there are three amino acid differences.
3. A comparison of the delta chain indicates that humans differ from chimpanzees and gorillas by one amino acid, and from gibbons by two.

Question 31

What does comparative anatomy involve?

Answer 31

It involves the structural features of related animals to ascertain the degree of similarity between them.

Question 32

What 3 areas will we be focusing on while discussing comparative anatomy?

Answer 32

1. Embryology - comparing the very early stages of the development of organisms
2. Homologous Organs - organs that are similar in structure but are used in different ways.
3. Vestigial organs - organs that may once have been important but have lost or changed their function.

Question 33

The embryonic gills in fish and reptile pouches and arches, similar to other vertebrates; So how the presence of such structures significant to fish and reptile?

Answer 33

• These structures are significant if the vertebrates are viewed as an evolutionary series that began with fish hundreds of millions of years ago.
• Over that time evolution resulted in their divergence into amphibians, reptiles, birds and mammals.
• In humans, one of the embryonic gill slits develops into the Eustachian tube, and tissue surrounding other gill slits develops into the thyroid gland and tonsils.

Question 34

Why are the forelimb bones described as homologous organs?

Answer 34

This is because they posses a similar structure.

• Organisms possessing organs that are similar in structure are likely to have a common ancestor.
• Therefore, the arrangement of the bones of the forelimb in such a range of vertebrates is convincing evidence that they have all evolved from a common ancestor.

Question 35

What are Vestigial Organs?

Answer 35

1. They are structures of reduced size that appear to have no function.
2. They are common in vertebrate species and form an intriguing aspect of comparative anatomy.
3. Humans may have as many as 90 of these structures.
4. Keep in mind that vestigial structures are largely or entirely functionless when their original role is being considered. Some may retain lesser functions or develop new ones.

Question 36

What is the nictitating membrane?

Answer 36

Aka transparent third eyelid, found in cats, birds, frogs and other vertebrates, os only represented in humans by a pinkish membrane located at the inner corner of each eye.

Question 37

Ear Muscles:

Answer 37

Muscles that move the external ears of many mammals are reduced to such an extent in humans that, in most individuals, they will not move the ears at all.

Question 38

Wisdom tooth:

Answer 38

• In most humans the thirds molars, or wisdom teeth, erupt abnormally and cannot be used in mastication.
• Frequently, they are removed before eruption so that they do not become painful.
• About one-fifth of the population are spared any discomfort because the third molars do not develop at all.

Question 39

Pyramidalis muscles:

Answer 39

• Likewise, about one-fifth of the population do not develop the muscles that lie above the pubic bone. the pyramidalis muscles.
• These muscles, if present, do not many any difference for a tail, fused to form the coccyx, and an appendix.

Question 40

Male nipples.

Answer 40

• Males also have nipples on their chests, although some would argue that these should not be termed vestigial, as they had no function in the first place.
• They appear to be retained in males because all human foetuses develop from the basic genetic form, and nipples do have an important function in females.

Question 41

How can evolutionary mechanisms be used to explain the existence of many of these structures (Vestigial organs)?

Answer 41

1. They are what remain of organs that were functional in ancestral forms.
2. Over time, and with changing environmental conditions, such organs were no longer essential to survival and were gradually reduced to vestigial remnants.
3. As these remnants are not harmful in any way, they have not been completely eliminated.
4. However, natural selection has reduced the organs to non-functional remnants because it would have been a waste of the organism’s energy and resources to maintain useless structures.
5. Such organs will probably disappear altogether as there is no selection pressure to retain them.

Question 42

How is further evidence for evolution found in the natural geographic distribution of related species?

Answer 42

1. Isolated land areas and island groups have frequently evolved their own distinctive plant and animal populations.

Question 43

For instance, before humans arrived in Australia, there were more than 100 species of kangaroos, koalas ad other marsupials but none of the more evolutionary advanced terrestrial placental mammals. Also, the only living representatives of the primitive egg-laying mammals, the echidna and platypus, are found in Australia and nowhere else in the world. Explain why.

Answer 43

The most likely explanation is that the unique species found in Australia had been evolving in isolation from the rest of the world for million of years.