chapter 10 p3

Question 1

sources to study the process of evolution

Answer 1

palaeontology - the study of fossils and the fossil record
comparative anatomy - the study of similarities and differences between organisms’ anatomy
comparative biochemistry - similarities and differences between the chemical makeup of organisms.

Question 2

Palaeontology:

Answer 2

• Fossils are formed when animal and plant remains are preserved in rocks.
• Over long periods of time, sediment is deposited on the earth to form layers (strata) of rock.
• Different layers correspond to different geological eras, the most recent layer being found on the top.
• Within the different rock strata the fossils found are quite different, forming a sequence from oldest to youngest, which shows that organisms have gradually changed over time.
• This is known as the fossil record.

Question 3

Evidence provided by the fossil record:
p1

Answer 3

Fossils of the simplest organisms such as bacteria and simple algae are found in the oldest rocks, whilst fossils of more complex organisms such as vertebrates are found in more recent rocks.
This supports the evolutionary theory that simple life forms gradually evolved over an extremely long time period into more complex ones.

The sequence in which the organisms are found matches their ecological links to each other.
For example, plant fossils appear before animal fossils.
This is consistent with the fact that animals require plants to survive.

Question 4

Evidence provided by the fossil record:
p2

Answer 4

By studying similarities in the anatomy of fossil organisms, scientists can show how closely related organisms have evolved from the same ancestor.
For example zebras and horses, members of the genus Equus, are closely related to the rhinoceros of the family Rhinocerotidae.
An extensive fossil record of these organisms exists, which spans over 60 million years and links them to the common ancestor Hyracotherium.
This lineage has been based on structural similarities between their skull (including teeth) and skeleton, in particular the feet (Figure 4).

Fossils allow relationships between extinct and living (extant) organisms to be investigated.

Question 5

Problem with the fossil record:

Answer 5

The fossil record is, however, not complete.
For example, many organisms are soft-bodied and decompose quickly before they have a chance to fossilise.
the conditions needed for fossils to form are not often present.
Many other fossils have been destroyed by the Earth’s movements, such as volcanoes, or still lie undiscovered.

Question 6

Comparative anatomy:

Answer 6

As the fossil record is incomplete, scientists look for other sources of evidence to determine evolutionary relationships.
Comparative anatomy is the study of similarities and differences in the anatomy of different living species.

Question 7

Homologous structures:

Answer 7

• A homologous structure is a structure that appears superficially different (and may perform different functions) in different organisms, but has the same underlying structure.
• An example is the pentadactyl limb of vertebrates.
• Vertebrate limbs are used for a wide variety of functions such as running. jumping, and flying.
• You would expect the bone structure of these limbs in a flying vertebrate to be very different from that in a walking vertebrate or a swimming vertebrate.
• However, the basic structures of all vertebrate limbs are actually very similar - the same bones are adapted to carry out the whole range of different functions.
• An explanation is that all vertebrates have evolved from a common ancestor therefore vertebrate limbs have all evolved from the same structure.
• The presence of homologous structures provides evidence for divergent evolution.

Question 8

Divergent evolution:

Answer 8

This describes how, from a common ancestor, different species have evolved, each with a different set of adaptive features.
This type of evolution will occur when closely related species diversify to adapt to new habitats as a result of migration or loss of habitat.

Question 9

Comparative biochemistry:

Answer 9

Comparative biochemistry is the study of similarities and differences in the proteins and other molecules that control life processes.
Although these molecules can change over time, some important molecules are
highly conserved (remain almost unchanged) among species.
Slight changes that occur in these molecules can help identify evolutionary links.
Two of the most common molecules studied are cytochrome c, a protein involved in respiration, and ribosomal RNA.

Question 10

Molecular Evolution and Neutral Evolution

Answer 10

The hypothesis of neutral evolution states that most of the variability in the structure of a molecule does not affect its function.
This is because most of the variability occurs outside of the molecule’s functional regions.
Changes that do not affect a molecule’s function are called ‘neutral’.
Since they have no effect on function, their accumulation is not affected by natural selection.
As a result, neutral substitutions occur at a fairly regular rate, although that rate is different for different molecules.

Question 11

Application in Determining Species Relationships:

Answer 11

To discover how closely two species are related, the molecular sequence of a particular molecule is compared.
(Scientists do this by looking at the order of DNA bases, or at the order of amino acids in a protein.)
The number of differences that exist are plotted against the rate the molecule undergoes neutral base pair substitutions (which has been determined through studies).
From this information scientists can estimate the point at which the two species last shared a common ancestor.
Species that are closely related have the more similar DNA and proteins, whereas those that are distantly related have far lewer similarities.
Ribosomal RNA has a very slow rate of substitution, so it is commonly used together with fossil information to determine relationships between ancient species.

Question 12

Evolutionary embryology: p1

Answer 12

Embryology is the study of embryos.
It is another source of evidence to show evolutionary relationships.
An embryo is an unborn (or unhatched) animal in its earliest phases of development.
Embryos of many different animals look very similar and it is often difficult to tell them apart.
This shows that the animals develop in a similar way, implying that the processes of embryonic development have a common origin and the animals share common ancestry but have gradually evolved different traits.

Question 13

Evolutionary embryology: p2

Answer 13

Many traits of one type of animal appear in the embryo of another type of animal.
For example, fish, and human embryos both have gill slits. In fish these develop into gills, but in humans they disappear before birth.
Darwin considered the evidence from embryology to be by far the strongest single class of facts in favour of his theory.
He studied a series of drawings produced by biologist Ernst Haeckel that depicted the growth of embryos from various classes of vertebrates.
The pictures show that the embryos begin looking virtually identical (suggesting common ancestry), but as they develop, their appearances diverge to take the form of their particular group.
As new organs or structures evolved, these features develop at the end of an organism’s embryonic development.
As a result, an organism’s evolutionary history can be traced in the development of its embryos.

Question 14

Types of variation:

Answer 14

The widest type of variation is between members of different species - these differences are known as interspecific variation.
For example, a mouse has four legs, teeth, and fur whereas a bird has two legs, two wings, a beak and feathers.
Every organism in the world is different - even identical twins differ in some ways.
Differences between organisms within a species are called intraspecific variation.
For example, people vary in height, build, hair colour, and intelligence.

Question 15

Causes of variation:
Two factors cause variation:

Answer 15

An organism’s genetic material - differences in the genetic material an organism inherits from its parents leads to genetic variation.
The environment in which the organism lives - this causes environmental variation.

Question 16

Genetic causes of variation:

Answer 16

Genetic variation is due to the genes (and alleles) an individual possesses.
There are several causes for genetic variation being present within a population:
Alleles (variants)
Mutations
Meiosis
Sexual reproduction
Chance

Points 3, 4 and 5 are all aspects of sexual reproduction.
As a result there is much greater variation in organisms that reproduce sexually than asexually.
Asexual reproduction results in the production of clones (individuals that are genetically identical to their parents).
Genetic variation can only be increased in these organisms as a result of mutation.
An example of a characteristic that is determined purely by genetic variation is your blood group.
The genes passed onto you from your parents determine if your blood group will be type A, B, AB or O.

Question 17

Alleles (variants)

Answer 17

genes have different alleles (alternative forms).
With a gene for a particular characteristic, different alleles produce different effects.
For example, the gene for human blood groups has three different alleles (A, B and 0).
Depending on the parental combination of these alleles (see point 4) four different blood groups can be produced (A, B, AB and O).
Individuals in a species population may inherit different alleles of a gene.

Question 18

Mutations

Answer 18

changes to the DNA sequence and therefore to genes can lead to changes in the proteins that are coded for.
These protein changes can affect physical and metabolic characteristics. If a mutation occurs in somatic (body) cells, just the individual is affected.
However, if a mutation occurs in the gametes it may be passed on to the organism’s offspring.
Both can result in variation.

Question 19

Meiosis

Answer 19

gametes (sex cells - ovum and sperm) are produced by the process of meiosis in organisms that reproduce sexually.
Each gamete receives half the genetic content of a parent cell.
Before the nucleus divides and chromatids of a chromosome separate, the genetic material inherited from the two parents is mixed up by independent assortment and crossing over.
This leads to the gametes of an individual showing variation.

Question 20

Sexual reproduction

Answer 20

the offspring produced from two individuals inherits genes (alleles) from each of the parents.
Each individual produced therefore differs from the parents

Question 21

Chance

Answer 21

many different gametes are produced from the parental genome.
During sexual reproduction it is a result of chance as to which two combine (often referred to as random fertilisation).
The individuals produced therefore also differ from their siblings as each contains a unique combination of genetic material.

Question 22

Environmental causes of variation:

Answer 22

All organisms are affected by the environment in which they live, although plants may be affected to a greater degree than animals due to their lack of mobility.
For example, two rose bushes are planted in different positions in a garden.
The one that has greater access to the sun will generally grow larger than one in a shadier position.
As the plant cannot move to gain sunlight, it is more affected by the environment than an animal, which could move to another area to look for food or shelter.
An example of a characteristic that is determined purely by environmental variation is the presence (or absence) of any scars on your body.
They will have occurred as a result of an accident or disease and have no genetic origin.
Scars cannot be inherited from a parent.

Question 23

Environmental and genetic causes:

Answer 23

In most cases variation is caused by a combination of both environmental and genetic factors.
If you have very tall parents, you have most likely inherited the genes to also grow to a tall height.
However, if you eat a very poor diet or suffer from disease you may only grow to below average height.
Another example of a characteristic that shows both environmental and genetic causes is your skin colour.
This is determined by how much of the pigment, melanin, it contains.
The more melanin present in your skin, the darker your skin is. Your skin colour at birth is determined purely by genetics - however, when you expose your skin to sunlight you produce more melanin to protect your skin from harmful UV rays.
This results in your skin turning darker.
As many characteristics are caused by a combination of both genetic and environmental causes, it can be very difficult to investigate and draw conclusions about the causes of a variation in any particular case (however, see the Application on studying variation in identical twins).
This is often referred to as the ‘nature versus nurture’ argument.
For example, many studies have investigated the primary cause of variation in intelligence - genetics or environment?
To date, no definitive conclusion has been reached.

Question 24

Studying variation in identical twins:

Answer 24

Many studies have been carried out on identical twins to determine how much of a characteristic is a result of genetic variation, and how much is a result of the environment in which a person lives.
Identical twins are produced when an egg splits after fertilisation.
At this point each twin contains identical genetic material, therefore they show no genetic variation.
If the twins are brought up in different environments, the results of environment on variation can clearly be seen.
Even within the same environment, as the twins grow they will show some variation.
The characteristics in which they show most variation must be influenced more greatly by the environment than by genes.
Those in which they show least variation are controlled more by genes than environment.

Question 25

case study Studying variation in identical twins:

Answer 25

One of the most famous case studies on identical twins is the Minnesota Study of Twins Reared Apart’.
This study looked at the lives of several pairs of identical twins. One pair was known as the Jim twins”.
Identical twins Jim Lewis and Jim Springer were four weeks old when they were separated, and adopted into different families.
They were later reunited, aged 39. At this point, the similarities the twins shared amazed researchers at the University of Minnesota. Many physical characteristics were shared; both twins:
were 6 feet tall
had a body mass of 82 kg (13 stone)
were fingernail biters
suffered from migraine headaches.
This is perhaps not too surprising. It was also discovered that the twins shared a number of other astonishing similarities. Both twins:
had owned a dog named Toy
had been married twice (where both first wives were called Linda, and the second wives both called Betty)
smoked the same brand of cigarettes
had studied carpentry and mechanical drawing.
- Of course, like other identical twins Jim Lewis and Jim Springer were not identical copies of each other.
- The two men styled their hair differently - one preferred the medium of speech to communicate, while the other preferred the written word.

Question 26

Representing variation graphically

Answer 26

When studying variation, scientists take measurements of different characteristics within a species.
To allow reliable conclusions to be formed, they need to collect measurements from large numbers of the population.
In order to analyse and interpret this data it can be represented graphically.
This allows any patterns to be seen clearly.
Based on the data collected, characteristics can be sorted into those that show discontinuous variation and those that show continuous variation.

Question 27

Discontinuous variation:

Answer 27

A characteristic that can only result in certain values is said to show discontinuous variation (or discrete variation).
There can be no in-between values.
Variation determined purely by genetic factors falls into this category.
An animal’s sex is an example of discontinuous variation as there are only two possible functional values - male or female.
An example of discontinuous variation in microorganisms is the shape of bacteria.
They can be spherical (cocci), rods (bacilli), spiral (spirilla), comma (vibrios) or corkscrew shaped (spirochaetes).
Discontinuous variation is normally represented using a bar chart, but i pie chart may also be used. Human blood groups show discontinuous variation.
Like most other characteristics that show discontinuous variation, it is controlled by a single gene, the ABO gene.

Question 28

Continuous variation:

Answer 28

A characteristic that can take any value within a range is said to show continuous variation.
There is a graduation in values from one extreme to the other of an characteristic - this is known as a continuum
The height and mass of plants and animals are examples of such characteristics.
Characteristics that show continuous variation are not controlled by a single gene but a number of genes (polygenes).
They are also often influenced by environmental factors.
Data on characteristics that show continuous variation are collected in a frequency table (Table 1).
These data are then plotted onto a histogram (Figure 1).
Normally a curve is then drawn onto the graph to show the trend.

Question 29

Normal distribution curves:

Answer 29

When continuous variation data are plotted onto a graph, they usually result in the production of a bell-shaped curve known as a normal distribution curve (Figure 3).
The data is said to be normally distributed.

Question 30

Characteristics of a normal distribution:

Answer 30

The mean, mode, and median are the same.
The distribution has a characteristic ‘bell shape’, which is symmetrical about the mean.
50% of values are less than the mean and 50% are greater than the mean.
Most values lie close to the mean value - the number of individuals at the extremes are low.