OCR A2 F215 VARIATION (PART 2)Specification

a) Explain, with examples, how environmental factors can act as stabilising or evolutionary forces of natural selection

b) Explain how genetic drift can cause large changes in small populations

c) Explain the role of isolating mechanisms in the evolution of new species, with reference to ecological (geographic), seasonal (temporal) and reproductive mechanisms

d) Explain the significance of the various concepts of the species, with reference to the biological species concept and the phylogenetic (cladistic/evolutionary) species concept

e) Compare and contrast natural selection and artificial selection

f) Describe how artificial selection has been used to produce the modern dairy cow and to produce bread wheat

Stabilising and Evolutionary (Directional) Selection

Genetic variation is the basis of natural selection

Within a population, there will be a range of different alleles known as the gene pool

Each individual in the population can have any combination of these alleles producing variation

The alleles that confer a survival advantage will become more common in the population. Disadvantageous alleles will become less common or even disappear

If the environment remains fairly stable, the same advantageous alleles will be selected for in successive generations. This is called a stabilising selection

If the environment changes, there may be a change in the selection pressures on the population. A selection pressure is an environmental factor that confers greater chance of survival of some members of the population.

1

A variation that was not advantageous may begin to confer better survival value than another, resulting in evolutionary/directional selection

Another example is the fur colour of wild rabbits. In UK habitats, the majority of rabbits have agouti coloured fur, because this coat colour camouflages them better in their environment. Agouti colouration is a stabilising selection and the alleles responsible are maintained from generation to generation

If the climate changes and the ground is covered in snow for most of the year, white coated rabbits would have a selective advantage. The frequency of the alleles resulting in white fur would increase resulting in an evolutionary/directional selection

2

Genetic Drift

Genetic drift refers to changes in the frequency of alleles in a population that have occurred by chance and not as a result of natural selection

Genetic drift is more likely to occur in small populations where selection pressures are not strong

When two heterozygotes produce two offspring, the frequency of alleles is likely to be very different from the parental frequency. In a small population, these changes in allele frequency are more significant.

Genetic drift is thought to occur frequently on islands with small populations. There is geographical separation of the individuals from other members of their species on other islands

Genetic drift explains why rabbits on offshore islands around Britain are more likely to have white or black coats

In 1775, in the Pingelap atoll in the western Pacific ocean, a storm and famine reduced the population to 30 people. Today, there are 2000 inhabitants, all descended from the 30 survivors. About 5% of them have an eye defect caused by a recessive allele. This condition is extremely rare in other populations

3

Speciation and Isolating Mechanisms

The formation of a new species from a pre-existing one is called speciation

Members of the new species cannot interbreed with members of the pre-existing species to produce fertile offspring

Speciation increases biodiversity

4

Time Scale for Speciation

This often takes a long time to occur, over many generations, and is usually studied by looking at populations existing today compared with those in the past

However, bacteria reproduce at a very fast rate, producing several generations in a few hours. Speciation can be identified much more easily in these species

Speciation Occurs because of Reproductive Isolation

A sub-group of a population are separated from the rest of the population

Changes may occur in the two separate groups, because of different mutations

In time, if the members of the original population are re-united, they can no longer interbreed. Speciation has occurred with a new species originating from the original species

Reproductive Barriers leading to Reproductive Isolation

1. Geographical Isolation leading to Allopatric Speciation

A population of a species may be subdivided into two population groups by a geographical barrier such as a mountain or river

Another example of geographical isolation would be a wild species separated from a domesticated species in the same genus. Grey wolves (Canis lupus) avoid human settlements and domesticated dogs (Canis familiaris) are confined by humans. This geographical isolation has resulted in speciation

The two groups will have different selection pressures acting on them and so different adaptations will be selected for

Over time, the inherited features of both groups or only one group will change (by mutations) and perhaps this will lead to members of the two populations being unable to interbreed

This type of speciation is called Allopatric Speciation

5

Allopatric speciation allowed the evolution of new species that Darwin studied in the Galapagos Islands

6

2. Sympatric Speciation

Sympatric speciation is the development of a new species from an existing one, that does not involve geographical isolation of members of the original species

Members of the original population remain in the same area

The isolating mechanisms involved include ecological, temporal and reproductive

Ecological Barriers

Members of two sub-groups of a population may live in the same area but rarely meet

An example is a fly in Canada and North America whose maggots feed on hawthorn berries and apples. Adult flies that fed on apples tend to mate with adults that also fed on apples. Those that fed on hawthorn berries tend to mate with other feeding on hawthorn berries. Perhaps over time, these two population sub-groups will not be able to interbreed and will form two distinct species

Seasonal/Temporal Barriers

Two subgroups of a species may share the same habitat but fail to interbreed because they are not reproductively active at the same time of day or do not reproduce at the same time of year

With the grey wolf/domesticated dog example. Wolves breed once a year whereas dogs breed all year round

Reproductive Barriers

Members of the same population may share the same habitat and reproduce at the same time but they cannot breed successfully because of the following reasons

Behavioural differences - members of the two subgroups have different courtship behaviour such that they are not stimulated to mate with each other

Mechanical differences – individuals of a subgroup may be much smaller than another or they have different shapes or sizes of genitalia

Gamete incompatibility – perhaps the gametes of the two subgroups have different chromosome numbers

7

What is a Species?

Definition of the Biological Species Concept

A biological species is a group of similar organisms that can interbreed and produce fertile offspring

Problems with the biological species concept:

Difficult to classify organisms that do not reproduce sexually

Some members of the same species look very different from each other eg males look very different from females of the same species – often seen in some bird species

Cannot be used to classify extinct organisms that are only known as fossils, old bones or skins

Definition of the Phylogenetic Species Concept

A phylogenetic species is a group of organisms that have similar morphology (shape), physiology, biochemistry, embryology (stages of development) and behaviour and occupy the same ecological niche.

A monophyletic group is one that includes an ancestral organism and all of its descendent species. Grey wolves and domesticated dogs share a recent common ancestor and form a monophyletic group

Closely related organisms have similar molecular structures for DNA, RNA and proteins. Grey wolves and domesticated dogs have very similar DNA.

With improvements in the techniques of DNA sequencing, biologists are able to compare the base pair sequencies of chromosomes. A particular base sequence is called a haplotype. Differences between species, caused by base substitutions are expressed as % divergence.

% divergence = (number of substitutions /number of base pairs analysed) x 100

Any group of organisms with haplotypes that are more similar to each other than to those in any other group is called a clade. Hence, using molecular analysis is a cladistic approach to classification.

8

Cladistics

Cladistics is the hierarchial classification of species based on their evolutionary ancestry

Cladistics is different from taxonomic classification systems since:

It focuses on evolutionary/phylogenetic relationships rather than on similarities between species

Molecular analysis is very important, including DNA and RNA sequencing

It uses computer programmes and the nucleic sequencing data to generate cladograms to represent evolutionary trees

It includes extinct and extant (existing) species

The taxons, kingdom, phylum and class (from Linnaean classification) are not used. Since the evolutionary tree is very complex, a fixed number of levels for classification is too simplistic

The cladistic approach has often confirmed the Linnaean approach but organisms have been reclassified. It has helped biologists to understand the evolutionary relationships between species

9

Comparison of Natural and Artificial Selection

Definition or Natural Selection

Natural selection is the process by which particular members of a population are selected for because they have favourable adaptations to their environment.

The environmental factors that cause the selection of individuals are called selection pressures.

The individuals that survive the selection pressures will reproduce to pass on their favourable alleles that were selected for, to their offspring

Natural selection is the basis of evolution

Definition of Artificial Selection/Selective Breeding

Humans select the animals or plants that they want to breed from

The individual organisms selected have desirable characteristics that are beneficial to humans

Historical Background of Selective Breeding

Humans have been selecting animals and plants for breeding, for thousands of years, since humans started agriculture (around 10,000 BC)

Darwin recognised selective breeding as artificial selection, in contrast to the natural selection processes that occur in wild type communities (without man’s interference)

10

In selective breeding, man applies the selection pressure

The procedure of selective breeding involves careful selection and controlled reproduction:

Select animals and plants with the desired characteristics

Allow the parental animals or plants to reproduce

Select the offspring with the best combination of characteristics and allow these to reproduce

Continue this selection and reproduction regime for many generations to exaggerate the desired characteristics

Detailed records are kept to prove the ancestry of individuals

before the chosen characteristic is expressed (seen)

Table of Comparisons of Artificial and Natural Selection

Natural selection Artificial selection

Agent of selection Environment Humans

Effect on allele frequencies

Alters it Alters it

Effect on evolution Contributes to evolution of the species

Contributes to evolution of the species

Speed Slow Fast

Genetic diversity High Low

Inbreeding or outbreeding

Outbreeding is common leading to hybrid vigour

Inbreeding is common leading to loss of vigour in offspring

11

Use of Artificial Selection to produce Bread Wheat (Triticum aestivum)

Wheat is grown to harvest the grains of wheat (seeds) so that flour can be produced

Modern bread wheat is Triticum aestivum

The genus Triticum includes wild and domestic species of wheat

The genus Aegilops (wild goat grass) has contributed its genome to Triticum aestivum

Most wild species of wheat are diploid with 14 chromosomes (n=7)

Grasses, like wheat, are able to undergo polyploidy – their nuclei can contain more than one set of diploid chromosomes

12

Modern bread wheat is hexaploid (6n), having (6x7) 42 chromosomes in the nucleus of each cell

Hexaploid cells are larger than diploid cells and the seeds therefore, contain more nutrients for making flour. Growing hexaploid wheat varieties produces a higher flour yield

T.aestivum is a hybrid containing three distinct genomes. The letters used below represent different sets of chromosomes (not alleles):

The genome AuAu from a wild wheat species, referred to as einkorn

The genome BB has come from wild emmer wheat

The sterile hybrid offspring is made fertile by a mutation technique that doubles the chromosome number, producing a tetraploid species

The genome DD has come from a wild goat grass

13

Current Improvements of Triticum aestivum

Breeders continue to carry out selection programmes to improve bread wheat such as:

Resistance to fungal infections

Increased protein content of wheat seeds (required for bread flour)

Increased flour yield

Resistance to logging (stems bending over in the wind and rain)

Artificial Selection to produce the modern Dairy Cow

Cattle have been domesticated for several thousand years

The main dairy cow bred for milk production Holstein-Friesian and Ayrshire

The original wild cattle that were first domesticated are thought to have looked like modern Chillingham White cattle

High milk yielding cows were selected and bred over many generations

Breeding Programme

Cows with high milk yields were chosen to breed. The bulls chosen to breed with them, were known to produce female calves that grew up to have high milk yields

The offspring calves (F1 generation) were selected for further breeding on the basis of their high milk yields. Again the cows were bred with the prize bulls

Offspring calves in the F2 generation were selected on the basis of high milk yields and breeding continued for further generations

Note that it is not necessary to allow the cow and bull to mate. Semen can be collected from prize bulls and frozen for long periods of time. Cows are artificially inseminated with the chosen semen

When breeding animals, the health and welfare of the animals is important. Cows that produce more milk have larger udders that are more prone to mastitis (udder infection). The cows are also more likely to suffer lameness.

14

Phenotypic traits selected for in Dairy Cows

Production of high volumes of milk

Long lactation periods (time that milk is produced for)

High milk quality

Large udders/correct udder shape for the milking machine

Resistance to disease such as mastitis

Calm temperament

Converts food to milk efficiently

Current Practices in Artificial Selection

Each cow’s milk yield is measured and recorded

The progeny of bulls is monitored – those that have fathered high milk yielding daughters are selected. Their semen is collected and used for artificial insemination

Current Practices involving Reproductive Technology

Some elite cows are hormone treated so that they produce many eggs

The eggs are fertilized in vitro (IVF)

The embryos are implanted into surrogate mothers

The embryos could also be cloned and divided into several embryos, to increase the number of elite cows

Genetic screening using gene probes to identify desired genes. Once the offspring are produced from the chosen parents, the offspring DNA is checked for the marker. This allows selection at an early age

Sex selection techniques – screening for X and Y sperm to prevent production of male offspring

15

16

17