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