5.4.1 Definition.

Syllabus definition: Evolution is the cumulative change in the heritable characteristics of a population.

Darwinian Evolution is not simply based on natural selection but was in fact composed of atleast five different 'sub theories:

1. Evolution: that all life is and has been perpetually changing. This contrasts strongly with notions that all forms of life are constant an unchanging.

2. Common descent: that all living things share a common ancestor if the traced back far enough.

3. Gradualism: that evolutionary change takes places slowly and gradually. This contrasts with saltation in which changes are sudden and extreme.

4. Multiplication of species: the diversity of life is a consequence of speciation. Populations adapting to locations and becoming reproductively isolated from other populations.

5. Natural selection: a two stage process in which:

• producing genetic variation

• selection

 

5.4.2 Evidence of evolution.

The theory of evolution requires evidence to show that organisms change overtime and even result in the production of new species of organism.

Types of evidence:

Fossil record: A fossil is the ancient preserved remains of an organism. The fossil can be dated from the the age of the rock formation. Sequences of fossil can show the gradual change of an organism over geological time. Continuous fossil records are rare with most containing large time gaps until subsequent discoveries are made.

Evolution and the fossil record This is an excellent site with links to all aspects of this section of the syllabus including the fossil record.

Homologous structures: All of life is connected through evolutionary history and consequently those organisms more closely connected might reasonably be expected to share common structures or homologous. Group of organisms closely related share a common form or derived trait which has been inherited from the common ancestor.

This classic example of homologous structures is the pentadactyl limb of the vertebrate.

a) Humerus

b) Radius

c) Ulna

In each example the bones are modified and adapted to the locomotion of the animal.

 

In homologous structures it is normal to find that parts of the structure will be modified, enlarged or reduced (vestigial).

Divergence: The pentadactyl limb structure shows adaptation and modification from a common limb (ancestor) structure.

Convergence: Two organisms with different ancestors have a limb structure that fulfills the same function but has evolved form different origins. Examples Wing of a bird and the wing of an insect.

 

Selective breeding: man has selectively breed animals and plants for thousands of years. If an animal posses a characteristic that is considered useful or valuable then this animal is selected for breading. The hope then is that this characteristic will be present in the next generation and at a higher frequency than before. In subsequent generations it may even then be possible to select from an even more advantageous characteristic.

A Thought experiment: Selection of high milk yields in cows.

• In a population of cows (generation 1 = G1) it is noticed that some produce more milk during lactation than others.

• These cows are selected for breeding and the other low milk yield cows are rejected for breeding.

• The calves of these high milk yields cows (G2) are then produced and once mature they themselves will have calves.

• These now mature cows (G2)will be producing on average, a higher yield of milk than G1 cows.

• The G2 population of cows will show variation in milk yield.

• The breeder will select higher yielding G2 cows for the next breeding population.

• The cycle is repeated until the cow population is producing very large yields of milk way beyond level seen in the G1 population.

 

The observations of artificial selection (selective breeding) suggested that natural populations would:

• show phenotypic variation

• be subject to natural selection pressures

• there would be selection of those individuals possessing the advantageous characteristic.

However selective breeding can work in another manner in which breeders selectively cull the weaker members of the population. Therefore selective breeding provides a model for nature suggesting that there could be selection two modes of selection.

• Selection for

• Selection against

Reading reference: Artificial selection

5.4.3 Population size and evolution.

• The population produces more offspring than the carrying capacity of the environment can support

• Offspring/population compete for limited resources (Intraspecific competition)

• Some individuals have characteristic (or combination ) that give them a competitive advantage.

• These individuals are consequently 'fitter' in terms of freedom from disease, food availability etc.

• These individuals are more likely to successfully reproduce (offspring survive)

• Through inheritance the frequency of these characteristics become greater in the next generation.

• By definition these characteristic have a genetic basis

• The alleles for the advantageous characteristic becomes more frequent in the population

 

Population modeling

 

5.4.4 Population size and survival

 

see section 5.4.3

• It should be noted that the 'struggle for survival' in this model is a consequence of over-population.

• The struggle takes the form of individuals in the population being 'selected for' or 'selected against' .

• Survivors form the new breeding population.

• The frequency of advantageous alleles has increased.

• The change in the heritable characteristics is by definition evolution.

 

5.4.5 Variation in a species.

• Populations of a species show variation.

• Variation means differences in phenotypes.

• Variation shows two basic patterns.

This type of variation is called discontinuous.

There are distinct classes of individual

e.g. Blood groups of a human population

Discontinuous variation usually indicates the condition is controlled by one to two genes.

 

This type of variation is called continuous variation with no distinct classes but a complete range of the characteristic

e.g. Height of a trees in a forest

Continuous variation like this normally indicates a polygenic condition or multiple alleles

 

5.4.6 Sexual reproduction and variation

Asexual and sexual populations both experience mutation which increases the variation within the members of a population. However sexually reproducing populations also experience significant additional sources of variation.

The sources of genetic variation in a populations :

• Meiosis and the independent assortment of chromosomes creates 2ⁿ new combinations of chromosome in the next generation n = haploid number of chromosomes

• Random fertilisation increases the variation in the population to 2²ⁿ again where n = haploid number of chromosomes

• The number of different genetic variations is increased further by cross-over in meiosis by an estimated 2³ in addition to the two above.

5.4.7 Natural selection

''........can we doubt (remembering that many more individuals are born than can possible survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind? On the other hand, we may feel sure that any variation in the least injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection' Darwin C. (1859) The Origin of Species

Natural selection is a two stage process:

• production of variation.

• selection

When a population evolves there is a cumulative change in the heritable characteristics of the population.

Natural selection can act on a population without speciation occurring.

In effect the genetic profile of the population is adapting to changes in local conditions.

Every phase in the process of evolution is affected by variation and by selection.

 

 

Natural selection models

Newbyte.com Frogs

Evolution Lab

Darwin-selection

Biology corner - Peppered moth

 

5.4.8 Examples of evolution.

Example 1: Staphylococcus aureus

This bacteria is associated with a variety of conditions including skin and lung infections. As an example of evolution it can be shown that the population of S. aureus has diverged into two forms.

 

The bacteria is in two forms;

• Methicillin-resistant Staphylococcus aureus (MRSA) also known as the epidemic MRSA against which Methicillin antibiotic has no effect.

• Methicillin Susceptible Staphylococcus aureus (MSSA). This form is still contained by the use of the antibiotic Methicillin.

• This image shows the increase in the increased frequency of MRSA from samples in USA hospitals

• The genome for S. aureus was completed at the Sanger Institute and published in June 2004.

• Comparison of the genomes for the two forms shows significant differences between MRSA and MSSA.

 

How MRSA evolved:

• Antibiotics selectively kill susceptible forms of the bacteria.

• The antibiotic is the selection pressure on the population of Staphylococcus aureus.

• Random mutation with the species produces a resistance gene at low frequency in the population

• The gene can be transferred via plasmids to other bacteria

• Frequent use of the antibiotic puts MRSA at a selective advantage to survive and MSSA at a selective disadvantage

• MRSA therefore survives to reproduce.

• The descendants of MRSA will also carry the resistance gene

• The resistance gene increases in frequency or there is a process of cumulative change in the heritable characteristics (resistance gene) in the population

• The species has evolved into two new forms

Currently this organisms is the subject of much concern amongst Health professionals particularly in the UK and USA. . This concern stems from the evolution of a new antibiotic resistant form of the species (eMRSA-16 or MRSA252). Figures from the USA indicate that some 51% of infections are contracted in hospital itself and 31% are contracted from within the community.

 

Example 2 New Zealand Kaka

• 1. Proto-kaka isolated from ancestral parrot family by the Tazmin sea formation 100 my ago.

• 2. Mountains form in New zealand (southern alps) creating alpine environment about 50 my ago.

• Proto-kaka diverges under selection pressures to form the alpine kea adapted to the mountain environment and the lowland kaka.

• 3. 0.5 M years ago New Zealand splits into two Islands. The Lowland Kaka diverges into two reproductively isolated populations of the North and South Island Kaka.

Evolutionary Biology research

Frontiers in Evolutionary Biology a publication by the National Science Foundation March 2005 (pdf 299kb).

Some reading for those with time. This document puts into perspective the kind of questions and tasks on which evolutionary biologists are working

 

 

 

 

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