Mutations and Population genetics

We have started Unit 4 by looking at a broad overview of the process of evolution over time – how the fossil record shows that species have changed over time. Embryology and comparative anatomy also provides evidence for evolutionary change. But what are the mechanisms that cause this change? The fundamental unit of phenotypical change is the gene, so in this part of the course we will investigate change at the genetic level.

In this section of Unit 4: Area of Study 1 we will learn about the following:

Population genetics is the study of distributions and changes in allele frequency in a population, as the population is subject to the four main evolutionary processes:

  • Natural selection
  • Genetic Drift
  • Mutation
  • Gene flow (by migration or distribution of pollen and seeds)

Variation between individuals and species and polymorphism: In any normal population there is some variation between individuals – a box of apples may have individual apples that are slightly different sizes and shapes with slightly different colour variations, even when they come from the same tree. So, not all variation is caused by genetic differences. It may be environmental differences that result in the variation – for example, the amount of light, temperature or soil nutrients in the case of plants or the nutrition and experiences provided to animals. Some characteristics , such as height or wing length might show continuous variation, while other characteristics can be clearly separated into different categories (discontinuous variation). An example of discontinuous variation in humans might be the ability to roll one’s tongue – you can either do this or you can’t – there is no blending, or in-between characteristic. Polymorphism occurs when two or more clearly defined phenotypes are present in the population. A classic example is the light and dark coloured morphs of the peppered moth. The ABO blood grouping is an example in human populations.

Inherited variations: We know that the DNA molecule, present in every cell of living organisms, codes for proteins that determine the phenotype of an organism. DNA is passed from one generation to the next through reproduction, which can occur asexually or sexually (with the production of gametes (meiosis) and fusion of sperm and egg). So, families of individuals tend to look more alike than non-related individuals due to these inherited characteristics. This was seen in our work with pedigree trees, showing how genetic diseases can be passed on through several generations.

Mutations – sources of variation: The source of these variations is genetic mutations. Genetic mutations may be spontaneous or may be induced by exposure to mutagenic agents (X-rays and Gamma rays and some chemicals, such as benzene and mustard gas).

Gene pool, gene flow, genetic drift (by chance): The gene pool is the set of all genetic information in a population, while gene flow describes how genes leave and arrive in a population by death and emigration or births and immigration. Genetic drift is the change of gene frequencies due to random sampling.

Polygenes, polygenetic traits: Polygenes are non-allelic genes that together influence a phenotypic trait – often the precise loci of these genes is unknown to biologists. Examples of polygenetic traits in humans are height, weight and skin colour.

Mitochodrial DNA: Mitochondrial DNA is a single, circular strand of DNA found in the mitochondria of eukaryotic cells. In most species, mitochondrial DNA is inherited solely form the mother. In humans, mitochondrial DNA was the first significant part of the human genome to be sequenced.

Founder and bottleneck effects:The founder effect is the lost of genetic variation that occurs when a new population is established by a small group of individuals from a larger population. The people of Easter Island and Pitcairn Island show limited genetic variability (small gene pools) due to this effect. A similar effect is the bottleneck effect, when large numbers of a population are removed, leaving a small gene pool.  As a species, cheetahs have famously low levels of genetic variation. In fact, cheetahs are so closely related to each other that transplanted skin grafts do not evoke an immune response. This can probably be attributed to a population bottleneck they experienced around 10,000 years ago, barely avoiding extinction at the end of the last ice age. However, the situation has worsened in modern times. Habitat encroachment and poaching have further reduce cheetah numbers, consequently snuffing out even more genetic variation and leaving cheetahs even more vulnerable to extinction.

 

Changes in the genetic make-up of a population

Image source

Good introduction from BBC Earth: How do we know evolution is real?

Evidence for evolution:

Tree of Life Resources:

YouTube Videos:

 

Unit 4: How does life change and respond to challenges over time?

 

Area of Study 1: How are species related? 

Outcome 1: “On completion of this unit the student should be able to (1) analyse evidence for evolutionary change, (2) explain how relatedness between species is determined, and (3) elaborate on the consequences of biological change in human evolution.”

Evidence for evolutionary change is abundant in the fossil record, biogeography (distribution of species across the earth), developmental biology and structural morphology. This area of study will allow us to investigate these four types of evidence.

Paleontologists have unearthed a wide range of evidence that provides a record of the change in the number of different species and the particular characteristics of those species over time. The fossil record demonstrates that living organisms first evolved in aquatic environments and gradually moved onto land. After plants had successfully colonized terrestrial habitats and converted a percentage of carbon dioxide into oxygen, animals were able to survive out of the oceans and freshwater environments. Evidence for these changes lies in the relative ages and absolute dating of fossils in different parts of the world.

Other evidence is provided by the similarities between living species – homologous structures in vertebrates, for example – and the similarities in developmental biology among different species. These similarities suggest that living organisms descended from a common ancestor and that species that are more closely related in the family tree have a more recent common ancestor. This evidence is supported by genetic research which confirms evolutionary relationships between species.

 

The Immune System – Like fighting invaders on the Great Wall

Image source

First line of defence (innate immunity): This is like a moat and castle walls, preventing invasion by foreigners. There are physical and chemical barriers to infection.

Second line of defence (innate immunity): If the foreign materials breach the first line of defence, an infection forms. This is the inflammation (heated battle) where invaders are being killed indiscriminately.

Third line of defence (active immunmity) : The last line of defence is the active, specific response by trained killer cells (ninjas!) that recognise their targets and actively seek them out and destroy them. They may be proteins or pathogens that have taken over the reproductive capacity of the cell (prions and viruses, for example), so the infected cell must be destroyed.

Gene Technology Access Centre’s Online courses – Active immunity

At the Gene Technology Access Centre for the “Body at War – Day of Immunology” seminar and workshops, you learned about pathogens, the human body’s response to antigens and how vaccines have been developed to reduce the spread of disease.  You conducted ELISA tests to identify infected individuals and observed diseased tissues through microscopy. A valuable activity was the Immunology Game, which demonstrated the response to antigens at a cellular level and gave you the experience of controlling the movement of white blood cells (dendrites, plasma cells, macrophages and B and T cells) around the body.

GTAC also have several online courses that I would like you to complete this week.

Action potential

“Vaccines – a scientific success story” is a well written article that explains how scientists have developed vaccines that have allowed small pox to be eradicated and measles to be restricted to small areas of the world.

The Action potential explained by Bozeman Science.  The distribution of sodium (Na), potassium (K) and chloride (Cl) ions is what changes when a nervous impulse is transmitted along the axon of a neuron.

Signalling molecules, antigens and the immune system

How do cells communicate? In this area of study students focus on how cells receive specific signals that elicit a particular response. Students apply the stimulus-response model to the cell in terms of the types of signals, the position of receptors, and the transduction of the information across the cell to an effector that then initiates a response. Students examine unique molecules called antigens and how they elicit an immune response, the nature of immunity and the role of vaccinations in providing immunity. They explain how malfunctions in signalling pathways cause various disorders in the human population and how new technologies assist in managing such disorders.

Outcome 2 On completion of this unit the student should be able to apply a stimulus-response model to explain how cells communicate with each other, outline immune responses to invading pathogens, distinguish between the different ways that immunity may be acquired, and explain how malfunctions of the immune system cause disease.

In Area of Study 2: How do cells communicate? we study cellular signals (signalling molecules, signal transduction and apoptosis); responding to antigens (including antigens, innate and adaptive immunity and the lymphatic system) and the immune system (including diseases of the immune system and cancer immunotherapy). The following links are some resources for study in these topics.

Chapter 6: Cellular Signals

Chapter 7: Responding to antigens

Chapter 8: Immunity, immune malfunctions and immunotherapy

Signal Transduction and Apoptosis

Last lesson we learned about the different types of signalling molecules (plant and animal hormones, neurotransmitters, cytokines and phermones). Today we will learn what happens when these signalling molecules reach their target cells, causing a response in the cell. Lipophobic (hydrophilic) signalling molecules cannot pass through the cell membrane, so they rely on complementary protein receptors that are embedded in the cell membrane. Once the signalling molecule – or ligand – binds to the receptor protein, secondary messenger molecules are released inside the target cell. Learn more about the specifics of signal transduction here:

Signal transduction from the Penguin Prof Channel

Signal transduction from Mr Bozeman (simple animated diagrams with commentary)

In the video above, observe one of the consequences of signal transduction – apoptosis or programmed cell death.

Cellular Respiration – aerobic and anaerobic

 

Cellular respiration is necessary to provide energy for cellular processes – repair and maintenance of cells, growth and cell division, active transport of substances across cell membranes and preventing disease.

Designing a scientific investigation poster

Pretty great poster for Grade 4 students!