Comprehensive Guide to Ecology and Genetics for Students

Posted on Jun 12, 2024 in Biology

Ecology

Basic Concepts

Ecosystem: All biotic (living) and abiotic (non-living) factors in an area.

Population: A group of organisms of the same species that live in a particular area and can interbreed.

Community: Different populations living in a particular area.

Niche: An organism’s place within an ecosystem, including its habitat, how it feeds, and its response to predation. No two species have the same niche, as interspecific competition would lead to one outcompeting the other.

Micro-habitat: A small, specialized habitat within a larger one.

Trophic Levels and Energy Flow

Autotrophs: Use simple inorganic molecules (CO₂, H₂O) to produce complex organic ones.

Heterotrophs: Consume complex organic molecules derived from other living organisms.

Food web: Represents the flow of energy through an ecosystem and shows feeding relationships between all organisms in an ecosystem.

Habitat: A particular area occupied by a population, characterized by abiotic and biotic factors that separate it from others.

Decomposers: Microbes, bacteria, and fungi that obtain nutrients from dead organic matter.

Detritivores: Organisms like annelids that feed on small fragments of organic debris (e.g., feces, dead leaves, and organisms).

Ecological Energetics

Ecological energetics: The study of energy flow through an ecosystem.

Producers (green plants/algae) trap solar energy and synthesize sugars from raw materials via photosynthesis.
The sun’s energy is passed on from one trophic level to the next within the ecosystem.
Some light energy is absorbed by H₂O in the atmosphere, reflected by dust particles, or may be transmitted through a leaf or not be the right wavelength for photosynthesis.
Energy is lost from plants via heat from respiration and uneaten parts.
Energy is stored as organic molecules (starch/glucose).
Only 10% of energy is transferred to primary consumers.
Energy is now stored as complex organic molecules (protein/fat).
10-20% of energy is transferred to secondary consumers, as protein is more digestible in meat.
Decomposers feed on and recycle dead decaying matter back into the ecosystem.

Photosynthetic efficiency = (energy incorporated into products / total light energy falling on plant) x 100

GPP (Gross Primary Productivity): Rate at which producers make photosynthetic products.

NPP (Net Primary Productivity): Potential food available to primary consumers = GPP – energy used in respiration.

Herbivores have lower energy conversion efficiency than carnivores because cellulose is consumed in large amounts and is difficult/inefficient to digest.

GEE (Gross Ecological Efficiency): Measure of how much energy is transferred from one trophic level to the next = (energy in trophic level / energy in the previous trophic level) x 100

Ecological Pyramids

Pyramid of numbers: Shows the number of organisms at each trophic level. Data is easy to collect but can be inverted due to large differences in size between organisms in the food chain.
Pyramid of biomass: Inversions are rarer but can occur due to temporal variations in populations (e.g., after a phytoplankton bloom, consumer ‘zooplankton’ reproduce more, leading to a larger biomass than phytoplankton for a short time before their death rate increases and biomass falls).
Pyramid of energy: Shows energy in each trophic level per unit area and time (kJm^-2yr^-1). Cannot be inverted, allowing comparison of the efficiency of energy transfer between levels. Collecting data is difficult.

Ecological Succession

Succession: Change in the composition of a community of organisms in an ecosystem over time due to a change in abiotic factors.

Primary succession: Introduction of plants/animals that haven’t previously colonized an area.

Secondary succession: Reintroduction of organisms into a bare habitat that may have previously been occupied by plants/animals (destroyed by a natural disaster) and all organisms were destroyed. The climax community may not be the same.

Seres: A series of stages within a succession.

Pioneer species (e.g., lichens and algae) colonize bare rock.
Erosion, death, and decay of pioneer organisms allow the development of simple soils with suitable conditions for mosses to grow.
Further decay allows grasses to grow with improved soil.
Shrubs later appear, and over a long period, trees grow.
Trees form a wood in an area and, after intermediate stages, form a climax community, a stable end to a succession.

As succession proceeds, species diversity increases, as does the overall stability of a community.

Human Impact on Ecosystems

Deforestation

Deforestation: Permanent removal of forest due to the need for agricultural land and timber. Implications include:

Reduction in biodiversity due to habitat loss.
Loss of potentially beneficial plants (e.g., medicinal) that may become extinct.
Soil erosion due to exposure to wind and removal of tree roots.
Changes in local rainfall patterns and flooding.
Climate change as a reduction in CO₂ fixation via photosynthesis leads to high atmospheric CO₂ levels.

Managed Sustainability

Coppicing: Trees are cut down to the trunk and re-grown. Timber is still produced, but the tree is not killed.
Selective cutting: Cutting the oldest, largest trees allows smaller, younger ones to grow and minimizes soil erosion as trees hold soil in place.
Planting trees an optimum distance apart: Reduces intraspecific competition, encouraging faster growth and reducing disease.
Planting fast-growing trees: Slower rotation times help sustainable management of forests.

Overfishing

Overfishing: Can cause knock-on effects to other organisms by disrupting food webs due to the removal of a large number of species.

Increase mesh size: Allows smaller, younger fish to escape and go on to reproduce.
Protected areas: Banned fishing in areas where fish reproduce.
Closed seasons: Banned fishing in parts of the year when fish reproduce.
Quotas: Fishermen are allowed to catch a certain number of fish.
Fish farming: Reduces pressure on wild stocks but can lead to high disease or pest levels, requiring the use of lots of antibiotics and insecticides. Runoff can lead to eutrophication in local bodies of water.

Agricultural Exploitation

Agricultural exploitation: Pressure on agricultural food production due to increases in population and wealth.

Increased mechanization: More machines used in farming = use of bigger fields. Hedgerows, important habitats, are removed, reducing biodiversity.
Monoculture: Growth of a single type of crop creates problems as they take up the same nutrients from the soil, which aren’t replaced. Causes an increase in pests and disease due to densely populated crops, leading to an increase in pesticide use.
Financial incentives are given to farmers to encourage sustainable agriculture to increase biodiversity and promote conservation.
H₂O availability will become a big issue, and drought-resistant crops will have to be developed to maintain food production.

Carbon Cycle and Climate Change

Carbon cycle: Burning fossil fuels and cutting trees releases CO₂ and reduces carbon fixation, contributing to the greenhouse effect and climate change.

Carbon footprint: Measure of the total CO₂ released by an individual, organization, or over the lifetime of a product.

Biofuels: Derived from plants. As they grow, they fix CO₂. When it is combusted, the CO₂ is released. However, plants being grown to provide biofuel continue to fix CO₂, and the cycle continues.

Disadvantages of biofuels: Land used to grow biofuel could be used for food production and is also monoculture.

Eutrophication

Eutrophication: Nitrate fertilizers are used to increase crop yield. It occurs when fertilizer enters a body of water from runoff from the surface or leaching through the soil.

Nitrates remove the limitation of phytoplankton (algae) growth, leading to an algal bloom.
A large number of algal cells form a layer on the water surface, blocking light.
Algae or plants living at the bottom cannot photosynthesize and die.
As they die and are decomposed by bacteria that have a high BOD (Biological Oxygen Demand), there is a fall in O₂ concentration in the water, leading to the death of other aquatic organisms, so the cycle repeats, resulting in a huge reduction in the biodiversity of the lake/pond.

Genetics and Evolution

Artificial Selection

Artificial selection: Breeding of plants and animals with desirable characteristics to produce offspring with the same characteristics over generations.

Leads to the same species having different breeds with different characteristics (e.g., dogs).
Cow’s milk: A cow that produces a large volume of milk is bred with a bull whose mother produced a large volume of milk. Some offspring will be female and produce a large volume of milk. This process is repeated over many generations.

Antibiotic Resistance

Antibiotic resistance in pathogenic bacteria: The wide use of antibiotics in medicine and agriculture has created selection pressures, causing bacteria to mutate and become resistant (e.g., MRSA).

Warfarin Resistance

Warfarin resistance: Warfarin is an anticoagulant used as rat poison.

A mutation, the codominant allele R, gives resistance to warfarin but confers a high demand for vitamin K.
The codominant allele N does not give resistance.
Individuals homozygous RR are immune but have a high vitamin K demand, a selective disadvantage in natural conditions.
Heterozygous RN individuals are still immune with a lower vitamin K demand.
As the R allele is a selective advantage against warfarin, its frequency increases in the population.
Heterozygous RN is most advantageous, but its high frequency in the population will lead to homozygous dominant rats.

Adaptive Radiation and Evolution

Adaptive radiation: Evolution leads to changes in organisms over time over many generations, giving rise to a new species from a pre-existing one.

Example: Finches on the Galápagos Islands. As the islands were newly formed, a wide range of ecological niches were available to birds, so beak shape and sizes changed to take advantage of different niches.

Natural Selection

Mutations of genes lead to adaptations to survive in an environment.
Individuals with advantageous adaptations are more likely to survive and reproduce, passing on these genes so offspring are more likely to survive.
Natural selection takes a long period but becomes widespread in a population.

Overproduction: When more offspring are produced than required to replace parents. Population size is relatively stable due to interspecific and intraspecific competition.

Genetic Drift

Genetic drift: Allele frequencies can change by chance and not due to a selective advantage or disadvantage that alleles confer.

Founder effect: When a small group from a population colonizes a new area, forming a new population. By chance, this may lead to a reduction in genetic diversity compared with the original population, and different allele frequencies within the gene pool of the new population can lead to large genotypic and phenotypic differences between both populations.

Natural disasters can lead to extreme genetic drift. A large number of a population is wiped out, causing a severe change in the allele frequencies of the population’s gene pool, causing a genetic bottleneck.

Speciation

Speciation: Occurs when two groups of organisms can no longer breed to form fertile offspring.

Allopatric Speciation

Allopatric speciation: Geographic separation (e.g., the formation of a river or blockage of a body of water by an earthquake) creates two groups called demes, local populations that interbreed and share a distinct gene pool.

For speciation to occur, there can be no exchange of genes between demes. Over time, they alter, and different selection pressures speed up natural selection:

There is variation in both demes.
Selection pressures lead to individuals with alleles that confer advantages to different selection pressures in both populations.
These individuals survive and breed, passing on advantageous alleles to offspring.
Repetition over many generations means the genes of both populations become so different that they are unable to interbreed to produce fertile offspring, as their homologous chromosomes won’t be able to pair up at prophase I of meiosis, so that organism is unable to create gametes, leading to the formation of two different species.

Sympatric Speciation

Sympatric speciation: Speciation without geographic isolation.

Mechanical isolation: Variation in the sexual organs of the same species means individuals are unable to mate successfully. Common in both insects and plants.
Behavioral isolation: Variations in mating behavior lead to the isolation of groups in a population (e.g., fruit fly (Drosophila)).
Gametic isolation: Fertilization doesn’t occur even though gametes of two different organisms can meet. Common in marine invertebrates.
Hybrid inviability: Fertilization occurs, but the embryo is unable to develop.
Hybrid sterility: A hybrid organism is formed but is sterile (e.g., mules). Problems arise during meiosis I.

Cloning

Clones: Organisms that are genetically identical to each other.

Micropropagation

Micropropagation: Plant cells are totipotent and can divide to form other plant cells.

A sample of cells is removed from the meristem and divided to form explants.
Explants are placed in a sterile, aerated medium.
Explant cells divide by mitosis to form a callus.
The callus is subdivided, and each piece differentiates into a plantlet.
Plantlets are transplanted into sterile soil.

Advantages:

Plants share the same desirable characteristics (e.g., high production yield).
High survival rate due to sterile, controlled conditions.
Storage and transport are more efficient as a large number of plants can be stored and transported together, leading to lower costs.

Disadvantages:

Contamination of the culture medium by fungi or bacteria can cause losses in large numbers of plants, so sterility is essential.
Increased risk of mutation amongst cloned plants, so they must be checked, and defective plants removed, leading to high costs.

Embryo Cloning

Embryo cloning: Splitting cells of an embryo, as they are undifferentiated, totipotent cells that can each form an embryo.

Organisms produced are genetically identical but not to their parents.
An embryo is produced by adding spermatozoa from an organism with desirable characteristics to the egg of a female.
After fertilization, the zygote is left to divide by mitosis to form a ball of cells.
The ball of cells is divided and implanted into several different surrogate mothers.

Nuclear Transfer Cloning

Nuclear transfer cloning: Produces a clone of a living animal.

A somatic (body) cell is taken from the animal to be cloned.
The nucleus is removed.
An oocyte from an animal of the same species is also taken, and its nucleus is removed.
The somatic cell nucleus and the enucleated oocyte are fused with an electric shock.
The resulting zygote develops and is implanted into a surrogate mother.

Cell Culture

Cell culture: Produces large numbers of identical cells.

Used in research, for example, cloning tumor cells to test anti-cancer drugs.
Cloning stem cells is used in tissue engineering. Stem cells are undifferentiated and can divide to form specialized cells and be used to form tissue or organs. There is an ethical debate as embryos are destroyed in the process.

Advantages of cloning:

Large numbers of organisms can be produced quickly.
Organisms are genetically identical.

Disadvantages of cloning:

Expensive and unreliable.
Clones may possess undesirable alleles, which could lead to long-term unforeseen effects.
Cloning of humans is seen as unethical.

In Vitro Fertilization (IVF)

IVF:

Ovulation is stimulated with hormones, causing several follicles to mature simultaneously, so several oocytes are released.
Oocytes are collected from the female using ultrasound to guide a tube through to the oviducts.
Semen from the male is stored in a nutrient-rich solution.
Each oocyte is placed in a separate container with sperm (around 7,500). If there is a low sperm count or low motility, sperm can be directly injected into the oocyte.
After 3 days, two oocytes that have formed zygotes are implanted into the woman’s uterus. Using two increases the chance of a successful implant.
IVF results in many unused embryos, which could be used as stem cells.

The Human Genome Project

HGP (Human Genome Project): An international project to determine the sequence of nucleotides that make up the DNA of humans and identify and map the genes it contained. The locations of genes on chromosomes were determined.

Benefits:

Allows the development of new and better-targeted medical treatments.
Increased opportunities for screening for genetic disorders by knowing the sequence of alleles that cause a genetically determined disease and so can determine whether a person will develop it.
Can also look for incidences of mutations in certain genes that may result in genetic disorders.

Genetic Counseling and Screening

Genetic counseling: Patients at risk of developing or transmitting a disease are advised on the consequences and risk of transmitting it to offspring. Genetic counselors must consider the number of people with the condition in the population, whether parents are closely related, and if parents have a family history of the condition.

Genetic screening: Used to determine if an individual is a carrier for a condition or used to screen unborn babies for genetic diseases.

Blood tests: Can be used to screen for a variety of genetic disorders.
Amniocentesis: A sample of amniotic fluid, which contains cells from the fetus, is removed and tested.
Chorionic villus sampling: A small volume of tissue is removed from the placenta and tested. Both amniocentesis and chorionic villus sampling can be used to test for cystic fibrosis.
Genetic testing can also be used to test for conditions that have not yet developed (e.g., Huntington’s disease).