Selection and Evolution
Variation is vital because it helps populations to survive unfavourable conditions, and in practice conditions are nearly always unfavourable. There is competition for resources; food, light, water, nutrients etc. In addition organisms have to survive draught, cold, heat, disease and a variety of other challenges.
Natural selection is a process in which those organisms whose genes give them a selective advantage are more likely to survive, reproduce and pass their genes onto the next generation.
Natural selection is often called survival of the fittest, but in a biological context fitness means reproductive success. Fit organisms will pass more of their genes on to the next generation than less fit ones.
A good example of natural selection is that of antibiotic resistance in bacteria. What the media call ‘superbugs’ are bacterial infections that resist treatment with antibiotics. Some strains of bacteria are resistant to virtually all of the antibiotics at our disposal and they represent a growing threat. Initially, the resistance probably arose as a result of a mutation. A single gene in a bacterium mutated so that it produced a protein that in some way made the bacterium resistant to the antibiotic.
It is important to appreciate that the antibiotic did not cause the mutation. However the mutation already existed in the population, or occurred in the right place at the right time, so it gave the individual a selective advantage. In areas of antibiotic use, the resistant bacteria multiplied while the susceptible ones were killed. In this way the frequency of the resistance allele would increase.
Defining a species is difficult because new species evolve out of existing ones and the process is gradual. However, for exam purposes a working definition of a species is…
A population or group of populations of similar individuals that can mate and produce fertile offspring.
The classic example of this concept is the horse and the donkey. A male horse can mate with a female donkey to produce a mule. This hybrid, however, is sterile and so the horse and the donkey are regarded as separate species
Note: - Domestic dogs come in all shapes and sizes, but a Jack Russell and a poodle can mate to produce perfectly fertile crossbreeds. We say that the poodle and the Jack Russell are different breeds, not different species.
The Evolution of new species: -
There are three key steps to the evolution of a new species...
1. Isolation: Part of a population becomes isolated so that it cannot breed with the rest of the population.
2. Natural selection: Natural selection acts in different ways according to the local situation so that the frequency of phenotypes and alleles changes
3. Speciation: Over the generations genetic differences accumulate so that the different populations can’t interbreed even if brought together again.
There are over two million known species of organism, and a large but unknown number yet to be classified. The science of classification is known as taxonomy, and it aims to produce a catalogue of living things on Earth.
All species are given a scientific name, which usually comes from Latin or Greek. This name is used across all language barriers, and avoids confusion when referring to a particular species
Scientific names are binomial – they have two parts. The first part, the generic name, is the name of the genus and has a CAPITAL LETTER. The specific name follows, and does not have a capital letter e.g. Canis familiaris – the domestic dog, or Gorilla gorilla – the gorilla.
Scientific names should be given in italics or underlined.
This system of classification is called phylogenetic – based on evolutionary history, like a family tree over millions of years. To construct a phylogenetic tree, scientists use anatomical/physical features, fossil records and biochemical analysis of DNA and proteins.
A Level Biology - Classification of Organisms: Phylogeny
A Level Biology - Classification of Organisms: Taxonomy
The taxonomic hierarchy is shown below.
The key features are:
It consists of a series of groups within groups, from the most general (kingdom) to the most specific (species)
There is no overlap between the groups. For instance, there is no organism that is part amphibian and part reptile – its either in one group of the other.
The groups are based on shared features. The more specific the group, the more shared features there are
The five kingdoms
Scientists have recently accepted the five kingdom system of classification. All organisms can be placed into one of these five kingdoms. Four kingdoms contain the eukaryotic organisms while a fifth contains the prokaryotes – the bacteria.
The essential features each of the five kingdoms are: -
1 Kingdom Animalia (the animals)
Includes all the vertebrates (mammals, birds, reptiles, amphibians, fish) and invertebrates (inc. insects and other arthropods, molluscs, segmented worms, roundworms, tapeworms/flukes and jellyfish)
Animals are eukaryotic and multicellular. Their cells have no walls
They cannot make food, so move in search of it. They have muscles and nervous systems.
2 Kingdom Plantae (Plants)
Includes flowering plants, conifers and ferns along with simpler plants such as mosses, liverworts. NB Not the algae/seaweeds
Eukaryotic and multicellular. Cells have walls made from cellulose. Usually a large vacuole in mature cells
Most plants have leaves, stems and roots
Most plants possess chlorophyll and make food by photosynthesis. A few are modified to a parasitic way of life.
3 Kingdom Fungi
Includes filamentous fungi and yeasts
Single celled fungi are called yeasts
Eukaryotic, but the multicellular fungi do not have separate cells – the nuclei are dotted around in the tissue.
Walls of fungal cells contain chitin
All fungi feed by extracellular digestion – secreting enzymes and absorbing the soluble products. Most feed on dead material, i.e. are saprophytes. Some are parasitic
4 Kingdom Protoctista
The members of this kingdom seem to have very little in common, more like an assembly of organisms that could be put into any other kingdom – a taxonomic dustbin. There is a sound scientific reason for this group, however, but I’ve forgotten it. Something to do with embryological development, or the price of cheese. You don’t need it.
Includes single cells protozoans such as amoeba and plasmodium (the malaria parasite, the algae (including seaweeds) and sponges.
Their cells are eukaryotic, but show a diversity of cell types
They feed in a variety of ways.
5 Kingdom Prokaryotae
Includes the bacteria and
Single celled, prokaryotic organisms. Cells much smaller than eukaryotic ones.
Most feed by extracellular digestion – secreting enzymes and absorbing the soluble products. Most feed on dead material, i.e. are saprophytes. Some are parasitic and some - the cyanobacteria – are photosynthetic.
Reproduce mainly by binary fission (splitting in half), but can reproduce sexually.
Types of variation
There are two basic types of variation; continuous and discontinuous.
Discontinuous variation refers to features that fall into one category or another. The human ABO blood group system is a classic example; individuals are either A, A AB or O – there is nothing in-between. Discontinuous features are usually controlled by single alleles, which the individual either has or it doesn’t. Monohybrid and dihybrid inheritance are concerned with features that show discontinuous variation.
Continuous variation refers to features that show a whole range of values. In humans most variation is continuous; height, shoe size, intelligence. For any discontinuous factor there is a range of factors and, usually, most individuals fall into the mid range, giving a normal distribution. Continuous variation is usually controlled by several genes – it is said to be polygenic.
Causes of variation: the causes of genetic variation can be summed up as...
1 Crossover in meiosis
2 Independent assortment in meiosis
3 Random fertilisation of gametes – in animals, all sperm and ova are genetically unique, and which sperm fertilises the egg is totally random.
4 Mutation. Gene mutations are changes in the base sequence of the DNA so that the amino acid sequence changes and a different protein is made (see AS module 2). This is usually harmful or irrelevant, but occasionally beneficial. Such beneficial mutations will give the individual a selective advantage – this is the driving force behind evolution.
When studying variation is difficult to determine the relative importance of genes and environment. In humans, a lot of work has been done on identical and non identical twins, and this is a popular exam topic. The key points are: -
1 Identical - or monozygotic – twins have exactly the same genotype so any differences between them must be due to environment i.e. their upbringing. Identical twins reared apart are especially interesting to geneticists.
2 Non-identical - or dizygotic - twins reared together have different genes but largely the same environment (though it can never be exactly the same). Any differences in these twins are largely genetic.
3 It is difficult to draw valid conclusions from twin studies in humans because there are so few subjects to study.
A Level Biology - Ecology
What is Ecology?
Ecology is the study of living organisms and their environment. The aim of ecological study is to explain why organisms live and interact the where they do. To do this ecologists study ecosystems - areas that can vary in size from a small pond to the whole planet.
Some key definitions: -
Abiotic: Any non-living or physical factor.
Biotic: Any living or biological factor.
Community: The living or biotic part of an ecosystem, i.e. all the organisms of all the different species living in one habitat.
Ecosystem: A reasonably self-contained area together with all its living organisms.
Habitat: The physical or abiotic part of an ecosystem, i.e. a defined area with specific characteristics where the organisms live, e.g. oak forest, deep sea, sand dune, rocky shore, moorland, hedgerow, garden pond, etc.
Population: The members of the same species living in one habitat.
Species: A group of organisms that can successfully interbreed
Energy and Matter
Before getting into the study of ecosystems, it is important to appreciate the difference between energy and matter.
Energy and matter are quite different things and cannot be inter-converted.
What is Energy?
Energy comes in many different forms, for example: heat, light, chemical, potential, kinetic, etc., which can be inter-converted, but energy can never be created, destroyed or used up.
If we talk about energy being “lost”, we usually mean as heat, which is radiated out into space.
Energy is constantly arriving on earth from the sun, and is constantly leaving the earth as heat, but the total amount of energy on the earth is constant.
What is Matter?
Matter comes in three states (solid, liquid and gas) and again, cannot be created or destroyed. The total amount of matter on the Earth is constant. Matter (and especially the biochemicals found in living organisms) can contain stored chemical energy, so a cow contains biomass (matter) as well as chemical energy stored in its biomass.
Energy is constantly arriving from the sun, passing through living organisms, and leaving the Earth as heat.
Matter cycles between living and non-living things. But no new matter reaches the Earth, and none leaves.
Food Chains and Food Webs
The many relationships between the members of a community in an ecosystem can be described by food chains and food webs. Each stage in a food chain is called a trophic level, and the arrows represent the flow of energy and matter through the food chain. Food chains always start with photosynthetic producers (plants, algae, plankton and photosynthetic bacteria) because, uniquely, producers are able to extract both energy and matter from the abiotic environment (energy from the sun, and 98% of their matter from carbon dioxide in the air, with the remaining 2% from water and minerals in soil).
All other living organisms get both their energy and matter by eating other organisms.
Although this represents a “typical” food chain, with producers being eaten by animal consumers, different organisms use a large range of feeding strategies (other than consuming), leading to a range of different types of food chain. Some of these strategies are defined below, together with other terms associated with food chains.
Producer: - An organism that produces food from carbon dioxide and water using photosynthesis. Can be plant, algae, plankton or bacteria.
Consumer: - An animal that eats other organisms
Herbivore: - A consumer that eats plants (= primary consumer).
Carnivore: - A consumer that eats other animals (= secondary consumer).
Top Carnivore: - A consumer at the top of a food chain with no predators.
Omnivore: - A consumer that eats plants or animals.
Vegetarian: - A human that chooses not to eat animals (humans are omnivores)
Autotroph: - An organism that manufactures its own food (= producer)
Heterotroph: - An organism that obtains its energy and mass from other organisms (=consumers + decomposers)
Plankton: - Microscopic marine organisms.
Phytoplankton: - “Plant plankton” i.e. microscopic marine producers.
Zooplankton: - “Animal plankton” i.e. microscopic marine consumers.
Predator: - An animal that hunts and kills animals for food.
Prey: - An animal that is hunted and killed for food.
Scavenger: - An animal that eats dead animals, but doesn't kill them
Symbiosis: - Organisms living together in a close relationship (= parasitism, mutualism, pathogen).
Mutualism: - Two organisms living together for mutual benefit.
Commensalism: - Relationship in which only one organism benefits
Parasite: - An organism that feeds on a larger living host organism, harming it
Pathogen: - A microbe that causes a disease.
So, be aware that food chains don’t need to end with a consumer, nor do they need to even start with a producer! for example: -
In general as you go up a food chain the size of the individuals increases and the number of individuals decreases. These sorts of observations can be displayed in ecological pyramids, which are used to quantify food chains. There are three kinds: -
1. Pyramids of Numbers.
These show the numbers of organisms at each trophic level in a food chain. The width of the bars represent the numbers using a linear or logarithmic scale, or the bars may be purely qualitative. The numbers should be normalised for a given area for a terrestrial habitat (usually Meter squared), or volume for a marine habitat (Meter cubed).
Pyramids of numbers are most often triangular (or pyramid) shaped, but can be almost any shape. In the pyramids below: -
(A) shows a typical pyramid of numbers for carnivores.
(B) shows a typical parasite food chain.
(C) shows the effect of a single large producer such as a tree.
2. Pyramids of Biomass
These convey more information, since they consider the total mass of living organisms (i.e. the biomass) at each trophic level. The biomass should be dry mass (since water stores no energy) and is measured in kg m-2. The biomass may be found by drying and weighing the organisms at each trophic level, or by counting them and multiplying by an average individual mass. Pyramids of biomass are always pyramid shaped, since if a trophic level gains all its mass from the level below, then it cannot have more mass than that level (you cannot weigh more than you eat). The “missing" mass, which is not eaten by consumers, becomes detritus and is decomposed.
3. Pyramids of Energy
Food chains represent flows of matter and energy, so two different pyramids are needed to
quantify each flow. Pyramids of energy show how much energy flows into each trophic level in
a given time, so the units are usually something like kJ m-2 y-1. Pyramids of energy are always
pyramidal (energy cannot be created), and always very shallow, since the transfer of energy
from one trophic level to the next is very inefficient The “missing” energy, which is not passed
on to the next level, is lost eventually as heat.
[insert pyramid of energy images]
Energy Flow in Ecosystems
Three things can happen to the energy taken in by the organisms in a trophic level: -
It can be passed on to the biomass of the next trophic level in the food chain when the organism is eaten.
It can become stored in detritus. This energy is passed on to decomposers when the detritus decays.
It can be converted to heat energy by inefficient chemical reactions, radiated by warm bodies, or in friction due to movement. The heat energy is lost to the surroundings, and cannot be regained by living organisms.
These three fates are shown in this energy flow diagram: -
[insert flow energy flow, flow diagram]
Eventually all the energy that enters the ecosystem will be converted to heat, which is lost to space.
Material Cycles in Ecosystems
Matter cycles between the biotic environment and in the abiotic environment. Simple inorganic molecules (such as CO2, N2 and H2O) are assimilated (or fixed) from the abiotic environment by producers and microbes, and built into complex organic molecules (such as carbohydrates, proteins and lipids). These organic molecules are passed through food chains and eventually returned to the abiotic environment again as simple inorganic molecules by decomposers. Without either producers or decomposers there would be no nutrient cycling and no life.
[insert simple material cycle image]
The simple inorganic molecules are often referred to as nutrients.
Nutrients can be grouped as: -
Major nutrients (molecules containing the elements C, H and O, comprising >99% of biomass)
Macronutrients (molecules containing elements such as N, S, P, K, Ca and Mg, comprising 0.5% of biomass)
Micronutrients or trace elements (0.1% of biomass).
Macronutrients and micronutrients are collectively called minerals.
While the major nutrients are obviously needed in the largest amounts, the growth of producers is usually limited by the availability of minerals such as nitrate and phosphate.
There are two groups of decomposers: -
Detrivores are animals that eat detritus (such as earthworms and woodlice). They digest much of the material, but like all animals are unable to digest the cellulose and lignin in plant cell walls. They break such plant tissue into much smaller pieces with a larger surface area making it more accessible to the saprophytes. They also assist saprophytes by excreting useful minerals such as urea, and by aerating the soil.
Saprophytes (or decomposers) are microbes (fungi and bacteria) that live on detritus. They digest it by extracellular digestion, and then absorb the soluble nutrients. Given time, they can completely break down any organic matter (including cellulose and lignin) to inorganic matter such as carbon dioxide, water and mineral ions.
Detailed material cycles can be constructed for elements such as carbon, nitrogen, oxygen, sulphur, phosphorus, or for compounds such as water, but they all have the same basic pattern as the simple image shown above.
Let's take a look at the carbon cycle and nitrogen cycles in detail.
The Carbon Cycle
[insert carbon cycle flow diagram]
The carbon cycle flow diagram (above) shows that there are really many carbon cycles, with time scales ranging from minutes to millions of years.
it’s import to know that microorganisms have a major role at all stages.
Far more carbon is fixed by microscopic marine producers (algae & phytoplankton) from CO2 dissolved in the oceans than by terrestrial plants from CO2 in the air.
During the Earth's early history (3000 MY ago) photosynthetic bacteria called cyanobacteria changed the composition of the Earth's atmosphere by fixing most of the CO2 and replacing it with oxygen. This allowed the first heterotrophic cells to use oxygen in respiration.
A large amount of the fixed carbon is used by marine zooplankton to make calcium carbonate shells. These are not eaten by consumers and cannot easily be decomposed, so turn into carboniferous rocks (chalk, limestone, coral, etc). 99% of the Earth's carbon is in this form.
The decomposers are almost all microbes such as fungi and bacteria. Most of the detritus is in the form of cellulose and other plant fibres, which eukaryotes cannot digest. Only a few bacteria posses the cellulase enzymes required to break down plant fibres. Herbivorous animals such as cows and termites depend on these bacteria in their guts.
Much of the CO2 that was fixed by ferns during the carboniferous era (300 MY ago) was sedimented and turned into fossil fuels. The recent mining and burning of fossil fuels has significantly altered the carbon cycle by releasing the carbon again, causing a 15% increase inCO2 in just 200 years.
The Nitrogen Cycle
Note! “Nitrogen" can confusingly mean nitrogen atoms (N) nitrogen molecules (N2), i.e. proteins contain nitrogen atoms but not nitrogen molecules.
[insert image of nitrogen cycle]
Microbes are involved at most stages of the nitrogen cycle!
Nitrogen Fixation. 78% of the atmosphere is nitrogen gas (N2), but this is inert and can’t be used by plants or animals. Nitrogen fixing bacteria reduce nitrogen gas to ammonia (N2 + 6H —> 2NH3), which dissolves to form ammonium ions. This process uses the enzyme nitrogenase and ATP as a source of energy. The nitrogen-fixing bacteria may be free-living in soil or water, or they may live in colonies inside the cells of root nodules of leguminous plants such as clover or peas.
This is an example of mutualism as the plants gain a source of useful nitrogen from the bacteria, while the bacteria gain carbohydrates and protection from the plants.
Nitrogen gas can also be fixed to ammonia by humans using the Haber process, and a small amount of nitrogen is fixed to nitrate by lightning.
Nitrification. Nitrifying bacteria can oxidise ammonia to nitrate in two stages: first forming nitrite ions then forming nitrate ions These are chemosynthetic bacteria, which means they use the energy released by nitrification to live, instead of using respiration. Plants can only take up nitrogen in the form of nitrate.
Denitrification. The anaerobic denitrifying bacteria convert nitrate to N2 and NOx, which is then lost to the air. This represents a constant loss of “useful” nitrogen from soil, and explains why nitrogen fixation by the nitrifying bacteria and fertilisers are so important.
Ammonification. Microbial saprophytes break down proteins in detritus to form ammonia in two stages: first they digest proteins to amino acids using extracellular protease enzymes, then they remove the amino groups from amino acids using deaminase enzymes.
Population Ecology in concerned with the question: why is a population the size it is? This means understanding the various factors that affect the population.
When a species is introduced into a new environment its population grows in a characteristic way. This growth curve is often seen experimentally, for example bees in a hive, sheep in Tasmania, bacteria in culture. The curve is called a logistic or sigmoid growth curve.
[insert diagram of growth curve]
The growth curve has three phases, with different factors being responsible for the shape of each phase. The actual factors depend on the ecosystem, and this can be illustrated by considering two contrasting examples: yeast in a flask (reproducing asexually), and rabbits in a field (reproducing sexually).
[insert table comparing population growth]
At the end of phase 3 the population is stable. This population is called the carrying capacity of the environment (K), and is the maximum population supported by a particular ecosystem.
Factors Affecting Population Size
Many different factors interact to determine population size, and it can be very difficult to determine which factors are the most important. Factors can be split into two broad group: abiotic factors and biotic factors.
You'll need to now about 7 different factors.
1. Abiotic Factors
The population is obviously affected by the abiotic environment such as: temperature; water humidity; pH; light/shade; soil (edaphic factors); mineral supply; current (wind/water); topography (altitude, slope, aspect); catastrophes (floods/fire/frost); pollution. Successful species are generally well adapted to their abiotic environment. In harsh environments (very cold, very hot, very dry, very acid, etc.) only a few species will have successfully adapted to the conditions so they will not have much competition from other species, but in mild environments lots of different species could live there, so there will be competition. In other words in harsh environments abiotic factors govern who survives, while in mild environments biotic factors (such as competition) govern who survives.
Many abiotic factors vary with the seasons, and this can cause a periodic oscillation in the population size.
[insert diagram explaining population cycles through the seasons]
This is only seen in species with a short life cycle compared to the seasons, such as insects. Species with long life cycles (longer than a year) do not change with the seasons like this.
3. Food Supply
A population obviously depends on the population of its food supply: if there is plenty of food the population increases and vice versa. For example red deer introduced to an Alaskan island at first showed a population increase, but this large population grazed the vegetation too quickly for the slow growth to recover, so the food supply dwindled and the deer population crashed.
4. Interspecific Competition
Interspecific competition is competition for resources (such as food, space, water, light, etc.) between members of different species, and in general one species will out-compete another one. This can be demonstrated by growing two different species of the protozoan Paramecium in flasks in a lab. They both grow well in lab flasks when grown separately, but when grown together P. aurelia out-competes P. caudatum for food, so the population of P. caudatum falls due to interspecific competition.
[insert graphs showing interspecific competition]
5. Intraspecific Competition
Intraspecific competition is competition for resources between members of the same species. This is more significant than interspecific competition, since member of the same species have the same niche and so compete for exactly the same resources. Intraspecific competition tends to have a stabilising influence on population size. If the population gets too big, intraspecific population increases, so the population falls again. If the population gets too small, intraspecific population decreases, so the population increases again.
[insert graphs showing intraspecific competition]
Intraspecific competition is also the driving force behind natural selection, since the individuals with the “best” genes are more likely to win the competition and pass on their genes. Some species use aggressive behaviour to minimise real competition. Ritual fights, displays, threat postures are used to allow some individuals (the “best”) to reproduce and exclude others (the “weakest”). This avoids real fights or shortages, and results in an optimum size for a population.
The populations of predators and their prey depend on each other, so they tend to show cyclical changes. This has been famously measured for populations of lynx (predator) and hare (prey) in Canada, and can also be demonstrated in a lab experiment using two species of mite: Eotetranchus (a herbivore) and Typhlodromus (a predator). If the population of the prey increases, the predator will have more food, so its population will start to increase. This means that more prey will be
eaten, so its population will decrease, so causing a cycle in both populations.
[insert graphs showing predator prey relationship]
7. Parasitism and Disease
Parasites and their hosts have a close symbiotic relationship, so their populations also oscillate. This is demonstrated by winter moth caterpillars (the host species) and wasp larvae (parasites on the caterpillars). If the population of parasite increases, they kill their hosts, so their population decreases. This means there are fewer hosts for the parasite, so their population decreases. This allows the host population to recover, so the parasite population also recovers.
[insert graphs showing parasitism & disease]
The Ecological Niche
A population’s niche refers to its role in its ecosystem.
This typically means its feeding role in the food chain, so a particular population’s niche could be a producer, a predator, a parasite, a leafeater, etc.
A more detailed description of a niche should really include many different aspects such as its food, its habitat, its reproduction method etc, e.g...
Gerbils are desert seed-eating mammals
Seaweed is an inter-tidal autotroph
Fungi are asexual soil-living saprophytes.
Identifying the different niches in an ecosystem helps us to understand the interactions between populations.
Members of the same population always have the same niche, and will be well-adapted to that niche, e.g. nectar feeding birds have long thin beaks.
Species with narrow niches are called specialists (e.g. anteater). Many different specialists can coexist in the same habitat because they are not competing, so this can lead to high diversity, for example warblers in a coniferous forest feed on insects found at different heights. Specialists rely on a constant supply of their food, so are generally found in abundant, stable habitats such as the tropics.
Species with broad niches are called generalists (e.g. common crow). Generalists in the same habitat will compete, so there can only be a few, so this can lead to low diversity. Generalists can cope with a changing food supply (such as seasonal changes) since they can switch from one food to another or even one habitat to another (for example by migrating).
The niche concept was investigated in the 1930s by Georgy Gause. caring out a series of now classic experiments Gause used flasks of different species of the protozoan Paramecium, which eat bacteria.
[insert images showing experiment 1]. Conclusion: These two species of Paramecium share the same niche, so they compete. P. aurelia is faster-growing, so it out-competes P. caudatum.
[insert images showing experiment 2]. Conclusion: These two species of Paramecium have slightly different niches, so they don’t compete and can coexist.
It is important to understand the distribution in experiment 2:
P. caudatum <