Biological sciences

There is a diverse range of living things that have evolved and will continue to evolve over short (bacteria) and long (most organisms) periods.

Classification 

The Linnaean classification system had as a major aim the general organisation of living things. This basis for classification essentially changed with the work of Darwin, who stated from the outset that classification systems should reflect evolution and heritage. That is, organisms within a common group should be related. Traditionally the classification of organisms has rested heavily on the analysis of morphological characteristics and has been organised within the familiar hierarchical structure (Kingdom, Phylum, Class, Order, etc.). The higher the rank, the larger the group and the more differences between organisms in that group. 

The development of DNA-based identification has led to the field of phylogenetics, which has transformed taxonomy and its techniques. Phylogenetic trees are much more detailed, with many more branches than traditional classifications ever produced. 

The field of taxonomy and the classification of living things is undergoing a period of rapid change, largely due to new genetic techniques. Within this confusion, it is most important that students gain a basic understanding that most biologists agree that any classification system be based on evolutionary relationships. 

Evolution 

Evolution by natural selection is a theory that explains how species change over time. It is worth noting the difference between facts, hypotheses, theories, and laws as this is often raised by students. Facts are observations that have been made. Hypotheses are proposed explanations for a phenomenon that can be tested. Theories are explanations for a phenomenon that has been repeatedly tested and confirmed through observations and experimentation. Theories can be used to make predictions about how things are, and how they will be in the future. Laws are descriptions of something that has been repeatedly observed (without an explanation). 

The theory of evolution by natural selection explains how:

• Genetic mutations cause variation in a species.
• Environmental conditions select some individuals to survive and some to die (bright colours make them more likely to be killed by a predator).
• Those individuals that survive mate and have offspring.
• The offspring have the same characteristics as the parents.
• The number of individuals with characteristics suited to the environment increases over time. 

Evolutionary change can happen over both long and short time periods. Organisms that reproduce rapidly can evolve quickly. An example of this is the development of antibiotic resistance in bacteria. Other forms of evolution through natural selection can take thousands or millions of years. 

‘Natural selection’ versus ‘Survival of the fittest’

Herbert Spencer (27 April 1820–8 December 1903) was an English philosopher and a contemporary of Charles Darwin. He read Darwin’s On the Origin of Species and was the first to coin the term ‘survival of the fittest’ as an alternative to Darwin’s ‘natural selection’. By ‘fittest’ he meant the individual that will leave the most copies of itself in successive generations. 

Alfred Wallace suggested to Darwin that he should adopt the term as an alternative to ‘natural selection’. Darwin agreed and incorporated it in his fifth edition of On the Origin of Species. 

While the phrase ‘survival of the fittest’ is often used to refer to ‘natural selection’, it is avoided by modern biologists, because the phrase can be misleading. The word ‘fit’ is frequently confused with a state of physical fitness. Many students misinterpret the term ‘survival of the fittest’ as meaning the survival of the ‘strongest’, rather than the survival of those most suited to their environment. For these reasons, it is encouraged to use the term ‘natural selection’ and not introduce the term ‘survival of the fittest’. 

Evidence of rapid evolutionary change includes fossil, biochemical, biogeographical, and anatomical evidence. 

Fossil evidence 

If several species are related, then it is expected that they will retain some characteristics in common. These features inherited from common ancestors are called homologies. Some homologies are easily recognised such as the pentadactyl limb, while others (like the variation in leaves, flower petals, and cactus spines) are more difficult to identify. 

Vertebrates such as humans, other mammals, and birds share a similar forelimb called the pentadactyl limb. Across these groups, the forelimb has the same bones (the humerus, radius, ulna, wrist bones, and fingers) suggesting that these groups share a common ancestor that had a pentadactyl limb. Though similar, the bones have been modified in different species depending on their environment e.g. the mole’s forelimb has changed (evolved) to become adapted for digging underground. 

Biogeographical evidence 

Scientists use biogeography as evidence for evolution, by looking at changes in populations in response to their geographical distribution.  

When supercontinents, like Pangaea and Gondwana, were broken up by plate tectonics, the environment changed as new continents moved relative to the equator. Species (like some members of the Embothriinae plant genus) that shared a common ancestor on the supercontinent have evolved/changed on modern continents in response to this changing environment. Examples of this include marsupial mammals that have pouches can be found in Australia (koala and kangaroos) and in the Americas (opossums), indicating that these two continents were once joined together. 

Biochemical evidence 

Comparisons of the biochemistry of different organisms can be used as evidence for evolution. All living organisms have DNA (genetic material). The structure of DNA and other molecules are similar in closely related species. Humans and chimpanzees have 95% shared DNA. This means that millions of years ago, both humans and chimpanzees shared an ancestor. For an unknown reason, two groups were formed, and mutations (small regular changes in the DNA) occurred that resulted in different species of humans and species of chimpanzees. The more similar the DNA, the more recently the common ancestor existed. Cladograms can be used to show evolutionary relationships based on biochemical comparisons. 

Modern examples of evolution 

The Galapagos finches vary in the size and shape of their beaks and some species are specific to some islands. However, on some islands, there are up to 13 species of finches coexisting.  

Different beaks are suited to different food sources. Birds with beaks that are better-suited to the food available on an island, will survive and reproduce better than birds with less-suited beaks. Over many generations, better-suited birds will become more common in the population—the population will have adapted to the conditions on the island. As bird populations adapt, they will become increasingly different from the original population on the mainland and populations on other islands. Evolution will have caused an increase in the variety of birds, i.e. an increase in bird biodiversity. 

All living things, from animals and plants to microorganisms interact with each other and their environment. These interactions mean that the world is interdependent around us.

Ecosystems 

An ecosystem is a community of living organisms (e.g. plants, animals, and microbes) that interact with each other and with the non-living components of their environment (e.g. air, water, minerals, and soil) and are influenced by physical factors such as temperature, pH (acidity) and wind. 

Most cells (plant, animal, fungi, bacteria) use the chemical process of respiration to provide the energy needed for normal cell functioning, growth, and repair. Cellular respiration is the chemical process where glucose and oxygen combine to produce water, carbon dioxide, and energy in the form of ATP molecules. Hence all these organisms need to obtain glucose. 

Plants and algae make their glucose by the chemical process of photosynthesis. Green leaves and stems contain the pigment chlorophyll. Plants collect energy from sunlight in chlorophyll and use it to combine carbon dioxide and water to form glucose. The glucose is transported (via phloem) to other cells of the plant for storage or use in cellular respiration. Because plants and algae can make their food (glucose) they are called producers. 

Animals cannot make their glucose. They eat or consume other organisms for both their nutrient and energy needs. Hence, they are called consumers. Animals can be classified according to their diets: 

• Herbivores eat plants.
• Carnivores eat other animals.
• Omnivores eat plants and animals.
• Parasites live on other organisms and derive nutrients from the host. 

Fungi and saprophytic bacteria grow on moist, nutrient-rich organic substrates like dead plants and animals. They excrete digestive enzymes which break the substrate into simpler nutrients (like sugars, amino acids, and minerals). These simple nutrients are absorbed into their bodies, providing the necessary nutrients for growth. Decomposers return minerals to the soil that can be used by other members of the ecosystem. 

Relationships within an ecosystem are described in terms of how the living things obtain their energy and nutrients and can be represented using food chains or food webs. 

Recycling matter 

Matter cannot be created or destroyed. Instead, it moves through an ecosystem in a cyclic manner, meaning it is continuously recycled and reused. Unlike energy, which flows in one direction, matter is conserved and reused by organisms in different forms. 

The movement of matter through an ecosystem also involves several biogeochemical cycles, where elements like carbon, nitrogen, oxygen, and phosphorus move between the living and non-living components of the environment.

For more information on these cycles, refer to Behind the science>Earth and Space sciences>Global systems.

Each organism has unique features that are related to the functions that are required to survive in their environment.

Plant structure and organs 

Plants have a hierarchical structure that is organised into different levels: cells, tissues, organs, and systems. These components work together to support the plant’s growth, reproduction, and survival. As students will learn in Year 8, plants have the following organs:

• Roots: Anchor the plant to the soil, absorb water and nutrients, and store food.
• Stems: Support the plant, transport nutrients (phloem) and water (xylem) between the roots and leaves, and can also store food.
• Leaves: The primary site for photosynthesis, where light energy is converted into chemical energy (glucose).
• Flowers (in flowering plants): Reproductive organs that enable sexual reproduction through pollination. The parts include sepals, petals, stamens (male reproductive organs), and pistils (female reproductive organs).
• Fruits: Mature ovaries containing seeds, which protect and disperse the plant’s offspring. 

Each organ has a specific function that supports the overall life cycle of the plant, from growth and energy production to reproduction and seed dispersal. 

Photosynthesis and coloured light 

Photosynthesis is the process by which plants use carbon dioxide and water to convert light energy into chemical energy (glucose) and oxygen.  

$$\require{mhchem}\ce{6H2O + 6 O2 -> C6H12O6 + 6 CO2}$$

This process occurs through the main pigment chlorophyll and other minor pigments such as carotenoids. 

The efficiency of photosynthesis is influenced by the amount or intensity of light and its colour. 

• Blue and red light are most effective for photosynthesis because chlorophyll absorbs these wavelengths efficiently.
• Green light is not absorbed as much by chlorophyll, so it has a lesser direct effect on photosynthesis, but still contributes in small amounts. The reflection of green light is the reason most plants appear green.
• The quality (colour) and intensity of light affect both the rate of photosynthesis and overall plant growth, with blue light promoting compact growth and red light encouraging flowering and fruiting.  

In controlled environments, such as indoor farming or research labs, artificial lights like LEDs are used to optimise photosynthesis. These lights are tuned to emit specific wavelengths (usually blue and red) to maximise plant growth and productivity. 

Cells 

Early classification systems of living things were based on general distinguishing features and scientists tried to categorise all organisms as plants or animals. However, with a more detailed study of cell structure, particularly with the use of electron microscopy, it soon became clear that cell differences were an essential part of classification. 

• Plant cells are surrounded by a rigid cellulose cell wall and have a regular, often box-like, shape. Cells have nuclei and can have large vacuoles containing cell sap. Cells in leaves or other photosynthetic parts contain chloroplasts; however, it is important that students understand that not all plant cells have chloroplasts, so that the presence of chloroplasts needs to be looked at, together with other cell features, for classification purposes.
• Animal cells have a nucleus but no cell wall and only small vacuoles (if present). Cells are surrounded by a membrane and are often irregular in shape.
• Fungal cells are box-like, with nuclei and cell walls, and so are similar to plant cells. Significant differences are that cell walls are made of chitin (the same material comprising exoskeletons of arthropods like insects and prawns), not cellulose. Chloroplasts are never present. Fungi are thus heterotrophic and placed in a separate Kingdom, the Fungi.
• Bacterial cells differ from other cells because they have no nucleus (prokaryotic). The nuclear material is free in the cytoplasm (as a circular chromosome) and is not visible under a light microscope. They are surrounded by a cell wall made of murein (peptidoglycan), a sugar and amino acid polymer. There are no cellular organelles (such as mitochondria, and chloroplasts) present. Bacteria are therefore placed in a separate Kingdom. 

Unicellular organisms cannot metabolise effectively if they grow past a critical size, at which point their surface-area: volume ratio (SA:V) becomes too small for them to efficiently take in and eliminate substances through the cell membrane. 

At the organ/system level, there are many good examples to illustrate this concept in the human body. Villi in the small intestine increase the surface area dramatically so that nutrients from digestion can pass into the blood efficiently, as do alveoli in the lungs, concerning oxygen and carbon dioxide transport. 

At the whole organism level, there are many examples of related species which are smaller in warm climates than in cold areas. The smaller animals are able to keep cooler in a hot climate by passing heat out through the surface of their smaller bodies, with relatively large SA:V, while the larger animals with smaller SA:V can conserve heat in colder climates. 

Further information will be provided in the future.