Classification and Keys
Classification is putting the millions of different species of plants and animals into groups according to their features. All living things are divided into one of the five kingdoms based on what an organism's cells are like: Animals (all multi-cellular animals), Plants (all green plants), Fungi (moulds, mushrooms, yeast), Monera (bacteria, blue/green algae - unicellular organisms without a nucleus) and Protoctista (amoeba, paramecium - mostly unicellular organisms that do not fit into the other Kingdoms). Kingdoms are then divided into phyla (singular phylum), class, order, family and genus.
For the animal kingdom, all animals can be divide into two major groups: vertebrates (with backbones) and invertebrates (without backbones). Vertebrates can be divided into cold-blooded (fish, amphibians, reptiles) and warm-blooded (birds, mammals) animals. A cold-blooded animal cannot use internal processes to control its body temperature and has a body temperature the same as its surroundings. A warm-blooded animal is able to keep its body temperature the same on hot/cold days.
Each of the vertebrate classes have their own characteristics:
Fish (~20000 species) - paired find, gills, slimy bony scales, 1000s of eggs and do not look after their offspring.
Amphibia (~1500) - slimy skin, fish tails when young/4 legs when adult, spend some of lives in water, 100s if eggs, do not look after their offspring.
Reptilia (~6000) - dry leathery scales, very small brains, several dozen eggs, lay eggs on land, some attention to their offspring.
Aves(birds) (~3600) - Feathers, 2 legs and 2 wings, a dozen or so eggs, no teeth but beak, lay eggs on land, look after their eggs and offspring.
Mammalia (3200) - Hair, 4 limbs, do not lay eggs, provide milk for young from special glands, look after their offspring.
The invertebrates can be classified in a similar way producing a number of different phyla:
- Protozoa (~30000) - made of one cell, e.g. amoeba
- Sponges (~4500) - animals made of cells loosely joined together
- Cnidaria (~10000) - body walls made of two layers of cells (e.g. jellyfish, sea anemones)
- Flat worms (~6000) - flattened worm-like shape
- Annelida (~7000) -worms made of segments, e.g. earthworms
- Arthropoda - joined legs, bodies made of segments. Includes spiders (Arachnida ~10000), insects (Insecta ~850000), centipedes (Myrapoda ~3000) and crabs (Crustacea ~25000)
- Mollusca (~100000) - no segments, a fleshly pad on which they crawl. e.g. slug, snail
- Echinodermata (~4000) - star-shaped pattern - spiny skin. e.g. starfish
The two main invertebrate classes are Insecta and Arachnida with these characteristics:
Insects (~850000) - 3-part body, 6 legs, 2 pairs of wings, 1 pair of antennae
Spiders (~10000) - 2-part body, 8 legs, no wings
For the plant kingdom, the two major phyla are plants that produce seeds (flowering plants, conifers) and plants without seeds (algae, mosses, fungi, ferns,lichens).
Keys are used to identify living things. A key is a series of questions which at each point in the key, the question divides the organisms into two groups depending upon whether or not they have a certain characteristic; the questions continue to divide the group up until there is only one left.
The scientific naming of species consists of two words. The first gives the genus (family) which the organism belongs to and the second gives the species. The scientific name for human being is 'homo sapiens' where homo means 'great apes' and sapiens means 'wise man' in Latin.
Inheritance
All the cells in our bodies, apart from red blood cells, contain nuclei. These nuclei contain chromosomes which are very long coils of DNA. These DNA molecules conatin genes (i.e. genetic codes) that control the function of the cell (e.g. cell reproduction) as well as genetic information that are pass-on to the next generation. Every human cell contains 23 pairs of chromosomes. Twenty-two pairs are ordinay pairs. The other pair are the sex chromosomes which determine the sex of the person. If the sex chromosomes are alike (XX), the sex is female, and if they are different (XY), the sex is male.
When cell divide, during growth or repair, they produce identical cells. This process is called mitosis where the chromosomes replicate themselves, then separate into two groups before the nucleus divides into two new nuclei. After the division, each cell contains the same number of chromosomes as the parent cell. However, sex cell reproduction is by a process called meiosis. The chromosomes make an exact copy of themselves but the parent cell divides into four new cells, each new cell has half the number of chromosomes of the original cell.
The male and female sex cells join together during fertilisation to produce a new cell (the zygote) which has characteristics of both parents and develops into the embryo. Each Gene in the chromosomes provide the instructions for a particular characteristic, such as eye colour or blood group. There are two copies of each gene in every cell, one in each chromosome. One gene comes from the father and one from the mother.
Variation
Variation is the differences between organisms. Variation is obvious between different species because they have different genes. However, there is also variation within the same species.
Variation is caused by new genes formed by mutation and different mixes of genes. There are two types of variation, discontinuous and continuous. Discontinuous variation shows clearly distinguished groups, such as blood group and eye colour. Another example is albinism where a person will never become tanned because of a complete lack of skin pigment caused by the difference of a single gene. Continuous variation does not show up clearly in different group, such as height and weight.
Generally, discontinuous variation is the result of genetic variation whereas continuous variation is caused/influenced by environmental variation and is therefore a mixture of genetic and environmental factors.
Mutations
Mutations are caused by faulty DNA where an error occurred during the copying of chromosomes when cells divide. The error can be due to radiation or some chemicals and these are known as mutagens. These mutagens are found in certain drugs, cigarette smoke and the fumes from certain types of plastics when burned.
Down's syndrome is caused by a mutation and children with Down's syndrome have an extra chromosome (i.e. 3 instead of a pair) in their cells. This occurs most frequently when the mother is older and cell division to produce eggs has not occurred properly, where both chrosomone (chromosome 21) go into the same egg cell leaving the other with none. If the egg with 2 chromosome 21s is fertilised the resulting offspring will have three chrososome 21s. The child will have a learning disability as well as more susceptible to certain diseases.
Most mutations are harnful and mutations often cause cancer. However, sometimes mutations are beneficial and evolution is due to mutations and natural selection.
Natural Selection
Natural selection is Charles Darwin's theory of evolution which suggests that only organims that have the characteristics to adapt to the environment will survive and, over a number of generations the population consists of the organisms that are best adapated to the environment. In essential, natural selection result in survival of the fittest.
The peppered moth is an example of natural selection. Moths are eaten by birds and light-coloured moths on light backgrounds have a better chance of survival than dark moths. The light-coloured moths survive to breed and so light moths predominate.
Selective Breeding
Selective breeding ys also known as artificial selection and can be used to improve plants and animals. This is achieved by selecting the plants/animals, from the natural population, with particular desirable characteristcs to bred together. This is repeated through several generations before all the offspring have the same desirable characteristics. In growing cereals, varieties which have sturdy stems and large number of seeds per stem may be chosen (i.e. selected) to produce a better variety for cultivation. The main disadvantage of selective breeding is the reduction in the gene pool.