Human activities are altering ecosystems. Tropical rain forests are being destroyed to make room for, and to support, the expanding human population. Globally, the rate of species being lost may be as much as 50times higher than at any time in the past 100'000 years.
Exctinction is a nural process, but it's the current rate of extinction that underlies the biodiversity crisis. The "background" rate of extinction is 1/100'000'000, but it is now estimated that human activity in tropical areas lone has increased the rates of extinction by 100-10'000 times. Massive destruction of habitas has been brought about by, argiculture, urban development, forestry, mining and environmental pollution.
The majority of Earth's earlier occupants, have been extinct largely as a result of climatic, geological and biotic changes. The main causes for decline in number now is loss of habitat, over-hunting and competition from introduced species.
Other species are also threatened by additional causes such as deforestation, pollution and drainage of wetlands.
The theory of evolution was put forward by Charles Darwin, during his visit to the Galapagos Islands. Darwin collected fossil evidence that supported the idea that life changes with time. In 1859 he proposed natural selection.
Darwin studied 14 different species of finches. These islands were recently formed an any animals found there must have reached the islands from the mainland, 600miles away. Finches are unable to fly long distances and Darwin suggested that one ancestral specieshad reched the islands with the help of the prevailing winds. As there were no other bird species, there was a variety of food available.
The main differences were in the size and shape of their beaks, and these were relted to the different type of food eaten.
It seemed that on each island, the characteristics that best suited a particular finch to its environment were inherited by the offspring. Darwin suggested that the finches had developed from a common ancestor and that the type of beak had developed over time and become specialised to feed on a particular food source. This is an example of adaptive radiation.
When describing plants and animals, taxonomists look for differences and similarities between them and place similar organisms closely together and dissimlar ones further apart. A classification system based on large groups being divided up into smaller groups is said to be hierarchical.
The natural classification used today was created by Linnaeus in the 18th century. A taxon is a level in the classification hierarchy and is a collection of organisms, sharing basic features:
Kingdom, Phylum, Class, Order, Family, Genus & Species.
In 1753, Linnaeus devised a common system of naiming organisms.
Each organism is given two names:
The generic name comes first and always has a capital letter
The specific name comes second and always has a small letter.
The system is based on using Latin as an international language.
The five kingdom classification
Prokaryotae: Unicellular organisms (bacteria and blue-green algae). Have no internal membranes, no nuclear membrane, no ER, no mitochondria and no Golgi body. They possess a cell wall but it isn't made of cellulose.
Protoctista: Small eukaryotic organisms, with membrane-bound organelles and a nucleus with a nuclear membrane. Neither plants/animals/fungi. (algae, water moulds, slime moulds)
Fungi: Eukaryotic, body consists of a network of hyphae forming mycelium. Cell wall of chitin. Have no photosynthetic pigments and feeding is heterotrophic.Reproduction is by spores. (mushroom, yeast)
Plants: Multicellular and carry out photsynthesis. Cells are eukaryotic and have cellulose cell walls, vacuoles containing cell sap, and chloroplasts containing photosynthetic pigments.
Animals: Multicellular, heterotrophic, eukaryotes with cells lacking a cell wall and show nervous co-ordination. Divided into 2 main groups, invertebrates and vertebrates.
Selected animal phyla- Annelids
8'000 named species.
Include: earthworms, leaches and lugworms.
- long, thin segmented body, the segments being visable as rings.
- a fluid-filled body cavity
- a hydrostatic skeleton
- a head end with a primitive brain and a nervous system, running the length of the body.
- thin permeble skin, through which gaseous exchange occurs
- a closed circulatory system containing an oxygen-carrying pigment
Selected animal phyla- Arthopods
Divided into 4 classes
- Myriopoda- many pairs of legs. 1 or 2 per segment. e.g. millipedes and centipedes.
- Crustacea- have between 10-20 pairs of legs. e.g. crabs.
- Spiders- have 4 pairs of legs
- Insects- have 3 pairs of legs.
The arthopods have the following common features: A body divided into segments and into a head, thorax and abdomen, a well-developed brain, a hard outer skeleton, paired jointed legs and an open circulatory system.
Two important evolutionary developments aire:
- Jointed legs. Modified to perform a variety of functions.
- Exoskleton. The outer most layer of the body secretes a thick cuticle which consists of mainly chitin. This performs many functions: protection of internal organs, a point of attachment for muscles, support and the waxy property reduces water loss.
Selected animal phyla- Chordata
60'000 named species. Vertebrates possess a vertebral column and a well-developed brain, encolsed in a cranium.
Divided into five classes.
- Fish- scales, fins and gills.
- Amphibians- partly terrestrial and partly aquatic. Soft, moist skin. The eggs are fertilised externally in water, where they also develop. Young are aquatic.
- Reptiles- mainly terrestrial and have dry skin with scales. They have lungs. The eggs are fertilised internally, covered with a shell and laid on land.
- Birds- they can fly and have feathers. They have lungs and their eggs have hard shells.
- Mammals- have skin with hair. Young are born alive and are fed on milk. They have lungs. They are subdivided into 2 groups:
- Marsupials- young are born at a very immature stage and develop in femals pouch.
- Placentals- young undergo considerable development in the mother's womb.
The phylogenetic relationships of different species can be represented by a phylogenetic tree. The closer the branches, the closer the evolutionary relationship.
Evidence of common ancestry
The theory of evolution suggests that widely seperated groups of organisms share a common ancestor. Therefore, it would be expected that they would share certain basic structural features. How similar they are should indicate how closely related they are in terms of evolution. Groups with little in common are assumed to have diverged from a common ancestor much earlier in geological history than groups which have a lot in common.
Using physical features
To decide how closely related organisms are, scientists need to look at structures, that have the same structure which suggests a common origin. Such structures are said to be homologous. A good example is the pentadactyl limb of the chordata. A pentadacytl limb has 5 digits.The structure is the same, but it varies in function.
By using information like this, it is possible to construct an evolutionary tree, where the end products of evolution have certain structural features in common with each other and with the ancestral stock from which they arise. The more similar 2 organisms are, the more recently they are assumed to have diverged.
However, there is a possible danger in assuming that just because two animals look similar, that they are related. This happens when structures have evolved to carry out similar functions to suit their environment. E.G. the fin of a dolphin and the fin of a shark. These limbs are called analogous.
Using genetic evidence- DNA analysis
DNA analysis has been used to confirm evolutionary relationships and can reduce the mistakes made in classification due to convergent evolution.
- The technique of DNA hybridisation involved the extraction and comparison of the DNA of two species. The sequence of bases is compared, the more alike the sequences, the closer the organisms are related.
- The sequence of amino acids in proteins is determined by DNA. Therefore the degree of similarity in the amino acid seqence of the same protein in different species will reflect how closely relaed they are.
- The proteins of different species can also be compared using immunological techniques. The principle behind this involves the fact that antibodies of one species will respnd to specific antigens on proteins in the blood serum of another. When antibodies respond to corresponding antigens a precipitate is formed.
How closely related?
To work out how closely related two species of primates are, the DNA strands from both species are extracted, seperated and cut into fragments. The fragments from the 2 species are then mixed and analysed. This technique gives resuls which show that chimpanzees and humans have 97.6% DNA in common.
Comparing the DNA and proteins of different species helps scientists to determine the evolutionary relationships between them.