How Have Organisms Changed Over Time?
The Outcomes Being Assessed:
22. Outline the theory of evolution by natural selection as proposed by Darwin and Wallace.
23. Describe the evidence for evolution including fossils, transitional forms, comparative embryology and homologous structures (the pentadactyl limb) and biochemical similarities.
24. Relate the fossil record to the age of the Earth and the time over which life has been evolving and identify that the fossil record supports major trends in evolution such as simple to complex and/or aquatic to terrestrial.
25. Explain how Darwin's observations of Galapagos finches and other groups of organisms enabled him to form his theory of evolution by natural selection.
26. Account for the changes in populations using the theory of natural selection E.g. Peppered moth, resistance to insecticides, antibiotic resistance.
27. Design an experiment to model the process of natural selection.
22. Outline the theory of evolution by natural selection as proposed by Darwin and Wallace.
23. Describe the evidence for evolution including fossils, transitional forms, comparative embryology and homologous structures (the pentadactyl limb) and biochemical similarities.
24. Relate the fossil record to the age of the Earth and the time over which life has been evolving and identify that the fossil record supports major trends in evolution such as simple to complex and/or aquatic to terrestrial.
25. Explain how Darwin's observations of Galapagos finches and other groups of organisms enabled him to form his theory of evolution by natural selection.
26. Account for the changes in populations using the theory of natural selection E.g. Peppered moth, resistance to insecticides, antibiotic resistance.
27. Design an experiment to model the process of natural selection.
What is the theory of evolution?
The theory of evolution states that life forms on earth have changed with time. New species have gradually developed and other species have become extinct in response to changing environments. These inheritable changes have occurred slowly over millions of years over many generations. There has been a general change of simple to more complex forms of life. However, an individual organism cannot evolve during its lifetime.
What is the evidence to support the theory of evolution?
Fossils
- A fossil is a preserved remnant or trace of a once living organism.
- Fossils are typically found in sedimentary rocks.
- Usually it is the ‘hard parts’ of plants and animals that are preserved e.g. wood or bones or shells, as the ‘softer parts’ e.g. flesh are usually decayed (decomposed by bacteria, etc.) or eaten.
- Fossils are often preserved by being covered by sediments or by falling into an environment where there is little oxygen (e.g. still water, as in swamps).
- In rarer cases, the remains of soft-bodied animals, such as jellyfish or even bacteria have been preserved.
- The fossil record is incomplete. It is likely to remain incomplete because: fossilization is a rare event; and although fossils of soft-bodied organisms are found, organisms with hard body parts are much more likely to be fossilized.
- Many fossils have been found which appear to have the characteristics of two different groups of organisms. These fossils are called transition fossils.
- Homologous structures are structures found in plants or animals that have the same origin, but not necessarily exactly the same form or the same function.
- The development of homologous structures is an example of divergent evolution. Divergent evolution occurs when a number of species develop from one common ancestor, becoming less and less alike over time due to their different habitats.
- The pentadactyl limb of vertebrate animals (mammals, reptiles and amphibians) is an example of an homologous structure. All of these groups have limbs with five digits (finger-like structures) which have the same basic structural plan, but have altered over time due to different environmental pressures and ‘life-styles’ (i.e. different methods of locomotion, feeding, etc). This suggests that all of these groups of vertebrates evolved from some original group of animals (the “common ancestor”).
- The embryos of different vertebrates can be compared to look for evidence that these animals evolved from a common ancestor.
- New biochemical techniques have made it possible to compare the biochemicals (such as proteins and nucleic acids (DNA)) in various species. The degree of difference between a specific protein or nucleic acid in different species provides a measure of the length of time since the two species shared a common ancestor.
- This is because mutations occur relatively regularly. So, the longer it is since evolution from a common ancestor, the greater the difference between molecules. It is this information that can be used in conjunction with fossils and other evidence to piece together evolutionary relationships.
- Cytochrome c is a protein involved in the process of respiration in all living things. Cytochrome-c first appeared in ancient bacteria about two billion years ago. It is present in all modern organisms. Cytochrome-c molecules each contain approximately 100 amino acids and the exact sequence of those amino acids is known for about 60 species. The number of differences in the amino acid sequence in this protein taken from different organisms can be used as an indicator of evolutionary relationships. The fewer the differences, the more closely related are the organisms.
The fossil record & trends in evolution
Fossils are typically found in sedimentary rocks. The age of rocks can be determined by using radioactive dating. The age of rocks can also be determined by correlating the fossils found in them with rocks of known age.The relative age of sedimentary rocks can be deduced by making the assumption that younger sedimentary rocks are above older sedimentary rocks (assuming no geological upheavals have occurred). If there have been no geological upheavals, older sedimentary rocks contain the older (earlier) fossils (because they were deposited earlier).The
oldest sedimentary rocks contain no fossils i.e. no evidence of life.· Older fossils are from organisms which were:
- simple
- aquatic
- both simple and complex
- both aquatic and terrestrial
Darwin's Theory of Natural Selection & his observations
Charles Darwin (1809-1882) was a naturalist and one of the originators (together with Alfred Russel Wallace) of the theory of evolution by natural selection. In 1831 he sailed on the voyage of the Beagle as a naturalist. Of all places visited, he was most impressed with the Galapagos Islands. The Galapagos Islands lay about 1000 km off the coast of Ecuador in the Pacific Ocean. On these recently formed volcanic islands, the animals and plants differed from those of the mainland as well as from one island to another in the archipelago; Darwin observed that many species that were clearly related differed slightly depending on where they were found. In his voyage to South America Darwin saw a world of
new creatures, many of which he collected.
He noticed that the curious fossil armour plated mammals of the South
American pampas, the Glyptodonts, were similar to the armadillos alive today in
the same region. On the Galapagos Islands, Darwin
became fascinated with a group of finches, now called “Darwin’s finches”. Darwin
discovered fourteen different species, none of which was found on the South
American continent nearby or elsewhere in the world (although they had
similarities to a species of finch found on the mainland). Darwin guessed that the finches had originated from
some common ancestor who had come from the mainland of South America, and the populations
of finches on the different islands had evolved, or changed due to their
different environments, with different food sources, to form separate species. This is
Darwin’s explanation for how the finch population had changed on each island:
- The original birds landing on each island from the mainland would have had some variation e.g. in the shape of their beaks.
- Those birds on each island which were most suited (best adapted – or “fittest”) to their environment (e.g. birds with favourable beaks best suited to the food resources on their island) would have survived.
- The birds with favourable beaks would survive and reproduced, passing on their favourable beak shape to their offspring.
- Thus, the favourable beak shape for each island would have become more common on each island. Eventually, each group of finches on the different islands would have become so different that they could be regarded as separate species.
Variation & Mutation
For evolution to occur by natural selection there must be VARIATION in a population.