Evidence for evolutionIB MYP Biology: Revision notes
Section 1
The fossil record
Fossils are the remains or traces of organisms that lived long ago, preserved in rock. Most form when hard parts such as bones or shells are buried in sediment and slowly replaced by minerals.
Fossils in sedimentary rock are arranged in layers. The deepest layers are the oldest, so the fossils form a timeline. Sequences of fossils, such as those of whale ancestors, show how organisms changed gradually over millions of years. Extinct species can be linked to living ones.
The fossil record is incomplete. Soft-bodied organisms usually decay without leaving fossils, fossilisation needs special conditions, many fossils are destroyed by geological activity, and many are not yet found.
If asked why the fossil record is incomplete, give two different reasons: few organisms fossilise, and fossils are destroyed or not yet found.
Section 2
Comparative anatomy and homologous structures
Comparative anatomy compares the body structures of different species. The forelimbs of a human, bat, whale and horse have different jobs but the same basic pattern of bones. These are homologous structures.
The best explanation is that these animals share a common ancestor that had this pattern of bones. Over time, natural selection adapted the limbs to different ways of life, such as flying, swimming and running.
Homologous structures have the same origin but may do different jobs. Do not confuse them with structures that just look alike or do the same job.
Section 3
DNA and protein similarities
Organisms store information in DNA, and proteins are made from chains of amino acids. Scientists compare DNA base sequences and amino acid sequences between species.
The fewer the differences, the more closely related the species are. They share a more recent common ancestor, so there has been less time for mutations to build up. For example, human cytochrome c is much more similar to a monkey's than to a fish's.
This is strong evidence because it can be checked against the fossil record and anatomy, and it can be done for any species with DNA.
Section 4
Lamarck and Darwin
Jean-Baptiste Lamarck (early 1800s) suggested that organisms change during their lifetime by use and disuse and pass these acquired characteristics to their offspring. A giraffe stretching to reach leaves would have a longer neck, and so would its young.
Darwin explained it differently. Giraffes vary because of their alleles, longer-necked ones reach more food, survive and reproduce more, and pass on their alleles. The population changes by natural selection.
Evidence supports Darwin. Characteristics gained in life, such as big muscles, are not inherited, because the alleles in the sex cells are unchanged.
Do not write that giraffes 'wanted' or 'tried' to grow long necks. In Darwin's idea, variation already exists and selection acts on it.
Must Know
- Fossils in rock layers show how life changed; the record is incomplete
- Homologous structures have the same bone pattern but different uses, which suggests a common ancestor
- Fewer DNA or protein differences means closer relationship
- Lamarck: acquired characteristics are inherited; Darwin: natural selection of inherited variation
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Evidence for evolution
- Palaeontologists have found fossils in layers of sedimentary rock in Pakistan and Egypt that record the history of whales. The oldest fossils, about 50 million years old, show a wolf-sized animal with four legs that lived partly on land. Younger fossils show animals with shorter back legs and paddle-like front limbs. The youngest fossils show animals with bodies like modern whales and only tiny hind-limb bones.Explain why the fossil record is incomplete.2 marks
- The front limbs of a human, a bat, a whale and a horse look very different and are used for different jobs: grasping, flying, swimming and running. However, when anatomists compare the bones, each limb contains one upper-arm bone, two lower-arm bones, a group of wrist bones and a series of finger bones, arranged in the same basic pattern.Explain how the limb bones support the idea that these animals evolved from a common ancestor, even though the limbs have different uses.2 marks
- Cytochrome c is a protein used in respiration by almost all living organisms. Scientists compared the order of amino acids in cytochrome c from different species with the human protein and counted the positions where the amino acids differ. For the chimpanzee there were 0 differences, for the rhesus monkey 1, for the dog 10, for the chicken 13 and for the tuna 21.Describe the pattern in the data and explain what it suggests about how closely related each species is to humans.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).