Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

6.3.1Inheritance, Variation and Evolution

Evidence for Evolution

5 min read 9 parts Lesson 12 of 16 Unit quiz

0%read

Section 1 of 9Why evidence matters

Evolution happens over very long periods, so most of the evidence is indirect. A theory is accepted when many independent lines of evidence point to the same conclusion. The main sources are fossils, rapid evolution in modern organisms, and similarities in the structure and genetic material of living things.

Section 2 of 9The fossil record

Fossils are the preserved remains or traces of organisms from millions of years ago.

1
Because rock layers are laid down over time, deeper layers are normally older.
2
Placing fossils in order shows how organisms have changed.
3
For example the fossils of horses show a gradual increase in size and a change in toe and tooth structure.
4
Fossils of transitional forms, such as Archaeopteryx with reptile and bird features, link major groups.
5
The fossil record shows that simple organisms appeared first and more complex ones later.

Section 3 of 9Evolution observed today

Bacteria reproduce so rapidly that evolution can be observed within years. Antibiotic-resistant strains such as MRSA arise when resistant mutants survive treatment and spread. Similarly, insects evolve resistance to pesticides and rats to certain poisons. Peppered moth populations changed in proportion as the environment changed. These examples show natural selection operating in real time.

Section 4 of 9Comparative anatomy

The forelimbs of humans, bats, whales, dogs and birds have different functions but the same basic bone pattern (humerus, radius and ulna, carpals, metacarpals and phalanges).

Pentadactyl limb
This is called the pentadactyl limb.
The simplest explanation is that these animals share a common ancestor and the limb has been modified by natural selection for different uses.
Such features, similar in structure because of shared ancestry, are called homologous structures.

Section 5 of 9Molecular evidence

All living things use DNA and a nearly universal genetic code, which suggests a common origin.

Molecular evidence
Comparing DNA base sequences or protein amino acid sequences allows scientists to measure how closely related species are: the fewer the differences, the more recently two species shared an ancestor.
Humans and chimpanzees differ by only a small percentage of their DNA.
Such data is used to build evolutionary trees and to classify organisms.

Section 6 of 9Interpreting evidence and its limits

Each type of evidence has strengths and weaknesses, and exam questions often ask you to evaluate them.

Interpreting evidence and its limits
Fossils show the actual forms of past organisms and their order in time, but the record has gaps.
Anatomical similarities show common ancestry but can sometimes be caused by similar environments, as in the streamlined bodies of sharks and dolphins,
  • which have evolved separately; this is called convergent evolution.
DNA evidence is powerful because it can be measured precisely and compared between any two species,
  • including those that do not look alike,
  • and it does not depend on the survival of fossils.

A worked example uses a table of DNA similarity.

Interpreting evidence and its limits
If a chimpanzee shares 98.8% of its DNA with humans, a gorilla 98.4% and an orangutan 96.9%, the ranking suggests that chimpanzees are the closest relatives of humans among these apes and orangutans the most distant.
Use phrases such as 'is more closely related to' and 'shared a common ancestor more recently'.
Do not claim that humans evolved from chimpanzees; both evolved from a common ancestor.
Interpreting evidence and its limits
Evidence is also found in the geographical distribution of species.
  • Marsupials are common in Australia but rare elsewhere,
  • which is explained by their evolution after Australia became separated from other land masses.
  • Islands often have species resembling those of the nearest mainland, suggesting that colonists arrived and evolved.
Observed evolution,
  • such as MRSA, is the strongest evidence of the mechanism,
  • because it can be repeated and measured within a human lifetime.
  • Taken together, the independent lines of evidence from fossils, anatomy, DNA and direct observation give high confidence in the theory, even though no single line is complete on its own.
If an exam question gives a table of amino-acid differences, rank the species from fewest to most differences, state that fewer differences mean a more recent common ancestor,
  • and use the numbers to support each comparison.
  • This structure earns marks reliably.

Section 7 of 9Exam-style questions

  1. Explain how fossils provide evidence for evolution. [3]3 marks

  2. Bats, whales and humans have forelimbs with the same arrangement of bones despite different uses. Explain what this suggests. [2]2 marks

  3. The cytochrome c protein differs by 1 amino acid between humans and chimpanzees and by 11 between humans and dogs. Explain what this shows. [2]2 marks

Section 8 of 9Key terms

TermMeaning
FossilThe preserved remains or traces of organisms from long ago.
Pentadactyl limbA limb with five digits and the same bone pattern, found in many vertebrates.
Common ancestorAn organism from which two or more species have descended.
Transitional fossilA fossil showing features of two different groups.
ResistanceThe ability of an organism to survive a drug or poison.
Homologous structureStructures similar due to shared ancestry, possibly with different functions.

Section 9 of 9Exam technique

1
Saying that fossils prove evolution on their own. They are one line of evidence, and the record is incomplete.
2
Thinking similar structures mean organisms are the same species. They show shared ancestry.
3
Forgetting to link antibiotic resistance to natural selection in the answer.
4
Saying deeper rock layers are always younger. Deeper layers are normally older.
\n