Evolution – SS3 Biology Lesson Note (Second Term)

Evolution is the gradual change in the features of living species over many generations.

• Darwin: explained evolution through natural selection.
• Lamarck: proposed that features acquired during life are passed on.
• Modern theory: adds genetics, mutation, gene flow and genetic drift.
• Evidence: comes from fossils, embryology and comparative anatomy.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Explain Darwin’s theory of evolution.
2. State Lamarck’s evolutionary laws.
3. Distinguish between Darwin’s and Lamarck’s theories.
4. Discuss the modern theory of evolution.
5. Identify the evidence of evolution.
6. Identify the forces responsible for evolution.

Instructional techniques and materials

Instructional techniques

• Identification
• Explanation
• Questions and answers
• Demonstration
• Videos

Instructional materials

• Videos
• Loudspeaker
• Textbook
• Pictures

What are the key terms in evolution?

Six terms are used throughout this topic, and each has a short meaning.

• Species: a group of organisms that can breed together and produce fertile offspring.
• Variation: the differences between individuals of the same species.
• Adaptation: a feature that helps an organism survive and reproduce in its environment.
• Natural selection: the survival and reproduction of the best-adapted individuals.
• Mutation: a sudden change in the DNA of an organism.
• Extinction: the complete disappearance of a species.

What is Darwin’s theory of evolution?

Charles Darwin’s theory states that species evolve by natural selection, as set out in “On the Origin of Species” (1859).

The steps of natural selection

1. Overpopulation: all living organisms produce more offspring than can survive.
2. Struggle for existence: because of overpopulation, the offspring compete constantly to survive.
3. Variation: the offspring show differences. No two individuals are the same.
4. Adaptive characteristics: some offspring are better fitted to survive the competition than others.
5. Inheritance of adaptive characteristics: the fittest pass their useful variations to their offspring, while poorly adapted ones die off.
6. Development of new species: over a long time, favourable variations build up and the group spreads (diverges) from the original stock to form new species.

What are Lamarck’s evolutionary laws?

Jean-Baptiste Lamarck’s theory, proposed in the early 19th century, states that organisms change because of need and pass on acquired features.

1. The theory of need

• Idea: an organism develops a new part or organ because it needs that part.
• Snake example: the early ancestors of snakes had short bodies and legs.
• Cause: as land formed, narrow places and dense vegetation appeared.
• Result: the snakes stretched their bodies until they became long and could crawl through the vegetation.

2. The theory of inheritance of acquired characteristics

• Idea: structures acquired during an organism’s lifetime are passed to the next generation.
• Giraffe example: the modern giraffe is believed to have come from a short-necked, short-legged giraffe.
• Cause: competition for low grasses among herbivores began.
• Result: the giraffe stretched its neck and legs to reach tree leaves, and the longer neck was passed on.
• Toad and duck example: they had no webs between their toes, but visiting water often created a need for webbed feet for swimming.

Why is Lamarck’s theory no longer accepted?

• Genetics: changes to body parts during life do not alter the DNA in the sex cells.
• Result: such changes are not inherited by offspring.

What is the difference between Darwin’s and Lamarck’s theories?

Darwin’s theory depends on natural selection acting on inherited variation. Lamarck’s theory depends on need and the inheritance of acquired features.

What is the modern theory of evolution?

The modern theory of evolution builds on Darwin’s ideas and adds genetics and other sciences.

• Natural selection: remains central, as individuals with traits suited to their environment survive and reproduce more.
• Genetics: DNA gives a molecular basis for inheritance and explains how traits pass between generations.
• Mutation: the source of new variation, which can produce new traits.
• Gene flow: the exchange of genes between populations.
• Genetic drift: the random change of gene frequencies in a population.
• Speciation: the formation of new species, often through reproductive isolation.
• Adaptation: natural selection acting on useful variations gives traits that suit a particular environment.

What is the evidence of evolution?

Evidence for evolution comes from fossils, embryology and comparative anatomy.

Fossil record

• Transitional fossils: they show features of ancestral and descendant groups, as Archaeopteryx shows features of reptiles and birds.
• Stratigraphy: fossils in rock layers (strata) appear in a time sequence that fits the expected evolutionary relationships.
• Extinct species: fossils of animals such as dinosaurs show that life has changed and many forms no longer exist.

Embryology

• Similar early stages: embryos of different vertebrates look alike early on, for example gill slits and tails, which suggests common ancestry.
• Vestigial structures: a tail is present in the early human embryo, and only a small remnant (the coccyx) remains in adults.
• Hox genes: these genes control body plan and development, and they are similar in many different organisms.

Comparative anatomy

• Homologous structures: parts with the same basic structure but different uses, such as the forelimbs of humans, bats and whales.
• Meaning: homologous structures point to a common ancestor.

What forces are responsible for evolution?

The forces responsible for evolution are mutation, gene flow, genetic drift and natural selection.

• Mutation: introduces genetic variation by changing the DNA sequence.
• Gene flow: moves genes between populations and increases genetic diversity.
• Genetic drift: causes random changes in allele frequencies, especially in small populations.
• Natural selection: favours individuals with useful traits so that these traits become more common.

Evaluation

1. State Lamarck’s first law of evolution.
2. Mention three forces responsible for evolution.
3. Discuss the modern theory of evolution.
4. Identify the evidence that supports evolution.
5. State two differences between Darwin’s and Lamarck’s theories.

Assignment

Answer all three parts in your notebook.

1. List the six steps in Darwin’s theory of natural selection.
2. Using the giraffe as an example, explain Lamarck’s theory of inheritance of acquired characteristics.
3. Describe two pieces of embryological evidence for evolution.