Covers selection pressures, adaptations, biodiversity, Darwin's observations and natural selection.
Lesson Summaries
A selection pressure is any biotic or abiotic factor that affects an organism's chance of surviving and reproducing. Pressures such as predators, disease, climate and competition act on the variation already present in a population, so the proportion of favourable phenotypes changes over generations. Populations evolve; individuals do not change to suit their environment.
An adaptation is an inherited feature that increases an organism fitness in a particular environment. Adaptations are structural, physiological or behavioural and arise through natural selection acting on heritable variation, not because an organism needs them within its lifetime.
Biodiversity is the variety of life measured at genetic, species and ecosystem levels. Species diversity depends on both richness and evenness, and Australia's high endemism reflects its long isolation since Gondwana. Biodiversity is not simply a count of species.
On the Beagle voyage Darwin observed variation, adaptation to local conditions, and patterns such as the Galapagos finches whose beaks diversified by adaptive radiation. Australian marsupials show convergent evolution with placental mammals. Gathered over decades from many independent lines, these observations built the evidence for natural selection.
Darwin and Wallace independently proposed natural selection. It requires variation, heritability, a selection pressure and differential survival and reproduction. Acquired traits are not inherited, and 'survival of the fittest' means greatest reproductive success, not merely strength.
1. Which of the following is an ABIOTIC selection pressure?
2. An adaptation is best defined as...
3. Species diversity in a community depends on:
4. Darwin's Galapagos finches are best described as:
5. Charles Darwin proposed that evolution occurs through...
6. Alfred Russel Wallace independently arrived at the same theory of natural selection as Darwin by...
7. Explain how a selection pressure can change the allele frequencies of a population over time. Use an example in your answer. 4 MARKS
1 mark: variation already exists in the population | 1 mark: selection pressure favours certain phenotypes | 1 mark: those individuals survive and reproduce more | 1 mark: allele frequency shifts over generations
8. Distinguish between an adaptation and an acquired characteristic, and explain why only one of them can drive evolution. 4 MARKS
1 mark: adaptation is inherited | 1 mark: acquired characteristic develops during the lifetime | 1 mark: only heritable variation is passed to offspring | 1 mark: so only adaptations can change allele frequencies
7. A population already contains heritable variation in a trait. A selection pressure (for example a predator, drought or antibiotic) means individuals with certain phenotypes survive and reproduce more than others. Because the favoured phenotype is heritable, those alleles are passed on more often, so their frequency in the population increases over generations. For example, when antibiotics are used, bacteria carrying pre-existing resistance alleles survive and reproduce, so the resistance allele becomes more common.
8. An adaptation is an inherited feature shaped by natural selection that increases fitness, whereas an acquired characteristic develops during an organism lifetime (such as larger muscles from exercise) and is not coded in the genes passed to offspring. Only heritable variation is inherited, so only adaptations can change the allele frequencies of a population over generations. Acquired characteristics are lost each generation and cannot drive evolution.