Identify and describe What is Evolution?, Natural Selection: The Driving Force of Evolution and Stages of Natural Selection in the context of evolution.
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GCSE Biology Revision
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GCSE Biology revision
Evolution
Variation and evolution
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Explain the relationships between What is Evolution?, Natural Selection: The Driving Force of Evolution and Stages of Natural Selection and how they contribute to evolution.
Apply and analyse knowledge of What is Evolution?, Natural Selection: The Driving Force of Evolution and Stages of Natural Selection to interpret unfamiliar information about evolution.
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What is Evolution?
- Evolution refers to a change in the alleles (different forms of the same gene) within a population of a species over time.
- Evolution is a slow process that occurs over thousands, millions, or even billions of years, leading to changes in the phenotypes of species.
- Charles Darwin proposed that all species of living things evolved from very simple life forms, a process that began more than 3 billion years ago.
- Darwin described his ideas in his book On the Origin of Species in the 1850s, forming the foundation of modern evolutionary theory.
Natural Selection: The Driving Force of Evolution
- Natural selection is the process by which organisms that are best adapted to their environment are more likely to survive and reproduce.
- The phrase 'survival of the fittest' summarises natural selection — the 'fittest' are those most adapted to their environment, not necessarily the strongest.
- Organisms with beneficial traits are more likely to find a mate, reproduce, and pass on their advantageous alleles to offspring.
- Natural selection leads to evolution, as over many generations the population shifts so that the majority of individuals carry the beneficial characteristic.
Stages of Natural Selection
- Variation exists within a population because individuals carry different alleles (versions) of the same genes.
- A selection pressure (e.g. limited food, predators) means that individuals with advantageous traits are more likely to survive.
- Individuals with beneficial alleles survive longer, successfully mate, and pass those alleles on to their offspring.
- Over time, the frequency of the beneficial allele increases within the population, while less advantageous alleles become rarer or disappear.
The Role of Mutations
- A mutation is a change in a gene that can alter an organism's genotype and potentially produce a new phenotype.
- If the new phenotype is advantageous, natural selection will cause it to spread through the population over successive generations.
- Beneficial mutations that confer a selective advantage will increase in allele frequency within the population over time.
Selection Pressure
- Natural selection requires a selection pressure — a factor in the environment that affects an organism's ability to survive and reproduce.
- Examples of selection pressures include limited food supply, predation, disease, and competition for mates.
- Without a selection pressure, there is no advantage to having one trait over another, so natural selection does not act on the population.
Example: Deer Mice in the Sandhills of Nebraska
- Deer mice that migrated to the Sandhills of Nebraska showed intraspecific variation — some were dark brown and some were lighter brown.
- In the sandy environment, lighter-coloured mice were better camouflaged from predators and therefore survived longer.
- Dark brown mice were more visible to predators, making them less likely to survive and reproduce.
- Over time, the allele for light brown colouring became more frequent in the population — a clear example of natural selection in action.
Example: Mountain Rams and Horn Length
- Within a population of mountain rams, variation exists in horn length due to different alleles of the same gene.
- Rams with longer horns can better defend themselves, compete for food and water, and successfully attract mates.
- Rams with shorter horns are less competitive, less likely to survive, and therefore less likely to reproduce and pass on their alleles.
- Over generations, the allele for longer horns increases in frequency within the population as short-horned rams reproduce less.
Speciation: The Formation of New Species
- Speciation is the formation of a new species, which occurs when two populations of the same species become so different that they can no longer interbreed to produce fertile offspring.
- A key definition: two organisms belong to different species if they cannot freely interbreed to produce fertile offspring.
- Geographical isolation (e.g. a river, mountain range) can separate a population, creating a reproductive barrier that prevents interbreeding.
- With no gene flow between the two isolated groups, natural selection acts independently on each population, causing them to diverge over time.
- Eventually, the two populations become so genetically different that they form two distinct species — this is speciation.
True or False: Common Misconceptions
- It is FALSE that different species can breed to produce fertile offspring — this is the very definition that separates two species.
- It is FALSE that geographical isolation is the only way speciation can occur — it is one example, but not an absolute requirement.
- It is FALSE that evolution is defined as a change in phenotype of a population — evolution is defined as a change in allele frequency within a population.
- It is TRUE that evolution requires a selection pressure, such as limited food or predation, to drive natural selection.