Populations change across generations
Definition
Evolution
Evolution is a change in the inherited characteristics of a population over successive generations.
- Evolution concerns changes in inherited characteristics across successive generations, so the population is the unit whose characteristics are compared over time.
- A population can evolve when the relative abundance of inherited variants changes, even if it still belongs to the same species.
- Natural selection can produce this change when individuals carrying particular inherited variants contribute more offspring to later generations.
- This links present-day populations to earlier populations through descent with modification, meaning inheritance accompanied by changes across generations.
- Growth, learning and changes caused only by the environment during one individual's life do not by themselves show evolution.
- For example, a person developing larger muscles through training is a change within an individual, without evidence that the population's inherited characteristics have changed.
- An increase in the total number of organisms is also insufficient evidence, because the relative proportions of inherited characteristics might remain the same.
Example
- In an illustrative population, an inherited colour occurs in 303030 out of 100100100 organisms in one generation and 120120120 out of 200200200 organisms several generations later.
- The initial proportion is 30100×100=30%\dfrac{30}{100}\times100=30\%10030×100=30%.
- The later proportion is 120200×100=60%\dfrac{120}{200}\times100=60\%200120×100=60%.
- The rise in proportion shows that the inherited colour became more common, while simply comparing the counts would mix this change with population growth.
- Representative data across generations would support an evolutionary change in the population.
New species can form
Definition
Species
A species is a group of organisms that can interbreed to produce fertile offspring.
Definition
Speciation
Speciation is the formation of a new species from an existing population.
- The fertile-offspring definition of species is useful for sexually reproducing organisms, although it cannot be applied directly to organisms that reproduce only asexually.
- Evolution may lead to speciation, but a change in one inherited characteristic does not automatically create a new species.
- One route begins when populations become isolated, for example when a geographical barrier prevents individuals from interbreeding.
- Separate populations then stop exchanging alleles through reproduction, allowing inherited differences to accumulate independently.
- Different conditions can favour different characteristics in each population, while new mutations can also introduce differences.
- Over many generations, the populations may become so different that individuals can no longer interbreed successfully to produce fertile offspring.
- They are then reproductively isolated, which marks the formation of distinct species under the fertile-offspring definition.
- Geographical separation alone does not show that speciation has occurred, because separated populations may still be capable of producing fertile offspring together.
Example
- A population of seed-eating birds is separated onto two islands, with no movement of birds between them.
- Different food supplies favour different inherited feeding characteristics on the two islands.
- After many generations, differences may accumulate in characteristics involved in mating as well as feeding.
- If the populations can no longer interbreed to produce fertile offspring, they have formed distinct species.
- Different beak shapes alone would be insufficient to establish that final step.
Branching ancestry and timescales
- A common ancestor is an ancestral organism or population from which two or more later lineages descend.
- When one ancestral lineage gives rise to separate descendant lineages, evolution forms a branching pattern rather than a single ladder from simple to advanced organisms.
- Present-day species at different tips of a tree may share an earlier ancestor without one present-day species having evolved directly from the other.
- The branching diagram represents inherited differences accumulating in descendant lineages, with branch points indicating shared ancestors.
- Evolution has no predetermined goal, and natural selection favours characteristics according to the conditions encountered by each population.
- Major differences between lineages can accumulate over very long periods, but evolutionary change does not always require millions of years.
- The speed at which a measurable change appears depends on factors such as generation time, available genetic variation and the strength of selection pressures.
- Organisms with short generation times can pass through many generations during a short period, allowing inherited changes to be detected sooner.
Exam technique
- Include inherited characteristics, population and successive generations when defining evolution.
- For new species, explain the loss of the ability to produce fertile offspring, rather than stopping at geographical separation or different appearances.
Self review
- Why is an individual's growth not evidence of evolution?
- Why are proportions useful when comparing inherited characteristics across generations?
- What does speciation mean?
- Why does geographical separation not immediately create a new species?
- How can short generation times make evolutionary change easier to observe?
