Evolution

· Evolution = the process by which new species form from pre-existing species over time.
· Evolution occurs because gene pools change from generation to generation.
· A gene pool = all the alleles present in a population at a particular time.
· Changes in allele frequencies may happen due to selection, mutation, genetic drift, founder effect, bottleneck effect and gene flow.
· Over many generations, accumulated genetic changes can cause populations to become reproductively isolated and form new species.
· Key exam phrase: evolution is a change in allele frequency in a population over time.

DNA Sequence Data and Evolutionary Relationships

· DNA sequence data can be used to compare evolutionary relationships between species.
· Species with more similar DNA base sequences are usually more closely related.
· Species with more DNA sequence differences are usually more distantly related.
· Differences in DNA sequences arise from mutations that accumulate over time.
· DNA comparisons are useful because DNA is inherited, so similarities can indicate shared ancestry.
· Exam link: DNA evidence can support evolutionary trees by showing which species share the most recent common ancestor.
· DNA sequence data is often considered stronger evidence than visible features because morphology can be affected by environment or convergent evolution.

Molecular data, including DNA sequences, can be used to construct phylogenetic trees. More sequence similarity suggests a closer evolutionary relationship. Source

Speciation

· Speciation = formation of a new species.
· Speciation occurs when populations become genetically isolated.
· Genetic isolation means allele flow between populations is reduced or stopped.
· Once isolated, populations experience different mutations, selection pressures and genetic drift.
· Over many generations, populations become genetically different enough that they can no longer interbreed to produce fertile offspring.
· This results in reproductive isolation and the formation of separate species.

Allopatric Speciation

· Allopatric speciation occurs due to geographical separation.
· A population is split by a physical barrier such as a river, mountain range, island formation, desert or glacier.
· The separated populations experience different environmental conditions and selection pressures.
· Different alleles become more common in each population.
· Over time, the populations become genetically different.
· If they can no longer interbreed successfully if reunited, they are now separate species.

This diagram shows allopatric speciation, where a physical barrier prevents gene flow between populations. Different allele frequencies develop over time, eventually producing separate species. Source

Sympatric Speciation

· Sympatric speciation occurs without geographical separation.
· Populations remain in the same general area but become genetically isolated due to ecological separation or behavioural separation.
· Ecological separation = groups occupy different niches, habitats, food sources or breeding sites.
· Behavioural separation = groups develop different courtship behaviours, mating signals or breeding times.
· Reduced interbreeding means reduced gene flow.
· Different selection pressures can lead to different allele frequencies.
· Over time, reproductive isolation may develop, producing new species.

Comparing Allopatric and Sympatric Speciation

· Allopatric speciation: caused by geographical separation.
· Sympatric speciation: caused by ecological or behavioural separation.
· Both require genetic isolation and reduced gene flow.
· Both involve changes in allele frequencies over many generations.
· Both can lead to reproductive isolation and formation of new species.

Exam Phrases to Use

· “Evolution involves changes in gene pools from generation to generation.”
· “DNA sequence similarity indicates close evolutionary relationship.”
· “More DNA sequence differences indicate a more distant common ancestor.”
· “Speciation requires genetic isolation followed by accumulation of genetic differences.”
· “Allopatric speciation occurs due to geographical separation.”
· “Sympatric speciation occurs due to ecological or behavioural separation.”
· “New species form when populations can no longer interbreed to produce fertile offspring.”