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17.3 Evolution

Syllabus
9700–2028–2029
Topic
17.3
Level
A2

Evolutionary gene-pool change can lead to new species

Evolution is change in the genetic composition of populations over generations. A gene pool contains all alleles in an interbreeding population; when allele frequencies change, the population evolves.

  1. Mutation creates new alleles and meiosis/sexual reproduction create new combinations.
  2. Natural selection and genetic drift change allele frequencies from generation to generation.
  3. Populations can accumulate different genetic and phenotypic changes, especially when gene flow between them is reduced.
  4. If divergence produces reproductive isolation, the populations no longer interbreed successfully and a new species has formed from a pre-existing species.

Individuals can acclimatise during life, but that is not evolution unless inherited allele frequencies change across generations. Evolution has no predetermined goal; species formation is a possible long-term outcome of diverging gene pools, not an immediate result of every frequency change.

DNA sequences reveal relationships

DNA sequence comparison provides evidence for evolutionary relationships. DNA from suitable samples can be sequenced and aligned; the more similar the nucleotide sequences of two organisms, the more closely related they are inferred to be.

  • Obtain DNA from an organism or fossil sample and determine its base sequence.
  • Compare homologous sequence regions between organisms.
  • Shared sequence similarities support common ancestry and closer evolutionary relatedness.
  • Greater sequence differences are consistent with a longer time since the groups separated, because differences can accumulate over generations.
  • Multiple sequence comparisons can be used as evidence for a phylogenetic relationship or tree.

Interpret the evidence comparatively: similarity supports a closer relationship relative to a less similar comparison, while differences support greater evolutionary distance. The inference concerns relative relatedness or relative recency of separation; it does not supply an exact time or, by itself, identify every mutation, pathway or ancestor.

Similar DNA does not mean that two organisms are identical or that sequence comparison alone proves a complete evolutionary history. A sequence comparison is evidence that must be interpreted with the sampling and comparison context. This card owns DNA evidence, not the general definition of evolution or the detailed genetic-isolation pathways of speciation.

Genetic isolation can produce new species

Speciation is the formation of a new species from a pre-existing population. Genetic isolation is required: the separated populations no longer exchange genes sufficiently for their gene pools to remain the same.

Allopatric pathway — geographic separation:

  1. A geographic barrier divides one population.
  2. The groups cannot interbreed across the barrier, so gene flow stops or is greatly reduced.
  3. Mutation, natural selection and genetic drift act independently in the two populations.
  4. Their allele frequencies and phenotypes diverge over many generations.
  5. Reproductive differences accumulate until the groups can no longer successfully interbreed; separate species have formed.

Sympatric pathway — separation in the same area:

  1. There is no geographic barrier, but ecological or behavioural differences separate groups within the area.
  2. The groups use different environments or behaviours and no longer exchange genes sufficiently.
  3. Different selection pressures and genetic changes drive divergence.
  4. Reproductive isolation develops, so the groups no longer successfully interbreed and can become separate species.

The starting separation differs, but the required logic is shared: isolation → reduced gene flow → independent gene-pool change → phenotypic divergence → reproductive isolation → speciation. A single visible difference or barrier is not, by itself, proof that two species have already formed; the change takes many generations and must include reproductive isolation.

In sympatric speciation, distinguish the factor that first separates the groups from the later differences that prevent interbreeding. Allopatric means geographic isolation; sympatric means no geographic barrier. This card owns the isolation-to-speciation mechanism, not the general evolution definition or DNA-sequence evidence.