17.3 Evolution

Evolution is a change in the gene pool of a population across generations. A gene pool is the collection of genes and their alleles in an interbreeding population of one species; a change in allele frequencies changes the genetic composition of that population over time.

• Variation: members of a population can carry different alleles, and their phenotypes reflect genotype together with environmental influence.
• Population process: mutation introduces new genetic differences; natural selection, genetic drift and founder effects can change allele frequencies across generations.
• Outcome: as the gene pool changes, the frequencies of phenotypes in the population may also change.
• Scale: the evolutionary change is measured across a population and generations, not as a purposeful change in one individual.

An individual can acclimatise or alter its phenotype during its lifetime without that being evolution. Evolution requires a heritable change in population allele frequencies across generations; organisms do not change their alleles because they “need” to adapt. “Better adapted” is environment-dependent, not a universal improvement.

Use the gene-pool model to explain population change without moving into DNA-sequence evidence or the detailed allopatric/sympatric speciation pathways. Species definitions depend on investigation, but the central claim here is the population-level change in allele frequencies over time.

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.

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.