A4.1 Evolution and speciation

Evolution is a change in heritable characteristics of populations across generations.

Mutation and recombination create variation, while selection, drift and gene flow change allele frequencies. Individuals do not evolve because a population-level frequency changes over time. This population-level definition is essential when separating evolution from an individual’s lifetime acclimation.

Trace the population process:

• heritable variation
• different survival or reproduction
• changed allele frequencies
• inheritance across generations

If a drought-resistant allele becomes more common after repeated dry years, the population has evolved; one plant becoming drought-resistant during its life has not.

Evolution is not progress toward perfection; it is change relative to a particular environment.

Similarity in DNA or protein sequences can provide evidence that organisms share a common ancestor.

Homologous genes retain inherited sequence features, while substitutions and insertions accumulate after lineages split. The strength of an inference depends on sequence choice, alignment, sampling and the possibility of gene transfer.

Use sequence evidence carefully:

• compare homologous regions
• align equivalent positions
• inspect shared derived changes
• report uncertainty and alternative histories

A conserved gene with fewer differences between species A and B than between A and C supports a closer relationship between A and B.

Sequence similarity supports ancestry; it does not prove that one modern species descended directly from another.

Artificial selection is evidence for evolution because humans repeatedly breed individuals with desired heritable traits, changing trait and allele frequencies across generations.

Domesticated animal breeds and crop varieties differ greatly from one another and from their original wild species. These changes can occur rapidly when strong selection acts on existing heritable variation.

A response requires variation in the trait, a heritable component, preferential reproduction of selected individuals and repeated selection. Correlated traits may also change because genes and traits are not always independent.

If farmers repeatedly propagate seeds from plants with a heritable high-yield trait, that trait can become more common over generations and the crop variety can diverge from its wild ancestor.

Selective breeding changes a population across generations; feeding one animal well can change its growth but is not inherited evolution. It also does not show that every trait is controlled by one gene.

Homologous and analogous structures answer different evolutionary questions. Homologous structures share a basic structural pattern and common ancestry, even when their functions differ. The pentadactyl limb in vertebrates has the same underlying arrangement of bones adapted for grasping, flight, running or swimming; it is evidence of divergent evolution.

Analogous structures have similar functions but different structural origins and ancestry. Bat wings and insect wings both support flight, but their underlying construction and evolutionary origins differ. They are evidence of convergent evolution: similar selection pressures produced similar functions independently. In an exam, compare structure and ancestry, not function alone.

Speciation is the splitting of one pre-existing species into two or more species; it increases the total number of species, whereas extinction decreases it.

Reduced gene flow lets separated gene pools diverge. Mutation supplies variation, differential selection changes allele frequencies under different conditions, and drift can add chance divergence until reproductive isolation prevents fertile gene flow.

Geographic isolation can begin the process. The Congo River separated populations ancestral to bonobos and common chimpanzees; different conditions on either side supported differential selection and divergence.

If two isolated populations accumulate different mating traits and eventually cannot produce fertile offspring when reunited, one ancestral species has split into two.

Geographic separation alone is not speciation, and gradual evolutionary change within one continuing lineage does not increase species number. The defining outcome is reproductive isolation after splitting.