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.

Allopatric speciation begins with a geographic barrier; sympatric speciation occurs while populations remain in the same geographic area. Both require reduced gene flow and eventual reproductive isolation.

In allopatry, physical separation stops or sharply reduces mating before differential selection and drift drive divergence. In sympatry, behavioural, temporal or ecological differences reduce mating within the shared area; chromosome changes can also isolate plant lineages.

Compare the routes by asking: Is there a geographic barrier? Which barrier reduces gene flow? What different selection pressures act? Is reproductive isolation established? Geographic, behavioural and temporal isolation can all contribute.

A river dividing a population is an allopatric starting point. Two populations breeding in different seasons in the same habitat can begin a sympatric route because temporal isolation reduces gene flow.

Sharing a location does not by itself demonstrate sympatric speciation, and physical separation does not by itself complete allopatric speciation; evidence of divergence and reproductive isolation is needed.

Adaptive radiation is rapid diversification of one ancestral lineage into multiple species adapted to different ecological niches.

When competitors are absent or new habitats become available, populations experience different selection pressures. Divergence in feeding, timing or habitat use can reduce gene flow and produce a cluster of related species.

Look for the combination:

• one ancestral lineage
• several ecological opportunities
• divergent traits
• reproductive separation

Finches colonizing islands may evolve different beaks for seeds, insects or nectar, with mating patterns that maintain the new lineages.

Many species in one area are not automatically an adaptive radiation; shared ancestry and niche divergence are required.

Reproductive barriers prevent gene flow and can act before or after fertilization. Prezygotic barriers prevent mating or fertilization, so no zygote forms: geographic separation, different habitats, different breeding times and different courtship behaviours are examples. Postzygotic barriers act after fertilization: the hybrid may be non-viable or may survive but be infertile, as in a mule.

Polyploidy is more than two complete chromosome sets and can produce abrupt sympatric speciation in plants. Autopolyploidy arises when chromosome number doubles within one species, often after meiotic failure; a fertile tetraploid can no longer produce fertile offspring with the original diploid population. Allopolyploidy begins with hybridization between two species followed by chromosome doubling, which can restore pairing and fertility in the hybrid.

If the new polyploid is viable and fertile but reproductively isolated from the parent population, it is a new species. Knotweeds/smartweeds in the genus Persicaria provide the named example. The scoring chain is chromosome change → incompatible meiosis with parent → reproductive isolation → speciation.