Notes on Evolutionary Change and Speciation

4.5 Evolutionary Change

Evolutionary changes are affected by factors beyond selection, including sexual reproduction and genetic drift.
Speciation may result from accumulation of genetic changes under different selection pressures or drift in geographically isolated populations.
Sexual reproduction increases variation; asexual reproduction can lead to rapid evolution if a favourable mutation arises (e.g., rapid rise of antibiotic-resistant bacteria).

Genetic drift: allele frequencies change by chance across generations, not due to fitness.
Bottleneck effect: drastic reduction in population size due to events (earthquakes, floods, drought, disease, etc.), reducing genetic variation.
Founder effect: a new population established by a small sample from a population; can lead to distinct genetic traits.

Northern elephant seals: bottleneck due to 19th-century hunting; population rebounded but genetic variation remains low compared to non-bottlenecked populations.
Amish: founder effect leading to higher frequency of certain rare traits (e.g., polydactyly) within the community.

Speciation is the process by which new species arise; exemplified by Galápagos finches diverging from a common ancestor after colonization and adapting to different food sources, resulting in multiple species.
Process involves changes in gene flow, geographic isolation, and reproductive barriers.

Speciation can involve geographic isolation or occur without it.
Barriers to gene flow can be prezygotic (before mating) or postzygotic (after mating).
Population A and Population B diverge with reduced gene flow over time, leading to subspecies and eventually new species.

Allopatric speciation involves a geographic barrier separating members of the original population, preventing gene flow.
Examples of barriers: oceans, mountains, rivers, deserts.
Geographic isolation leads to divergence and eventual reproductive isolation.

Sympatric speciation occurs without geographic isolation; one population gives rise to two or more species in the same region.
Mechanisms:
- Polyploidy: organisms possess more than two chromosome sets; usually arises from meiotic nondisjunction; instantaneous speciation because polyploids cannot interbreed with the parent ploidy level. Common in plants and some fish/amphibians. Expression: >2 sets of chromosomes.
- Occupying different micro-habitats: subgroups specialize in different niches, promoting divergence.

Polyploidy: organisms with more than two chromosome sets.
Plants: polyploidy is common; hexaploid wheat as an example.
Animals: relatively few polyploid species (e.g., some salamanders, goldfish, salmon).
Polyploidy often leads to increased size, disease resistance, and vigor; a major driver of plant speciation.
Some polyploid lineages can reach very high chromosome counts (e.g., pentaploid, hexaploid, septaploid, octaploid, up to 84-ploid; 1260 chromosomes in the fern Ophioglossum reticulatum).

Visual and revision resources cover Allopatric speciation, geographic barriers, and phylogenetic concepts (e.g., Amoeba Sisters).