4.3 Classification, Speciation, and Extinction

Classification is a model of relationships

Classification is the system scientists use to organize living things. In older courses, students often memorized kingdom, phylum, class, order, family, genus, and species. For Regents Life Science: Biology, the more important idea is that classification is a model based on evidence. Scientists revise classification when new evidence, especially DNA evidence, shows that organisms are more or less closely related than appearance alone suggested.

A species is commonly described as a group of organisms that can interbreed and produce fertile offspring, but real biology has complications. Asexual organisms, fossils, hybrids, and geographically separated populations can make species boundaries harder to define. Regents questions usually keep the situation clear: if two populations no longer successfully interbreed, or if gene flow between them has stopped and differences accumulate, speciation may be occurring.

From population split to new species

Speciation is the formation of new species. It usually requires reproductive isolation, which means separated populations no longer exchange genes successfully. Isolation can be geographic, behavioral, temporal, mechanical, or genetic.

Once gene flow is reduced, populations can diverge. Mutations introduce new alleles. Natural selection favors different traits in different environments. Genetic drift changes allele frequencies by chance, especially in small populations. Over many generations, enough differences may accumulate that the separated populations become distinct species.

Regents data example: island lizards

Imagine a storm carries a few mainland lizards to an island. The island has thicker shrubs, fewer ground predators, and different insects. After many generations, the island population has shorter legs and stronger climbing toes than the mainland population. Breeding tests show island and mainland lizards rarely choose each other as mates, and hybrid offspring have low fertility.

A strong explanation would identify a founder event, isolation, different selection pressures, and reduced gene flow. The island did not "need" a new species. A small sample of mainland genetic variation arrived, mutations and drift changed the island gene pool, shrub habitat favored climbing traits, and reproductive isolation later limited interbreeding. That is a speciation story built from evidence.

Classification and phylogeny

A taxon is a named group, such as a family or genus. A clade is a group that includes a common ancestor and all of its descendants. Modern classification tries to reflect clades because they show evolutionary relationships. This is why a DNA comparison can change a classification. Two animals may look similar because they live in similar environments, but DNA may show they are not close relatives. Another pair may look different because they adapted to different niches, yet DNA and anatomical evidence can show shared ancestry.

When reading a tree, find the branch points. Species that share the most recent common ancestor are closest relatives on that model. Do not count how close the names look on the page unless the branches support it. Rotating a branch around a node does not change relationships.

Extinction and biodiversity

Extinction is the disappearance of all members of a species. Extinction can occur when environmental change is too rapid, a habitat is destroyed, food sources disappear, diseases spread, invasive species outcompete natives, or reproduction falls below replacement. Extinction is not the same as one organism dying, and it is not always caused by human activity. However, human activities can increase extinction risk by fragmenting habitats, overharvesting species, introducing invasive species, polluting ecosystems, and altering climate.

Extinction reduces biodiversity, but its ecosystem impact depends on the species role. Losing a keystone predator, major pollinator, reef-building organism, or dominant plant can change many interactions. Losing a species with a narrow niche may still matter because it can remove genetic information, ecosystem functions, and future adaptation potential.

Bottlenecks, founder effects, and small populations

Small populations are vulnerable because chance has a larger effect. A bottleneck occurs when a population is sharply reduced, leaving only a small sample of the original genetic variation. A founder effect occurs when a few individuals start a new population. Both can reduce genetic diversity. Low diversity can make populations less able to respond to disease or environmental change because fewer inherited variants are available for selection.

Regents traps to avoid

• Classifying only by appearance when molecular data are provided.
• Saying two species are close relatives because they appear next to each other on a tree without checking branch points.
• Treating geographic isolation alone as proof of a new species; reproductive isolation or accumulated divergence is needed.
• Saying extinction means a species moved away from an area. Local disappearance is extirpation; global disappearance is extinction.
• Assuming small populations are safe because they have fewer competitors. Small populations often have lower genetic diversity and higher risk.

For constructed response, use evidence language: identify the barrier or isolation, cite the trait or DNA differences, and explain how reduced gene flow plus selection, mutation, and drift could lead to speciation or extinction risk.