Learning Goal

Examine how life continues to evolve within a changing environment

Speciation and extinction have occurred throughout the Earth’s

history.SpeciationThe process of species formation from ancestral populations.Speciation rates can vary, especially when adaptive radiation occurs when new habitats become availableAdaptive radiation is the result of clusters of closely related species that are each adaptively specialized to a specific habitat or food source.

ExtinctionSpecies extinction rates are rapid at time of ecological stress.There have been 5 major extinctions throughout Earth’s history. They resulted in the loss of most life on the planet

Some scientists believe we are in the midst of the 6th extinction, this time due to human impact.The background extinction rate throughout Earth’s history has been about 7-8 species per year.It is estimated that extinction rates are about 1000 times the background rate, with thousands of species becoming extinct each year.

Speciation may occur when two populations become reproductively isolated from each other.Speciation results in a diversity of life forms.Species can be physically separated by a geographic barrier such as an ocean or a mountain range, or various pre- and post-zygotic mechanisms.These can maintain reproductive isolation and prevent gene flow.

Geographic Speciation•

• Prezygotic Isolating MechanismsExert their effects before the production of a zygote (fertilized egg). There are five types:Ecological Isolation – species live in the same geographical region but occupy different habitats.

Temporal Isolation – Species live in the same habitat but mate at different times of the year.

Behavioral Isolation – Mating signals for one species are not recognized by another.

Mechanical Isolation – Species have differences between reproductive organs or other body parts.Gametic Isolation – There is a mismatch between the sperm of one species and the eggs of another.

Postzygotic Isolating Mechanisms

Hybrid Inviability – Hybrids (offspring of mixed species) die as embryos or at an early age.Hybrid Sterility – Hybrids develop into adults but cannot reproduce.

Hybrid Breakdown – First generation hybrids are healthy and fertile, but subsequent breeding produce chromosomal abnormalities in future generations.

New species arise from reproductive isolation over time, which can involve scales of hundreds of thousands or even millions of years.Speciation can also occur rapidly through mechanisms such as polyploidy in plants.

Populations of organisms continue to evolve.Scientific evidence supports the idea that evolution has occurred in all species and continues to occur.Observable examples of evolution:Chemical resistance to things like antibiotics and pesticides.Observed directional phenotypic change in a population such as the Grant’s study of the finches on the Galopagos.

Vocab Terms

1. Speciation2. Extinction3. Background rate4. Adaptive Radiation5. Reproductive isolation6. Geographic speciation7. Temporal isolation8. Behavioral isolation9. Prezygotic isolating mechanisms10. Postzygotic isolating mechanisms