A Galpagos Islands tortoise Millions of species inhabit the Earth. Speciation (the formation of new species) is not a rare event! Macro-evolution Chapter 24: The Origin of Species Anopheles albimanus Anopheles bellator Anopheles maculipennis What is a species? Biological Species Concept (Ernst Mayr 1942). Species are groups of actually or potentially interbreeding populations that produce fertile offspring and are reproductively isolated from other such groups. The biological species concept is based on interfertility rather than physical similarity Eastern and western meadow lark Figure 24-2 The biological species concept is based on interfertility rather than physical similarity Figure 24-2 Proposed timeline for selected hominin species. Most of these fossils come from sites in eastern and southern Africa Fossil of Homo ergaster (ca. 1.7 myrs old) Cladogenesis The formation of new species by branching of the evolutionary tree. It forms a clade (a group of species that includes their ancestor and all descendent species) Allopatric Speciation: Geographic barriers Sympatric Speciation: No geographic barriers 24.3 Exploring reproductive barriers between closely related species. Cladogenesis The formation of new species by branching of the evolutionary tree. It forms a clade (a group of species that includes their ancestor and all descendent species) Allopatric Speciation: Geographic barriers Sympatric Speciation: No geographic barriers 24.5 Two main modes of speciation 24.6 Allopatric speciation of 2 species of antelope squirrels in the Grand Canyon A model for adaptive radiation on island chains Long-distance seed dispersal Inquiry Can divergence of allopatric fruit fly populations lead to reproductive isolation? Brownfield s Mine drainage s Islands are often called "natural laboratories for evolutionary studies." Why is this so? A. Islands are isolated from other land masses. B. Islands are geologically very young. C. Islands have very low speciation rates. D. Islands are all ecologically similar. E. Islands always have small numbers of species on them. Sympatric speciation by autopolyploidy in plants (page 495) (an individual that has more than two chromosome sets, all derived from a single species) Botanist Hugo de Vries The new primrose species of botanist Hugo de Vries 24.11 One mechanism for allopolyploid speciation in plants. (resulting from two different species interbreeding and combining their chromosomes) Hybrid: Offspring from genetically dissimilar parents Because there are no physical or geographical barriers separating the species, many biologists believe that speciation of flowers and their specific pollinating insects is thought to occur by A.sympatric speciation. B.parallel speciation. C.allopatric speciation. D.hybridization. E.prezygotic isolation. Haplochromine cichlids [endemic] 24.12 Mate choice in two species of Lake Victoria cichlids Mate choice based on coloration is the main reproductive barrier Normal light Monochromatic light The breakdown of reproductive barriers. Increasing cloudy water in Lake Victoria over the past 30 years may have weakened reproductive barriers between these two species of cichlids. They now hybridize extensively. Hybrid zones for two frog species in Europe. Pattern of allele frequency changes across the width of the zone (a counter-example to the common definition of species which states that members of different species are not able to breed and produce fertile offspring) What happens if allopatric populations come back in contact with one another? Two models for the tempo of speciation Punctuated pattern: Most rapid change as new species branch from an ancestor (followed by little change Gradual pattern: Species diverge gradually from one another over time Chapter 24 Summary of Key Concepts (p.505) Biological species concept (reproductive isolation) Speciation with or without geographic separation (allopatric vs. sympatric) Factors that cause reproductive isolation The time course of speciation (punctuated vs. gradual). Which of the following would NOT produce a reproductive barrier between two populations that are reunited following geographic isolation? A.The two populations have different breeding seasons. B.There are physiological differences between the two populations so that they cannot produce viable offspring. C.Members of one population do not find members of the other population attractive as mating partners. D.The two populations have different courtship behaviors. E.The two populations produce successful hybrids, which in turn can cross with either population.