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Diversity of Life Chapter 24 The Origin of Species Speciation • Happens when microevolution causes reproductive isolation • What is a species? • What mechanisms are at work as populations diverge? • How does this lead to macroevolution? Evolutionary change can lead to: -Anagenisis; heritable changes within a species -Cladogenesis; heritable changes that lead to branching (speciation) events Biological species concept • A species is a population whose members can interbreed and produce viable offspring Biological Species Concept • A species is a population whose members can interbreed and produce viable offspring • Reproductive isolation – Prezygotic barriers – Postzygotic barriers Speciation details • Biological species concept and asexual organisms: – Need new definition of species • Life classification schemes can be based on : – – – – Morphological species concept (subjective) Paleontological (fossil record only) Ecological (similar niche) Phylogenetic (unique genetic history) Speciation and Geographic Separation • Allopatric speciation – geographic barrier separates a population • Sympatric speciation – speciation within a population – Chromosomal changes – Non-random mating – Exploiting different food sources Adaptive Radiation • Movement of a few individuals to new environment (founder effect) – Eg Hawaii • or extinction event opens new niches (dinosaurs and mammals) Speciation tempo: Punctuated equilibrium: change may occur in relatively short time Macroevolution and speciation • Evolution of complex structures – Small speciation events + geological time = incredible diversity now and in fossil record – Even a partial structure can provide an advantage – If a change is heritable then selection can act on it – Eg. eyes Evolution and development (evo-devo) • Development and growth from embryo to adult based on genes that control rate, timing, and spatial events • Allometric growth – how body proportions alter during development Evolution and development (evo-devo) • Development and growth from embryo to adult is based on genes that control rate, timing, and spatial events • Allometric growth – how body proportions alter during development • Heterochrony – change in rate or timing of development Homeotic genes • Control placement and spatial organization of body parts • Eg. Hox gene expression and development of limbs – location of expressed genes and timing play critical role in final morphology So, how does evolution work anyway? • Does each change lead in one direction? • NO! Change environmental conditions and trends can change or end. So, how does evolution work anyway? • Evolution may seem random but it is dependent on changing conditions