Transcript Speciation
17 Speciation Chapter 17 Speciation Key Concepts • 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life • 17.2 Speciation Is a Natural Consequence of Population Subdivision • 17.3 Speciation May Occur through Geographic Isolation or in Sympatry • 17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact Chapter 17 Big Idea #1 Can evolution be observed in the lab or field? Concept 17.1 Species Speciation—divergence of biological lineages and emergence of reproductive isolation between lineages. Species—groups of organisms that mate with one another & produce fertile offspring. Members of the same species do not allows look alike Figure 17.2 Cryptic Species Look Alike but Do Not Interbreed Concept 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life Reproductive isolation: two groups of organisms can no longer exchange genes. Reproductive isolation is necessary for lineages to remain distinct over evolutionary time. It is also responsible for morphological differences—mutations that result in morphological changes cannot spread between reproductively isolated species. Concept 17.2 Speciation Is a Natural Consequence of Population Subdivision Not all evolutionary changes result in new species. Speciation requires interruption of gene flow. How can one lineage ever split into two reproductively isolated species? Concept 17.2 Speciation Is a Natural Consequence of Population Subdivision Types of speciation in link The Dobzhansky–Muller model: A population is subdivided and then the two groups evolve independently. In each lineage, new alleles become fixed at different loci. The new alleles at the two loci are incompatible with one another. Genetic incompatibility between the two isolated populations will develop over time. Figure 17.3 The Dobzhansky–Muller Model Concept 17.2 Speciation Is a Natural Consequence of Population Subdivision In bats of the genus Rhogeessa, chromosomal rearrangements in different lineages have led to speciation. Hybrids between some types will not be able to produce normal gametes in meiosis. Figure 17.4 Speciation by Centric Fusion Figure 17.5 Reproductive Isolation Increases with Genetic Divergence Two Types of Speciation Concept 17.3 Speciation Allopatric May Occur through Geographic Isolation or in Sympatry An example in lizards link Allopatric speciation—when populations are separated by a physical or geographic barrier. Barriers like geology -continents drift, sea levels rise and fall, glaciers advance/retreat, or climates change. Polar bears are starting to eat other food sources other than seals due to climate change. The populations evolve through genetic drift and adaptation to different environments in the two areas. Figure 17.7 Allopatric Speciation among Darwin’s Finches Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry Some members of a population may cross an existing barrier and establish an isolated population. Finch species in the Galápagos Islands evolved from a single South American species that colonized the islands. The islands are far apart, and have different environmental conditions. Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry Sympatric speciation—speciation without physical isolation. May occur with disruptive selection—individuals with certain genotypes prefer distinct microhabitats where mating takes place. Or through mutation This appears to be taking place with apple maggot flies. One group prefers to lay eggs on hawthorne fruits, the other group lays eggs on apples. They are partially reproductively isolated. Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry Animation Sympatric speciation most commonly occurs by polyploidy—duplication of sets of chromosomes within individuals. Autopolyploidy—chromosome duplication in a single species Allopolyploidy—combining of chromosomes of two different species Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry A tetraploid can result if two accidentally unreduced diploid gametes combine. Tetraploid and diploid individuals are reproductively isolated because their hybrid offspring are triploid. But tetraploids can self-fertilize or mate with another tetraploid. Thus polyploidy can result in complete reproductive isolation in two generations. Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry Hybridization between closely related species can disrupt normal meiosis and result in chromosomal doubling. Allopolyploids are often fertile: each chromosome has a partner to pair with in meiosis. Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry Speciation by polyploidy is common in plants; about 70% of flowering plant species and 95% of fern species. It is more common in plants because many can self-fertilize. It has also occurred in animals (e.g., tree frogs). Pre-Post Zygotic Barriers to creating species Concept 17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact Prezygotic isolating mechanisms—prevent hybridization from occurring. Postzygotic isolating mechanisms—reduce fitness of hybrid offspring. Postzygotic mechanisms result in selection against hybridization, which leads to reinforcement of prezygotic mechanisms. Concept 17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact Prezygotic isolating mechanisms Mechanical isolation—differences in sizes and shapes of reproductive organs. In plants, it may involve pollinators. Concept 17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact Prezygotic isolating mechanisms Temporal isolation—species may breed at different times of year or different times of day. Example: closely related leopard frog species. Figure 17.9 Temporal Isolation of Breeding Seasons (Part 1) Figure 17.9 Temporal Isolation of Breeding Seasons (Part 2) Concept 17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact Prezygotic isolating mechanisms Behavioral isolation—individuals reject or fail to recognize mating behaviors of other species. Examples: mating calls of male frogs Let’s look at a research site on frogs.