Chapter 16 The Origin of Species 1 Species The basic unit of taxonomic classification a population that evolves independently of other populations The biological species concept Kingdom Phylum Class Order Family Genus Species 16 1 2 The Biological Species Concept Based on the observation that reproductive isolation ensures evolutionary independence Defining a species can be confusing Appearance can be misleading Some organisms look very similar but are different species Some organisms look different but are the same species 16 1 3 Species 16 1 Fig 16 1 4 Species 16 1 Fig 16 2 5 How is reproductive isolation between species maintained Isolating mechanisms traits that maintain reproductive isolation Premating isolating mechanisms Traits or mechanisms that prevent mating between species Postmating isolating mechanisms mechanisms that prevent the formation of vigorous fertile hybrids between species 6 16 2 Premating Isolating Mechanisms 1 Geographic isolation 2 Ecological isolation 3 Temporal time related isolation 4 Behavioral isolation 5 Mechanical incompatibilities 16 2 7 Premating Isolating Mechanisms Geographic Isolation Interbreeding is prevented because species live in different areas Does not necessarily mean they cannot interbreed So geographic isolation is usually considered more of a mechanism that allows new species to form 16 2 8 Premating Isolating Mechanisms Geographic Isolation Kaibab squirrel lives only on the north rim of the Grand Canyon Abert squirrel lives only on the south rim of the Grand Canyon Geographically isolated but are they a separate species 16 2 9 Fig 16 3 Premating Isolating Mechanisms Ecological Isolation Different species may occupy Different habitats within the same General area White crowned sparrow inhabits Fields and meadows White throated sparrow inhabits Dense thickets 16 2 10 Premating Isolating Mechanisms Temporal Isolation Different species may breed at different times Bishop pine releases pollen in the summer Monterey pine releases pollen in early spring 16 2 Fig 16 5 11 Premating Isolating Mechanisms Behavioral Isolation Different species may have different courtship signals and behaviors Colors mating calls etc 16 2 Fig 16 6 12 Premating Isolating Mechanisms Mechanical Isolation Differing sexual organs may foil mating attempts 16 2 Fig 16 7 13 Postmating Isolating Mechanisms Serve to limit hybrid offspring Gametic incompatibility sperm from one species may not be able to fertilize eggs of another Hybrid inviability hybrids either abort early in development due to genetic differences or have difficulties performing tasks like carrying nest material 16 2 14 Postmating Isolating Mechanisms Hybrid infertility Most commonly because due to inability of chromosomes to pair up properly during meiosis so gametes fail to develop 16 2 Fig 16 8 15 16 2 16 So how do new species form Speciation the process by which new species form Speciation depends on two factors Genetic divergence Isolation Isolation followed by genetic divergence can occur in several different ways 16 3 17 So how do new species form There are two broad categories of pathways to speciation Allopatric speciation when two populations are geographically separated from one another Sympatric speciation when two populations share the same geographic area 16 3 18 Geographic separation can lead to allopatric speciation Organisms may colonize isolated habitats Landscape changes rising sea levels formation of mountain ranges etc may isolate populations Genetic differences between two isolated populations may become sufficient to prevent interbreeding 16 3 19 Geographic separation can lead to allopatric speciation 16 3 Fig 16 9 20 Genetic isolation without geographic separation can lead to sympatric speciation Can occur in an area with two distinct types of habitats Biologists are watching such an event right now in a species of fruit fly 16 3 21 Genetic isolation without geographic separation can lead to sympatric speciation R pomonella is a fruit fly that lives in hawthorn trees Lays eggs in the fruit Maggots hatch and eat the fruit About 150 years ago scientists notices that r pomonella had started to infest apple trees introduced from Europe 16 3 22 Genetic isolation without geographic separation can lead to sympatric speciation today is appears that r pomonella is splitting into two separate species One breeds in hawthorn trees and one breeds in apple trees 16 3 23 16 3 Fig 16 10 24 Genetic isolation without geographic separation can lead to sympatric speciation But apples and hawthorns typically grow in the same areas so how is reproductive isolation maintained female flies tend to lay their eggs in the same type of fruit that they developed Males also prefer the same fruit 16 3 25 Under some conditions many new species may arise in a relatively short amount of time This is called adaptive radiation Can occur when a species encounters a location that has a wide variety of habitats Ex Ancestral silversword plants and the Hawaiian islands 16 3 26 About 30 different species of silversword plants inhabit the Hawaiian Islands and they all descended from a single ancestral species 16 3 Fig 16 12 27 Under some conditions many new species may arise in a relatively short amount of time Fig 16 11 16 3 28 What causes extinction Extinction is the death of all remaining members of a species 99 9 of all species that have ever existed are extint The immediate cause of extinction is almost always environmental change 16 4 29 What causes extinction Habitat change and habitat destruction are the leading causes of extinction Present day habitat destruction by humans is proceeding at a rapid pace Many biologists believe that we are in the midst of the most fast paced widespread episode of species extinction in the history of life As many as 50 of the species on earth face extinction as we continue to clear the rainforest 16 4 30 What causes extinction Lived only in the Golden Toad monteverde cloud Has been recently Declared extinct Source Mother Nature Network 16 4 31 What causes extinction Interaction with other species can also cause extinction Competition Organisms compete for limited resources predation 16 4 32 Some species are at a high risk for extinction Localized distribution makes species vulnerable to extinction Devil s Hole pupfish Found in only one Spring fed water hole In the Nevada desert 16 4 Fig 16 13 33 Some species are at a high risk for extinction Overspecialization makes species
View Full Document
Unlocking...