Chapter 14. Speciation DEFINING SPECIES. What is a Species? Taxonomy & Classification. Several ways to define a species

Transcription

1 Chapter 14 Chapter 14: Big Ideas Defining Species Mechanisms of What is a Species? DEFINING SPECIES If we all come from one ancestral stock (prokaryotes), why are there so many species? Before we examine this let s learn what a species is. Species = Latin for kind or appearance. Seems intuitive need formal definition. How similar are members of same species? What keeps one species distinct from others? Taxonomy & Classification Taxonomy = science of dividing / classifying / naming org.s Carl Linnaeus (Swedish) Wanted to classify everything on Earth Devised system of binomial nomenclature (i.e. Genus species ) Began by describing phenotypes (the way things looked) Several ways to define a species Morphological species concept Defines species by observable physical traits Apply to asexual organisms Fossils Homo sapien = wise man Homo erectus = upright man Homo habilus = handy man

2 Several ways to define a species Ecological species concept defines a species by ecological role / niche focuses on unique adaptations to particular roles in a biological community e.g. two species may be similar in appearance but distinguishable based on what they eat The most common & functional way to define a species Biological species concept: a group of populations members have potential to interbreed and produce fertile offspring Therefore, members of a species are similar because they can reproduce with each other. where they live Biological Species Concept in Process = process by which one species splits into two or more species. 1. Geographic Isolation 2. Reproductive isolation Every time speciation occurs, diversity of life increases. Scientific evidence shows: all current species (millions!) evolved from ancestral life form (3.5 billion years ago). 3. Being separated by a physical barrier members of different species prevented from mating with each other Therefore, species become distinct from each other because they no longer share the same gene pool. The biological species concept can be problematic Some pairs of clearly distinct species occasionally interbreed and produce hybrids. e.g. grizzly bears and polar bears may interbreed and produce hybrids called grolar bears. Melting sea ice may bring these two bear species together more frequently and produce more hybrids in the wild. The biological species concept can be problematic Reproductive isolation can t be determined for extinct organisms known only from fossils. Reproductive isolation does not apply to prokaryotes or other organisms that reproduce only asexually. Therefore, alternate species concepts can be useful.

3 Barriers keep species separate Reproductive barriers keep species separate 1. Physical Geographic Barriers Reproductive barriers e.g. River forms between groups, rock slide separates groups, etc. Isolate gene pools of species & prevent interbreeding 2. Biological (Reproductive) Barriers keep closely-related species from interbreeding, even when species overlap e.g. mating just doesn t work Happen either before or after zygotes form, reproductive barriers categorized: Prezygotic (premating) or Postzygotic (postmating). Figure 14.3A Prezygotic Barriers Individuals of different species Prezygotic Barriers Habitat isolation Five types of prezygotic barriers prevent mating or Temporal isolation Behavioral isolation Terrestrial vs. aquatic garter snakes fertilization between species. Mechanical isolation 1. In habitat isolation, two species live in same area but not in same kind of place. Gametic isolation Fertilization Postzygotic Barriers Reduced hybrid viability 2. In temporal isolation, two species breed at different times (seasons, times of day, years). Reduced hybrid fertility Hybrid breakdown Viable, fertile offspring Eastern & Western Spotted Skunks breed in different seasons Prezygotic Barriers, continued 3. In behavioral isolation, mating rituals differ e.g. fireflies recognize flash pattern of mate 3. In mechanical isolation, female and male sex organs are not compatible. Postzygotic Barriers firefly-closeup.jpg Three types of postzygotic barriers operate after hybrid zygotes have formed. %20Salamander/spoted4.jpg 1. In reduced hybrid viability, most hybrid offspring do not survive. 2. In reduced hybrid fertility, hybrid offspring are vigorous but sterile. Horse+ Donkey = Mule. Mule = infertile 4. In gametic isolation, female and male gametes are not compatible. 3. In hybrid breakdown, first-generation hybrids are viable and fertile but offspring of hybrids are feeble or sterile. 14/278858/hlCmY.jpg

4 Figure 14.UN01 Zygote Gametes Prezygotic barriers Postzygotic barriers Habitat isolation Reduced hybrid Temporal isolation viability Behavioral isolation Reduced hybrid Mechanical isolation fertility Gametic isolation Hybrid breakdown Viable, fertile offspring MECHANISMS OF SPECIATION Isolated islands are often showcases of speciation Isolated island chains offer best evidence of speciation via geographic isolation. events most likely on: islands with physically diverse habitats, islands far enough apart to permit populations to evolve in isolation, and islands close enough to each other to allow occasional dispersions between them. Galapagos Islands The Galápagos Archipelago located ~560 miles west of Ecuador one of world s great showcases of adaptive radiation formed naked from underwater volcanoes colonized gradually from other islands and South America mainland has many species of plants and animals found nowhere else in the world Galapagos Islands Galapagos Finches = great example of Adaptive Radiation Adaptive Radiation = evolution of many diverse species from a common ancestor, to fill niches Galápagos islands: 14 species of closely related finches, called Darwin s finches (Darwin collected them during his around-theworld voyage on the Beagle). These finches share many traits differ in feeding habits beaks specialized for what they eat All finches evolved from single small ancestral population of colonists Adaptive Radiation

5 Figure 14.8 can occur rapidly or slowly Cactus-seed-eater (cactus finch) There are two models for the tempo of speciation. 1. Punctuated equilibria model draws on fossil record, where species change most dramatically as they arise from an ancestral species and then Tool-using insect-eater (woodpecker finch) experience relatively little change for the rest of their existence. 2. Other species appear to have evolved more gradually Gradual Model. Seed-eater (medium ground finch) Figure Punctuated pattern Generally, is SLOW (using human time scale) What is the total length of time between speciation events*? Gradual pattern In a survey of 84 groups of plants and animals, the time ranged from 4,000 to 40 million years. Overall, the time between speciation events averaged 6.5 million years. * All depends on how define a species Time