Transcript Speciation
Speciation Evolution creates (and destroys) new species, but … What is a species? Its not as straightforward a question as most believe. What is a Species? The definition we’ll use is this: A species is a group of individuals capable of interbreeding to produce fertile offspring. How Many Species Are There? We don’t know. About 2 million species have been described. Estimates of existing species number range from 4 million to 100 million (with 10-15 million being a more commonly considered upper estimate). How did this diversity of life come to be? Speciation is an event that produces two or more separate species from an original species. Species = basic unit Continuous lineage - information passed through genes Speciation - rise of new species How Do Species Arise? The key to speciation is reproductive isolation of populations. There are extrinsic and intrinsic reproductive isolating mechanisms. Geographic isolation is the primary extrinsic reproductive isolating mechanism. Geographic Isolation Geographic isolation is the physical separation of members of a population. Geographic isolation is the primary extrinsic isolating mechanism. Allopatric Speciation Allopatric (different home lands) speciation occurs when geographic isolation creates a reproductive barrier (an extrinsic mechanism). Once a populations have been separated, Natural Selection cause the two new Populations to genetically Diverge. Allopatric Speciation Harris’ antelope squirrel White-tailed antelope squirrel Two species of ground squirrel are postulated to have descended from a common ancestral population that was separated by formation of the Grand Canyon. Sympatric Speciation Sympatric speciation occurs when a reproductive barrier is created by something other than geographic isolation (intrinsic mechanisms). Intrinsic mechanisms involve changes to organisms that prevent interbreeding. There are many different causes for intrinsic speciation. Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation (different habits within an overlapping range) Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation Courtship rituals, like these, are critical for mating within a species, but ineffective for attracting members of other species. Behavioral Isolation Mechanisms Courtship rituals, like these, are critical for mating within a species, but ineffective for attracting members of other species. Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation Hybrid Infertility Was the reeson for Cloning Mules Speciation Occurs at Widely Differing Rates A slow rate of speciation evidenced by a living horseshoe crab (13 extant species) and a 300 million year-old fossil species A rapid rate of speciation evidenced by Galapagos finches which have diversified into 13 species within the last 100,000 years. Speciation Dynamics - Gradualism or Punctuated Equilibrium? Gradualism states that speciation Occurs at a regular gradual rate. Punctuated equilibrium states that organisms evolved in a relatively short period of time. Short bursts. Scientists still argue over this one! Species Come and Go Best estimates from the fossil record indicate that greater than 99% of species that have exited are now extinct. A typical “lifetime” for a species is about 1 million years. Classifying Life’s Rich Diversity Why bother? An intrinsic reason is that modern classification systems tell who’s related to whom and how we all came to be. Classifying Life’s Rich Diversity Why bother? A practical reason is that if we want to preserve an environment compatible with human life, we’d better know what’s out there. The Linnaean Hierarchical Classification System How Do We Classify Organisms? Ideally, classification is based on establishing the evolutionary relationships between organisms. The evolutionary relationship between organisms is their phylogeny. Cladistics is the method of classification based on establishing phylogenies (i.e. getting at evolutionary relationships. Cladistics proceeds by comparing shared ancestral and shared derived characters between sets of organisms. Cladistics A phylogeny (cladogram) for vertebrates. each node indicates a common ancestor The greater the number of derived characters shared by a pair of organisms, the closer their degree of relationship. The closer the degree of relationship, the closer the most recent common ancestor. It’s Critical (and often difficult) To Distinguish Homology from Analogy Homologous structures, like the bat wing and gorilla arm, are similar because they are derived by modification of a shared ancestral structure. Homology is the key to establishing phylogenies. Another Set of Analogies Created by Convergent Evolution Ocotillo of the US southwest Allauidia of Madagascar Results of Cladistic Analyses Sometimes Run Counter to Classical Classification Schemes Which pair is more closely related? A lizard/crocodile or bird/crocodile? Cladistic analysis indicates that the bird/crocodile pair is more closely related. DNA Hybridization: uses DNA similarity between species Which species are the closest living relatives of modern humans? 15-30 MYA Gorillas Humans Chimpanzees Chimpanzees Bonobos Bonobos Orangutans Gorillas Humans Orangutans 0 The pre-molecular view was that the great apes (chimpanzees, gorillas and orangutans) formed a clade separate from humans, and that humans diverged from the apes at least 15-30 MYA. 0 14 MYA Mitochondrial DNA, most nuclear DNA-encoded genes, and DNA/DNA hybridization all show that bonobos and chimpanzees are related more closely to humans than either are to gorillas.