Lecture 02 - Origin of Species PDF

The Origin of Species Chapter 22 Learning Outcomes Biological species concept vs. Morphological species concept vs. Ecological species concept Prezygotic vs. Postzygotic barriers Drivers of allopatric vs. sympatric speciation Hybrid zones and factors that cause reproductive isol...

The Origin of Species Chapter 22 Learning Outcomes Biological species concept vs. Morphological species concept vs. Ecological species concept Prezygotic vs. Postzygotic barriers Drivers of allopatric vs. sympatric speciation Hybrid zones and factors that cause reproductive isolation Rate of evolutionary change On the Origin of Species Only 1.2 - 1.8 million species of ~4 to 100 million existing species have been named, most of which are insects More than 99.9% of all species that ever existed are now extinct Blue-footed booby (Sula nebouxii) Spang & Ettema (2016) Nature Microbiology What is a Species? Linnaeus used the morphological species concept The classification of plants and other organisms into separate species based on their Carolus Linnaeus 1707 – 78 visible structural Father of Taxonomy differences, such as feathers or number of (1700s) flower parts But this is not ideal!  The Biological Species Concept A species consists of one or more populations whose members interbreed in nature to produce fertile offspring, and do not breed with members of other species Reproductive barriers keep a given species genetically distinct from other species New species evolve when populations become reproductively isolated from one another Held together by gene flow Reproductive isolation Limitations to the Biological Species Concept Applies only to sexually reproducing organisms Does not apply to bacteria and other asexually-reproducing organisms, or to extinct organisms, which are classified by structural and biochemical characteristics Individuals assigned to different species (such as coyotes and wolves) sometimes do successfully interbreed Hybridization Between Two Species of Bears in the Genus Ursus  The Morphological Species Concept Distinguishes a species by body shape and other structural features Applies to sexual and asexual organisms Does not rely on information about gene flow Disadvantage is that it relies on subjective criteria Morphological Species Concept is particularly useful for studying fossils. Ecological Species Concept Defines a species in terms of its ecological niche Applies to sexual and asexual organisms Emphasizes role of disruptive selection as organisms adapt to different environments Disadvantage: Geographically widespread populations that don’t interbreed may be considered the same species Closely related species of salamander split into different ecological niches and continued to evolve. Reproductive Isolation Various reproductive isolating mechanisms prevent interbreeding between two different species whose ranges overlap Gene flow between two species may be prevented by prezygotic or postzygotic barriers The environment can impose a barrier to reproduction that can lead to speciation over time Prezygotic Barriers Reproductive isolating mechanisms that prevent mating or fertilization from taking place No interspecific zygote is produced Prezygotic barriers include temporal isolation, habitat isolation, behavioral isolation, mechanical isolation, and gametic isolation  Temporal Isolation Genetic exchange is prevented because two groups reproduce at different times of the day, season, or year (a) (c) (b) Habitat Isolation Two closely related species in the same geographic area live and breed in different habitats in that area Example: Five species of small birds (flycatchers) have overlapping ranges but, during breeding season, each species stays in a particular habitat, so potential mates from different species do not meet Behavioral Isolation Also known as sexual isolation, many animal species exchange a distinct series of signals before mating (species-specific courtship behaviors) Mechanical Isolation Structural differences in reproductive organs prevent successful mating between species (a) (b) Gametic Isolation If mating takes place, molecular and chemical differences between species prevents fertilization Aquatic animals with external fertilization Postzygotic Barriers Fertilization sometimes occurs between gametes of two closely related species despite their prezygotic barriers When this happens, postzygotic barriers increase the likelihood of reproductive failure Generally, the embryo of an interspecific hybrid spontaneously aborts = Hybrid inviability If an interspecific hybrid lives, it may not reproduce = hybrid sterility Temporal Isolation Hybrid inviability Habitat Isolation Hybrid sterility Behavioral Isolation Hybrid breakdown Mechanical Isolation Gametic isolation 22.2: Speciation Speciation is the evolution of a new species Begins when a population becomes reproductively isolated from other populations and their gene pools diverge Has occurred when the populations are different enough that there is no genetic exchange between them Happens in two ways: allopatric and sympatric speciation  Allopatric Speciation Speciation that occurs when one population becomes geographically isolated from others of the species and evolves by natural selection and/or genetic drift ( a ) ( Allopatric Speciation (cont’d.) Allopatric speciation also occurs when a small population migrates or is dispersed (as by a storm) and colonizes a new area away from the original species Oceanic islands such as the Galápagos Islands and Hawaiian Islands provide the geographic isolation required for allopatric speciation – hundreds of unique species are presently found on each island Allopatric Speciation in Snapping Shrimp (Alpheus) Sympatric Speciation Speciation between two populations in the same geographic range Occurs when reproductive isolating mechanisms evolve at the start of the speciation process Common in plants Occurs in three ways: a change in ecology, a change in ploidy, and sexual selection  Sympatric Speciation in Maggot Flies © Cengage Learning 2015  Sympatric Speciation in Maggot Flies (cont’d.) © Cengage Learning 2015 Polyploidy Having more than two sets of chromosomes A major factor in plant evolution Reproductive isolation occurs in a single generation when a polyploid species with multiple sets of chromosomes arises from diploid parents Two kinds of polyploidy: Autopolyploid and allopolyploid Autopolyploid Contains multiple sets of chromosomes from a single species Allopolyploidy in Plants Multiple sets of chromosomes derived from two or more species As many as 80% of all flowering plant species are polyploids Allopolyploidy provides a mechanism for extremely rapid speciation Primula Primula Primula floribunda kewensis verticillata  Sexual selection can drive sympatric speciation Inquiry: Does Sexual Selection in Cichlids Result in Reproductive Isolation? For example, sexual selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria 22.3: Hybrid Zones Black capped Carolina An area of overlap chickadee chickadee between two recently diverged populations in which interbreeding takes place and hybrid offspring are common Hybrids often have reduced fitness compared to parent species A Narrow Hybrid Zone for Bombina Toads in Europe Hybrid Zones and Environmental Change Changes in environmental conditions can Cause new hybrid zones to form Shift hybrid zones by altering where habitats of interbreeding species meet Introduce new alleles into gene pools of species involved that can help species cope with environmental change Hybrid Zones over Time Reinforcement Occurs when hybrids are less fit than the parent species Natural selection strengthens (reinforces) reproductive barriers, and over time the rate of hybridization decreases Where reinforcement occurs, reproductive barriers should be stronger for sympatric than for allopatric species Fusion Gene flow in hybrid zones can weaken reproductive barriers resulting in fusion of the parent species into a single species Stability If hybrids continue to be produced, then the hybrid zone will remain stable This may occur if hybrids are at least as likely to survive and reproduce as their parent species Even if hybrids are selected against, migration from parent populations outside the hybrid zone can lead to continued production of hybrids 22.4: The Rate of Evolutionary Change Two models explain evolution as observed in the fossil record: 1. Punctuated equilibrium: long periods of stasis are interrupted by short periods of rapid speciation – perhaps triggered by changes in the environment 2. Phyletic gradualism: evolution proceeds continuously over long periods  Slow, gradual Extinction of changes Stasis original species Stasis Stasis Time Time Divergence is sudden, with rapid changes Stasis (little Divergence is change) gradual Structural changes Structural changes (a) (b)  Speciation Rates The existence of a punctuated pattern in the fossil record combined with evidence from lab studies suggest that speciation can be rapid In a survey of data from 84 groups of plants and animals, researchers found that The interval between speciation events ranged from 4,000 years (cichlids) to 40 million years (beetles) The average time between speciation events was 6.5 million years and was rarely less than 500,000 years Learning Outcomes Biological species concept vs. Morphological species concept vs. Ecological species concept Prezygotic vs. Postzygotic barriers Drivers of allopatric vs. sympatric speciation Hybrid zones and factors that cause reproductive isolation Rate of evolutionary change