23. Species and Their Formation

Species and Their Formation23 Species and Their Formation23.1 What Are Species?23.2 How Do New Species Arise?23.3 What Happens when Newly Formed Species Come Together?23.4 Why Do Rates of Speciation Vary?23.5 Why Do Adaptive Radiations Occur?23.1 What Are Species?Species literally means “kinds.”We recognize most species by their appearance.Many species change little over large geographic ranges.Figure 23.1 Members of the Same Species Look Alike—or Not (A)23.1 What Are Species?Linnaeus described species based on their appearance—the morphological species concept.Members of species look alike because they share many alleles.He originated the binomial system of nomenclature.23.1 What Are Species?But males and females may not look alike.Immature individuals may not look like their parents.Other types of information must be used to determine species.Figure 23.1 Members of the Same Species Look Alike—or Not23.1 What Are Species?Species can be thought of as branches on the tree of life.Speciation: The process by which one species splits into two or more daughter species, often gradually.Figure 23.2 Speciation May Be a Gradual Process23.1 What Are Species?Speciation involves reproductive isolation—when individuals of a population mate with each other, but not with individuals in another population, they are a distinct evolutionary unit.23.1 What Are Species?The biological species concept: proposed by Ernst Mayr:“Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.”This does not apply to asexually reproducing organisms.23.2 How Do New Species Arise?Darwin called speciation the “mystery of mysteries.”Not all evolutionary change results in new species.But if two populations are isolated, over time their genetic structure may change enough so that interbreeding is no longer possible: gene flow must be interrupted.23.2 How Do New Species Arise?Allopatric speciation occurs when populations are separated by a physical barrier. Also called geographic speciation.Thought to be the dominant mode of speciation.Figure 23.3 Allopatric Speciation23.2 How Do New Species Arise?Barriers can form as continents drift, sea level changes, glaciers advance and retreat, climate changes.The environments in which the isolated populations live are different, and so the populations evolve differently.23.2 How Do New Species Arise?Allopatric speciation can also occur if some individuals cross a barrier to form a new, isolated population.The 14 species of finches on the Galapagos (Darwin’s finches) arose from a single species that colonized the islands from South America.The islands have different environments, and are sufficiently far apart for speciation to occur.Figure 23.4 Allopatric Speciation among Darwin’s Finches (Part 1)Figure 23.4 Allopatric Speciation among Darwin’s Finches (Part 2)23.2 How Do New Species Arise?The Hawaiian Islands have 800 species of Drosophila, many restricted to one island.Many of these species have resulted from founder events—species are descendents of individuals that dispersed among the islands.Figure 23.5 Founder Events Lead to Allopatric Speciation23.2 How Do New Species Arise?The effectiveness of physical barriers depends on the size and mobility of the organisms.Example: An 8-lane highway would be impossible for a snail to cross, but easily crossed by a bird.23.2 How Do New Species Arise?Sympatric speciation does not require physical isolation.Disruptive selection is required, such as in black-bellied seed crackers.Figure 22.15 Disruptive Selection Results in a Bimodal Distribution23.2 How Do New Species Arise?Sympatric speciation may be occurring in a fruit fly in New York state.The flies previously deposited eggs only on hawthorn fruits.When large apple orchards were started in the region, some began laying eggs on apples.Figure 23.6 Sympatric Speciation May Be Underway in Rhagoletis pomonella23.2 How Do New Species Arise?The flies are somewhat reproductively isolated because the populations are ecologically isolated—they feed on different resources. They also emerge from their pupae at different times—apple-feeding flies develop faster.23.2 How Do New Species Arise?Sympatric speciation most commonly occurs by polyploidy—duplication of the whole set of chromosomes.Chromosome duplication in a single species is autopolyploidy; combining of chromosomes from two species is allopolyploidy.23.2 How Do New Species Arise?Autopolyploidy can occur if a cell accidentally duplicates the chromosomes, resulting in a tetraploid individual.Tetraploid and diploid populations are soon separated because their hybrid offspring are triploid and sterile—the chromosome do not properly synapse during meiosis.Figure 23.7 Tetraploids Are Soon Reproductively Isolated from Diploids23.2 How Do New Species Arise?Allopolyploids can arise if two closely related species mate, or hybridize.Allopolyploids are often fertile.Many species of flowering plants and ferns are polyploids. Most have arisen through hybridization followed by self-fertilization.23.2 How Do New Species Arise?Speciation by allopolyploidy has occurred in the salsifies (Tragopogon). These weedy plants have been carried around the world by humans.Three species were introduced into North America. Two tetraploid hybrids have been produced.The hybrids have formed several times and are now more widespread than the parental strains.Figure 23.8 Polyploids May Outperform Their Parent Species 23.3 What Happens when Newly Formed Species Come Together?When populations have been isolated, differences can accumulate that reduce the probability that members of the two populations could interbreed.Partial reproductive isolation has been shown for Phlox drummondii.Plant breeders created many strains by selecting for many characteristics, and inadvertently reduced reproductive compatibility.23.3 What Happens when Newly Formed Species Come Together?Geographic isolation does not always lead to reproductive isolation.American sycamores and European plane trees have been geographically isolated for 20 million years;but they can form fertile hybrid offspring.Figure 23.9 Geographically Separated, Morphologically Similar23.3 What Happens when Newly Formed Species Come Together?Mechanisms of reproductive incompatibility fall into two categories:Prezygotic reproductive barriersPostzygotic reproductive barriers23.3 What Happens when Newly Formed Species Come Together?Prezygotic reproductive barriers operate before fertilization occurs.Habitat isolation—e.g., Rhagoletis flies in the Hudson River valley.Temporal isolation—mating periods do not overlap.Mechanical isolation—differences in size and shape of reproductive organs; common in insects.23.3 What Happens when Newly Formed Species Come Together?Gametic isolation—eggs of one species don’t have appropriate chemical signals for sperm of another species; or sperm is not able to attach to and penetrate the egg.Behavioral isolation—individuals reject or fail to recognize potential mating partners.23.3 What Happens when Newly Formed Species Come Together?Floral traits of plants can influence the behavior of pollinators, and thus whether plants can hybridize.Two species of columbines (Aquilegia) in California can produce fertile hybrids, but flower structure determines that one species is pollinated by hummingbirds, the other by hawkmoths.Figure 23.10 Hawkmoths Favor Flowers of One Columbine Species (Part 1)Figure 23.10 Hawkmoths Favor Flowers of One Columbine Species (Part 2)23.3 What Happens when Newly Formed Species Come Together?Postzygotic reproductive barriers: survival and reproduction of hybrid offspring are reduced.Low hybrid zygote viability—fail to mature or have severe abnormalities.Low hybrid adult viability—lower survival rate.Hybrid infertility—e.g., mules.23.3 What Happens when Newly Formed Species Come Together?If hybrid offspring survive poorly, natural selection may favor prezygotic barriers.Strengthening of prezygotic barriers is known as reinforcement.Where two species of Phlox are sympatric, P. drummondii (normally pink flowers) has red flowers.An experiment indicated reinforcement. Figure 23.11 Prezygotic Reproductive Barriers (Part 1)Figure 23.11 Prezygotic Reproductive Barriers (Part 2)23.3 What Happens when Newly Formed Species Come Together?If reinforcement is occurring, related species should evolve prezygotic barriers faster than allopatric pairs of species.This has been shown in Agrodiaetus butterflies. Wing color in males has diverged much faster in sympatric than allopatric populations.23.3 What Happens when Newly Formed Species Come Together?If populations are reunited before complete reproductive isolation has developed, interbreeding can occur.If hybrid offspring are fit and interbreed with both populations, gene pools are combined and no speciation occurs.If hybrid offspring are less fit, reinforcement may result in more prezygotic barriers.23.3 What Happens when Newly Formed Species Come Together?A hybrid zone can develop, and may persist for a long time while reinforcement develops.Hybrid zones make good natural laboratories for the study of speciation.Example: two species of European toads; hybrids have many defects. Hybrids are only half as fit as pure-bred individuals.Figure 23.12 Hybrid Zones May Be Long and Narrow23.3 What Happens when Newly Formed Species Come Together?The hybrid zone is narrow, because there is strong selection against hybrids.Pure-bred individuals move only a short distance into the hybrid zone, so there are few encounters with the other species, and reinforcement has not evolved.23.3 What Happens when Newly Formed Species Come Together?Human activities can change hybrid zones.Habitat disruption has created a hybrid zone between two Banksia species in Australia. The flowering seasons have expanded and overlapped.23.4 Why Do Rates of Speciation Vary?Rates of speciation among groups vary greatly.Many factors influence the likelihood of speciation.Species-rich groups are more likely to speciate faster than species-poor groups.23.4 Why Do Rates of Speciation Vary?Speciation rates are likely to be faster in species with poor dispersal abilities, which can be separated by even narrow barriers.Example: There are several thousand species of land snails in the Hawaiian Islands, many restricted to a single valley.23.4 Why Do Rates of Speciation Vary?Populations with specialized diets are more likely to speciate.Groups of closely related true bugs (hemipterans) have a common ancestor that was a predator on other insects.Herbivory has evolved twice in this group, and these lineages have many more species. Herbivores tend to specialize on one or a few plant types.Figure 23.13 Dietary Shifts Can Promote Speciation23.4 Why Do Rates of Speciation Vary?In plants, speciation rates are higher in insect-pollinated than wind-pollinated plants.Speciation rate in columbines (Aquilegia), has been three times faster than lineages that lack nectar spurs.Long spurs limit the number of possible pollinator species, leading to reproductive isolation.Figure 23.10 Hawkmoths Favor Flowers of One Columbine Species (A)23.4 Why Do Rates of Speciation Vary?Sexual selection also appears to increase rates of speciation.Examples: birds with promiscuous mating systemsBirds of paradise have strong sexual dimorphism; males assemble at display grounds, females come and choose mates. Females then build nests and care for young alone. Males remain and mate with more females.23.4 Why Do Rates of Speciation Vary?In closely related manucodes, males and females resemble each other, form monogamous mating bonds, and both parents participate in raising young.Manucodes—Five speciesBirds of paradise—Thirty-three species Figure 23.14 Sexual Selection in Birds Can Lead to Higher Speciation Rates23.4 Why Do Rates of Speciation Vary?Animals with complex sexually selected behaviors are likely to form new species because of the high degree of discrimination in mate selection.Discrimination on subtle differences in color, size, appearances, and behaviors can lead to rapid evolution of species.23.5 Why Do Adaptive Radiations Occur?An evolutionary radiation is the proliferation of a large number of species from a single ancestor.If the resulting species live in a wide array of environments, it is called an adaptive radiation.23.5 Why Do Adaptive Radiations Occur?Adaptive radiation is likely to occur in environments with abundant resources. A population may encounter underutilized resources when colonizing a new area that contains few species, such as islands.Adaptive radiations have followed mass extinctions.23.5 Why Do Adaptive Radiations Occur?Many adaptive radiations have occurred on the Hawaiian Islands.Native biota included many bird, insect, and flowering plant species, but no terrestrial reptiles or amphibians, and only one mammal (a bat) species.23.5 Why Do Adaptive Radiations Occur?The 100 species of birds are thought to have arisen from only seven colonizing species.More than 90 percent of plant species are endemic—they are found nowhere else.Twenty-eight species of silverswords have evolved diverse morphological differences. Figure 23.15 Rapid Evolution among Hawaiian Silverswords (Part 1)Figure 23.15 Rapid Evolution among Hawaiian Silverswords (Part 2)23.5 Why Do Adaptive Radiations Occur?The original silversword colonizers arrived on islands that had very few plant species.There were few trees and shrubs, because such large-seeded plants rarely disperse to islands.Many silverswords developed tree and shrub forms.23.5 Why Do Adaptive Radiations Occur?Adaptive radiations can occur in other places as well:1,563 species of ice plants in southern Africa radiated within the last 9 million years. Ice plants are succulents with CAM metabolism.The region is known as the Succulent Karoo.