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Chapter 26

BIOLOGY 1M03 Chapter 26: Biology Chapter 26

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Ben Evans

Chapter 26: Speciation  If mutation, selection and genetic drift cause isolated populations to diverge sufficiently, distinct types or species form, a process called speciation  Speciation is a splitting event that creates two or more distinct species from a single ancestral group  When speciation is complete, a new branch has been added to the tree of life  In essence, speciation results from genetic isolation and genetic divergence o Isolation results from lack of gene flow and divergence occurs because selection, genetic drift and mutation proceed independently in the isolate populations How Species are Defined and Identified  Species are distinct types of organisms and represent evolutionarily independent groups  Species are distinct from one another in appearance, behaviour, habitat use or other traits  These characteristics differ among species because their genetic characteristics differ  Genetic distinctions occur because mutation, selection and drift act on each species independently of what is happening in other populations  Allele frequencies in populations and thus the populations’ characteristics, become more alike when gene flow occurs between them  If gene flow between populations is extensive and continues over time, it eventually causes even highly distinct populations to coalesce into species  Conversely, if gene flow between populations stops, then mutation, selection and drift begin to act on the populations independently  If a new mutation creates an allele that changes the phenotype of individuals in one population, there is no longer any way for that allele to appear in the other population  As a result allele frequencies and other characteristics in the populations diverge  When allele frequencies change sufficiently over time, populations become distinct species  A species, is then defined as an evolutionarily independent population or group of populations  The three criteria for identifying species are o The biological species concept o The morphospecies concept o The phylogenetic species concept  The Biological Species Concept o According to the biological species concept, the critical criterion for identifying species is reproductive isolation o No gene flow occurs between populations that are reproductively isolated from each other o Specifically, if two different populations do not interbreed in nature or if they fail to produce viable and fertile offspring when matings take place, then they are considered distinct species o Groups that naturally or potentially interbreed and that are reproductively isolated from other groups belong to the same species o Reproductive isolation can result from a wide variety of events and processes o To organize the various mechanisms that stop gene flow between populations, biologists distinguish ▪ Prezygotic isolation • Prevents individuals of different species from mating • Reproductive isolation occurs before mating can occur • Temporal isolation o Populations are isolated because they breed at different times o Ex. Bishop pines and Monterey pines release their pollen at different times of the year • Habitat isolation o Populations are isolated because they breed in different habitats o Ex. Parasites that begin to exploit new host species are isolated from their original population • Behaviour isolation o Populations do not interbreed because their courtship displays differ o Ex. To attract male fireflies, female fireflies give a species- specific sequence of flashes • Gametic barrier o Matings fail because eggs and sperm are incompatible o Ex. In sea urchins, a protein called bindin allows sperm to penetrate eggs • Mechanical isolation o Matings fail because male and female genitalia are incompatible o Ex. In many insects, the male copulatory organ and female reproductive canal fit like a “lock and key” (changes in either organ initiate reproductive isolation) ▪ Postzygotic isolation • In which offspring of matings between members of different species do not survive or reproduce • Interspecies mating does occur, but any hybrid offspring produced have low fitness • Hybrid viability o Hybrid offspring do not develop normally and die as embryos o Ex. When ring-necked doves mate with rock doves, less than 6% of eggs hatch • Hybrid sterility o Hybrid offspring mature, but are sterile as adults o Ex. Eastern meadowlarks and western meadowlarks are almost identical morphologically, but hybrid offspring are largely infertile o Although the biological species concept has a strong theoretical foundation, it has disadvantages o The criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce asexually o In addition, it is difficult to apply when closely related populations do not happen to overlap with each other geographically  The Morphospecies Concept o Under the morphospecies concept, researchers identify evolutionarily independent lineages by differences in size, shape or other morphological features o The logic behind the morphospecies concept is that distinguishing features are most likely to arise if populations are independent and isolated from gene flow o The concept is widely applicable o It is a useful criterion when biologists have no data on the extent of gene flow and it is equally applicable to sexual, asexual or fossil species o Its disadvantage is that the features used to distinguish species are subjective o In the worst case, different researchers working on the same populations disagree on the characters that distinguish species o Ex. Some researchers who work on the fossil record of humans argue that Homo habilis and Homo rudolfensis actually belong to the same species  The Phylogenetic Species Concept o The phylogenetic species concept is based on reconstructing the evolutionary history of populations o The reasoning behind this concept begins with Darwin’s claims that all species are related by common ancestry o A monophyletic group (AKA a clade or lineage), consists of an ancestral population, all of its descendants and only those descendants o Under the phylogenetic species concept, a species is defined as the smallest monophyletic group in a phylogenetic tree that compares populations o On a tree of populations, each tip is a phylogenetic species o Two distinct advantages ▪ Can be applied to any population (fossil, asexual or sexual) ▪ It is logical because populations are distinct enough to be monophyletic only if they are isolated from genetic flow and have evolved independently o A distinct disadvantage ▪ Carefully estimated phylogenies are available only for a tiny subset of populations on the tree of life  The Case of the Dusky Seaside Sparrow o Seaside sparrows live in salt marshes along the Atlantic and Gulf Coasts of the US o Subspecies are populations that live in discrete geographic areas and have distinguishing features, such as colouration or calls, but are not considered distinct enough to be called separate species o Because salt marshes are often destroyed for agriculture or oceanfront housing, biologists began to be concerned about the future of some seaside sparrow populations o A subspecies called the dusky seaside sparrow was in particular trouble, by 1980 only six individuals remained and all were males o The government created the Endangered Species Act ▪ The law uses the biological species concept to identify species and calls for the rescue of endangered species through active management o Because current populations of seaside sparrows are physically isolated from one another and because young seaside sparrows tend to breed near where they are hatched, researchers believed that little to no gene flow occurred among populations o Under the biological species concept and morphospecies concept, there may be as many as six species of seaside sparrow o Scientists launched a conservation program to preserve as much genetic diversity as possible by re-establishing a healthy population of dusky-like birds o However, a different group of biologists found out that seaside sparrows represent just two distinct monophyletic groups o Under the phylogenetic species concept, only two species of seaside sparrow exist o The phylogeny showed that the dusky sparrow is part of the same monophyletic group that includes the other Atlantic Coast sparrows o Further, officials had unwittingly crossed the dusky males with females from the Gulf Coast lineage o Because the goal of the conservation effort was to preserve existing genetic diversity, this was the wrong population to use o The researchers who did the phylogenetic analysis maintained that the biological and morphospecies concepts had misled the conservation program o Under the phylogenetic species concept, they claimed that officials should have allowed the dusky sparrows to go extinct and concentrate their efforts on simply preserving one more populations from each coast o In this way, the two monophyletic groups of sparrows would be preserved o Under the morphospecies concept, officials did the right thing by preserving distinct types o They argue that dusky seaside sparrows had distinctive, heritable traits like colouration and songs that are now lost forever Isolation and Divergence in Allopatry  Speciation begins when gene flow between populations is reduced or eliminated  Genetic isolation happens routinely when populations become physically separated  Physical isolation, in turn, can occur in one of two ways, dispersal or vicariance o A physical splitting of habitat is called vicariance  Speciation that begins with physical isolation via either dispersal or vicariance is known as allopatric speciation  Populations that live in different areas are said to be in allopatry  Dispersal and Colonization Isolate Populations o Start with one continuous population, then colonists float to an island on a raft o Island population begins to diverge due to drift and selection o Finish with two populations isolated from one another o Characteristics of a colonizing population are likely to be different from the characteristics of the source population due to founder effects o Subsequent natural selection may extend the rapid divergence that begins with genetic drift  Vicariance Isolates Populations o If a new physical barrier such as a mountain range or river splits the geographic range of a species, vicariance has taken place o An example of speciation by vicariance involves the group of large, flightless birds called the ratites (ostrich, kiwis, emus, rheas) o The earliest ratites in the fossil record lived about 150 million years ago, on a landmass called Gondwana o Gondwana was made up of a number of physically distinct landmasses ▪ Landmasses make up continental plates o Continental drift describes the ongoing motion of continental plates through time o About 140 million years ago, the continents in Gondwana began to drift apart o Since ratites are flightless, each vicariance event isolated distinct populations o Initially, each ratite population would have diverged via genetic drift o These differences would have been extended by natural selection as environments changed on each plate, leading to the evolution of the species observed today Isolation and Divergence in Sympatry  When populations or species live in the same geographic area or at least close enough to one another to make interbreeding possible, biologists say that they live in sympatry  Many researchers believe that speciation cannot occur among sympatric populations because gene flow is possible o The gene flow would overwhelm any differences among populations created by genetic drift and natural selection  Can Natural Selection Cause Speciation Even when Gene Flow is Possible o Recently, several examples have upset the traditional view that sympatric speciation (speciation that occurs even though gene flow is possible) is rare or nonexistent o The key realization is that even though sympatric populations are not physically isolated, they may be isolated by preferences for different habitats o Ex. Speciation in soapberry bugs ▪ A species of insect, native to the south-central and south-eastern United States ▪ Feed on plants in a family called Sapindaceae, including the soapberry tree, serjania vine and balloon vine ▪ Bugs feed by piercing fruits with their beaks, reaching in to penetrate the coats of the seeds located inside the fruit and then sucking the contents of the seeds through their beaks ▪ Bugs also mate on their host plants ▪ Horticulturists brought three new species of sapindaceous plants to North America from Asia ▪ Soon after these plants were introduced, soapberry bugs began using them as food ▪ The fruits of the non-native species are much smaller than the fruits of the native species ▪ In soapberry bugs that feed on native host plants, beak length corresponds closely to the size of the host fruit ▪ Correlation between fruit size and beak length is logical because it should allow individuals to reach the seeds inside the fruit efficiently ▪ One biologist asked, in populat
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