Download Evolution: Macroevolution

Evolution: Macroevolution
 Microevolution: changes on the small scale
 Changes in gene frequencies in a population
 Macroevolution: changes on the large scale
 Earth is home to numerous species
Species Formation
 Estimates range between 10 and 25 million species
 4 million species is lowest estimate
 Present species are survivors or newcomers
 99% of all species that have ever lived on Earth are now extinct
What Is a Species?
 All of the species on Earth share a common ancestor ~3.8 billion years ago
 Initial type of organisms branched into two types of organisms
 New “species”
Species Formation
 Process continued, producing all of the species that have ever lived on the planet
 These species branched further
 New species are formed after populations of a single species stop interbreeding
What Is a Species?
Biological species concept
 Species are groups of actually or potentially interbreeding natural populations which are
reproductively isolated from other such groups
What Is a Species?
 Some separate species may be able to interbreed in captivity, but do not do so in nature
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e.g., Lion ♂ + tiger ♀  liger
e.g., Lion ♀ + tiger ♂  tigon
Since this interbreeding does not occur in nature, lions and tigers are separate species
 Mythical powers
What Is a Species?
 The biological species concept is not always sufficient in defining species
 Many bacteria reproduce asexually, not sexually
 There is sometimes limited very limited gene flow between two species
How Do New Species Arise?
 “Speciation” is the development of new species through evolution
 Branches from parent species, while parent species continues to exist
 Speciation results from the same processes operating in microevolution (changes in
allaele frequencies in the population)
How Do New Species Arise?
 Evolution within a population involves a change in the population’s allele frequencies
 Two interbreeding populations will share any changes in allele frequencies
 These populations will evolve together and remain a single species
How Do New Species Arise?
 Two populations that do not interbreed will not share changes in allele frequencies
 Changes will add up over time
 Ultimately, a new species could be formed
How Do New Species Arise?
Allopatric speciation
 Geographic separation can restrict gene flow between populations
 Glaciers can move into an area
 A river can change course
 Ponds can dry up
 Part of a population may migrate into a remote area (e.g., Galápagos Islands, Hawaiian
Islands, etc.)
How Do New Species Arise?
Allopatric speciation
 Restricted gene flow between two populations can ultimately result in the formation of a new
species
 “Allopatric speciation”
How Do New Species Arise?
 During their geographic separation, allele frequencies of two populations will change
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differently
 These populations will evolve differently
 Physical or behavioral changes may result
How Do New Species Arise?
When two geographically separated populations are reunited, they may or may not be able to
interbreed
 If not, then speciation has occurred
How Do New Species Arise?
 Mechanisms preventing interbreeding are central to speciation
 Mountains and rivers are extrinsic isolating mechanisms
 Characteristics of the organisms are intrinsic isolating mechanisms
How Do New Species Arise?
Intrinsic reproductive isolating mechanisms
 Any factor that prevents interbreeding of individuals of the same or closely related species
Allopatric speciation involves extrinsic isolation (geographic separation) followed by the
development of intrinsic isolating mechanisms
How Do New Species Arise?
Intrinsic reproductive isolating mechanisms
 Ecological isolation
 Temporal isolation
 Behavioral isolation
 Mechanical isolation
 Gametic isolation
 Hybrid inviability or infertility
How Do New Species Arise?
Ecological isolation
 Two species may feed, mate, and grow in different habitats within a common area
 e.g., Ranges of lions and tigers overlapped
 Lions preferred the open grasslands
 Tigers preferred the deep forests
 No interbreeding occurred
How Do New Species Arise?
Ecological isolation
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 Two species may feed, mate, and grow in different habitats within a common area
 e.g., Ranges of lions and tigers overlapped
 Lions preferred the open grasslands
 Tigers preferred the deep forests
 No interbreeding occurred
How Do New Species Arise?
Behavioral isolation
 Individuals choose their mating partners
 This choice is often dependent upon courtship rituals
 Closely related species may have incompatible courtship rituals
 e.g., Songs of birds and crickets, fiddler crab claw waving, etc.
How Do New Species Arise?
Mechanical isolation
 Reproductive organs of two closely related species may have incompatible sizes or shapes
 e.g., Different butterfly species have genitalia that differ in shape
How Do New Species Arise?
Gametic isolation
 Mating may occur, but the sperm is incompatible with either the egg or the female
reproductive system
How Do New Species Arise?
Gametic isolation
 e.g., Sperm in pollen of one plant species cannot reach egg of related species
 e.g., Sperm of one animal species is killed in reproductive system of related species
 e.g., Sperm of one species cannot bind to receptors on egg of related species
How Do New Species Arise?
Hybrid inviability or infertility
 Offspring resulting from a mating between closely related species may be unhealthy
 Offspring resulting from a mating between closely related species may be infertile
 e.g., Horse + donkey  mule
 Mules are healthy, but infertile hybrids
Sympatric Speciation
The fruit fly Rhagoletis pomonella provides one of the best-studied examples of sympatric
speciation
Sympatric Speciation
 R. pomonella
 Originally existed solely on hawthorn trees
 “Haw flies”
 Some moved to apple trees newly introduced from Europe
 Flies colonizing apple trees are becoming a new species
 “Apple flies”
 Haw fly life cycle
Sympatric Speciation
These flies winter underground as larva
Adult flies emerge in the summer
Flies fly to their host trees, mate, and lay their eggs in the fruit
Adult flies live for approximately one month
Sympatric Speciation
 A mutation or new combination of existing rare alleles arose in the ancestral haw flies
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Sympatric Speciation
 Mutant flies emerged earlier in the summer
 These flies were attracted to apples as well as hawthorns
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 Apples mature slightly earlier than hawthorn fruit
 These early emerging flies interbred amongst themselves to a high degree
 Limited gene flow between these populations
Sympatric Speciation
Mating periods of “haw flies” and “apple flies” do not fully overlap
 Temporal isolation
These two types of flies occupy different habitats in the same area
 Ecological isolation
These two intrinsic reproductive isolating mechanisms have occurred without geographical
separation
When Does Speciation Occur?
Some species remain relatively unchanged for long periods of time
 e.g., Horseshoe crabs have changed little in 300 million years
Other species change dramatically over relatively short periods of time
 e.g., The 13 species of Darwin’s finches arose from an ancestral species within the past
100,000 years
When Does Speciation Occur?
 Horseshoe crabs are generalists
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 Extremely diverse diet
 Eat plants, animals, scavenged debris
 Shifts from one food source to another depending on availability
 Do not adapt in response to changes in food source
When Does Speciation Occur?
Plants were established on the Galápagos Islands prior to arrival of finches
 No similar birds preceded the finches
There was very little competition for the resources the islands offered
 Many niches were unoccupied
 Populations could specialize to fill one of many available niches
When Does Speciation Occur?
Finches could fly between the 25 islands
Water between the islands did represent a geographical barrier
 Reduced gene flow between populations on different islands
 These populations evolved into multiple species
When Does Speciation Occur?
Darwin’s finches exemplify an “adaptive radiation”
 Rapid emergence of many species from a single species introduced into a new
environment with unfilled ecological niches
Two conditions conducive to speciation
 Specialization
 Migration to a new environment
Categorization of
Earth’s Living Things
A taxonomic system is used to classify every known species on Earth
 Organisms are classified into various groups based on their evolutionary relationships
 Further classified according to physical characteristics
 Currently undergoing revision
 May rely solely on DNA analysis
Categorization of
Earth’s Living Things
 Eight basic categories are used
 Species, genus, family, order, class, phylum, kingdom, and domain
 Species is the most specific grouping
 Domain is the broadest grouping
Categorization
 Taxonomy gives a specific (Latin) scientific name to every species
 e.g., Homo sapiens, modern humans
 e.g., Rhagoletis pomonella, a fruit fly species
 e.g., Drosophila melanogaster, another species of fruit fly
 Specific scientific names allow scientists to know which type of fruit fly (for example) they are
talking about
Categorization
 Closely related species are combined in a larger group called a “genus”
 The first word in a scientific name is actually the name of the genus
 e.g., Canis lupus, the gray wolf
 e.g. Canis familiaris, the domestic dog
 Both of these species belong in the same genus (Canis)
Categorization
 A variety of techniques are used to construct evolutionary histories
 Comparative morphology, etc.
 Comparisons of DNA, RNA, and protein sequences provide the bulk of this information
today
 Evolutionary trees can be constructed
 “Phylogenetic” trees
Categorization
 Homologous structures provide evidence for the occurrence of evolution
 Similar structure due to common descent
 Structures may arise independently in multiple evolutionary lineages
 “Analogous structures” arise through “convergent evolution”
 Similar environmental pressures lead to similar adaptations
Categorization
 Analogous structures can be misinterpreted as homologous structures
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 One would conclude that organisms share evolutionary ancestry when in fact they do not
Taxonomy and Relatedness
There is a well-established system for classifying organisms
 Evolutionary relatedness is the most important factor used in placement of organisms
 Taxonomy sometimes recognizes other factors when placing organisms
Taxonomy and Relatedness
Class Reptilia includes organisms such as snakes, lizards, crocodiles, and dinosaurs
Class Aves includes all birds
Dinosaurs and birds are more closely related than dinosaurs and lizards
 Birds split off of the dinosaur lineage long after dinosaurs split from other reptiles
Birds are arguably different enough from modern reptiles to have their own class
Polyploidy
 Diploid species possess paired homologous chromosomes
 e.g., 23 pairs in humans, 4 pairs in Drosophila (fruit fly), etc.
 These homologous chromosomes are separated during meiosis
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 Gametes receive only one copy of each chromosome
Polyploidy
Many plants and some animals can produce hybrids
 Products of fertilization between two different species
Most hybrids are sterile
 Lack pairs of homologous chromosomes
 Homologues cannot pair in meiosis
 Generally cannot produce functional gametes
Polyploidy
A hybrid zygote may double its DNA in preparation for mitosis, but fail to divide
 The chromosome number has doubled
 This zygote now possesses pairs of homologous chromosomes
 “Polyploid”
Mitosis will produce a multicellular individual whose cells all possess this doubled number of
chromosomes
Polyploidy
In a polyploid individual
 Sperm and egg can be produced through meiosis
 Self-fertilization is possible
 Fertile offspring are produced
 Fertilization of either parent species will produce infertile offspring
 This individual is reproductively isolated from both parent species
Polyploidy
Polyploidy produces a new species
 Reproductively isolated from parent species
 Able to perpetuate itself through self-fertilization
Maize
Maize
Wheat
Polyploidy
Triploid crops: banana, apple, ginger, citrus
Tetraploid crops: durum or macaroni wheat, maize, cotton, potato, cabbage, leek, tobacco,
peanut, kinnow, Pelargonium
Hexaploid crops: chrysanthemum, bread wheat, triticale, oat
Octaploid crops: strawberry, dahlia, pansies, sugar cane
 Humans can make use of polyploidy
Polyploidy
 Polyploidy can be chemically induced in watermelons
 The polyploid individual is then crossed to a normal diploid watermelon plant
Polyploidy
 Humans can make use of polyploidy
 Offspring from this cross have three copies of each chromosome
 They cannot form functional gametes or functional embryos
 True seeds cannot be produced
 “Seedless” watermelons