What Species Are and How They Arise

Sketches of four species of Galapagos Finches from Darwin’s “Journal of Researches”

Large Ground Finch Medium Ground Finch

Small Tree Finch Warbler Finch

The divergent pattern of evolution Related lineages tend to “evolve away” from each other, producing a tree-like pattern. Darwin proposed a process underlying this pattern; competition between closely related lineages

A page from “On the Origin of Species” (Darwin 1859)

(after Ridley 1993)

Outline of Topics

I. Species Concepts

•Morphological species concept

•Biological species concept

II. Speciation in sexually reproducing lineages: Origin of new species from existing ones

•Basic Elements of Speciation

• interrupted gene flow among subunits of a species

• evolutionary processes that proceed within those subunits

•Allopatric speciation

•Sympatric speciation

III. If a new species persists, processes by which that occurs

TIM

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PHENOTYPIC (AND UNDERLYING GENETIC) CHANGE

Anagenesis CladogenesisThe fossil record reveals two patterns of speciation

•Anagenesis (phyletic speciation);

•Accumulation of heritable changes in a population associated with speciation

•Cladogenesis (branching speciation);

•New species arises from a population that buds from a parent species

•Basis for biological diversity

Speciation:Origin of New Species from Existing Species

(after Campbell 2000)

Species Concepts

•Morphological Species Concept

•Group of individuals united by similarities that distinguish them from all other individuals.

•Aristotle, Carolus Linnaeus…

•Biological Species Concept

•Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such populations

•Theodosius Dobzhansky, Ernst Mayr (1940)...

Carol Linnaeus (1707-1778)

Ernst Mayr

Biological Species Concept: elements and issues

•“…actually or potentially interbreeding…”

• regard geographically separated populations as same species if presumed to be capable of interbreeding

•Concept is restricted in its generality, in that it does not apply to asexually reproducing species

•“…natural populations…”

•species may be interbred in captivity, but that is irrelevant to evolutionary processes underlying speciation and the biological species concept in nature

II. Speciation: Processes by which new species arise

Barrier Genetic Reproduction

to Gene Flow Differentiation Isolation

Basic Elements of Speciation

Barrier to gene flow is established

Populations diverge genetically but are still reproductively compatible

Sexual incompatibility is established

Interbreeding population (parent species)

Daughter species A

Daughter species B (after Purves 1999)

Gene flow is interrupted

Agents of evolutionary change variously operate on the isolated gene pools

•Selection (Adaptation)

•Mutation, Migration (gene flow), Drift

•Non-random mating

Degree of reproductive isolation evolves

After repatriation, reinforcement of reproductive isolation by natural selection

Reproductive isolation may be a consequence of independent evolution

Two Modes (processes) of Speciation

Allopatric Speciation

•Etymology: “allo-patric” = “different-country”

•Gene pools isolated via geographic separation

•Prevalent mechanism of speciation in animals

Sympatric Speciation

•Etymology: “sym-patric” = “same-country”

•Gene pools isolated via process other than geographic separation

•via polyploidy, particularly in plants

•via behavior, ecology and to a lesser extent polyploidy in animals

•Common in plants (>70% of species via sympatric speciation)

•Growing understanding of this process as it applies to animals

Allopatric Speciation: Geographic Isolation

Isolation by dispersal: arrows indicate movement of individuals

Isolation by vicariance: arrow indicates an encroaching physical feature such as a river, glacier, lava flow, or new habitat

(Freeman and Herron 1998)

Peripheral Isolates may be predisposed to (Allopatric) Speciation

Peripheral Isolates may be predisposed to (Allopatric) Speciation

Peripheral Isolate; small, isolated population at edge of species’ range

•Environment, and therefore selection pressures may differ substantially compared to parent population

•Gene pool likely to show incipient differentiation

•Genetic drift will drive random changes in genetic structure

Founder Events (dispersal, in the example to follow)

May Lead to Speciation

Ten species of Darwin’s Finches from Isla Santa Cruz, each with different bill size and feeding habits.

Dispersion model for evolution of Darwin’s Finches

•Founding population inhabits San Cristobal

•Period of disperals and differentiation

•Secondary contact; immigrants from Santa Cruz are reproductively isolated from the ancestral stock on San Cristobal

•The consequence of repeated speciation events, such as is thought to have occurred with Darwin’s finches, in referred to as Adaptive Radiation; Emergence of numerous species from common ancestor

Model for speciation and adaptive radiation on island chains

•dispersal and colonization

•adaptation of colonizing population to environment on colonized island

•genetic differentiation underlies phenotypic adaptations

•if, following repatriation of ancestral and descendant lineages, those lineages are reproductively isolated, we recognize those lineages as distinct species

Hypothetical Example of Allopatric Speciation by Vicariance

Angel's Window, Cape Royal, north rim

Looking northwest across the Grand Canyon, from the south rim of the in the vicinity of Hopi Point – Colorado River a mile below

Visit these two websites; http://www.edu-source.com/GCpages/CVOpage5.htmlhttp://www.edu-source.com/GCpages/CVOpage4.html

Allopatric Speciation via Vicariance: A Real Example

Harris’ antelope squirrel (Ammospermophilus harrisi)

White-tailed antelope squirrel (Ammospermophilus leucurus)

http://animaldiversity.ummz.umich.edu/accounts/ammospermophilus/a._harrisii.html

Today: complete lecture 3 and begin lecture 4

Conclude lecture on speciation

Begin Lecture 4. Macroevolution

Variation in speciation rates (ch 22:420-422)

Evolutionary radiations (ch 22: 422-423)

The significance of speciation (ch 22; 423-424)

Rates of Evolutionary change (ch 20: 390-392)

Patterns of evolutionary change (ch 20: 392-393)

The future of evolution (ch 20; 393-394)

Allopatric Speciation: Important Considerations

•Inference for the importance of allopatric speciation in animals;

•The effectiveness of a geographic barrier to impede or eliminate gene flow depends on the locomotion and other characteristics of individuals

•We consider speciation to have ensued if and when two gene pools have diverged to the point that, should secondary contact occur, the individuals of each are reproductively isolated from each other; can no longer interbreed and produce fertile offspring.

Sympatric Speciation

New species evolves withiin geographic range of parent species

•Common in plants, via polyploidy

Hugo DeVries with new species of primrose, Oenothera gigas

(2n=28)

(2n=14)

Recall the breach between “Mendelism” and “Darwinism”…

Example of Autopolyploidy through nondisjunction and self-fertilization

Meiotic nondisjunction of a diploid (2n) cell results in gamete with unreduced chromosome number of 6

Self-fertilization, as depicted below, by such an in individual gives rise to a new species; individuals are capable of sexual reproduction with complete set of homologous chromosomes –required for successful meoisis

•Autopolyploid Species

•Auto-polyploid species arise from single parent species

•Can arise through a meiotic error – a non-disjunction event that constitutes a mutation -- results in gametes with one or more extra sets of chromosomes compared to chromosome complement in normal gametes

Sympatric Speciation in Plants via Polyploidy

Polyploidy Chromosome complement with one or more extra sets of chromosomes; an increase in the number of chromosomes

There are two distinct mechanisms by which polyploid species of plants arise

•Allopolyploid Species

•Allo-polyploid species arises through interbreeding of two different species

•A number of different routes to allopolyploidy are known – all involve arriving at a chromosome complement that is functional in terms of meiotic reduction division

•Regarded as being much more common than autopolyploidy

Examples of Allopolyploidy

Modern Bread Wheat is a Hexaploid Plant that probably originated about 8000 years ago as a spontaneous hybrid of a cultivated wheat and a wild grass (Campbell 2000)

Sympatric Speciation in Animals

•Animals may become reproductively isolated if genetic factors cause them to depend on different resources than parent population

•(Polyploid speciation in animals is rare)

•Mechanisms not well-understood, but probably not common

Mouth-brooding Cichlids surrounded by swarms of fry in Lake Tanganyika. Young are periodically released to feed but gathered up into parents mouth at first sign of danger.

Non-Random mating in a polymorphic species may have led to sympatric speciation the Genus Pundamilia in Lake Victoria

two closely related species of Cichlids in the genus Pundamilia

Reproductively isolated in nature and in captivity under natural light conditions – females only choose conspecific males.

Under monochromatic orange light, males look similar (presumably) to females – and females mate indiscriminantly with males of either species

Inference from experiment that speciation occurred relatively recently and that color is the main, perhaps only “reproductive barrier”

Four species of Haplochromis cichlids in Lake Victoria that occupy different ecological niches, although they are similar in appearance.

Reproductive Isolation;Prezygotic and Postzygotic Barriers that Isolate Gene Pools of Biological Species

•As an incipient new species diverge behaviorally, physiologically, morphologically from the parent species, those very differences may preclude the two from reproducing successfully; i.e., the two may become “good biological species”, or not!!

•Reproductive barriers; Evolved traits that preclude production of fertile, viable hybrid offspring

•Prezygotic Barriers; reproductive isolating mechanisms that operate before fertilization, some before mating

Spatial, temporal, mechanical and gametic isolation

•Postzygotic Barriers; reproductive isolating mechanisms that operate after fertilization

•Problems with hybrids – including developmental abnormalities, infertility and low viability

•In “hybrid zones” or “areas of secondary contact” where hybridization takes place, if there is selection against hybrids, we may expect evolution of stronger prezygotic barriers (demonstrated in some laboratory populations, not well-supported in observations of natural populations)

Barriers to reproduction can arise without having been favored directly by Natural Selection, as a consequence of adaptive divergence

Adaptive divergence of two populations

•populations diverge evolutionarily (think in terms of the genetic structure of each)

•divergence is consequence of populations experiencing different selective forces; divergence is “adaptive” in that sense (can be true for sympatric or allopatric)

•populations may diverge so much (morphologically, physiologically, behaviorally, etc) that interbreeding is not possible; reproductive isolation

•complete reproductive isolation, but not as a consequence of selection for isolation

Reproductive Isolation can arise as a consequence sexual selection operating within one or the other population, or both

•Sexual Selection is a form of natural selection; selection that occurs when individuals vary in their ability to acquire mates (less successful individuals are “selected against”)

•In many species of animals, it’s the males that experience substantial sexual selection

•This selection pressure drives evolution of traits that make individuals more successful at acquiring mates

Male great frigate bird shows “ornament” that evolved through sexual selection

Male white-tail deer shows “armament” that evolved through natural selection

Solomon 1999 Raven and Johnson 1999

Habitat Isolation. Populations live in different habitats and do not meet

Temporal Isolation. Mating or flowering occurs at different seasons or times of day

Behavioral Isolation. Little or no sexual attraction between males and females

Spatial Isolation = Habitat or Ecological Isolation

Behavioral Isolation-- Blue-footed Boobies on Galapagos Islands

Behavioral Isolation -- Song in Eastern and Western Meadowlarks

Distinct songs help prevent interbreeding among these sibling species

Hybridization (one definition): interbreeding among individuals from two divergent populations

Hybrid Zone: region where two related populations that diverged after becoming geographically isolated make secondary contact and interbreed

Yellow-rumped (Audubon’s) warbler

Yellow-rumped (Myrtle) warbler

What can we say about “speciation” when areas of secondary contact exist where hybrids do not have reduced fitness?

The yellow-rumped warbler diverged into two distinct races: Eastern populations were separated from Western ones during the Wisconsin glaciation, and probably came into secondary contact about 7500 years ago. Populations are reproductively isolated over most areas of secondary contact, except in some regions in the Canadian Rockies, where hybrids do not have reduced fitness; yellow-rumped genes are introgressing west, and Myrtle genes are migrating east.

Macroevolution; Evolutionary Patterns and Proceesses among species and higher taxa

Three major faunas have dominated animal life on Earth

Cambrian Explosion – all major animal lineages arose Paleozoic and Triassic Explosions – many new families, genera and species, but not new fundamentally new body plans

Reason for difference in pattern of diversification (no new phyla) may relate to the ecological conditions; low competition and predation may have fostered evolution of major body plans

millions of years ago

num

bers

of f

amili

es

The size and complexity of organisms have increased

Early Eukaryotes were larger and substantially more complex than Prokaryotes from which they arose (and modern Prokaryotes as well)

Multicellularity allowed greatly increased size, which facilitated homeostasis, specialization….

Co-evolution among predators and prey is probably partly responsible for increasing complexity, paricularly in the form of highly developed nervous and muscular systems, and for capture and avoidance traits in general

Evolution of shell morphology indicates increasing predation rates on snails over evolutionary time.

Most Evolutionary Novelties are Modified Versions of Older Structures

Eye complexity in Molluscs Complex eyes evolved from simple ones many times in evolutionary history

limpet (Patella)slit shell mollusk (Pleurotomaria)

Nautilusmarine snail (Murex)

squid (Loligo)

Descent with modification extends to major morphological transformations

Complex structures often evolve incrementally from simple ones

Exaptation Evolutionary novelty can arise through gradual refinement of existing structure for new function

Homologous structures in the forelimbs of mammals. Wing of a bat and flipper of a whale are examples of exaptations of terrestrial forelimbs.

Significant evolutionary change leading to the origin of new species may be gradual or may occur in spurts

•Fossil record does not bear many forms transitional between species; suggests that significant morphological (and underlying genetic) change occurs quickly relative to the life of a species.

•Gould and Eldridge developed and published this idea in the 1970’s, referring to the process as “Punctuated Equilibrium”

•species undergo most morphological change shortly after diverging from parent stock

•No reason to regard these hypthotheses as mutually exclusive among lineages

long periods of evolutionary stasis puncuated by episode of morphological change that reflects speciation

Punctuated Equilibrium

•Speciation happens rapidly; most of the morphological differences evolve rapidly in a new species, as that new species first buds from its parent species

•Support for theory in fossil record; Darwin acknowledged that fossil record didn’t seem to show the gradual change he expected

•Allopatric speciation thought to occur relatively rapidly; natural selection and genetic drift can cause significant change in a few hundred to a few thousand generations

•If a species survives (leaves fossils!) for five million years, first 50,000 years of its existance would be only 1% of its existance, 1% of its fossil-producing time

•Mutation in genes that regulate embryonic development may be associated with changes that can generate new species..,

•Will return to some topics from “Origin of Evolutionary Novelty” in the next chapter….

Developmental genes have strong influence on basic body plans and therefore, potentially, on macroevolutionary change

•Developmental genes control the rate, the timing (eg onset) and spatial pattern of changes in form as an organism grows and develops

•The specific form a body takes on depends in part on proportioning or allometric growth (“other measure”); growth rates of different body parts relative to eachother

•Slight change in growth rate of one body part relative to the others can have substantial effect on adult form

•Heterochrony refers to evolution of morphology through modification in allometric growth; heterochrony is driven by developmental genetics

Arms and legs grow faster than head and trunk (different aged individuals all rescaled in drawing to same height)

Comparison of chimp and human skull growth. Fetal skulls are similar in shape. Sloping skull of adult chimp compared to human is due to faster growth of the chimps jaw than other parts of the skull (compared to humans)

Heterchrony may involve a change in the timing of reproductive development of reproductive relative to the timing of somatic (non-reproductive) development

Paeodmorphosis If reproductive rate accelerates, may contribute to evolution of new species that when sexually matures, retains structures that were characteristic of the juvenile form in the ancestral species

Axolotl – a salamander that retains certain larval (tadpole characteristics, including gills, after it has grown to full size and is sexually mature

Mutations in genes that control spatial organization and location of body parts can contribute to macroevolutionary change

Hox mutations and the origin of vertebrates

Rates of Evolutionary Change Vary

Gradualism Punctuated Equilibrium

Species Show Geographic Variation

Pacific Coast Subspecies of the Song Sparrow (Melospiza melodia)

Species Concept

•Biological species concept is prevalent

•Most species described based on morphological species concept

•Value, relevance of concept depends on

•questions your asking

•what you want in a species concept and classification scheme!!!

Haploid chromosome set, as might occur in a gamete

Diploid chromosome set, as might occur in a somatic cell or primordial germ cell in a plant

Polyploid chromosome set – triploid; six of each chromosome; individual could

Evolutionary Stasis of Large Successful Populations

Haploid chromosome set, as might occur in a gamete

Diploid chromosome set, as might occur in a somatic cell or primordial germ cell in a plant

Polyploid chromosome set – hexaploid; six of each chromosome

Sympatric Speciation by Allopolyploidy in Plants

The Tug of Selection: Endler’s work on guppies in Trinidad

•Males in dangerous (high-predation) streams had colors that blended with sand on stream bottom, for camoflage

•Males in safe (low-predation) streams had spots that were colorful and bigger than sand grains

•“Natural lab experiment”; transplanted males from dangerous stream to safe stream.. within 20 generations, descendent males had larger more colorful spots

•See page 16 about other experiments on Trinidad guppies by Endler and Reznik