Chapter 16: Evolutionary Biology

What processes cause micro and macro evolution?

Mutation, natural selection, genetic drift, and migration, the time scales for micro and macro differs but the mechanisms are identical

What do all species trace their ancestry back to?

The first life on earth 3.8 billion years ago, earth was estimated to have formed 4.6 billion years ago

What is organism grouping typically based on why is this not ideal?

Apparent similarity, biologists must use more objective criteria to name and classify organisms,

What are species concepts?

Theoretical definition of what constitutes a species, principles of how to draw the lines between species

What are species criterion?

The practical manner in which to determine what is a species

What is the best evidence that each species was no specially created?

The existence of intermediate variants, races, and subspecies

What did Ernst Mayr propose in 1942?

The first formal species concept (biological species concept), which said species is defined as a freely breeding population and must have reproductive isolation from all members of other species, concept is used by the endangered species act and it is used by many biologists

Why is the Ernst Mayr proposal imperfect?

It does not apply to unisexual species, hybrid species exist, and species may be isolated by distance; it requires subjective judgement about whether species would interbreed if they had the chance

What is the Morphospecies or Typological species concept?

Morphological similarity determines species membership, this is the species concept that non-scientists have been using still the most widely used species concept because it is based on practical species criterion

What are the problems with the Morphospecies species concept?

Unclear how much difference is too much, existence of sexual dimorphism, polymorphism & intraspecific variation, and cryptic species

What is the phylogenetic species concept?

Proposed by Joel Cracraft in 1983 states that a species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent, requires a phylogenetic tree where patterns of descent can be seen

What are monophyletic groups?

Groups composed of taxa that contain an ancestor and all of its descendents

How does the phylogenetic species concept relate to monophyly?

It is based on the monophyly criterion and if a group is not monophyletic it is not a recognized taxon

What does the phylogenetic species concept advocate?

A very high number of species since any monophyletic clade could be a species

What are the issues with the phylogenetic species concept?

Advocates of the concept demang that species must be monophyletic, in current classification there are many non-monophyletic species, difficult to implement because most taxa do not have phylogenies

What is the evolutionary species concept?

Defined by Wiley in 1978 because biological species concept could not be applied to asexual organism or temporal sequences of species where they changed over geological time

What is an evolutionary species defined as?

A single lineage of ancestor descendent populations of organisms which maintains its identity from other such lineages ( in space and time) and which has its own evolutionary tendencies and historical fate

What is the biological species concept?

Species are groups of interbreeding natural populations that are reproductively isolated from other such groups, defined by Ernst Mayr

How is the evolutionary species concept applied?

Can be applied to biparental or uniparental species, based on how a lineage changes over time, if the lineage is distinct and stable over time then it is a distinct species

What is the ecological species concept?

Defined by species occupying different adaptive zones and evolving separately

What is the recognition species concept?

A specific mate recognition system (SMRS)

What is the cohesion species concept?

Focuses on mechanisms that maintain genetic and phenotypic identity

What did De Queiroz (1998) do to synthesize all of the species concepts?

He wished to show how they are not so different from each other and to avoid arguments, he combined most of the newer species concepts and separated concept from criteria

What is the general lineage species concept?

Species are metapopulations that exchange alleles frequently enough to comprise the same gene pool, created by De Queiroz and combined previous concepts, this is a good framework but like most concepts is not perfectly correct

What are the three steps to the process of speciation?

- Population isolation

- Divergence of phenotypes and genotypes

- Reproductive isolation

What is physical isolation?

Migration tends to homogenize genotypes, when gene flow stops speciation may occur, migration selection balance of water snake in lake erie ( banded and unbanded)

What was the water snakes in lake erie example?

Migration- selection balance of water snakes in lake Erie, populations did not diverge into banded and plain forms because migration kept gene flow inactive, if snakes were prevented from immigrating to the island, the two forms might diverge into separate species by selection, mutation, and drift

Allopatric speciation

A model by Ernst Mayr, populations are separated by a physical barrier and diverge, considered the most common speciation mechanism (evidence for this model exists)

Vicariance

A new barrier to gene flow arises such as a mountain range or river

What is gene flow?

movement of alleles from one population to another

What is the evidence for allopatric speciation?

Regions withmany geographic barriers typically have more species than regions with fewer barriers

Sympatric speciation

The formation of new species in populations that live in the same geographic area

Parapatric speciation

speciation pattern in which populations speciate while in contact along a common border

Peripatric speciation

occurs when peripheral populations become geographically isolated from the main population and undergo genetic divergence and speciation

How does peripatric speciation occur?

Genetic drift may cause initial changes in allele frequencies especially when founder event occurs, then selection may act to further diverge colony from parent population

What is speciation?

Formation of new species

What is an example of Peripatric speciation?

Hawaiian Drosophila

How many species of Hawaiian Drosophila are there?

850 species

What do Hawaiian Drosophila exploit?

A vast number of niches

What is a characteristic of Hawaiian Drosophila's body forms?

They have highly varied body forms

What hypothesis explains the speciation of Hawaiian Drosophila?

Founder hypothesis of speciation

How did the Hawaiian islands arise?

They vary in age and arose from west to east

Where did the founder flies of Hawaiian Drosophila inhabit?

The western island

What direction did the founder flies emigrate as other islands arose?

Eastward

How was the founder fly hypothesis tested?

with geology of Hawaiian islands and phylogeny of flies, used mtDNA to estimate phylogeny and hypothesis was supported

What are the predictions of the founder hypothesis? (Hawaii flys)

Closely related species will occupy adjacent islands, branching sequence should partially coincide with island emergence sequence

The nature of peripatric speciation in founder populations

Genetic change can be very rapid in localized populations founded by few individuals cut off from gene flow, differences in ecological selection can play a role because the environment of a small area is often more homogenous than the larger area, conflicting pressures that act on a widespread species may be less numerous and more directed at certain adaptations in a founder population

How does peripatry differ from allopatry?

Most allopatric events are thought of as a major split leaving two large populations on either side, whereas a peripatric division typically results in the nearly intact parent population and a small founder population ( thus drift is powerful in the founder)

How does parapatric speciation occur?

Evolution of reproductive isolation between populations that are continuously distributed in space, so that there is substantial movement of individuals and gens between them

Note on parapatric speciation

The observation that two sister species currently have parapatric or sympatric ranges does not mean that was how they were speciated it could be allopatry followed by secondary contact of ranges that were previously distinct

The nature of parapatric speciation

Divergent selection is greater than gene flow over time, generally a hybrid zone forms, the degree of selection and gene flow depends on the size of the character clines

How does sympatric speciation occur?

Complete panmictic mating and a reproductive isolating mechanism evolves within the population via either polyploidy or disruptive selection

Which form of speciation is the most controversial in biology?

Sympatric

The nature of sympatric speciation

Natural selection may be more important than drift in speciation, small populations can lead to divergence but they are not necessary for speciation

What is an example of sympatric speciation?

Apple and hawthorn maggot flies

What do apple and hawthorn maggot flies have in common?

They have identical body forms.

What is a key difference in the diet of apple and hawthorn maggot flies?

They eat different food.

What caused different adaptations in apple and hawthorn maggot flies?

Natural selection based on their respective foods.

What is a difference in the seasonal activity of apple and hawthorn maggot flies?

They have different seasonal activity periods.

Where do apple and hawthorn maggot flies live?

They live in sympatry on adjacent trees.

Why did apple and hawthorn maggot flies naturally diverge?

Apples are not native to the US and were introduced 200 years ago.

What did a mark recapture molecular study reveal about apple and hawthorn maggot flies?

They do form distinct populations.

How did hawthorne and maggot flies diverge?

There are other modes of speciation besides allopatric, through assortative mating there is only 6% gene flow among populations, separated in time by pupating at different times of year, they maintain distinct populations even with gene flow because of strong natural selection

What causes divergence?

-Mutation (slow process)

-Drift (small populations)

-Inbreeding ( genotype freq change but not allele frequency unless coupled with drift or selection)

-Selection ( especially with selective pressure differences between populations)

-NOT migration ( gene flow causes populations to converge no diverge)

Speciation mechanisms

-Sympatric Speciation (popul diverge in same area)

-Parapatric Speciation (popul diverge over enviro gradient)

-Peripatric Speciation ( peripheral section of popul breaks off to form distinct popul)

What form of speciation is likely dominant?

Allopatric speciation is most dominant based on current data but many studies have shown other modes are common as well

How do changes in chromosome number lead to isolation?

Polyploidization can create barriers to gene flow that are not geographic, more common in plants much less common in animals but occasionally important in barrier creation, sometimes smaller scale chromosomal changes can cause speciation as well

Are changes in chromosome number important to plants?

Yes, very important mechanism of isolation in plants, 43-70% of extant plant species may have arisen by polyploidization

How do barriers lead to divergence?

Genetic drift and natural selection a t on mutations to cause divergence in isolated populations after barrier forms

Why is drift most important in small populations?

Most species are formed from few individuals, drift may rapidly cause divergence because of bottlenecking, over time populations become different and begin to speciate

How does sexual selection cause divergence?

If females begin to prefer different traits than other females population may diverge, the weight of evidence is that sexual selection is a major source of pre-zygotic isolation, leads to assortative mating

What are example of sexual selection causing divergence?

- Hawaiian Drosophila have elaborate courtship behaviors and secondary sexual characteristics

- Males meet in leks and fight to attract females, males have bizarre head shapes to help them win fights

Two species of Drosophila that diverged

-D. heteroneura have hammerheads and butt heads during fights

-D.silvestris have headsjust like females and grapple during fights

What was the evolutionary scenario in the diverged drosophila?

Ancestor of both species had normal heads, mutation occured to cause head butting behavior, mutants won more contests, mutation increased to fixation in a subpopulations and additional mutations caused change in head shape for better fighting (natural selection)

What is secondary contact?

When populations come back into contact after steps one and two they may still interbreed, need isolating mechanisms to ensure they don't interbreed (hybridization = secondary contact)

What are reproductive isolating mechanisms?

Prezygotic isolating mechanisms prevent fertilization such as, mate choice, time of breeding, genetic incompatibility, and geographic or ecological mechanisms

In what species is hybridization common?

Plants and birds

What did Theodosius Dobzhansky formulate?

The reinforcement hypothesis about the third stage of speciation, if populations sufficiently diverged in allopatry hybrids should have reduce fitness, should be strong selection for assortative mating

What is reinforcement of speciation?

Selection that reduces frequency of hybrids and completes reproductive isolation

What is the reinforcement hypothesis?

Predicts that pre mating isolation will evolve in species in secondary contact, prezygotic isolating mechanisms prevent fertilization ( mate choice, breeding time, genetic incompatibility), post zygotic isolation mechanism also prevent hybridization ( offspring sterile (mules) or not viable, reinforcement is not necessary in these cases

In what mode of speciation is prezygotic isolation common?

According to a study by Coyne and Orr prezygotic isolation occurs more often in sympatric species than allopatric

The nature of hybridization

Hybrids should have reduced fitness by reinforcement hypothesis, some have normal or increased fitness

What is Sorghum?

Sorghum is an important crop.

What is Johnsongrass?

Johnsongrass is a weed closely related to Sorghum.

Why were agriculturists concerned about Sorghum and Johnsongrass?

They were worried that genetically engineered herbicide resistance would be transferred from Sorghum to Johnsongrass due to hybridization.

What significant event occurred between Sorghum and Johnsongrass?

Significant gene flow occurred, leading to herbicide resistance introgressing into Johnsongrass.

How were sunflowers formed?

Biologists attempted to recreate hybridization event that lead to formation of sunflowers, certain crosses had higher fitness and others had lower fitness, high fitness hybrids became new species

Where do hybrid zone occur?

Where recently diverged populations overlap, may occur by secondary contact or during parapatric ( or peripatric) speciation, often present where hybrids have equal fitness to parental species, size of zone depends on fitness of hybrids

What are hybrid zones?

regions in which members of different species meet and mate, producing at least some hybrid offspring

What type of sagebrush is found at low elevations in the western US?

Basin sagebrush

What type of sagebrush is found at high elevations in the western US?

Mountain sagebrush

Where do basin sagebrush and mountain sagebrush make contact and hybridize?

At middle elevations

Who compared several fitness measures of hybrids and parentals in sagebrush?

Graham

What did Graham find about the fitness of hybrids in transitional habitats?

Hybrids have superior fitness

Can new species be formed through hybridization?

Yes, new species may be formed

Example of hybrid formed species (Audubon Warbler)

Warbler was thought to have a hybrid origin, another similar geographically proximate warbler species was sampled for nuclear and mtDNA, found to have mtDNA of one species and nuclear DNA of another (other additional species contributed as well)

What is the fate of hybrid zones?

1) May persist indefinitely with selection maintaining steep clines at some loci ( if zone is a tension zone it may move around)

2)Selection may favor alleles that enhance prezygotic isolation, resulting ultimately in full reproductive isolations (concept of reinforcement)

3) Alleles that improve the fitness of hybrids may increase in frequency. In the extreme, the postzygotic barrier to gene exchange would break down, and two populations would become one species

4) In at least part of the hybrid zone the hybrids themselves could evolve reproductive isolation frm the parent forms and become a third species

What changes in the genome are necessary for speciation?

They would need to not be sterile, F1 hybrids are sterile ( postzygotic isolation), if one sex is sterile it is usually the heterogametic sex ( human males have Y bird females have W)

What is haldane's rule?

Pattern of sterility is in heterogametic sex, regardless of which is male or female

Why does Haldane's rule work?

-Consider an autosomal locus A and an X-linked locus B in a mammal species

-Individuals from one species are fixed for A1 and B1

-Sister species fixed for A2 and B2

-A2 and B1 interact to cause inviability (but B2 is dominant to B1)

-If females from first species mate with second: A1A2B1 males and A1A2B1B2 females

-Males are inviable, females are viable