Biology Unit 3 Macroevolution

If gene flow stops, allele frequencies in isolated populations can…

diverge and populations began to evolve independently (speciation)

Divergence will occur as a result of:

• Mutation

• Natural Selection

• Genetic Drift

• may result in speciation

Speciation

a splitting event that creates two or more distinct species from an ancestral species

• can occur gradually or abruptly

Approaches for identifying species:

• The biological concept

• The morphological species (morphospecies) concept

• The phylogenetic species concept

According to the Biology species concept, what is the main criterion for identifying species?

reproductive isolation

What is the mating pattern in the Biological Species Concept?

Members of the population do not interbreed

OR

They will fail to produce viable, fertile offspring after mating

Biological Species Concept

reproductive isolation between populations

(they do not breed and do not produce viable, fertile offspring)

Prezygotic Isolation- Biological Species Concept

individuals of different species are prevented from mating successfully

Postzygotic Isolation- Biological Species Concept

hybrid offspring do not survive or reproduce

Disadvantages of Biological Species Concept

• reproductive isolation cannot be evaluated in fossils and asexually reproducing species

• or species that do not overlap geographically

Temporal Isolation- prezygotic isolation mechanism

• populations are isolated because they breed at different times

• Ex. Spotted skunks that mate at different times of the year

Habitat Isolation- prezygotic isolating mechanism

• populations are isolated because they breed in different habitats

• Ex. Garter snakes live in different habitats

• Ecology is important

Behavioral Isolation- prezyogtic isolation mechanism

• populations do not interbreed because their courtship display differs

• Ex. Male songbirds sing species-specific songs to attract females

Gametic Isolation- prezygotic isolating mechanism

• matings fail because eggs and sperm are incompatible

Ex. differences in the bindin protein determine whether sea urchin sperm will penetrate eggs

Mechanical Isolation- prezygotic isolating mechanism

• matings fail because male and female reproductive structures are incompatible

Hybrid Viability- Postzygotic isolating mechanism

• hybrid offspring do not develop normally and die as embryos

• Ex. when some species of

  salamanders mate their offspring

  have much lower viability

Hybrid Sterility- postzygotic isolating mechanisms

• hybrid offspring mature but are sterile as adults

• Ex. Horses and donkeys can mate and produce viable offspring, but their hybrid offspring (mules) are usually sterile

State Haldane’s Rule

“When in the F1 offspring of two different animal races one sex is absent, rare or sterile, that sex is the heterogametic sex.”

Morphospecies Concept

• individual lineages differ in size, shape, or other morphological features

• distinguishing features likely arise if populations are independent and isolated from gene flow

Disadvantages of Morphospecies Coencept

• Subjective

• misidentifies polymorphic species (species with same genes but different physical forms)

• misses cryptic species (genetically distinct species that appear similar) by not showing obvious physical difference

The Phylogenetic Species Concept

identifies species based on evolutionary history

Monophyletic group

• an ancestral population and all descendants

• species are defined as smallest monophyletic group on tree of life

• also called a clade or lineage

Synapomorphy

trait unique to a monophyletic group (“unique forms”)

Advantages of Phylogenetic Species Concept

• widely applicable and based on testable criteria

• logical because different species have different synapomorphies due to lack of gene flow and independent evolution and independent evolution

Disadvantages of Phylogenetic Species Concept

• few well-estimated phylogenies available

• may lead to recognition of many more species than either of the other species concepts

Biological species concept Advantages

Reproductive isolation=evolutionary independence

Morphospecies concept Advantages

Advantages: widely applicable and useful when there is no data on the extent of gene flow

Modes of Speciation

• Allopatric speciation

• Parapatric speciation

• Sympatric speciation

Allopatric Speciation

• genetic isolation occurs when populations become geographically separated

• populations living in different areas are in allopatry

• MOST COMMON MODE

Speciation that begins with geographic isolation is called

Allopatric Speciation

Dispersal- of allopatric speciation

• a population moves to a new habitat, colonizes it, and finds a new population

• colonization events can lead to speciation

Vicariance- of allopatric speciation

the physical splitting of a habitat

• a physical barrier like a mountain range uplifting or a river splitting the geographic range of species

Ex. researchers compared DNA sequence of trumpeters from different areas of the Amazon basin to the geological events that occurred (isolated populations due to vicariance)

Biogeography

• study of how species and populations are distributed geographically 

• can tell us how dispersal and vicariance events occur

In Parapatric Speciation, how does divergence (genetic isolation) occur?

• gradually along an environmental gradient (cline)

• latitude or elevation impose a gradient of selection pressures which changes phenotypes and genotypes as result (Ex. House mice increase in body size in the north)

Sympatry

• populations or species that live in same geographic area or close enough to interbreed

Sympatric Speciation can be initiated by two types of events:

(1) External events: Disruptive selection based on different ecological niches or mate preferences

(2) Internal events: Chromosomal mutations (polyploidy)

The apple maggot fly adapted to different host fruits within the same area. This is an example of:  

sympatric speciation via disruptive selection

• sympatric populations may be reproductively isolated by adapting to different habitats

• can cause rapid divergence, affects gene flow via prezygotic isolation

A niche

range of ecological resources that a species can use and the range of conditions it can tolerate

Conclusion for Disruptive Selection in flies

• hybrid individuals do not orient to fruit scent as well as parents leading to them having a lower reproductive success

• thus, disruptive selection has led to sympatric speciation

Sympatry

populations or species that live in the same geographic area— close enough to interbreed

Polyploidy

• a mutation that causes instant speciation

• results in more than two sets of chromosomes

Sympatric Speciation by Polyploidy -Autopolyploid

Autopolyploid (oneself)

• doubles chromosome number

• chromosomes are all from the same species (self-fertilization)

• reproductively isolated from original population

Sympatric Speciation by Polyploidy- Allopolyploid

Allopolyploid (different)

• different species mate and there are different chromosome numbers resulting in viable, nonsterile offspring

• new tetraploid species can be created when two diploids hybridize (usually offspring are sterile)

What is “instantaneous speciation”?

autopolyploids cannot interbreed with original species after a single generation of self-fertilization (instantaneous speciation)

How are Allopolyploid species created?

• new tetraploid species created when two diploid species hybridize resulting in sterile offspring

• if error occurs during mitosis creating diploid gametes in adult offspring, meiosis proceeds, gametes fuse, and create tetraploid individuals

Advantages of being polyploid

• higher levels of heterozygosity than diploids

• tolerate high levels of self-fertilization w/o inbreeding depression

• genes on duplicated chromosomes can diverge independently

If divergence and prezygotic isolation exist, then

mating between the populations is rare, populations continue to diverge

When prezygotic isolation does not exist, then

• populations may interbreed which may erase distinctions between the two populations

• can lead to reinforcement and hybrid zones

According to reinforcement, if two populations have diverged extensively and are genetically distinct, what will happen?

• the fitness of hybrid offspring will be lower than parents’ fitness

Reinforcement

• natural selection for traits that prevent interbreeding among populations (selection for prezygotic isolating mechanism)

• is shown for species that overlap geographically

Hybrid Zones

a geographic area where interbreeding between two populations occurs

can be narrow or wide, long or short lived, and stable in one place or move over time

Example of Hybrid Zones

• Townsend Warblers and Hermit Warblers hybridize extensively where their ranges overlap

• hybrid offspring have intermediate traits relative to two parental species

Data from Mitochondrial DNA in Warblers show

• most hybrids form when Townsend Warbler males’ mate with Hermit Warbler females

• Townsend males invade Hermit territories, drive off Hermit males, and mate with Hermit females

Phylogenetic Trees

• shows evolutionary relationships among genes, species, and higher taxa

• most universal tree is the tree of life which depicts evolutionary relationships among all organisms

Phylogenetic Tree Symbols and Definitions

• Branch: represents a population through time

• Node: represents a point where a branch splits— most recent ancestor

• Tip: represents endpoint of branch— either a living or extinct taxon

Phylogenetic Symbols and Definitions continued

Root: most ancestral branch

Outgroup: a taxon that diverged before taxa of interest, “roots” tree

Polytomy: node that divides into 3 or more branches

Taxa are always located where on phylogenetic trees?

• branch tips: never within tree because they are not ancestor of others

• closely related taxa depicted as sister groups that share a recent common ancestor

On a phylogenetic tree, what do nodes represent?

• speciation events

• tree can rotate around any node— resultant trees are equivalent

A Character

• A character or trait is any genetic, morphological, physiological, or behavioral characteristic that is studied

• has two possible states: present (1) or absent (0)

Outgroup

• is a sister group sharing a recent common ancestor with taxa being studied but is not part of study group

• establishes whether a trait is ancestral or derived

• can evolve, do not represent ancestors of other taxa

Ancestral trait

a character that existed in an ancestor

Derived trait

modified form of ancestral trait, found in a descendant (mutation, selection, genetic drift occurs)

Homology

similarity in organisms due to common ancestry

(trait A is homologous among the red branches)

Synapomorphy

shared, derived trait shared by 2 or more descendant taxa

(trait A occurs only in red branches)

Paraphyletic group

includes ancestral population but not all of descendants

Homoplasy

similarity not due to common ancestor and therefore require >1 evolutionary step

(trait B arose twice independently)

Polyphyletic group

does not include the most recent common ancestor, arise from convergent evolution (have independent traits)

(shown in purple)

What is the difference between traits that are Homologies vs ones that are Homoplasies?

• Homologies are traits shared among related species

• Homoplasies are traits that unrelated species shared (Ex. wings in birds and flies drawn by convergent evolution)

Parsimony

• implies the least number of evolutionary changes most accurately reflects what occurred during evolution

Characters on Trees

show branching patterns

A repeated trait that is the result of independent evolution and not shared ancestry is referred to as:

homoplasy

When evaluating DNA sequences between species, the fewest sequence differences suggests

a closer relationship (parsimonious)

Phylogeny DNA Sequence Example

summarize data by differences

• Land — Green: 1

• Green — Brown: 2

• Land — Brown: 3

• most closely related are land pants and green algae, sharing a recent common ancestor

Males are sterile in XY taxa, which are

flies and mammals

Females are sterile in ZW taxa, which are

birds and butterflies

Sympatric Speciation

speciation that occurs among populations within the same geographical area

• (1) No geographic isolation

• (2) Divergence due to selection

• (3) Genetic Isolation

When using DNA for Phylogenetics, what occurs more frequently?

1) Transitions (A-G or C-T) more frequent than Transversions (other combos)

2) Synonymous (does not change protein) are more frequent than Nonsynonymous (may change protein, subjected to natural selection)

What is the Molecular Clock?

• rate at which mutations accumulate in DNA within a species.

• used to estimate nodes on tree by using geological events

• most changes are ‘neutral’

• species that are more divergent at the sequence level share a more distant common ancestor

What is the divergence rate for the cox1 gene in a beetle?

3.5% My^-1, the DNA sequence of the cox1 gene changes by 3.5% every million years between lineages

What is the ‘S’ region and why is there so much nucleotide diversity?

It is the Spike protein; which facilitates virus entry into healthy cells

Fossil

physical evidence from an organism that lived in the past

How are fossils formed?

when part or all of an organism is buried in sediment

Intact fossils

form when decomposition does not occur and organic remains are preserved 

Compression Fossils

• form when sediments accumulate on top of the organism and become cemented into rocks

• sediments’ weight compresses organic material below into a thin, carbonaceous film

Cast Fossils

form when remains decompose after burial and dissolved minerals create a cast in remaining hole

Permineralized fossils

form when remains decompose slowly, dissolved minerals slowly infiltrate the cells’ interior and harden into stone

Trace fossils

form when sedimentation and mineralization preserve indirect evidence of an organism 

Ex. Footprints, tracks, burrows, feeding marks, and feces

How can the age of species be estimated if it is new?

its age can be estimated based on the age of nearby rock layers

Fossils form under ideal conditions, which means:

• they must be buried rapidly

• they must decompose slowly

fossilization is a rare event

Habitat bias- limitation of fossil record

• organisms living where sediment is actively being deposited (beaches and swamps) are more likely to fossilize than are organisms in other habitats

• burrowing organisms are more likely to fossilize than aboveground organisms

Taxonomic and Tissue Bias- limitation of fossil record

• some organisms with bones or shells are likely to decay slowly and leave fossil evidence

• tissues with a tougher outer coat that resists decay (pollen) fossilize more readily

Temporal Bias- limitation of fossil record

recent fossils are more common than ancient fossils

Abundant Bias- limitation of fossil record

organisms that are abundant, widespread, and present for a long time leave evidence more often than species that are rare or local

Ex. crinoids

When did Earth start to form? When did life start?

4.6bya/3.5-3.8bya

How is Earth’s history divided into?

• Precambrian

• Paleozoic era

• Mesozoic era

• Cenozoic era

Precambrian Era

• from Earth’s formation (4.6 bya) to appearance of most animal groups (543 mya)

• unicellular until late

• oxygen absent for 2 billion years, until evolution of photosynthetic bacteria

Describe the three eras of the Phanerozoic Eon?

Paleozoic- most major animal lineages and diversification of land animals (fungi and land plants), ending with removal of almost all multicellular life

(543-248 mya)

Mesozoic- begins with paleozoic extinction ends with extinction of dinosaurs, abundance of dinosaurs and gymnosperms

(252-66 mya)

Cenozoic- mammals and gymnosperms 

(66 mya-present)

The history of life on earth consisted of a series of 

Adaptative Radiations <—> Extinctions

Adaptative Radiations

the rapid production, from a single lineage, of many descendant species