Evo Bio Test 4

Mechanisms of Divergence

- After a barrier has been formed, genetic drift and natural selection act on populations to cause divergence in isolated populations. - Drift is most important in small populations. - Most species are formed from few individuals. - Drift may rapidly cause divergence because of small population size. - Over time, populations become different and begin to 'speciate'.

Sorghum is an important crop and Johnsongrass is a closely related weed

- Agriculturalists worried that if they genetically engineered herbicide resistance into sorghum it might be transferred to johnsongrass because of hybridization. - Significant gene flow occurred and herbicide resistance introgressed into johnsongrass, making a 'superweed'. - Hybrids have increased fitness.

Homo floresiensis probably descended from what?

full-sized Homo erectus that colonized Flores as much as 900,000 years ago. These early humans dodged komodo dragons and storks (5.9ft tall) and hunted the giant rats and dwarf elephants. - Individuals from this population became, on average, smaller each generation, consistent with the well-known phenomenon of endemic dwarfing in small, inbred island populations with limited resources, and evolved into new species - Homo floresiensis. - Homo floresiensis represents an evolutionary reversal in hominin brain and body size in an insular environment. - Even more pronounced dwarfism in newly discovered 700,000 year old Flores fossils.

Prevention of gamete fusion

gametes of one species function poorly with the gametes of another species or within the reproductive tract of another species.

Ancestral polymorphisms

genetic variants that arose prior to the speciation event that generated the species in which they are currently found.

Robust Australopithecines

had enormous teeth and very strong jaw muscles for powerful chewing (eating grasses and chewing on bark?). Ex: Paranthropus

H. ergaster

more recent ancestor from Africa (H. erectus?)

Introgression

the integration of genetic material from one species into another through repeated backcrossing.

Species concept

theoretical definition of what constitutes a species. - The difficulty in defining species points to the grey area that exists between species (i.e. hybrids, variants, races, subspecies). - This may be among the best evidence that each species was not independently created.

The Phylogenetic Species Concept advocates a very high number of species

this species concept advocates a very high number of species because any clade that is monophyletic could be considered to be a species. - Could GREATLY increase the number of recognized species. - Complicated because phylogenetic trees can be constructed using just about any trait (morphology, genetic, physiology, etc) and different traits will often result in different tree topologies! - Difficult to implement because most taxa do not have phylogenies.

H. heidelbergenesis

thought to be a descendent of H. erectus.

A way to test the issue of gene versus species trees: Ruvolo (1995, 1997)

use as many genes as possible and see which ones are congruent. In 14 independent datasets: - 11 showed humans and chimps together. - 2 showed gorillas and chimps. - 1 showed humans and gorillas. * Human/chimp relationship is most likely.

H. erectus

was first to leave Africa.

Does mutation cause divergence?

yes, but this is an incredibly slow process on its own.

Does drift cause divergence?

yes, especially when populations are small.

The beginning of Language

- Brain size increased steadily in Homo. - Enabled early humans to speak. - Costly because large brains require a ton of energy to maintain. - Endocasts reveal similar brain structure in extinct human species (Homo) compared to modern humans (Including speech centers). - Language may be 2 My old.

An example of hybrid speciation

- If hybrids have high fitness and occupy a different habitat than either parental species they can form a new species. - Biologists attempted to recreate the hybridization event that lead to the formation of desert dune sunflowers, Helianthus anomalus. - Certain crosses of the progenitors had higher fitness than either progenitor in the desert dune environment. - Hybrids with similar genetic admixture likely gave rise to the actual species in nature.

Dobzhansky formulated the reinforcement hypothesis about the third stage of speciation

- If populations sufficiently diverged in allopatry, hybrids should have reduced fitness. - Should be strong selection for assortative mating, and any mechanisms that promote it (prezygotic mechanism). - Selection that reduces frequency of hybrids is Reinforcement. Completes reproductive isolation.

Sarich and Wilson (1967) Phylogeny based on similarity of serum albumin proteins

- Injected rabbits with the protein from humans, gave them time to make antibodies. - Then mixed the rabbit serum (with antibodies to the human protein) and mixed with the serum of primates. - Strength of the immune reaction is measure of similarity of proteins.

Until a major volcanic eruption around 12,000 years ago, Flores was a textbook example of what?

- Island gigantism (absence of large carnivores and preditors). Komodo dragons, even larger giant lizards, giant tortoises, rats, and storks. - Dwarfism (limited resources and space and competition). Tiny elephants. And as we now know, tiny humans.

How can you objectively tell the difference between one species and other?

Must apply to all species: - Sexual and asexual species. - Prokaryotic and eukaryotic.

It is very important to remember that the same forces that cause microevolution also cause macroevolution

Mutation, natural selection, genetic drift, migration. The mechanisms are identical, but the timescale is longer.

Does migration cause divergence?

NO. Gene flow causes populations to converge, not diverge. It will negate other differentiating forces (drift and selection). Populations must be isolated to evolve as independent units.

T or F: Based on genomic data, we infer that humans interbred with Neanderthals and Denisovans because patterns of SNP excess indicate gene flow into non-African human lineages.

True

T or F: Human populations typically show as much genetic diversity within a population as among populations from different regions.

True

T or F: Humans share a more recent common ancestor with Homo ergaster than with Australopithecus anamensis.

True

Discussion: What is the "thrifty gene hypothesis" and to which human disease do the author's apply it?

Type 2 diabetes- due to common variants that were adaptive for the efficient conversion of food into energy in the past when resources were scarce, but is maladaptive in modern environments.

How does peripatric speciation differ from Allopatry?

Typically, 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 very powerful in the founder population).

Discussion: What is the proposed reason for skin color differences in high vs low latitude human populations?

Ultraviolet radiation- selection pressure for darker skin at low latitudes for UVR protection, and lighter skin at higher latitudes to maintain vitamin D photosynthesis.

Parapatric Speciation

A peripheral portion of population moves into a new niche and diversifies due to different environmental conditions.

Peripatric Speciation

A small portion of population breaks off to form a distinct population (special case of allopatry). - Genetic change can be very rapid in localized populations founded by few individuals cut off from gene flow. - Differences in selection driven by environment can play a role.

Sexual Selection of divergence

A type of natural selection where one sex chooses mates and/or the other sex competes for access. - If some females begin to prefer different traits than other females, populations may diverge. - The weight of evidence is that sexual selection (choosiness) is a major source of prezygotic isolation! - Leads to assortative mating.

Allopatric speciation is a mechanism that produces new species when: A. A physical barrier emerges separating the populations. B. Two species have a contact zone where hybrids can be produced. C. Ecological changes in the environment drive population to diverge. D. Few individuals colonize a new area.

A. A physical barrier emerges separating the populations.

Which characters are used most often in building phylogenies of Homo sapiens and its recent ancestors? A. Characters of skulls and teeth B. Characters of skulls and feet C. Characters of thumbs and throat D. Characters of legs and feet

A. Characters of skulls and teeth

What do you call populations that are similar in morphology but are strongly divergent in traits like songs, temperature or drought tolerance? A. Cryptic species B. Secret species C. Sister species D. Clandestine species

A. Cryptic species

What is the most obvious genetic difference between humans and the African great apes? A. Gorillas and chimps have 24 chromosomes, whereas humans have 23 chromosomes B. percent sequence divergence of introns C. percent sequence divergence of exons D. humans lack functional copies of some genes found in chimpanzees and gorillas

A. Gorillas and chimps have 24 chromosomes, whereas humans have 23 chromosomes

Although many different definitions of "species" exist, the essence of speciation is established as A. Lack of gene flow B. Differential mutation C. Natural selection D. Populations of different size

A. Lack of gene flow

Under the biological species concept, what is the criterion for identifying evolutionary independence? A. Reproductive isolation B. Monophyletic groups C. Morphological differentiation D. Separate evolutionary trajectory

A. Reproductive isolation

Snapping shrimp were separated when the isthmus of Panama formed. Now there are closely related species on either side of the isthmus. What are these pairs called? A. Sister species B. Cryptic species C. Secret species D. Daughter species

A. Sister species

What is it called when an existing species range is split by a new physical barrier? A. Vicariance B. Secondary contact C. Primary speciation D. Secondary speciation

A. Vicariance

Does selection cause divergence?

Yes, especially if environment (selective pressure) differs between two populations.

Under the morphospecies species concept, what is the criterion for identifying evolutionary independence? A. Reproductive isolation B. Monophyletic groups C. Morphological differentiation D. Separate evolutionary trajectory

C. Morphological differentiation

The precise relationship between humans and other Great Apes has been uncertain, but considering the sum of the data humans most likely share a most recent common ancestor with which of the following genera? A. Gorilla B. Pongo C. Pan D. Hylobates

C. Pan

Does inbreeding cause divergence?

Changes genotype frequencies, but not allele frequencies unless coupled with drift or selection.

What is a monophyletic group?

Contains all the known descendants and their single common ancestor. - If a group is not monophyletic, it is not a recognized taxon.

Which statement is TRUE? A. All paleontologists agree that Sahelanthropus tchandensis is the earliest known relative of modern humans. B. All paleontologists agree that Ardipithecus ramidus is the earliest known relative of modern humans. C. Although paleontologists disagree in which fossil came first, they agree on the names given to all known fossil hominins. D. Paleoanthropologists disagree about the most appropriate names for some of the specimens of early hominins.

D. Paleoanthropologists disagree about the most appropriate names for some of the specimens of early hominins.

What is it called when two divergent populations come back into contact after they have speciated? A. Ecological speciation B. Sympatric speciation C. Genetic isolation D. Secondary contact

D. Secondary contact

Evolution of uniquely human traits

Oldest known tools from Gona, Ethiopia 2.5- 2.6 My old. - Who made these tools? - Oldest Homo fossil is 2.3 My old (Homo habilis). - Maybe it was a robust australopithecine (genus Paranthropus). They had three hand muscles that chimpanzees lack (similar to members of Australopithecus). - Similar to modern humans. - Increased grasping ability. - Based on fossil thumb bones both H. erectus and P. robustus were toolmakers.

Origin of Homo sapiens

Paleontologists disagree about the taxonomic status of H. ergaster (Same species as H. erectus or different?) - New H. erectus fossil recently found in Georgia from ~1.75 Mya - There is also disagreement about status of H. neanderthalensis (Species or subspecies of H. sapiens?) - H. antecessor may be ancestor of H. sapiens and H. neanderthalensis.

What does parapatric speciation require?

Parapatric Speciation requires divergent selection to be GREATER than the affect of gene flow over time. - Generally a hybrid zone may be formed. - The degree of selection and gene flow depend on the size of the character clines.

Allopatric Speciation

Populations are separated by a physical barrier and diverge. Considered the most common mode of speciation.

Sympatric Speciation

Populations diverge in the same area. - May inhabit different microhabitats, niches, times. - Or polyploidization/genetic changes!

Multiregional Evolution model

H. sapiens evolved concurrently in Europe, Africa, and Asia with sufficient gene flow to maintain their continuity as a single species. Current gene pools are a mixture of all of these regional variants.

African Replacement (Out of Africa) model

H. sapiens evolved in Africa, migrated to Europe and Asia, and replaced H. erectus and H. neanderthalensis without much interbreeding.

Reinforcement Hypothesis

Predicts that prezygotic (or premating) isolation will evolve in species in secondary contact. - NOT Postzygotic isolation where offspring are sterile or not viable. This is wasted energy and lower fitness of those who do cross-mate. Means selection will promote premating/prezygotic isolation.

Hybridization

Hybrids often have reduced fitness but some have normal or increased fitness. Ex: Sorghum and Johnsongrass.

Secondary Contact

If populations/species come back into contact after steps 1 and 2 (Secondary Contact), they will have the opportunity to interbreed and they may converge again. - Hybridization is common in plants and birds.

Larynx in mature humans

In mature humans, larynx is lower to allow greater diversity of sounds (Increases choking risk). - Some archaeologists argue that language cannot be proven without abstract symbols. - Cave paintings in Europe are 32,000 years old. - But humans built boats and traveled to Australia by 40,000 ya. Must they have had language to accomplish this? Are H. sapiens the only species to use language?

Is peripatric speciation a special case of allopatric speciation via dispersal?

In some cases yes. But doesn't have to be via dispersal away from parent population per se.

Modern human structure is most similar to which body form of Australopithecines?

Modern human structure more similar to Gracile australopithecines.

Australopithecines

Several genera including: Australopithecus, Paranthropus, Ardipithecus, Kenyathropus. - Two main body forms: Robust and Gracile. - Bipedal like modern humans: known from limb structure and fossil footprints. - Australopithecines were short with small brain cases.

Characteristics of H. Floresiensis

Skeletal remains show that these hominins were: - only one meter tall (~3.5 ft). - had a brain one-third the size of that of modern humans. - lived on an isolated island long after Homo sapiens had migrated through the South Pacific region.

How does speciation occur?

Speciation may be viewed as a three-step process: 1) Population isolation- physical/genetic/behavioral/ecological 2) Divergence of phenotypes and genotypes 3) Reproductive isolation

Behavioral isolation

Species differ in their mating rituals.

Temporal isolation

Species reproduce in different seasons or at different times of the day.

How do we determine the relatedness of human species?

Strait constructed a hypothesis of descendent relationships. - Based mostly on teeth and skull morphology. - Cladogram has no information about time while the hypothesized phylogeny makes best guesses.

Mechanical isolation

Structural differences between species prevent mating.

Ardipithecus ramidus

The best specimen, 'Ardi' is a 4.4 million year old female, found in modern-day Ethiopia in 1994 (published fully in 2009). - Ardi was up to 4 ft tall and likely weighed around 110 lbs. She had a brain slightly smaller than a modern chimpanzee, approximately 20% the size of the modern human brain.

Important note about the mechanisms of speciation

The observation that two sister species currently have overlapping (parapatric or sympatric) ranges does not mean that they speciated parapatrically or sympatrically! - this may instead be allopatry followed by secondary contact of ranges that were previously distinct. Most speciation is older than forces that dramatically reshape ranges.

Ingman (2000) tested models with mtDNA genome

- 53 people produced phylogeny which lead back to a single mitochondrial haplotype. - All non-African haplotypes were nested within African haplotypes. - mtDNA analysis supports the African Replacement model. However, because mtDNA is effectively all a single gene (and maternally inherited), we need independent estimates of the age of the split.

Tishkoff examined nuclear allelic variation on Chromosome 12

- African populations had highest allelic diversity. - Dated split to be 102,000- 450,000 ya. - Consistent with African Replacement.

Paleoanthropologist are divided about taxonomic status of Homo ergaster and H. erectus

- All hominins before H. ergaster/erectus were confined to Africa with the oldest examples appearing (nearly simultaneously) in Africa, Europe, China and Java about 1.6-1.9 mya. - There is general agreement that modern humans are the descendants of some or all of these populations. - However the how and where of the transition is still a matter of debate.

What is a species?

- All species trace their ancestry back to the first life on earth 3.8 billion years ago. - Conservative estimates propose 9 million species living today. - Some estimates go as high as 100 million - 99% of all species that have ever existed have gone extinct! This means that speciation has happened billions of times throughout the history of life!

Speciation Mechanisms

- Allopatric (physical isolation)/ Peripatric speciation is likely the dominant mechanism in animals based on current data. - But isolation and genetic drift alone would take a loooooong time. - Many studies have shown if populations of the same species are found in different environments they will diverge phenotypically and genetically. Doesn't require a complete cessation of gene flow

Morphological Species Concept

- Also called Morphospecies or Typological Species Concept - Oldest species concept - Morphological similarity determines species membership. - Widely applicable - Can be used for individuals that are living or extinct - Although not formally stated, this is the species concept that non-scientists have been using since the beginning of time. - Probably still the most widely used species concept by taxonomists because it is based on a practical species criterion.

Recent Human Ancestry

- Anatomically modern Homo sapiens appeared by 100,000 years ago. - Prior to the emergence of Homo sapiens, as many as five human species existed simultaneously (that we know of), some even in the same regions. - While the phylogeny is not yet known with certainty we do know evolutionary history of humans has been complicated (not linear). - We are the sole surviving species of a much larger human radiation!

A 60,000 year old Neanderthal skeleton from Israel had an intact hyoid bone

- Anchors throat muscles in larynx allowing for a wider range of tongue, pharyngeal and laryngeal movements. - Neanderthal hyoid is nearly identical to modern humans and completely different from chimpanzees.

Water snakes of Lake Erie: Why is the unbanded allele not fixed on islands?

- Banded snakes migrate from mainland to island bringing banded allele to island gene pool. - If migration stopped (or selection was strong enough) these two populations would diverge under selection, mutation, and drift to form two species.

Study on sagebrush in western US

- Basin sagebrush found at low elevations. - Mountain sagebrush found at high elevations. - Species make contact at middle elevations and hybridize. * Study compared several fitness measures of hybrids and parentals and found that hybrids have superior fitness in transitional habitats.

Which hominid first used language?

- Because language is a behavior, it cannot fossilize. - Language is a complex adaptation involving neural circuitry in the brain. - Appears to be an innate trait in humans. - Also involves physical adaptations of throat. - In apes and human babies, larynx (voice box) is high in the throat and seals with nasal cavity to prevent choking.

What is the evolutionary scenario as to why these 2 species of drosophila are so different?

- Common ancestor of both species had normal heads. - Mutation in subpopulation caused head-butting behavior. - Mutants won more contests and mutation increased to fixation. - Flies with largest heads won more fights and/or additional mutations led to changes in head shape.

Relationship between humans and African apes: Criticisms

- Criticism of these reconstructions because they are gene trees and not species trees. - If ancestor was genetically variable for a particular locus, the gene tree may not accurately represent the species tree. - Ancestor may pass on a subset of its alleles for a gene to each descendent species. - This is called Incomplete Lineage Sorting (ILS) - That gene will not give an accurate estimate of species phylogeny.

Problems with the morphological species concept

- Cut offs can be arbitrary or idiosyncratic - Sexual dimorphism - Intraspecific variation = polymorphism - Cryptic species

General Lineage Species Concept

- De Queiroz (1998) determined that all modern species definitions either explicitly or implicitly equate species to distinct evolutionary lineages. - He formulated the 'General lineage species concept' which states that a species is a lineage, held together by some force, evolving through time, and notes that all the existing species concepts already recognize this. - Basically, speciation is a process, and the 'criteria' used in species concepts accrue over time as species diverge from one another.

Bowcock examined 30 nuclear loci from 14 human populations

- Deepest split is African and non-African populations. - Split occurred 75,000- 287,000 ya. - Dates consistent with the African Replacement model.

What is parapatric speciation?

- Evolution of reproductive isolation between populations that are continuously distributed in space, so that there is movement of individuals and genes between them. - No extrinsic barrier to gene flow. - Individuals are more likely to mate with someone nearby. = reduced gene flow + selection in a new niche

Other species concepts

- Evolutionary Populations- which maintain their identity in space and time and which has its own evolutionary tendencies and historical fate. - Ecological- defined by species occupying different niches and thus evolving separately. - Recognition- Specific Mate Recognition System (SMRS). - Cohesion- focuses on mechanisms that maintain genetic, demographic, and phenotypic identity.

The beginning of the Homo lineage

- First Homo was Homo habilis = first species of human! - Homo is latin for "human being" - Homo had larger brain cases and smaller teeth than Australopithecus. - Homo was also taller with less sexual dimorphism in size. - H. habilis is associated with stone tools and bones bearing the marks of butchery.

Why is polyploidization different from other forms of speciation?

It skips steps 1 and 2. Goes directly to reproductive isolation.

Conclusions about the origin of Homo Sapiens

- Gene flow occurred between Neanderthals and humans in secondary contact after speciation. - Non African genome is 2.5% Neanderthal! (and Melanesians are 4.8% Denisovan!). - Despite slightly different evolutionary histories all humans are extremely closely related.

Relationship between humans and African apes

- Gorillas and chimpanzees share knuckle walking, which humans lack. - Humans and both chimpanzee species share features of skull, teeth, limbs, and delayed sexual maturity. - Some molecular analyses place humans and gorillas as sister taxa. Conflicting evidence makes determining the correct branching order difficult.

Hybridization and Assimilation model

- H. sapiens evolved in Africa and migrated to Europe and Asia with some amount of hybridization. - Some non H. sapien genes were assimilated and persist into modern humans.

Founder hypothesis of speciation

- Hawaiian Islands vary in age. - Hawaii Island is youngest. - Founder flies inhabited western island and emigrated eastward as other islands arose.

Recent Ancestry of Humans

- Humans and chimps last shared a common ancestor about 5-7 Mya. - Based on fossils, the most recent common ancestor walked on its knuckles, ate mostly fruit, lived in many habitats, had complex social groups, and made tools.

Hybrid zones

- Hybrid zones often occur where recently diverged populations overlap. - Usually occur by secondary contact following divergence in allopatry. - Hybrid zones are often present where hybrids have equal fitness to parental species.

Possible outcomes of secondary contact

- Hybrids thrive and backcross with parental species and eventually erase divergence between them. - Hybrids have distinct characteristics and they form a population on their own..maybe eventually a new species. - Hybrids have reduced fitness should lead to assortative mating and evolution of reproductive isolating mechanisms.

Lieberman and Waddle predict cladograms based on African Replacement and Multiregional Evolution models

- Predicts the relationships of modern humans. - Can be tested with molecular evidence.

Apple and hawthorn maggot flies

- Live in sympatry on adjacent trees. - Common ancestor laid their eggs only on hawthorns. - Apples were introduced to the U.S. 200 years ago and some flies began to use the new habitat. - Females generally choose to lay their eggs on the type of fruit they grew up on, and males tend to look for mates on the type of fruit they grew up on. - A mark-recapture and molecular study revealed they do form distinct populations.

Hawaiian Drosophila have elaborate courtship behaviors and secondary sexual characteristics

- Males meet in leks (aggregation of males that engage in competitive displays) and fight to attract females. - Males have modified head shapes to help them win fights. - D. heteroneura have hammerheads and butt heads during fights. -D. silvestris have heads just like females and grapple during fights.

Speciation along an ecological gradient: Sweet vernal grass in Eurasia (Anthoxanthum odoratum)

- Metal pollution occurs in a gradient away from mines. - Plants display variation in metal tolerance. - Strong selection has created divergence for metal tolerance. - Hybrids are less fit in either environment. - Selection has also pushed flowering times apart (reducing hybridization).

Physical Isolation

- Migration tends to homogenize genotypes. - When gene flow is stopped, speciation, given enough time, will (book says "may") occur. - Migration-selection balance of water snake in Lake Erie ( Banding pattern due to natural selection).

Snail example of genetic isolation

- Single gene mutation causes left- to right-handed shift and genitals no longer align. - Instant reproductive isolation (skip steps 1&2)!

Genetic Mechanisms of Isolation

- Polyploidization creates barriers to gene flow that are not related to geography! - VERY important mechanism in plants, causes instant reproductive isolation. - 40-70% of extant plant species may have arisen by polyploidization. - Much less common in animals. - Sometimes smaller scale chromosomal changes or mutations that dramatically change phenotypes can cause 'instant' speciation as well. - Can cause differences in flowering times, which leads to the isolation.

Allopatric Speciation model of Ernst Mayr

- Populations are separated by a physical barrier and diverge. - Considered the most common speciation mechanism (we have the most evidence for this model). - Can happen through Vicariance or dispersal.

Test founder hypothesis with geology of Hawaiian Islands and phylogeny of flies

- Predictions of founder hypothesis- Closely related species will occupy adjacent islands. Branching sequence should partially coincide with island emergence sequence. - Used mtDNA to estimate phylogeny. - Founder hypothesis was supported. - older taxa found on older islands and sister taxa found on same or adjacent islands.

Researchers can (mostly) test models using only modern human sequences

- Problem: the prediction is the same for African Replacement and Multiregional Evolution. - The only difference is the time scale. - Out of Africa model says regional differentiation began 200,000 ya. - Multiregional Evolution model says it began 1.8 Mya.

Phylogenetic Species Concept

- Proposed by Joel Cracraft in 1983 - Under this definition the criterion for identifying evolutionary independence is monophyly. - Monophyletic groups contain all known descendants and their single common ancestor. - Species are identified by estimating the phylogeny of closely related populations and finding the smallest monophyletic group.

3 different possible pathways for hybrids

- Selection against hybrids and reinforcement leads to prezygotic isolation (reinforcement or genetic incompatibility). - Hybrids are fit and occupy a separate niche leads to hybrid speciation (novel habitat not occupied by parent species). - Selection favors hybrids in transitional habitats and leads to stable hybrid zone.

Relationship between humans and living apes

- The details of taxonomic classifications are not universally accepted. - It is accepted that humans belong to the same clade as African great apes (chimp, gorilla, bonobo) - Synapomorphies (shared derived characters) like: Elongated skulls, enlarged brow ridges, shortened canines, wrist bone fusion, reduced hair, etc. - Our closest relatives are Gorilla gorilla, Pan troglodytes, and Pan paniscus. Gorilla, standard chimpanzee, bonobo (+ humans = African great apes).

2004 - Homo floresiensis (the Hobbit)

- The discovery of H. floresiensis in a cave on the island of Flores in Indonesia (100k to 60k ya). - Implies that we coexisted with another species until much more recently than had been thought.

Humans have many traits that distinguish us from other primates

- walk bipedally - very large brains - use complex tools - Communicate via complex language - Complex culture Chimpanzees and other animals make and use tools and have culture, but only humans make and use complex tools and have complex cultures.

New piece in the human evolution puzzle: H. Floresiensis

- The species' diminutive stature indicates that humans are subject to the same evolutionary forces that made other mammals shrink to dwarf size when in genetic isolation and under ecological pressure, such as on an island with limited resources. - Homo erectus was found to have lived on the nearby island of Java as long as 1.6 million years ago; the team suggests that the Flores hominins may be their descendants. - These astonishing tiny humans made tools, lived in caves, hunted tiny elephants and lived at the same time as modern humans who were colonizing the area.

What is sympatric speciation?

- There is complete panmictic mating and then a reproductive isolating mechanism evolves within the population - Has been shown to occur via: Polyploidization and 'Instant' mutation - E.g. shell handedness, flowering time Can also occur via: Disruptive (diversifying) selection with assortative mating. But this mechanism is highly controversial and many believe they don't happen.

How have Apple and hawthorn maggot flies diverged without initial isolation? Did they skip Step 1?

- 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 to match host fruiting period and ability to thrive on different diets.

Characteristics of Ardi

- Unlike the common ancestor of humans and chimps, Ardi was a facultative biped (her foot bones indicate that she walked erect on the ground but used all four limbs when climbing trees). Her teeth suggest omnivory and frugivory, with a more generalist diet than modern non-human apes. - She had smaller canine teeth than chimps, equal in size between males and females. Based on patterns in other primates, this suggests reduced male-male conflict, pair-bonding, and increased parental investment. This suggests fundamental social and behavioral changes in early Hominids long before major increases in brain size.

Audubon's warbler example of hybrid speciation

- thought to have a hybrid origin. - Other similar, geographically proximate warbler species were sampled for nuclear and mitochondrial DNA. - Found to have mtDNA of one species and nuclear DNA of another. Other additional species contributed as well.

Postzygotic isolation

Offspring are sterile or have lower viability. Ex: male donkey and a female horse.

Fate of hybrid zones

1) The postzygotic barriers to gene exchange might break down and two species merge. 2) Hybrid zones may be stable. 3) Selection may favor alleles that enhance prezygotic isolation, resulting ultimately in full reproductive isolation (Reinforcement). 4) In at least part of the hybrid zone, the hybrids themselves could evolve reproductive isolation from the parent forms, and become a 3rd species (Hybrid speciation).

Paabo recovered genome sequences of three H. neanderthalensis bones

Also a Denisovan specimen (An undescribed human species from Denisova Cave in Siberia from 30,000-40,000 ya). - Also sequenced 663 modern humans, 7 chimpanzees, and 2 bonobos. - Found that modern humans from Europe, Africa, Asia, the Americas, Australia, and Oceania are more closely related to each other than any is to H. neanderthalensis or the Denisovan fossil. - Estimated divergence of Neanderthal to modern humans to be 317,000-741,000 ya. - Supports African Replacement model.

Incomplete Lineage Sorting (ILS)

Ancestor may pass on a subset of its alleles for a gene to each descendent species. May not give an accurate representation of species phylogeny.

Discussion: Based on the introduction which of the "Origins of Homo sapiens" models is supported by the authors?

Assimilation and hybridization because they diverge from Africa, but continue to interbreed.

Sahelanthropus tchadensis

At 6-7 million years old, this is the earliest known hominid. This is the older end of the scale based on molecular evidence! Discovered in Chad in Central Africa, this skull comes from a crucial yet little-known interval when the human lineage was becoming distinct from that of chimpanzees. Has a combination of primitive and advanced features. Perhaps a transitional fossil between great apes and hominids.

Under the phylogenetic species concept, what is the criterion for identifying evolutionary independence? A. Reproductive isolation B. Monophyletic groups C. Morphological differentiation D. Separate evolutionary trajectory

B. Monophyletic groups

When two closely related species come back into contact after separation, what is it called if selection reduces the frequency of hybrids because selection favors individuals that choose to mate with individuals of the same species? A. Postzygotic isolation B. Reinforcement C. Ecological speciation D. Bottlenecking

B. Reinforcement

You mate a male donkey and a female horse and obtain a mule. Mules are hard working, but usually sterile. What mechanisms separate gene flow between donkeys and horses? A. prezygotic mechanisms B. postzygotic mechanisms C. isozygotic mechanisms D. none, they will eventually become a single species

B. postzygotic mechanisms

Biological Species Concept

Ernst Mayr proposed the first formal species concept in 1942 - Under this definition the criterion for identifying evolutionary independence is reproductive isolation. - Specifically if populations do not hybridize or when they do their offspring are infertile they are separate species. - Inherently does not apply to unisexual or uniparental species. - Requires subjective judgments about whether species would interbreed if they had the chance. - The BSC is the species concept employed by the Endangered Species Act. Although seemingly simple, it is operationally difficult to apply.

T or F: Language most likely arose after the evolution of modern humans (Homo sapiens).

False

T or F: The wealth of evidence of early hominin evolution is that the pattern of evolution leading from our last common ancestor to present day Homo sapiens has been a steady transformation of a single lineage.

False

Two hypotheses about the pattern of human evolution

Fossils provide the only data for distinguishing the stead transformation (progressive evolution of a single lineage) versus evolutionary radiation (many lineages) hypotheses.

Reich did a SNP study with the same data but a model that could detect gene flow

Found that modern humans contained SNPs from Neanderthals and Denisovans! - Supports Hybridization and Assimilation model ("Leaky replacement").

Discussion: To what habitat is the "pygmy" stature an adaptation? What environmental factors are thought to drive this trait?

In the tropical rainforest. Factors were limited resources, resistance to heat stress, immune response, and a trade-off between early onset of reproduction and cessation growth. H. Floresiensis was under selection pressures like limited resources, space and competition as well. Same evolutionary pressures.

Dispersal

Individuals migrate to new area and are separated from their source population. - 850 species of Hawaiian Drosophila! Exploit a vast number of niches and have highly varied body forms.

Vicariance

a new barrier to gene flow arises. - Mountain range or river. - Snapping shrimp from Pacific vs. Caribbean side of Isthmus of Panama

What do most scientists agree is a species?

a species is the smallest evolutionarily independent unit. When mutation, selection, gene flow and drift work independently on populations.

What is a genetic difference between humans and apes?

humans have one less chromosome than other apes. Fusion of two chromosomes after the split from our common ancestor with chimps.

Homo rudolfensis

lived at the same time as H. habilis and in the same area so there is debate whether they are the same species but with some minor skeletal differences.

H. neanderthalensis

may have been a sister species that died out; evidence of interbreeding with H. sapiens.

Prezygotic (or premating) isolating mechanisms

prevent fertilization, like mate choice, ecological, time of breeding, morphological or genetic incompatibility. - Prezygotic isolating mechanisms prevent fertilization. - Changes in gamete recognition proteins lead to gamete incompatibility (red and purple sea urchin). - Coyne and Orr showed that prezygotic isolation occurs more often in sympatric than allopatric species.