Macroevolution - Lecture notes 1 PDF

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Summary

MacroevolutionAdaptive Radiation Event in which a lineage rapidly diversifies, with the newly formed lineages evolving different adaptations  Different factors may trigger adaptive radiations, but each is a response to an opportunityGeorge Gaylord (‘GG’) Simpson American palaeontologist  Perhaps...

Description

Macroevolution Adaptive Radiation  

Event in which a lineage rapidly diversifies, with the newly formed lineages evolving different adaptations Different factors may trigger adaptive radiations, but each is a response to an opportunity

George Gaylord (‘GG’) Simpson     

American palaeontologist Perhaps the most influential paleontologist of the 20th century Major participant in the modern evolutionary synthesis Dispelled the myth that the evolution of the horse was a linear process An adaptive radiation with only one extant (surviving) lineage

Evolution of Horses     

Equus Pliohippus Merychippus Miohippus Hyracotherium

Hyracotherium 

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Earliest horse-like forms: - Actually more doglike than horse-like (25-50cm at the shoulders / pads for feet / 4 distinct front toes / only 3 hind toes) Small brain case Teeth included flattened, grinding molars & pre-molars, indicating herbivory Shortness of tooth crowns indicates browsing not grazing Grasses rare in the Eocene, & became common in the Miocene

Location     

Hyracotherium was common in Northern Hemisphere in the early Eocene The 2 continents (which were joined as Laurasia) soon separated – so the descendants took different paths In N. America the phyletic line led to the horse genus Equus In Eurasia the line led to modern tapirs, rhinoceroses + other kinds of odd-toed Separate evolutionary paths – consequence of the isolation of continents resulting from continental drif

Miohippus      

By the late Oligocene in N. America Hyracotherium had evolved into Miohippus Still not quite a horse Miohippus a larger animal Big brain case / different forefeet - loss of the little digit, lateral digits reduced Several branches/lineages existed in the Miohippus part of the tree One branch gave rise to true horses in the Miocene

Merychippus

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True horse line: arose in Miocene Much larger-bodied animal: suggestive of exploitation of more open habitat Teeth (premolars + molars) modern horse-like Long / high-crowned / greatly lengthened enamel ridges / hollows between the ridges filled with cement Major change in tooth structure coincided with extensive development, around the World, of grasslands

What selective forces might have shaped horse evolution?      

Grasses - Very tough, with a high silica content Apical meristem (the shoot’s growing point) is close to ground Both are adaptations against herbivory High-crowned teeth – adaptation against wear from the silica Merychippus therefore a grazer Merychippus had a deeper skull, no doubt to accommodate greatly lengthened upper & lower teeth

The Spring Foot      

The feet were hoofed – conforms to the spring-foot of modern horses (Equus) Strong elastic ligaments bound toe bones to main leg bones Merychippus also larger-bodied The spring-foot is a more efficient mechanism/structure in larger-bodied animals, compared with pad-foot With the spring-foot, the foot is bent upon impact with the ground, then straightened when the horse takes its weight off its leg The suspensory ligament provides lif to the limb

The Descendants of Merychippus      

At least 6 separate lines developed from Merychippus (i.e. radiation) Only one led to modern horses: gave rise to Pliohippus, which had single-toed feet Pliohippus soon gave rise to Equus - Rapidly invaded Europe, Asia and Africa, as well as S. America N. America had become connected to Asia via Bering land bridge S. America had become connected to N. America Horses eventually became extinct over the whole of the New World

Evolution (radiation) within the Genus Equus        

The domestic horse (Equus caballus) – an example of hybridisation Does not have a single, simple origin mtDNA research (2002): the modern horse originated from 77 wild mares These mares were quite different in their mtDNA (i.e. were from different populations) Horse domestication: several distinct populations were captured from the wild We do not know where or when this happened Perhaps on the Eurasian steppes in the Near East (Turkey?) 6.5 to 4.5 kYBP Horses used for riding, eating & milking

Among the best examples of adaptive radiation at the level of Class

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Class Angiospermae (the flowering plants) Class Mammalia Within the Mammalia: - Australian & South American marsupials - Placentals generally

Level of Subfamily  

Butterflies of the subfamily Satyrinae (the Browns) Radiation accompanied radiation of grasses

Among the best-known examples at the level of genus   

Darwin’s finches (Geospiza & other genera) in the Galapagos Islands Honeycreepers (several bird genera) & Drosophila in the Hawaiian Islands Cichlid fishes (Haplochromis & others) in Lake Victoria & other Great Rif Valley lakes in Africa

Colonising populations are very likely to evolve / radiate rapidly  

Speciation: both anagenesis & cladogenesis [evolution within, vs. splitting of a lineage] Founding populations are initially small therefore are likely to undergo genetic drif

Founding populations are also usually subject to very different selection pressures compared to the parent (mainland) population   

There are no / few competing, related species There are no / few predators Larger islands & lakes are spatially more heterogeneous: allopatric speciation is therefore more likely within them

Darwin’s Finches    

Beak form correlated with diet Divergences in beak form: character displacement among secondarily sympatric populations Evolution driven by competition for shared, sometimes limited resources? (David Lack) Genetic drif also important?

Placental & Marsupial Mammal Faunas  

Australian marsupial fauna radiated in isolation from the placentals Convergent evolution between members of the 2 lineages – a response to similar habitat selection pressures

Extinction vs. Change vs. Persistence 

Persistence isn’t hard to explain unless the duration is very, very long

Coelacanths    

Big (up to 1.5 m) carnivorous fish Arose in Devonian (> 400 Mya) Prevalent in Triassic period Lobed fins suggest link to early tetrapod lineage Believed extinct by late Cretaceous (80 Mya)

Extinction Vs Mass Extinction

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Mass Extinction = loss of ≥75% of spp in ≤ 2my

Inbreeding and Disease in Sea Lions 

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Asking whether sea lions rescued from wild due to disease are more inbred than those rescued due to trauma and whether different diseases have different amounts of evidence for effect of inbreeding The higher the relatedness the higher the disease As population declines inbreeding increases

Global Mass Extinction   

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5 major global extinctions in the past: End of Ordovician (450 Mya) - Ice age or a gamma ray burst End of Permian (250 Mya) - The Great Dying - Wiped out 95% of spp - Attributed to massive asteroid impact (on Antarctic plate?) or massive undersea release of methane or massive release of volcanic gases End of Triassic (200 Mya) - Wiped out a whole animal class (conodonts) - 20% of all marine families & all large crurotarsans (non-dinosaurian reptiles) - Many of the large amphibians: >50% of spp - Attributed to widespread volcanic eruptions (massive basalt flooding) & subsequent global warming End of Cretaceous (65 Mya) - K-T event - wiped out dinosaurs, pterosaurs, plesiosaurs, mosasaurs, ammonites & belemnites

The K-T Event   

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K-T Event at end of Cretaceous / start of Tertiary 65 Mya Extinction of the dinosaurs, pterosaurs, plesiosaurs, mosasaurs, ammonites & belemnites Causes - Impacts of a shower of giant meteorites: esp. those near S.E. Mexico & Boltysh Crater in Ukraine &/or Mantle plume eruption on Indian subcontinent (Deccan Traps) - Relatively short -term acid rain - Long -term atmospheric barrier to solar radiation that followed Early mammals were likely able to survive these adverse conditions due to burrowing habit (feeding on roots & invertebrates in soil?) By the Eocene, a major radiation of the mammals was taking place (competitive release?)

Extinction  

Widespread species are less likely to go extinct than endemic species Depends on what species you feed on

6th Mass Extinction

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Currently under way Due to human activities - E.g. over-hunting, deforestation, habitat fragmentation, introductions of alien species, pollution, climate change

North Atlantic Fishes   

Populations of these 5 fish spp. fell 87-98% during 1978-1994 Due to over-fishing Another sp., Atlantic (fish-shop) Cod is on the endangered list along with the Orang Utan

American Bison 

By 1890, as few as 750

Passenger Pigeon  

Several billion individuals in the USA in 1800 Extinct in the wild by 1900, extinct in captivity by 1914

Deforestation  

E.g. Borneo & Sumatra Home of the Orang Utan + 1000s of other spp. of animals & plants

O’ahu tree snails in Hawaiian archipelago    

A large group of colourful tropical tree-living airbreathing (pulmonate) land snails - genus Achatinella Endemic to Hawaii – all extant spp. are endangered Main threat is from the predatory snail Euglandina which was introduced to control the Giant African Land Snail (GALS) – one of the World’s worst invasive pest species Many species of Achatinella went extinct afer GALS’s introduction

Global movements of animals & plants  

Alien species – become pests (arrive without their natural enemies) Pathogens - alien diseases - the spread of chytridiomycosis has been attributed to the trade in amphibians

Climate Change     

Implicated in decline of amphibians Global warming predicted to result in rising levels + acidification of oceans ~100% of coral reef communities predicted to become extinct if global temperatures reach 2oC ormore Coral reef communities also threatened by collectors: commercial dealers of tropical fish &co The vast majority of lineages that have ever existed are now extinct - I.e. extinction is the norm - But so is replacement by new spp. (but humans are limiting this)

Continental Drifft 

Nothofagus (Southern Beech) occurs only in patches around the Antarctic, Chile, New Zealand, Australia & New Guinea

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Fossils abundant on Antarctica Fossils of Glossopteris fern occur in Antarctica, Australasia & India Fossils of the extinct reptile Mesosaurus occur only in a small regions on East coast of Brasil and West coast of Africa Palaeontologists assumed the prior existence of land bridges between continents But evidence for most of these is lacking (Panama Isthmus & Bering Bridge are exceptions)

General observations on the effects of Continental Drif    

Land masses split (e.g. S. America & Africa ) & join up with others (e.g. India & Asia) Terrestrial faunas+ floras become isolated / fuse with others (mixing) Isolated seas join up with Oceans – affecting marine faunas + floras Stretches of continental shelf created & lost, affecting marine faunas + floras

Ice Ages   

Ordovician / early Silurian & Carboniferous / early Permian Ordovician ice age is believed to have been triggered by the rise of mosses – perhaps because the ‘chemical weatheringof rocks they caused reduced atmospheric CO2 More recently (starting 40 Mya, cycling every 10,000-40,000 years)

Effects of ice ages on faunas   

Extinction of spp. (mass extinctions in some cases) Formation of barriers to movement (glaciers in high latitudes, arid areas in low latitudes) Splitting of populations & subsequent speciation

Glacial Refugia  

There are many examples of speciation (or near-speciation) occurring through splitting of a previously widespread species The species sub-populations became isolated because of: - Extension of a glacial finger - Formation of arid areas - Lowered sea levels - Water locked up in ice

Southern Refugia      

Species distributions contract in cold phases Rufugia in the south Expansion northwards following climate warming Diversity gradient along colonization route (more in the south than in the north) Repeated multiple times during Pleistocene Extant structuring is enhanced over time

Wallace’s Line    

An imaginary line used by biogeographers (¾ ) to delineate faunas among islands - SE Asia & Melanesia West of line, the bird fauna has more in common with mainland Asia East of line, it has more in common with Australia Similar lines have been applied to mammals

Why do we get these distinctive distributions?   

Australia & New Guinea (= Australasia) were previously part of Gondwanaland But Asia, Indonesia & Malaysia were part of Laurasia During ice ages Australia & New Guinea became connected, due to lower sea levels

Symbiosis Definition  

Living together A physically close (long- or short-term) association between different species

Types of Symbiosis      

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Mutualism: +/+ Parasitism: -/+ Commensalism: 0/+ Plus & minus symbols refer to the net effects on the fitness of the partner (‘ pay-offs’) Net effects, because the association can involve fitness costs as well as benefits With mutualism & parasitism (but hardly so with commensalism) we expect there to be a degree of coevolution between partners - Due to 2-way selection Selection is mainly 1-way in the case of commensalism

A crazy (tentative) example of symbiosis       

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Sloth diet is difficult to digest and nutrient-poor Algae in sloth fur are highly digestible and lipid-rich Sloths consume these algae-gardens from their fur, presumably to augment their limited diet Adult moths are specialised to colonise sloth fur Moths are carriers of nutrients, increasing nitrogen levels in sloth fur – fuels algal growth By descending a tree to defecate, sloths transport moths to their oviposition sites in sloth dung This facilitates reproduction of the moth - Mutualism between moths, sloths and algae appear to aid the sloth in overcoming a highly constrained lifestyle (and the algae, moths) Both three and two toed sloths harbour a diverse ecosystem in their fur The more sedentary three toed sloths (black bars) possessed: - A greater number of moths - More inorganic nitrogen in the form of NH4 - Greater algal biomass

Mutualism  

2-way selection resulting in coadaptation (a marriage) Plant & pollinator / plant & seed-disperser

Parasitism  

2-way selection resulting in counteradaptation (an arms race ) Host & parasite

Obligate Mutualisms 

Pollinator-plant mutualisms:

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- Yucca & moth - Figs & fig wasps Frugivore (seed-disperser)-plant mutualisms

Coarse-scale (diffuse) coevolution 

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Angiosperms & insects - Massive radiation, in terms of angiosperm floral architecture, esp. from Cretaceous onwards - Coincided with radiations of some flower-visiting insects Bees - ~20,000 known spp. of bee, in 9 families - The actual number of spp. is probably higher - Bees would not have become bees without flowering plants – they would still be relatively unspecialised wasps - They could not have diversified without angiosperms also diversifying - However, insect pollination not the only driving force in angiosperm diversification - Herbivory & abiotic factors (climate, habitat characteristics) were also very important

Most bee-flower relationships are non-obligate  

There is a general rarity of 1 sp. / 1 sp. Relationships Therefore there have been few cases of cospeciation - Little phylogenetic congruence

Fine-scale coevolution: cospeciation 

As the host speciates, so does its associated specialist mutualist (e.g. pollinator) or parasite

Phylogenetic congruence: evidence for cospeciation  

Tree topologies of mutualists or hosts / parasites match perfectly There has been complete co-cladogenesis in this case

Which systems are most likely to exhibit cospeciation?   

Systems in which parasites/mutualists are physically very closely related to their hosts Especially obligate parasites and mutualists More particularly vertically transmitted parasites/mutualists - Foamy viruses (parasites) and their primate hosts - Buchnera bacteria (mutualists) and aphids

Pocket gophers & chewing lice 

Some congruence, but also some incongruence

Functional & comparative functional genomics    

Functional genomics is the study of the function of genes + other parts of the genome, i.e. what they do & how they are expressed in the phenotype Comparative genomics is the analysis & comparison of the genomes of different species E.g. to identify common functions of genes One aim is to gain insights into how organisms have evolved

How has comparative genomics helped understanding of macroevolution? Regulatory (homeotic) genes 

Hox genes in animals – master control genes – conductors of the gene orchestra – an example of epistasis: - Are involved in in embryogenesis - Control differentiation along the longitudinal body axis in most spp - Control differentiation of limbs along the axis

Role of a Hox Gene    

Know whereabouts in the body it is Inform (activate) the builder genes in that region Builder genes can then construct the structures – antennae, legs or wings That are appropriate to that specific body part

Hox Gene     

Mutations in these genes occurred during the evolutionary past They continue to occur very occasionally The mutations can be highly maladaptive (pictured, below) Would be selected against in the wild Thus, Hox & MADS are highly conserved They vary very little across species & even across Kingdoms

Homology between fly & mouse homeotic genes  

In many cases, distant ancestors had most or all of the homeotic genes that are now involved in the developmental processes of their distant descendants These genes were in some cases put to new uses

Hox gene of Amphioxus (a non-vertebrate)       

Neural crest cells are absent in Amphioxus (they are characteristic of vertebrates only) Researchers implanted Amphioxus Hox DNA fragments into mice & chick embryos These regulated development of the neural crest in mouse & chick embryos So – the phenotypic expression of a homeotic gene depends on what body it lives in The homeotic genes themselves are being regulated Maternal mRNA – Gap & pair-rule genes – Homeotic genes I.e. there is a transcription cascade

But some homeotic genes have retained their original (or at least early) overall function         

E.g. the gene controlling eye development Human version of the gene could, theoretically, be inserted into a fly and still trigger building of an insect eye E.g. gene controlling heart development Identical in insects & humans The use of homeotic genes is an example of the economy of nature It gave animals great flexibility for evolving drastically new body plans / organs There is no one gene-one trait rule in the building of an organism Despite the body plan / organ diversity that has been achieved, the tool kit is conservative It doesn’t allow the evolution of some body forms

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E.g. there ...