1.1 Introduction into Mammals and Evolution - Summary Flashcards
What is natural selection?
Natural selection: descent with modification
Selection leads to the accumulation of favoured variants, which over a long period produce new life forms, the origin of species
Pattern and process in evolution
Define Pattern
Define Process
in the context of evolution
Draw the diagram
Pattern: is what we measure and record what we see in the fossil record, in DNA sequences (comparative genomics) –
By looking at these and quantifying them we can infer evolutionary patterns…, which we can then use to infer about evolutionary process
Fossilisation: chapter 2 in Benton goes through fossilisation
Define fossilisation?
To convert into a fossil; replace organic with mineral substances in the remains of an organism
Give three examples of fossilisation and what they represent?
- *1. Early bird, Archaeopteryx:** transitional fossil between dinosaurs and birds from the late Jurassic period, the fossil can be used to infer ability of flight
- *2. Echolocating (microchiropteran), Icaronycteris**; bat that lived in the early Eocene,
Both these fossil examples can be used to infer powered flight, these examples illustrate an important point regarding analogue (powered flight in this case) and homology (limb bones have evolved but in different ways to enable flight)
3. Reptile Ichthyosaur birth
Fossils can tell us a great deal about the life style of these great extinct animals, classic one bellow, the marine reptile ichthyosaur birth. Baby being born tail first because these are marine reptiles, they might look like a shark but are closer to dolphins because they are air breathing these ichthyosaur are born tail fist as dolphins are so that the young can swim up to the surface to take a breath
**Fossilisation **
**What are they used for? **
**Where are they found? **
**What are the two main fossilisation methods? **
- Fossils are crucial (Darwin used it to confirm his ideas) to our knowledge of evolution
- Fossils only occur in sedimentary rocks, normally only the ‘hard parts’
- This normally occurs in aquatic environments: the sea, in rivers, and in lakes, or after floods Fossil record
1. Permineralisation: Organic material in the bones decays, and is replaced by minerals (silica, calcite, iron pyrites) from water percolating through the sedimentary rocks. When minerals replace the entire bony structure, the process is known as petrification
2. ‘Natural cast’ process: The bones dissolve to leave a hollow mould which may be filled later with minerals to form a solid replica of the bone
- Soft parts are rarely preserved: recent some soft fossils from China
- Preservation of ancient material may also occur in amber, and by mummification for more recent stuff
**Darwin recognized the limitations of the fossil record **
**List the limitations **
- The fossil record is incomplete
- Fossilisation is a chance event
- Species with small population sizes may be poorly represented (or not at all)
**Dating methods **
**What are the two main types? **
**What do they mean? **
Absolute dating – the item itself is dated (NOT USUALLY DONE)
Relative dating – strata above (younger) and below (older) are dated and the item expressed relative to these
**Dating methods **
** Initially, most periods were named after the locations of characteristic deposits: **
**What are they known as? **
– Cambrian from Cambria (the Roman name for Wales)
– Permian from Permia in Russia
– Devonian from Devon
– Jurassic from the Jura Mountains (France, Switzerland and Germany)
Summary of dating methods and there periods?
< Summary of the dating methods, radio-metric methods are based on the decay of naturally occurring isotopes, when an organism dies the clock is set at zero
What can cause evolution?
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- <!--[endif]-->Evolution may also arise from chance factors other than adaptation, like genetic drift
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- <!--[endif]-->Natural selection and adaptation are tightly linked in NeoDarwinism – emphasises gradualism. Example = horse changing in size
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- <!--[endif]-->1970s gradualism question due to the variation identified, but we now know a lot of DNA isn’t doing anything (most mutations = silent)
Punctuated equilibrium: long periods where nothing is happening, then sudden rapid changes ***paper to read **
**Evolutionary processes: further thoughts to ponder on **
**Define microevolution?
Define macroevolution? **
- *Microevolution:** formation of different populations/species
- *Macroevolution:** where you get whole new higher taxa diverging
**Morphological VS molecular evolution = Phenotype VS Genotype **
**What can be said about this? **
What is functional continutiy?
Might not always perfectly correlated because molecular evolution is neutral a lot of the time
Functional continuity: how are morphological integration maintained, look at things like developmental genes
What are the main drivers of evolution?
What are the consequences of this?
The Geological Timescales
Focus on the level of the Era
1.2Introduction into mammals and evolution – Summary
Continental drift: tectonic plates
The surface of the earth is divided into tectonic plates. The Earth formed about 4.5 billion years ago – it is a solid outer crust floating on molten layer. The molten layers below the plates have convection currents run through them – those convection currents drive the plates – junctions between the plates moving – subduction one under the driver
These plates are moving about the same rate fingernail grows – over thousands of years the whole surface of the earth has changed dramatically.
What happened?
Late Proterzoic – 650 million years ago
Cambrian explosion
Late Cambrian (514 mya)
- *Late Proterzoic – 650 million years ago**
- Polar region is frozen over, most of the globe is covered in sea
- *Cambrian explosion
- ** Seen as the point of explosion for multicellular life
- Recently fossils found to predate this – Edicaran funa – jelly like multicellular organism, but nothing like them exists today
Late Cambrian (514 mya)
Beginning of the Palaeozoic Era
Cambrian explosion, where many species suddenly appear, like molluscs, arthropods like crustaceans.
Trilobites common
Late Cambrian (cont)
• Pikaia (sub phylum Cephalochordata)
What happened?
Middle Ordovician (458 mya)
Middle Silurian (425 mya)
Middle Ordovician (458 mya)
Increase in land mass
Northern and Southern Hemisphere
Oceans separate Laurentia, Baltica, Siberia and Gondwana
Important time because we get the first vertebrates – jawless fish some still survive till today in lampreys and hagfish
Middle Silurian (425 mya)
• Continents begin to collide as Paleozoic Oceans close
what happened?
Early Devonian (390 mya)
Late Carboniferous (306 mya)
Late Permian (255 mya)
End Permian extinction
- The Age of Fish
- Jawless fish
- First Tetrapods (amphibians) also appeared during the Devonian, about 350 mya e.g Ichthyostega
Late Carboniferous (306 mya)
- Pangea begins to form during Early Carboniferous
- Amniotes - Vertebrates living on the land, have an egg that resists drying out
- Earliest ancestors of mammals start to appear
Late Permian (255 mya)
• As a result of continental movement, massive outpouring of larva and this huge volcanic event caused mass extinction because of the effect on climate • Mammal-like reptiles emerge and dominate the Permian e.g **Dimetrodon (Order Pelycosauria, sub-class Synapsida)**
End Permian extinction
• At the end of the Permian was greatest extinction of all time; an estimated 96% of marine species and 70% of terrestrial vertebrates became extinct
What happened?
**Early Triassic (237 mya) **
Late Jurassic (152 mya)
** Late Cretaceous (94 mya)**
Early Triassic (237 mya)
- Dinosaurs emerged in the late Permian, and ruled through the Triassic and Jurassic ‘age of reptiles’
- However, mammal-like reptiles were hanging on… (eg Thrinaxodon)
- … and by the late Triassic, early Jurassic, early mammals are on the scene (eg Megazostrodon)
Late Jurassic (152 mya)
• Continental break up
Example: Megazostrodon
Late Cretaceous (94 mya)
• First placental mammals appear in the Cretaceous, about 100 mya
What is the KT boundary?
As we leave the cretaceous, there is another mass extinction event: KT boundary
Large impact on the gulf of Mexico – Chicxulub event
KT Marks the end of cretaceous
Then move into Eocene and Last Glacial Maximum
**Mass extinctions **
**How many are there? **
**Which ones affected mammal evolution? **
- Major radiations have occurred following mass extinctions
- The ‘big five’ extinctions are:
- End Ordovician
- Late Devonian
* *3. End Permian** - Late Triassic
* *5. End Cretaceous (K-T-boundary)**
• Mass extinctions that are particularly significant from the point of view of mammal evolution are:
- End Permian, as mammal-like reptiles are superseded by the dinosaurs. A few mammal-like reptiles remained and evolved into true mammals
- End Cretaceous, when we see the rise of mammals following the extinction of the dinosaurs
• More recently, cycles of ice ages and glaciation have also dramatically affected evolution, with particular implications for hominids
Summary
- Changes in the physical environment have had major influences on the history of life in earth
- These changes – acting via natural selection – have shaped the evolution of life on earth, by mechanisms which involve gradualism, punctuated equilibrium, or both (most likely)
- The mass extinctions at the end of the Cretaceous paved the way for the radiation of mammals, and we’ll begin with this in the next lecture
What are the main characteristics of mammals?
Main characteristics of mammals
- Mammary glands
- Characteristic teeth (heterodont dentition)
- Single lower jawbone
- Three bones in the middle ear
- Possess hair, sweat to cool, warm blooded
Summary
• Mammals are morphologically a very diverse group, but share common features (endothermy, lactation etc.).
• The MAJOR adaptive radiation of mammals apparently began after the K-T event and the extinction of dinosaurs.
• Some of the main lineages may have been diversifying before this (see later lectures).
• A major challenge is establishing the divergence times and super-ordinal relationships of extant taxa.
