Lecture 21: The Principles of Evolution - PDF

Lecture 21: The principles of evolution, or, what’s it all about? ● Many scientists use language inappropriately. ● “People talk about what does evolution do?”, “Are things shaped by evolution?”, Does evolution solve problems? Does evolution come up with ingenious solutions? Does evolution “find” wa...

Lecture 21: The principles of evolution, or, what’s it all about? ● Many scientists use language inappropriately. ● “People talk about what does evolution do?”, “Are things shaped by evolution?”, Does evolution solve problems? Does evolution come up with ingenious solutions? Does evolution “find” ways to adapt living things? ● All this manner of speaking is wrong. ● Evolution is not a thing. There is no object. But it is an abstract idea. ● But a process that living beings change over time. Not a thing per se ● Not a force like gravity. It is a description of how life changes. o Species do not compete, but individuals o Individuals do not evolve, only a population or lineage over generations For centuries people have been puzzled about living things. What are they? How do they work? Where did they come from? Here is the story they wanted to discover… STAGES OF DEVELOPMENT OF LIFE 3.5 Billion years ago… our planet was formed ● Simple organic molecules ● Single celled organisms with DNA ● All species now share DNA. ● Our planet starts about as a molten globe. Just like all other planets. They all started as a solid state. ● Earth is like a giant bowl of ‘larva’. Entire surface is molten. ● Formed over 4 billion years ago. A short time after that the exterior core cools down substantially. From fossil evidence, at about 3.5 billion, there were living things, life started. ● Early earth covered with water. Still is today. Full of organic compounds. The primeval or primordial soup. Chemicals but not yet life. In this complex mixture life emerged. Not yet been replicated in laboratory. ● Organic compounds – Building blocks of life. ● The world is now full of life, any place where life could emerge will be devoured by the living things today. ● The earliest things were simple organic molecules. 2.5 billion years ago. After single molecules ● Multicellular organisms appear ● Oxygen made by photosynthesising cyanobacteria or blue-green algae. ● The first cells arise. ● The next thing, typical, things that have already appeared, get tagged together to make new things. ● Cells appear then cluster of cells appear. Leading to things like us. We are multicellular organism. Multicellular: Cyanobacteria ● Capable of photosynthesis ● Waste product: oxygen o Cyanobacteria caused the biggest catastrophe in the history of the world. o They poisoned the entire atmosphere on earth with poison gas and killed everything else. ● It is still here. The poison gas is called oxygen. It killed the previous kinds of bacteria that cannot live in oxygen. Oxygen is poison to many living things. ● But animals later evolved and adapted to oxygen Multicellular: Stromatolites ● Colonies of bacteria ● Some of the bacteria forms these cluster called stromatolites. o Looks like rocks Fast forward 2 billion years, 500 million years ago ● Jellyfish appear ● Jellyfish are very simple organisms. They have a very interesting life cycle –they go through a phase from egg to larvae to polyp to medusa to adult jellyfish, etc. Hydra ● Similar lifecycle/system as Jellyfish. ● Not plant but primitive animal. ● The myth of multithreaded hydra ● These early creatures like hydra and jellyfish, one thing to remember their adult form. We must remember these swimming stage. Sponges – very primitive/simple animals ● First animal to think of that are primitive and visible to naked eye ● Have a free swimming larval form ● No body symmetry ● Hermaphrodites that reproduce with eggs and sperm ● Cells are so simple that you can put them in a blender and mix them 🡪 will rearrange back into a sponge cell Flatworms (start very similar to free swimming sponge larvae) ● Move with a front side, sensors (intentionally) ● Bilateral symmetry ● Organs emerge, brains, stomachs ● Animal. ● Flatworm moves. Bilateral symmetry. The first sort of animal with specialised structures. ● They start of very similar to the free-swimming sponge larva, where they are descended from ● The flatworms move. ● The sponge is basically not specialised, just a bunch of cavities. ● They can absorb all the oxygen they need from their patterns. ● Their body is so thin they do not need a circulatory system to transfer the oxygen. ● They have no gills or blood. Ocean sediments gradually accumulate ● Round worms appeared after flatworms (evolved) ● Worms with shell ends evolved ● Shells without worm end sticking out: molluscs o Clams o Snails ● Relationship between worms and those with shells. ● Flatworms – the most early primitive things. Nautilus & Ammonites ● Ammonites extinct but cousin, nautilus is still around ● Move by pumping fluid in and out of their body. ● Have sensory organs ● They used to have these little bits sticking at the end, but it dropped off, rotted, and died. ● Octopus, squid do not have clams or shell but they are molluscs. [close relatives to nautilus] ● The Argonauts are without a shell like most octopus. They show the relation between the nautilus/ammonites and octopus/squid Segmented worms ● What happens after worms – Segmented worms. Segmented body animals become more common. ● 1) Crustaceans (segmented): Crabs can walk out of the water without collapsing on the floor ● 2) Insects (segmented) – first creatures to walk on dry land c. 400 million years ago o Insects also happen to have holes in them where the oxygen can just enter the body. Only works if the body is small. o The first creature to walk out of the sea are the ancestors of today’s insect, 400 million years ago. Wood lice – looks identical to sea creatures o More complicated forms than normal one. o Develop external structure, protects. Provide strength to support its body weight outside water. ▪ Rigity of structure allows them to walk on ground o Millipedes are the first to leave a footprint on the surface of the earth. Sea squirts. ● Their larval form has a simple notochord, which swim like tadpoles ● Another very primitive animal. ● Look like a plant or some sort of seaweed. They are just like jelly. ● Do not let the adult form fool you. The larval form resembles things to come. ● Notochord – very primitive spinal column ● This is where the advent of creatures with spines and backbones. All descended from this early creature. Vertebrates! – Lancelets, Cartilaginous Fish, Bony Fish ● These early spinal creatures evolve ● ● ● ● ● ● o Then later create vertebrates Lancelets – not a fish. Very primitive things. No bones, eyes or structures of fish. Very primitive creature. These creatures give rise to fish. Cartilaginous fish – no bones yet but cartilage o Cartilage appeared before bones (material in ears) Some of cartilaginous fish is still around today. Sharks, stingrays. Stingrays are very different from fish. These things give rise to modern bony fish. Earliest true fish is spinal cord 🡪 vertebrates Fish like lungfish acquired the first lungs to help with poorly oxygenated water ● Some of these modern fish, small subset develop ability to take oxygen out of the air. ● There are many places in the world where the water or lake dries up. Breathing oxygen out of water is soon gone so become crucial to get oxygen. o They live in places where oxygen is limited in water so must go up land to take air o Some fish (koi, carp) develop the ability to gulping in the air to get the oxygen which then diffuse across the membrane of the swim bladder. ▪ Lung bags to extract oxygen out of the air ▪ Live at the edge of poorly oxygenated water ● Swim bladder is the ancestor of lungs. ● Also, limb like fins Tiktaalik 375 million years ago ● Evolved from creatures like lung fish ● These creatures found. Fill the gap in the fossil record. o Fins became more robust to skittle from one muddy pond to another muddy point ● These creatures clearly have structures that can support them in the mud or in the land. ● First creature with backbone to walk out of water ● Likely candidate of ancestor of all tetrapod. 4 limbs (have 5 appendages in those fins, collection of smaller bones) o Just like us and other Tiktaalik Evolution of the tetrapod ● Eusthenoptetron 🡪 panderichthys 🡪 tiktoalik 🡪 arcanthostego 🡪 ichthyostega o All called tetrapods ● Tetrapod, creatures with 4 limbs. ● (Early) Maybe just living in very shallow waters, running away from predators like the mudskipper Which reminds one of modern Mudskippers ● This is a fish that can walk. They are really fast. ● These little fish, for safety go out of the water. Most of their predators are in the water. Fossil records is very complete and understood. ● In their fossil bones – become dominant form of tetrapod. The arrangement of bones ● Why there are limbs at the front or at the back? Because they are fins from the early fish? ● 5 digits on the limbs. Why 5? Amphibians ● Some still have to remain wet because they still absorb oxygen through skin. If the skin dried up, they die. o Skin is semi permeable ● They also have to go back to the water to lay eggs o Tired to water at some part of their lives Reptiles (retain scales, fish body shape, lay eggs) ● Waterproof skins ● From such creatures we got the reptiles. ● How did snakes lose their legs? o Leg lost is a result of burrowing lifestyle, in sand or soil. But eventually many members of these family, came back out. (sensitive to vibrations, no sense of hearing) 🡪 because underground initially 🡪 was blind too as it was dark so scales grew over their eyes o Later some of them came out to surface so scales didn’t grow over eyes anymore (can see again) ● Change from a lizard to a slithering snake opened many possibilities. ● Dinosaurs (which are birds) is a reptile o Feathers o Meteorite killed all dinosaurs except 1 family of birds o *seems like smaller creatures survived but big ones didn’t ● Tortoise and crocodiles are much more ancient than dinosaurs Mammal-like reptiles (from one branch of reptiles) ● The mammals are descended from reptiles. Before the time of dinosaurs, called mammal-like reptiles. And eventually evolved to mammals. ● Reptiles when they move, their spine move side to side like a snake. This comes from their fish ancestor. Fish propel themselves that way. o Dogs and cheetah move up and down unlike fish but like dolphin. o ● ● ● ● ● ● Difference between mammals and reptiles, mammals when move their spines moves up and down. They are the first to evolve hair. Control their body temperature. They continue to lay eggs. Mammals Primates 🡪 Lemurs (good sense of smell) o Became Monkeys o Became Apes ▪ Hominids ▪ Humans The platypus and echidna are the only mammals left that still lay eggs o Primitive o Produce milk through fatty sweat off mother’s babies. No nipples o Being able to nurse their baby was a huge advantage Later these mammals kept their eggs inside to nurse their baby Then evolved to this 🡪 Marsupials (carried in a pouch and drinks from nipples – kangaroo) VS Placental mammal (those that produce live young – humans) The recurrent laryngeal nerve is an example of a structure that is not ideal, but good enough to work. Its counterintuitive pathway is the result of ancestral species – all the way back to ancient fish During all this time that life has been evolving, the surface of the planet has been changing ● The earth has also been changing too at the same time. E.g. plate tectonics. ● Creating and destroying life at the same time What would evolution look like if you could watch it or see it? ● A misunderstanding to say that I do not see evolution now. ● If you could take any animal, if you were to lay every ancestor end to end, all the way back, what will you see? You will be seeing evolution, or nothing because you will be seeing the same thing. You must walk for very long to see any substantial change. Because it is a slow gradual process that takes such a long time. o Can see the tree of life o This is what Darwin is saying in the origin of species, 1859 How and why did the birds of the Galapagos (and other isolated islands) become so tame? ● From becoming fearless to becoming sensitive and scared at the slightest of movement o Waste energy, waste time ● No land predators in these islands o Will have those that have more dull senses, less afraid o Have advantage because no need waste energy ● Flightless cormorant of the Galapagos – there were flightless mega-ducks, geese and rails in Hawaii, flightless Moas in New Zealand etc. o When the birds hang around on the ground longer, they will no longer need their flight muscles and there will be disuse of it. Some birds can also be born with lesser flight muscles. (more advantage to just be a good swimmer) How did the collared dove get to Britain? 🡺 Before ww2, don’t have any in Britain 🡺 Decade by decade flew across Europe, finally to Britain naturally 🡺 Shows that the world is not static 🡪 animals come and go, change and they die off Obvious examples of evolution 🡺 Land crabs that drown in water but must lay eggs in the sea o Initially ancestors’ live in sea 🡺 Aquatic mammals (dolphins, whales), sea snakes and turtles. Need air. Turtles must still lay eggs on land. o Still tied to the land 🡺 Tusks amongst Africa elephants 🡺 Caesarean births amongst humans o Increasing the population of women who cannot give birth through natural birth Peppered moths ● Example of natural selection ● In Victorian times, Wallace, and other people were collecting insects. ● The pepper moth originally looked like it had pepper shaken on it. ● With industrial revolution, large amount of black soot, large tract of UK turned black with soot, ash, dust 🡪 major change in the environment. This particular moth became a different colour, it became dark. A particular species that evolved right through our eyes. o To prevent being eaten by birds o Later the soot cleared up and those dark ones got eaten again. ● Articles claiming that this is a fabrication. Peppered moths did change, only in regions of UK that are black and sooty. When the environment cleaned up, it became white again. Have polar bears over eaten penguins 🡺 No because polar bears live in the artic, penguins live in Antarctica 🡺 Both are adapted to cold weather 🡺 Showed principle that even if they are adapted to the environment, they cannot get there because there’s no natural way o Salty ocean bearer Some basic principles of evolution ● Everything comes about gradually. Every step has to be advantageous/make sense in its own terms - there is no reference or mechanism for future needs. ● There is no such thing as evolving for ‘the good of the species’- only individuals live or die. ● Ancestry usually explains similarity o Analogy vs. ancestry… wings of bats & birds are convergent evolution… ● Isolation leads to difference … Galapagos or any isolated place o Divergence. o Gives a false impression of progress. If you look at a small subset, you will be mistaken. o It is not a story of progress ● “Progress” is not inevitable, although of course: o Complex things can only come about after simpler ones. But sometimes more simple things follow more complex. ▪ Cave fish, crabs etc. have lost their complex eyes. (live in underground, don’t need eyes anymore) ● New territory or niches opening up tend to draw in new species ● Many things have happened again and again. o Big extinction events. o Explosions of diversity such as the Cambrian explosion. o Evolution of carnivores/predators from herbivores- in fish, reptiles, dinosaurs, marsupial mammals, placental mammals... Aldabra tortoises perhaps… ▪ Next time if not enough dead meat, scavengers may become carnivores ● But life on earth has always been mostly microorganisms (bacteria etc.) What is life anyway? ● Living matter is real stuff. Physical stuff. Living cells. Basically, organic compounds. ● Life is a very complex chemical reaction. For the last 3.5 billion years, chain chemical reaction has been ongoing in the way we describe as evolution. Last word from Wallace 🡺 Attenborough in Paradise (1996) Wallace, loving the natural world Last page of origin of species 🡺 He offered a vision, unlike visual scientific book 🡺 Elaborately constructed form, so different. Endless forms beautiful and are being evolved. 🡺