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Evidence for Evolution. SBI3U0. Darwin’s Evidence . Darwin spent several years after returning from South America gathering data, consulting other scientists, conducting experiments, and reflecting on his findings before publishing his work
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Evidence for Evolution SBI3U0
Darwin’s Evidence • Darwin spent several years after returning from South America gathering data, consulting other scientists, conducting experiments, and reflecting on his findings before publishing his work • Some of the evidence he considered in coming to his revolutionary theory are; • Biogeography • Homologous and analogous features • Vestigial features • Competition
Biogeography • Biogeography • The observed distribution of species around the globe • If evolution occurred, we would expect to see a few things • Remote islands would only contain species whose ancestors could have swam/flown there • Species on islands would closely resemble those on nearby continents, but NOT be the same • No amphibians and few mammals would exist on remote oceanic islands • There is no reason for these things to be true if evolution did not occur
Biogeography • These observations are exactly what we see in all remote islands (Madagascar, Galapagos, Hawaii, Australia, New Zealand, etc...) • These islands do not naturally contain any amphibians and few (if any) mammals • These species’ ancestors could not swim/fly them across thousands of kilometres of ocean to get to the islands • The species inhabiting these islands are similar to species on their surrounding land masses, but not identical • Lemurs only live in Madagascar, but there are lemur-like fossils found in India
Biogeography • Darwin observed these things mainly in the Galapagos islands and Australia • But the predictions of his hypothesis have been tested and verified for all oceanic islands
Homologous Features • Another observation that struck Darwin is that many different types of species have very different features, but the structure of those features is incredibly similar • For example, the human hand, horse hoof, cat’s paw, bat wing, and whale flipper all have the same number of bones in the same positions • If evolution did NOT occur, there is no reason that this should be the case • Who would design a wing, flipper, and hand with the same underlying design?
Homologous Features • Similar relationships are true for other structures • Eg: all mammals have exactly 28 skull bones and 7 neck bones, regardless of the size of the head/neck • A giraffe has the same number of neck bones as a mouse • If you compare the bone structure of any two mammals, they are strikingly similar • Evolution accounts for this because through slow accumulation of changes to pre-existing structures, new features can be made • Eg: paws turned into wings or flippers
Embryology • The development of embryos is also strikingly similar between species • Eg: at early stages, human embryos have tails and gill-like structures, even though mature humans do not possess these things • The gill-like structures at the early stages are gradually altered into other structures (like the larynxand ear). • Evolution has acted on these structures over time to morph them into other things • Note: These are not fully developed gills, just the structures that develop into gills in fish embryos
Analogous Features • In many cases, there are species with similar features, but those features are constructed in very different ways • Eg: Shark and Dolphin fins, Bat/Bird/Insect eyes and wings, the eyes of vertebrates/cephalopods • These organisms are only distantly related, but their structures are very similar
Analogous Features Vertebrate Cephalopod
Vestigial Features • Many complex organisms possess structures that are rudimentary and non-functional (or only marginally functional) • These structures are homologous to similar structures in other organisms • We call these structures vestigial features • Eg: A fish with eye-sockets, but no eyes • Eg: an animal with a few toes that do not touch the ground • Vestigial features are structures that have become useless over evolutionary time because they were not important to the organisms survival/reproduction
Vestigial Features Tail bone in apes
Analogous Features Pelvis in whales Appendix vs. Caecum
Competition • In 1838, Thomas Malthus released a mathematical paper on population growth called “Essay on the Principle of Population” that greatly influenced Darwin • Malthus came to the conclusion that population growth is regulated by the amount of resources available • The more resources, the large the population • Food supply was particularly important for this regulation • Darwin considered that individuals among a species must compete for resources • This is a driving factor for which individuals survive and which do not
Evolution by Natural Selection • Darwin recognized that within a population, there was a wide variation in traits • Those traits that convey advantages for survival and/or reproduction are called adaptations • Darwin’s main idea was that evolution occurred because of natural selection • Organisms with beneficial adaptations had a higher chance to survive, reproduce, and pass on those traits to their offspring • Organisms with less beneficial adaptations had a lower chance of surviving to reproduce
Evolution by Natural Selection • Darwin also recognized that even though more offspring are produced than there are adults, population sizes do not vary greatly over time • Meaning that several of the offspring do not survive to adulthood • This, along with Malthus’ idea about competition, led him to conclude that organisms compete for resources • Those that are more successful survive • Those that are not successful die
Evolution by Natural Selection • Natural Selection • The organisms that have adaptations giving them a better chance at survival/reproduction are favoured by nature • Many factors in nature act as “selecting factors” • Competition for food • Competition for space • Competition for mates • Climate • “Survival of the Fittest” • “Fit” means “better able to survive and reproduce”
Evolution by Natural Selection • Natural selection explains many of the evolutionary observations we have discussed • Eg: Why did giraffe’s grow long necks? • Natural selection suggests that at some point, the ancestors of giraffes had short necks • Individuals with naturally longer necks would have an advantage, being able to eat food that is higher up in trees • Thus individuals with long necks were more likely to survive and reproduce (pass on their long neck trait) • Long necks were being “selected for” naturally • Over time, this process would result in long-necked giraffes
