Evolution: How Change Occurs

Evolutionary Theory = collection of carefully reasoned and tested hypotheses about how evolutionary change occurs

Early Views on the Origin of Species Hypothesis I: “fixity of species” – all plants and animals had been placed on the Earth at its beginning and none have ever changed Aristotle believed species were fixed creations arranged by their complexity This idea lasted 2000 years

Hypothesis II: Life changed over time HOW? Lamarck’s Theory B. Darwin’s Theory 1809 1859

How did Darwin come up with his theory? 1. James Hutton (English Geologist) -suggested changes in the Earth’s crust due to slow continuous processes (erosion, earthquakes, volcanoes, etc.) -called Gradualism

How did Darwin come up with his theory? 2. Georges Cuvier (French Zoologist) -studied fossils, saw drastic changes across short periods of time -suggested species disappeared in response to a catastrophic event (volcano or earthquake…) -called Catastrophism

How did Darwin come up with his theory? 3. Charles Lyell (English Geologist) -geologic processes that affect Earth’s crust are uniform (the same) across long periods of time -proposed the Earth was millions of years old not thousands -called uniformitarianism

How did Darwin come up with his theory? 4. Thomas Malthus (English Economist) -noted # of births > # of deaths -suggested that high birth rates and limited resources would force life and death competition -each species struggles for existence -death rate will increase to balance population size and food supply

Finally… Charles Darwin -English Geologist & Biologist -travelled around the world on the HMS Beagle (5 years long) -studied distribution of wildlife and fossils he saw across his journey

Jean-Baptiste Lamarck (Lamarck’s Theory of Acquired Inheritance, 1809) “Law of Use and Disuse” – parts of organisms that are used more develop more than those that are not “Inheritance of Acquired Characteristics” – developed traits are passed onto their children

Jean-Baptiste Lamarck (Lamarck’s Theory) Example: Stretching a piece of anatomy during the parent’s life results in stretched parts in the children Disproving Lamarckism: In 1891, August Weismann cut the tail off ~20 generations of parent mice resulting in no change in their children

Darwin’s Theory (1859) i. “Struggle for Existence” = competition between living things to survive Idea inspired by Malthus’ Principle of Population Always more young produced than the environment can support

Darwin’s Theory (1859) ii. Variation within a population – all organisms of a species differ slightly (through artificial or natural selection) Canis lupus

Darwin’s Theory (1859) iii. Survival of the fittest – the most “fit” survive the competition organisms with less favorable traits do not reproduce as quickly/often and in time, they become extinct

Darwin’s Theory (1859) iv. Inheritance = fittest reproduce and pass on their special characteristics on to their children

Darwin’s Theory (1859) v. over very long periods of time, small changes accumulate and populations change this is evolution Evolution: change in the relative frequency of alleles in a population’s gene pool

Publication of “On the Origin of the Species” Darwin did not publish his results until he read an essay from Alfred Wallace Fellow naturalist Independently developed the same theory After 25 years, someone else had come to the exact same conclusions of nature

Why didn’t Darwin publish right away? Darwin knew that his theory would be extremely controversial His theory challenged established religious and scientific beliefs, particularly about the creation of humankind

Common misconceptions about evolution Individuals do not evolve, populations do Individuals that are more successful have an increased chance to reproduce and pass on their genes (fitness!) These individuals do not adapt to their environment, some are just better at surviving in their environment (variation within a population) Evolution is very slow, nothing changes over one or two generations Evolution is random, there is never a goal to be reached (feathers and wings did not evolve for the sole purpose of flight)