A critical phrase: variation precedes adaptation selection only works on genetic variation that is already present in a population acquired traits are not passed on to offspring, so do not evolve in response to selection Make sure you can explain the difference between what Darwin proposed and what Lamarck proposed for how species adapt over time to their environment
Selection on color in guppy fish streams with predatory fish streams without predators Females Males What kinds of selection are acting on guppies? Which is stronger?
Controversies: the politics of knowledge According to a poll by the New Scientist, U.S. citizens lag way behind the rest of the western world in their acceptance of evolution Only 15% of US adults believe humans evolved over millions of years
Controversies: the politics of knowledge 1. Evolution is “just a theory” 2. You should “teach the controversy” 3. Intelligent Design
Controversies..? 1. Evolution is “just a theory” In science, theory = a general expression of a hypothesis that is well substantiated by lots of experimental data - the best explanation for all the data - a hypothesis that has withstood testing in thousands of experiments over many decades This is not how “theory” is used in common parlance; that is what we call a hypothesis, a notion that needs to be tested
Controversies..? 1. Evolution is “just a theory” 2. You should “teach the controversy” There isn’t one. Evolution is the central principle of biology; nothing in biology makes sense except in light of evolution via natural and sexual selection. There is no evidence that contradicts the theory of evolution There is no controversy in biology about this!
Controversies..? 1. Evolution is “just a theory” 2. You should “teach the controversy” 3. Intelligent Design Says it is “too unlikely” life could evolve by descent with modification, so organisms must have been created by God, as a product of deliberate design This is untestable, therefore it is not science Only testable ideas are subject to the scientific method; other ideas are a matter of opinion or belief, but are not science
“Evolution Has Failed” - oh really Intelligent Design proponents frequently claim: “Evolutionists have had 150 years to explain everything that has ever happened in the history of life on earth, but they still cannot explain everything, so toss evolution out” In science, we work by disproving hypotheses with evidence - ideas are not rejected because they cannot explain the entire universe in one sentence Example: Newton’s physics vs. Einstein’s Theory of Relativity
Adaptation versus Design Constraints of Ancestry: Natural selection modifies traits that were present in an ancestor; radically new features rarely appear There are no examples of clear “design” in nature, but endless examples of imperfect adaptation The human body is adapted to walk upright, but all of physiology and medicine demonstrates it is not well designed for upright movement
Reconstructing Evolutionary Relationships: Taxonomy and Phylogeny If all living things are descended from a common ancestor, how do we differentiate among the different kinds of life? - A phylogeny is a hypothesis of how organisms are related to each other by descent from a common ancestor - often presented as a branching tree (ancestors not shown)
Showing Evolutionary Relationships on a Cladogram - at each branch point, a new characteristic appeared in a common ancestor of all later branches - shared, derived characteristics are called synapomorphies
Showing Evolutionary Relationships on a Cladogram - every group that is decended from that common ancestor inherits this trait, unless it was later lost - shared, derived characteristics are called synapomorphies
Feathers: a synapomorphy uniting all birds Scales (snakes) Scales (bony fish) Feathers (turkeys) Feathers (parrots) Feathers (dinosaur) Scales(early reptile) Scales(ancestral fish)
Taxonomy & Phylogeny Taxonomy is how we group related organisms together, using a system of hierarchical classification - smaller groups are nested within larger groups Panthera pardus or P. pardus The binomial name combines the genus and speciesname of the organism
Monophyletic group: includes ALL descendants of an ancestor Paraphyletic: leaves out some descendants Most biologists think only monophyletic groups should get names, because only they are “true” groups Paraphyletic Monophyletic
Clade= monophyletic group Birds = clade Reptiles: paraphyletic group (not a clade) Mammals = clade
Why are paraphyletic groups given names? Taxonomists used to name groups that were morphologically distinctive, such as birds and cetaceans (whales & dolphins) - does not reflect their evolutionary status: both are nested within other groups, which they no longer resemble What do you think: should scientists consider birds and reptiles classes of vertebrates, if birds are a subset of reptiles? Another reason is, we are often fooled by convergent evolution - natural selection can make unrelated organisms look similar, if they are adapted to the same environment
Convergent Evolution: Homoplasy Both birds and bats have wings; does this mean they are related? - similar traits can stem from a common ancestor, OR from convergent evolution when the two organisms live in similar habitats, or fill a similar ecological role Homoplasy is when a trait makes us think two organisms are related, when in fact they are just similar because of convergent evolution
Homologoustraits: structurally similar, but functionally different - example: human arm, dolphin flipper, bat wing - reflects descent with modification
Homoplasy: structurally different (unrelated), but functionally similar - example: streamlined shape of sharks + killer whales - Recognizing the difference between homology and homoplasy allows us to sort out the phylogeny, or evolutionary relationship, among living things based on their morphology (= physical traits)
Species concepts What do biologists mean when they refer to a “species” ? Many different definitions have been proposed to explain what a “species” is - in different kinds of organisms, different criteria seem useful for defining what constitutes a species Note – “species” is both singular (one species) and plural (two species) there is no such word as “specie” !!
Definitions for discussing speciation Big question: what can cause one population to split into 2 new populations that do not reproduce with each other? Migration Dispersal Gene flow = movement of alleles between populations Reproductive isolation= two groups of organisms do not produce offspring together - either they won’t mate, or their hybrid offspring are infertile/dead movement of individuals between populations
Biological Species Concept (Mayr, 1942) Species are defined as groups of actually orpotentially interbreeding individuals If two organisms cannot produce fertile offspring together, then they are different species - if a horse + a donkey mate, their offspring is a mule, which is sterile - therefore, horses and donkeys are 2 different species under this definition (they are not interbreeding) The boundaries between species are defined by reproductive isolation, the inability of two organisms to successfully produce offspring together
Reproductive isolation initially results from: - mate preferences (what’s sexy to you?) - when do you spawn (season, or time of day) - where do you mate pre-mating isolation: sperm never meets egg Natural selection favors pre-mating isolation, because it stops closely related organisms from trying to interbreed - hybrid offspring are usually less fit than their parents -
Reproductive isolation initially results from: - mate preferences (what’s sexy to you?) - when do you spawn (season, or time of day) - where do you mate pre-mating isolation: sperm never meets egg After a long period of separation, different populations become too genetically different to hybridize anymore (3) developmental incompatibilities produce infertility or death in hybrid offspring.... post-mating isolation: genetic incompatibility results in dead or infertile offspring....
How do species form? Step 1 – gene flow between 2 populations is interrupted Step 2 – populations start becoming different, by random mutation or by adapting to different environments Step 3 – mating preferences change (they don’t make babies) Step 4 – post-mating isolation eventually results (they can’tmake babies) The populations have now speciated, or become distinct species What can initially interrupt gene flow between 2 populations?
Allopatric speciation Edges of a species’ range are often defined by a geographical boundary (a river, a mountain range) - species distributions can be determined by geological features This is the result of allopatric speciation – physical barriers prevent migration between two populations, leading to speciation
Allopatric speciation Consider a species found in a desert and a neighboring forest Cool, rainy forest Hot, dry desert
Allopatric speciation Selection will favor different alleles in the desert and forest Cool, rainy forest Hot, dry desert Natural selection will favor forest-adapted individuals Natural selection will favor desert-adapted individuals
Allopatric speciation After selection, the two populations will be genetically different Cool, rainy forest Hot, dry desert forest-adapted individuals have survived here desert-adapted individuals have survived here
Allopatric speciation But, migration will keep mixing alleles between the populations Cool, rainy forest Hot, dry desert
Allopatric speciation But, migration will keep mixing alleles between the populations Cool, rainy forest Hot, dry desert Natural selection won’t result in desert-adapted & forest-adapted populations when gene flow is high; remains one population
Allopatric speciation Now: say a barrier to gene flow arises between the 2 habitats mountains Cool, rainy forest Hot, dry desert
Allopatric speciation Now: say a barrier to gene flow arises between the 2 populations mountains Cool, rainy forest Hot, dry desert Natural selection will favor forest-adapted individuals Natural selection will favor desert-adapted individuals
Allopatric speciation Each population may evolve into a distinct, well-adapted species mountains Cool, rainy forest Hot, dry desert Forest-adapted population Desert-adapted population Species #1 Species #2