Key Questions: • How do existing species give rise to new species? • How do species diversify? • What does the “family tree” of species look like? • Are there any challenges to the idea of evolution?
How do existing species give rise to new species? • When populations in an existing species CAN NO LONGER give birth to fertile hybrid offspring under natural conditions • Why does this happen? REPRODUCTIVE ISOLATING MECHANISMS make it impossible for populations to produce viable offspring
Categories of Reproductive Isolating Mechanisms: • ECOLOGICAL isolation = Different populations (of the same species) adapt to different environmental niches (ex. Hominid speciation) • SEASONAL isolation = Different populations mate at different times of the year • SEXUAL isolation = Different populations have different courtship behaviors • MECHANICAL isolation = Different populations have incompatible organs of reproduction
DIFFERENT POLLINATOR isolation = In flowering plants, different populations attract different insects, birds, or bats to facilitate pollination • GAMETE isolation = Different populations have different cells of reproduction; no fertilization • HYBRID INVIABILITY = Different populations can mate and become fertile, but the hybrid zygotes do not survive • HYBRID STERILITY = Different populations produce living hybrids, but they are sterile (ex. mules)
When does SPECIATION occur? • When ANY of these reproductive isolating mechanisms evolve! • HOW do these reproductive isolating mechanisms evolve? • Through EVOLUTIONARY PROCESSES of: Mutation, Natural Selection, Genetic Drift, Gene Flow
How do species diversify? ADAPTIVE RADIATION = Spreading out of related species into new niches
When does Adaptive Radiation occur? (3 instances) 1. When an environment supports no similar, competing species Ex. 3mya: A small group of finches migrated from South or Central America. They radiated into different environmental niches and evolved into 14 different species!
When does Adaptive Radiation occur? 2.When Extensive Extinction wipes out competing species in a set of environments Ex. 65 mya: Mammals survive Cretaceous/Tertiary (K/T) extinction of dinosaurs
When does Adaptive Radiation occur? 3. When a new group of a related species is adaptively GENERALIZED, it can disperse successfully into different niches, displacing species already there Ex. 40 mya: Monkeys more generalized than Prosimians: larger brains, diurnal, arboreal, mixed diet, so…radiated to Madagascar & the New World, displacing most prosimians in Old World
Interpretations of Speciation2 Theories: 1. DARWINIAN GRADUALISM: Evolution occurs in slow changes in species over time, so “family tree” of species has few, gracefully diverging branches. 2. PUNCTUATED EQUILIBRIUM: Species tend to remain stable, experiencing “oscillating selection.” Evolution occurs in spurts of relatively rapid change.
What does the “family tree” of species look like? A BUSH WITH MANY TWIGS! Twigs = evolution’s experiments, potential new species Natural Selection = “Editor” of Evolution, maintains adaptation of a species to its environment Ex. Grants’ finches beaks changed back & forth as environ. conditions changed
Evidence for Evolution 1. FOSSIL RECORD: Fossils and the order in which they appear in layers of sedimentary rock (strongest evidence) 2. BIOGEOGRAPHY: Geographical distribution of species 3. Presence of TRANSITIONAL fossils 4. TAXONOMY: Classification of life forms
Evidence for Evolution • HOMOLOGOUS STRUCTURES: Structures that are similar because of common ancestry (comparative anatomy) • COMPARATIVE EMBRYOLOGY: Study of structures that appear during embryonic development • MOLECULAR BIOLOGY: Shared DNA
Evidence for Scientific Creationism • FAITH in a literal translation of the Book of Genesis in the Bible • What do you think about Creationism?
Brief Evolutionary Timetable 15 bya Universe forms (“Big Bang”) 12 bya Galaxies form 5 bya Solar System forms 4.5 bya EARTH forms 3.8 bya LIFE on Earth (single-celled organisms) 543 mya CAMBRIAN EXPLOSION! (all major body plans of complex multi-cellular organisms evolve!)
The Burgess Shale Site about 550 mya • An "avalanche" of fine mud sliding down from the submerged reef top carried off any animals living in the shallow reef waters above • The hard parts of all these animals caught in the mudslide were preserved as fossils, like the process at any other Cambrian site • However, here the fine mud also penetrated and filled all available spaces within the animals, thus preserving the shapes and locations of all the soft parts. This is a rare event and has made these fossils extremely valuable
Cambrian Explosion • Three definite body segments: a head with two prominent tentacles, an unsegmented trunk with stubby side fins, and a flattened tail • Fins and tail suggest this was an active swimmer (also suggested by its rare appearance in the Burgess Shale formation) Amiskwia
Cambrian Explosion (cont.) • Fearsome-looking beast is the largest known Burgess Shale animal. Some related specimens found in China reach a length of six feet! • Giant limbs in front, which resemble shrimp tails, were used to capture and hold its prey • Mouth on the undersurface of the head had a squared ring of sharp teeth that could close in like nippers to crack the exoskeleton of arthropods or other prey Anomalocaris
Cambrian Explosion (cont.) • Unusual assembly of spines and grasping arms at the head end. Its mouth lies in the center of that ring of six finger-like projections • Thought to have been a parasite living on sponges since it is commonly found in association with their remains • Presumably, the spiny parts at its head were designed for grasping and feeding on its prey Aysheaia
Cambrian Explosion (cont.) Hallucigenia • Even today, scientists can't be sure which end is the head! • When originally discovered at the Burgess Shale site in Canada, the Hallucigenia fossils were squashed flat within the shale layer (like every other Burgess Shale fossil) with two sets of "spines" appearing to stick out in one direction and one set of "tentacles" in the other Hallucigenia
Cambrian Explosion (cont.) Opabinia Opabinia is thought to have lived in the soft sediment on the seabed, although it presumably could have swum after prey using its side lobes. On the bottom, the proboscis could have plunged into sand burrows after worms. Sizes ranged up to 3 inches, plus that unique, amazing 1 inch proboscis!
Cambrian Explosion (cont.) Pikaia • Earliest known representative of the phylum to which we ourselves belong • Averaging about 1 1/2 inches in length, Pikaia swam above the seafloor using its body and an expanded tail fin • Note the characteristic muscle blocks lying along the centrally important feature, the notochord
Brief Evolutionary Timetable 425 mya Fish evolve, Plants & Animals colonize land 400 mya Insects evolve 350 mya Reptiles evolve 250 mya MASS EXTINCTION (volcanic eruptions in Siberia?, 95% marine & land species extinct!) 256 mya Mammal-like Reptiles evolve 235 mya Dinosaurs evolve
Brief Evolutionary Timetable 220 mya True Mammals evolve 150 mya Small Dinosaurs w/feathers evolve, ancestors of Birds 100 mya Flowering Plants evolve 65 mya CRETACEOUS/TERTIARY (K/T) EXTINCTION (6 mile asteroid crashes through Earth in the MX Yucatan, dinosaurs extinct!) 55 mya PRIMATES evolve
Chimpanzees! to be continued…
Dettwyler, Chs. 7 & 8 • How did Dettwyler collect information concerning traditional beliefs about infant feeding? Do you think group interviews of this kind provide biased data? How does this compare to the older ethnographic practice of relying on one or two key informants for information about cultural beliefs? 2. Compare the advantages and disadvantages of life in Mali and in the US for children with Down syndrome. What advantages do children with Down syndrome have in the US? What advantages do they have in Mali? What advantages do pregnant women have in the US? In Mali?