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University of Texas at Austin
ANT 301
Chris Kirk

Macroevolution Wednesday, March 06, 2013 2:27 PM So far, we've been talking about populations (level at which evolution occurs) •Study of evolution in populations: MICROEVOLUTIONS How do you get new species? •One of the fundamental questions in MACROEVOLUTION What is a species? •You know: A species is the smallest fundamental unit in the Linnaean hierarchy - all individuals of a given specific type •BUT: Species means different things to different biologists. •Biological Species Concept o Biological species is a group of organisms that are (1) actually or potentially interbreeding and (2) are reproductively isolated from other groups o If you CAN or COULS mate and produce fertile offspring, you're in the same species •What is reproductive isolation? o Prevention of gene flow between populations by genetically determined differences between them. o Something that prevents act of mating/production of viable offspring other than physical separation. o Parts font fit o Different body sizes o Different courtship rituals o Genetic differences • Cause offspring to be sterile (ex: mule) • Male: mule donkey x female horse o Problems: Some very different groups CAN produce viable offspring. • Liger: M Lion x F Tiger - sterile • Tigon: M Tiger x F Lion - sterile o Some guenon species can produce fertile offspring even though they look and behave differently AND even have different number of chromosomes… •Ecological species concept o Emphasizes the role of natural selection in maintaining species boundaries o Gene flow CAN occur, but separate species are recognizes because natural selection acts against hybrid individuals. o Ex: Baboon species are ecological species but not biological species How different species arise •Three modes of speciation •Mode #1: ALLOPATRIC speciation: occurs when a population is divided by some type of physical barrier (river, mountain range, ocean) and then the separated populations diverge over time. •Mode #2: PARAPATRIC speciation: occurs when two populations are physically adjacent, but adapt to different environmental conditions over time. Hybrid zones occur between different species ranges. •Mode #3: SYMPATRIC speciation: individuals of a single population in one habitat gradually diverge over time (no large-scale spatial separation or physical barrier- rare) o Tree hopper: six sympatric species. Each lays eggs on different genus of host plant in same forest. Eggs hatch at different times in response to spring sap flow. Diff. species reach sexual maturity and mate at different times. • In each of these three modes of speciation, one ancestral species gave rise to two or more descendant species. o Cladogenesis: speciation by splitting of lineages • Adaptive Radiation: Special type of cladogenesis, Rapid series of multiple cladogenic events. Often occurs following colonization of new landmass.  Primate Example: Lemurs. o Anagenesis: gradual transformation of one species into another. How you decide where to recognize a species boundary is a problem… (How much change do you need in order to tell which is a different species?) Adaptations • What is an Adaptation? o Any trait that increases an organism's fitness (lifetime reproductive potential) • Do adaptations have to arise by natural selection? o No, but they must be maintained by natural selection (because adaptations by definition beneficial/increase fitness) o
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