G falls drops some species of flowering plants have

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Unformatted text preview: ific example) Shape of flower/position of pollen Lightweight pollen floats on water Anther/stigma mature at same time Anthers above stigma Animal vectors (e.g., transfer, carry) Water (e.g., transfers, carries) Gravity (self-pollination) (e.g., falls, drops) Some species of flowering plants have evolved mechanisms to prevent self-fertilization. (c) Discuss an evolutionary advantage of preventing self-fertilization. (2 points maximum) • Maintains/increases genetic variability of the population (not at individual level) • Variability in action—explain or give an example (e.g., more material for natural selection, avoids effects of inbreeding, allows population to cope with changing environment) • Hybrid vigor © 2008 The College Board. All rights reserved. Visit the College Board on the Web: www.collegeboard.com. AP® BIOLOGY 2008 SCORING GUIDELINES Question 4 (continued) (d) Describe TWO mechanisms that prevent self-fertilization. (3 points maximum) 1 point for a description of each mechanism as suggested by the bullets below (2 points maximum); 1 point for an appropriate specific example or detailed description Self-incompatibility • Pollen fails to germinate (stigma epidermal cells prevent germination of pollen through signal transduction pathway). • Pollen tube does not complete development (due to destruction by RNAses). • Sperm fails to unite with egg. • S-genes must be different (allele incompatibility). o If pollen grain and stigma have matching alleles at the S-locus then the male gametophyte fails to begin process of fertilization. Structural adaptations • Stigmas are higher than anthers or vice-versa (pin and thrum) (heterostylous). • Separate male/female flowers (monoecious)/separate sexes/stamens OR carpels (dioecious). • Temporal separation of maturation of male/female parts (dichogamy/protogyny/ protandry). • Nectar production at different times. • Mechanical isolation: difference in size of pollen grains and stigma papillae. © 2008 The College Board. All rights reserved. Visit the College Board on the Web: www.collegeboard.com....
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This note was uploaded on 11/23/2009 for the course BIOLOGY 1101 taught by Professor Keith during the Winter '05 term at The University of Oklahoma.

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