Mechanisms of evolution Natural selection acts on the range of phenotypes in a population. Phenotypes are largely determined by genotypes.

Section 15.2 Summary– pages 404-413 Populations, not individuals, evolve Individuals cannot acquire new phenotypes within their lifetimes and pass on those acquired traits to offspring Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Populations, not individuals, evolve Gene pool – all of the alleles of all the individuals in a population. Allelic frequency – the percentage of any specific allele in the gene pool. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Populations, not individuals, evolve Genetic equilibrium – allele frequencies remain stable over many, many generations. Evolution occurs as a result of changes in allele frequencies within a population. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Genetic (Hardy-Weinberg) equilibrium 5 conditions must be met in order for a population to achieve equilibrium Large population size Random mating No gene flow No mutations No selection Does this ever happen in nature? Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Changes in genetic equilibrium Mutations introduce new alleles. Mutations are therefore the ultimate source of genetic variability within populations. What are mutations and what causes them? Are mutations always bad? Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Changes in genetic equilibrium Genetic drift – fluctuations in allele frequencies due to random events. Effects of genetic drift are especially acute in small, isolated populations Gene flow – the gain or loss alleles due to migration of individuals between neighboring populations. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Natural selection acts on variations Disruptive selection Three types of natural selection: Stabilizing selection Directional selection Disruptive selection Stabilizing selection Directional selection Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Natural selection acts on variations Stabilizing selection favors average individuals in a population. Selection for average size spiders Normal variation Section 15.2 Summary– pages 404-413 9

Section 15.2 Summary– pages 404-413 Natural selection acts on variations Directional selection favors one of the extreme variations of a trait. Selection for longer beaks Normal variation Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Natural selection acts on variations In disruptive selection, individuals with either extreme of a trait’s variation are selected for. Selection for light limpets Normal variation Selection for dark limpets Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 The Evolution of Species Speciation – the evolution of new species due to significant changes in the gene pool of a population over time. Occurs when members of similar populations no longer interbreed to produce fertile offspring. Gradualism vs. Punctuated Equilibrium. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Physical barriers can prevent interbreeding Geographic isolation - a physical barrier divides a population. The formation of a river may divide a single population of frogs into two populations. Over time, these two populations could evolve into two, distinct species. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Reproductive isolation can result in speciation As populations become increasingly distinct, reproductive isolation can arise. Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 A change in chromosome numbers and speciation Polyploid – an individual or species with a multiple of the normal set of chromosomes. New polyploid species Abnormal gametes (2n) Fertilization Zygote (4n) Nondisjunction Sterile plant Fertilization Parent plant (2n) Meiosis begins Normal meiosis Normal gametes (n) Zygote (3n) Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Diversity in new environments Divergent evolution - populations change as they adapt to different environmental conditions, eventually resulting in new species. Adaptive radiation - type of divergent evolution in which an ancestral species evolves into an array of species to fit a number of diverse habitats Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Diversity in new environments Amakihi Extinct mamo Crested honeycreeper Possible Ancestral Lasan finch Kauai Niihau Molokai Oahu Maui Lanai Akialoa Kahoolawe Akepa Hawaii Apapane Maui parrotbill Akiapolaau Liwi Grosbeak finch Akikiki Palila Ou Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Different species can look alike Convergent evolution – distantly related organisms evolve similar traits. Convergent evolution occurs when unrelated species occupy similar environments in different parts of the world. Section 15.2 Summary– pages 404-413

PARALLEL NICHES Tasmanian wolf MARSUPIALS PLACENTALS Banded anteater Giant anteater Koala Tree sloth Sugar glider Flying squirrel Wombat Woodchuck Tiger cat Ocelot Marsupial mole Common mole Dunnart Tasmanian wolf Gray wolf Shrew