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Transcript
Some Useful Terms • Species – Discrete groups differing in appearance, behavior, ecology, genetics, etc. • Population – a localized group of individuals • Gene – a sequence of nucleotides producing a protein • Allele – alternate form of a gene • Gene pool – all alleles at all gene loci in all individuals in a population Species, population, gene pool, allele, gene Why is genetic variation important? Darwin and the ingredients for evolution in response to natural selection 1. Individuals within species vary (phenotypic variation) 2. Some of this variation is heritable (genetic variation) 3. Survival and/or reproduction are non-random (natural selection) The individuals that survive & reproduce the most are MORE LIKELY TO BE those with variations most suited to their environment Genetic variation is essential for evolution to occur; and population dynamics is about likelyhood (normal distribution) Microevolution vs. Macroevolution Microevolution: change in allele frequencies within a population Macroevolution: evolutionary change at the species level or higher Microevolution • A generation to generation change a populations allele or genotype frequency • Causes include: – Genetic drift – Gene flow – Mutations – Non-random mating – Natural selection Genetic Drift Change in gene frequencies of a population due to chance Genetic Drift • Bottleneck Effect – Disaster reduces population size so gene pool is reduced and unlikely to represent the original population – By chance, some alleles will be over- or under-represented and in some cases eliminated Genetic Drift • Founder Effect – A few individuals colonize an isolated island, lake etc. – Small gene pool unlikely to reflect variety of original population Gene Flow • Genetic exchange between populations Compare & Contrast • Gene flow – Less likely when populations are isolated – Movement of genes across the landscape – Increases genetic diversity within populations – Populations become similar • Genetic drift – More likely when populations are small and isolated – Random change in allele frequencies = direction is unpredictable – Decreases genetic diversity within populations – Populations can become different Mutations • A change in DNA – Point – single base pair – Chromosomal – Polyploidy - The doubling of the whole sets of chromosmes in the nucleus of a developing egg or seed Non-Random Mating Natural selection • Differential reproductive success – Alleles are passed on to the next generation in numbers disproportionate to current generation • Only mechanism likely to adapt a population to its environment Natural selection • Heritable Variation – idea is central to Darwin’s theory – Within a population • Polymorphism – variation in a discrete characteristic – Among populations • Geographic clines – graded change in some trait along a geographic axis Natural Selection • Evolutionary (Darwinian) Fitness – the contribution an individual makes to the gene pool of the next generation • Selection acts on phenotypes – indirectly on genotypes Prerequisites for natural selection • Trait variation • Difference in fitness among traits • Inheritance • Sources of variation – Polymorphism – occurrence of more than one form of a species – Heterozygosity – the fraction of loci that have multiple alleles Hamlet Modes of natural selection Stabilizing – Average individuals have highest fitness Directional – Shift in mean value or frequency Disruptive – Average individuals have lowest fitness Effect of Selection: Stabilizing Effect of Selection Directional • Most common during periods of environmental change • Shifts the frequency curve in one direction or the other by favoring what are initially rare types – E.g. size of black bears during glacial and interglacial periods Effect of Selection Disruptive • Selection favors individuals on both extremes • Size of prey species – Small fit in smaller holes than predator – Large – too big to swallow Constraints to adaptations • Genetic forces prevent “perfect” adaptations – Mutation – Gene flow • Environments are constantly changing Constraints to adaptations • Historical constraints • Adaptations involve tradeoffs Macroevolution - A Basic Observation • The living world is wildly diverse. – Bacteria to whales (size) – Antarctic ice to oceanic thermal vents (habitat) • All organisms now alive shared ancestors 3500 mya. • Basic unit of classification for life forms is the species. – Discrete groups differing in appearance, behavior, ecology, genetics, etc. • Bridge between micro- and macroevolution is speciation. Result = diversity of life. Mechanisms of Speciation • Barriers that prevent interbreeding – Prezygotic – impede mating or prevent fertilization – Postzygotic – mating and fertilization occur, but hybrid has low viability and fertility Mechanisms of Speciation Prezygotic • Habitat Isolation – Species in different habitats in the same area may not encounter each other • Garter snakes – thamnophis – same area but 1 aquatic, 1 terrestrial Mechanisms of Speciation Prezygotic • Behavioral Isolation – Fireflies blinking pattern – Bird songs – Courtship rituals Mechanisms of Speciation Prezygotic • Temporal Isolation – Breed at different times of day, seasons – Frogs Mechanisms of Speciation Prezygotic • Mechanical – Anatomically incompatible • Pollination of flowering plants Species Separation by Pollinator Recognition Columbines Hawkmothpollinated Hummingbirdpollinated Character A. formosa A. pubescens Flower position nodding erect Spur and sepal red Pale yellow or white Petal blade Deep yellow Pale yellow or white Spur length 10-17 mm 29-37 mm Petal blade length 2-4 mm 9-12 mm Mechanisms of Speciation Prezygotic • Gametic Isolation – Gametes may meet but fail to form a zygote • External fertilization of many marine species Mechanisms of Speciation • Reduced hybrid viability – a zygote is formed, but is never born • Reduced hybrid fertility – hybrid dies before reaching reproductive age, or is infertile • Hybrid breakdown – hybrids can mate and produce offspring, but offspring are weak and/or sterile Postzygotic Geographic Modes of Speciation Allopatry vicariance Ratites Ratites = rhea, ostrich, emu Geographic Modes of Speciation Allopatrydispersal Galapagos finches Geographic patterns • Parapatric • Evolution of reproductive isolation between geographically contiguous (adjacent) populations Geographic patterns Sympatry Sympatric speciation in animals can work when disruptive selection coevolves with assortative mating. 3 spine sticklebacks