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 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 Directional

Effect of Selection Selection favors individuals on both extremes Size of prey species –Small fit in smaller holes than predator –Large – too big to swallow Disruptive

Constraints to adaptations Genetic forces prevent “perfect” adaptations –Mutation –Gene flow Environments are constantly changing

Constraints to adaptations Historical constraints Adaptations involve trade- offs

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 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 Prezygotic

Mechanisms of Speciation Behavioral Isolation –Fireflies blinking pattern –Bird songs –Courtship rituals Prezygotic

Mechanisms of Speciation Temporal Isolation –Breed at different times of day, seasons –Frogs Prezygotic

Mechanisms of Speciation Mechanical –Anatomically incompatible Pollination of flowering plants Prezygotic

Species Separation by Pollinator Recognition CharacterA. formosaA. pubescens Flower positionnoddingerect Spur and sepalredPale yellow or white Petal bladeDeep yellowPale yellow or white Spur length10-17 mm29-37 mm Petal blade length2-4 mm9-12 mm Columbines Hummingbird- pollinated Hawkmoth- pollinated

Mechanisms of Speciation Gametic Isolation –Gametes may meet but fail to form a zygote External fertilization of many marine species Prezygotic

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 Allopatry- dispersal Galapagos finches

Geographic patterns Parapatric Evolution of reproductive isolation between geographically contiguous (adjacent) populations

Geographic patterns Sympatry 3 spine sticklebacks Sympatric speciation in animals can work when disruptive selection coevolves with assortative mating.