• 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: change in allele frequencies within a population
Macroevolution: evolutionary change at the species level or higher

• A generation to generation change a populations allele or genotype frequency
• Causes include:
– Genetic drift
– Gene flow
– Mutations
– Non-random mating
– Natural selection

Change in gene frequencies of a population due to chance

• 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

• Founder Effect
– A few individuals colonize an isolated island, lake etc.
– Small gene pool unlikely to reflect variety of original population

• Genetic exchange between populations

• 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

• 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

• 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

• 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

• Evolutionary
(Darwinian) Fitness – the contribution an individual makes to the gene pool of the next generation
• Selection acts on phenotypes – indirectly on genotypes

• 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

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

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

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

• Historical constraints
• Adaptations involve tradeoffs

• 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.

• Barriers that prevent interbreeding
– Prezygotic – impede mating or prevent fertilization
– Postzygotic – mating and fertilization occur, but hybrid has low viability and fertility

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

Prezygotic
• Behavioral Isolation
– Fireflies blinking pattern
– Bird songs
– Courtship rituals

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

Prezygotic
• Mechanical
– Anatomically incompatible
• Pollination of flowering plants

Species Separation by Pollinator Recognition
Columbines
Hawkmothpollinated
Hummingbirdpollinated
Character
Flower position
Spur and sepal
Petal blade
Spur length
Petal blade length
A. formosa nodding red
Deep yellow
10-17 mm
2-4 mm
A. pubescens erect
Pale yellow or white
Pale yellow or white
29-37 mm
9-12 mm

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

• 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

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

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