Evolution – Take 2
Why do monarch butterflies have orange and black wings?
• Proximate answer: “Because of special pigments in their wings” (functional process)
• Ultimate answer: “Because bright colors provide an advantage by advertising toxicity”
(evolutionary process)
Genetics review
Individuals have 2 alleles for each gene
Variation in phenotype can be due to genes AND environment:
A population has a frequency of alelles and genotypes
Hardy Weinberg Equation
• Original proportions of genotypes in a population will remain constant from
generation to generation
Hardy Weinberg Equilibrium
• Calculate genotype frequencies with a binomial expansion
(p+q)2 = p2 + 2pq + q2
• p = individuals homozygous for first allele
• 2pq = individuals heterozygous for alleles
• q = individuals homozygous for second allele
Agents of evolutionary change
Mutations
• Mutations are random and spontaneous
• Provide raw material for evolutionary change
Gene flow
• Introduces new alleles into a population or changes allele frequencies
• Decreases differences between populations and usually counteracts natural selection
Small population size
• Genetic drift
– Change in a gene pool happens by chance due to small size
• Founder effect
– Small population not representative of original population
• Population bottleneck
– Restriction in population size
Non-random mating
• Decreases the frequency of the heterozygote
• Inbreeding and self-pollination are examples
Inbreeding
• Nonrandom mating between related individuals leading to increased homozygosity
and a lack of genetic variation
• Can lower fitness when deleterious recessive alleles are expressed
Ellis-van Creveld Syndrome
Inbreeding
Evolution = Change in allelle frequencies over time
What are the necessary conditions for evolution due to natural selection to occur?
Evolution by natural selection happens…
• … and leads to adaptation!
Phenotypic variation due to environment
Ecotypes and common garden experiments
Species evolve together
Speciation
• Process of creating new species or “kind” of organism
Species are created through genetic divergence
• Gene flow reduced in isolated populations
• Gradual accumulation of differences in the gene pools of populations through natural
selection, genetic drift, and mutation
• Reproductive isolation occurs as a by-product of genetic change
Genetic Divergence
Biological species concept
• Species are groups of interbreeding natural populations that are reproductively isolated
from other such groups
– Ernst Mayr
What is a Species?
• A species is one or more populations of individuals that …
– Interbreed under natural conditions
– Produce fertile offspring
– Are reproductively isolated from other such populations
Separate species may hybridize
Mechanisms of Speciation
– Allopatric speciation
– Sympatric speciation
– Parapatric speciation
Mechanisms of Speciation
Allopatric Speciation
• Populations are first geographically isolated, then become reproductively isolated
• Species separated by geographic barriers will diverge genetically
• If divergence is great enough it will prevent inbreeding even if the barrier later
disappears
• Effectiveness of barrier varies with species
Geographic Isolation
• With the members (and their gene pools) isolated from one another, they become
TWO UNIQUE populations
Mechanisms of Speciation
Parapatric Speciation
Adjacent populations evolve into distinct species while maintaining contact along a
common border
Mechanisms of Speciation
Sympatric speciation
• Speciation by polyploidy is a common event in plants
• Polyploids have more than two sets of chromosomes
Speciation by Polyploidy
• Change in chromosome number
(3n, 4n, etc.)
• Offspring with altered chromosome number cannot breed with parent population
– In autopolyploids, offspring have a doubling of chromosome number from parents
– Allopolyploids are interspecific hybrids
Evolution of wheat
Sterile hybrids
Rates of speciation
• Gradual model
– Species emerge through many small morphological changes that accumulate over a
long time period
• Punctuation model
– Most changes in morphology are compressed into brief period near onset of
divergence
– Adaptive radiation
Evolutionary Trees