2. Last week.... Heredity
• How traits are passed from one generation to the next
• Considered individual organisms & families
Today.... Mechanisms of Evolution
• Changes in the frequency of traits over time
• Causes of differences among populations in traits
• Scaling up to populations
3. Origin of species & diversity of form
Species are fixed
• ~2.500 years ago
• Aristotle & the ancient Greeks
• Judeo-Christian culture
• Earth is ~6,000 years old (very young!)
Life evolves, descent from common ancestors
• Buffon, mid-1700’s, earth older than 6,000 years
• Lamarck, early 1800’s, Inheritance of acquired characteristics
• Darwin & Wallace, 1858
• Descent with modification, or, natural selection
Lamarck
Wallace
4. 1. Individuals compete for resources
2. There is phenotypic variation among individuals
3. Some of this variation is heritable
4. Those individuals with traits best suited to current conditions
leave more progeny than others (natural selection)
*Leads to changes in allele frequencies over time, or, EVOLUTION
Evolution by Natural Selection
The Origin of Species - Darwin 1859
“I have called this principle, by
which each slight variation, if
useful, is preserved, by the term
Natural Selection.”
5. • Overproduction & competition
• Phenotypic variation
• Some variation is heritable
• Differential survival & reproduction
*Leads to changes in allele frequencies over time
6. The Modern Synthesis
• Mid-1900’s
• Fusion of the work of Mendel & Darwin
Mendelian genetics can help us
understand the patterns of phenotypic
variation observed by Darwin.
Gregor Mendel Charles Darwin
7. Terminology
Evolution – changes in allele frequencies over time
Natural selection – a mechanism for evolution, differential fitness
among phenotypes present in a given environment
Fitness – survival & reproduction, the contribution an individual
makes to the gene pool of the next generation
Gene pool – all gametes produced by a population in a generation
Population – a group of individuals of the same species living in the
same area at the same time
Allele frequency – # of particular allele/total # of alleles in a
population
Genotype frequency – # of particular genotype/total # of individuals
in a population
8. The Equilibrium Population:
Assumptions of Hardy-Weinberg
• No mutation (no new genetic variation)
• No genetic drift (infinitely large population)
• No migration (no individuals entering or leaving the pop)
• No selection (genotypes have equal fitness)
• Random mating (dealing with a single population)
9. p2
+2pq + q2
= 1
Hardy-Weinberg Equilibrium
p + q = 1
For a single locus with two alleles:
• p = frequency of the recessive allele (a)
• q = frequency of the dominant allele (A)
• Allele & genotype frequencies sum to 1 within a population
Genotypic and allelic frequencies are stable & predictable
freq. aa = p2
, freq. AA = q2
, freq. Aa = 2pq
10. p2
+2pq + q2
= 1
Hardy-Weinberg Equilibrium
p + q = 1
For a single locus with two alleles:
• p = frequency of the recessive allele (a)
• q = frequency of the dominant allele (A)
• Allele & genotype frequencies sum to 1 within a population
Genotypic and allelic frequencies are stable & predictable
freq. aa = p2
, freq. AA = q2
, freq. Aa = 2pq
Editor's Notes
TL- marine iguana of the Galapagos
Bottom-examples of camouflage as evolutionary adaptation, related mantid species
L = leaf mantid in Costa Rica, C = flower mantid in Malaysia, R = trinidad tree mantid mimics dead leaves
Scaling up to talk about populations now – not just individuals
Because how an individuals traits interact with the environment determines if they are selected for or against,
But evolution can only occur in populations!
Before we can talk about what causes evolution, we first have to discuss what evolution is, where it came from.
View persisted until early 1800’s when French naturalist Lamarck suggested that present species are similar but different from fossil forms and that evolution must have occurred, had his mechanism wrong though
Until Darwin, Wallace and Mendel come along...
In 1859, Darwin publishes the Origin of Species with two bold statements
Species on earth today descended from ancestral species
this descent is accompanied by modification due to natural selection
*So he proposes natural selection as the mechanism for descent with modification (evolution)
4 primary arguements to his reasoning....
Remember: selection happens to individuals, but evolution happens to populations!!!!
Natural selection does not create variation! It just works with what is there already
NS favors characteristics for a certain time/place – what it favors changes as conditions change!
But Darwin had no understanding of genetics – this was filled in with Mendel’s work
Mendel and Darwin were contemporaries (mid 1800’s) but their work wasn’t sythesized and appreciated until the mid 1900’s (decades later!)
The fact that Mendel’s genetics nicely fit the hole in Darwin’s reasoning (which lacked an understanding of heritability of traits) meant we could use Mendel’s rules to understand the phenotypic variation among and within species that Darwin had observed.
This is POPULATION GENETICS, and what we’ll spend the rest of this talk discussing.
Need some new terms....
Selection happens to individuals!
Evolution happens to populations!!!
To explore the forces that changes allele frequencies over time, we’re going to start with an idealized population at equilibrium....
This means that the population is not experiencing any changes in allele frequ over time –
which alleles the pop has and how rare/frequent they are is the same in every generation (no evolution)
A population in this condition is said to be in Hardy-Weinberg equilibrium (named after two guys developed it in 1908)
A population that is in H-W Equ (experiencing no evolution) meets these 5 assumptions
We can understand H-W by using what we learned of basic Mendelian genetics & probability
Where do the p2, 2pq & q2 come from????
Recall 2 basic principles – 1) alleles at same locus segregate independently at meiosis and fuse randomly at fertilization, 2) alleles at different loci segregate independently