1. Lecture 3
Evolution

2. A unifying theory in Biology:
‘Nothing in biology makes sense, except in the light of evolution.’ Dobzhansky

3. What is: evolution?
What is a theory?
Why is it important for species to evolve?
‘Evolution is a population thing’: Individuals do not evolve, evolution is a process which occurs over generations in a population.
Population is evolutionary unit

4. Evolution has a genetic basis:
Genes are heritable units that determine traits
Variation in genes occurs naturally: due to mutation
Allele: genes may occur in variant forms; these variant forms are known as alleles
Gene Pool: the total collection of gene within a population

5. What forces or factors may result in an evolutionary change in a population?
Genetic drift: ‘chance variation in a population’
Natural selection: Selection in a genetically variable population for those with traits which favor survival and reproduction

6. Natural selection: drives evolution by favoring those with the best fitness
Results in a gradual change in the population/gene pool
Adaptation: a trait that is possessed by an individual enables an enhanced ability to survive and reproduce
Fitness: ‘the ability to survive and leave reproductively viable offspring’

7. Examples of Evolution
1. Gartersnake-Oregon Newt interaction
2. modification of beak size in Galapagos finches – studies of Peter and Rosemary Grant
No new species arose in either case
Species: ‘A group of similar appearing organisms that share the same gene pool’

9. Fig. 5-8

10. Fig. 5-9

11. Selection pressure within a species
Selection for one extreme
Selection for the ‘average’
Selection for two extremes (bimodal)

12. Sexual selection: an aspect of natural selection
A process in which (usually) females select amongst potential mates (p 674)
Example - Peacocks: female selects males with large, bright, colorful tails- why?
Handicap hypothesis (p 704)

13. Sexual selection – an aspect of natural selection:
includes two aspects
Intrasexual: males compete - selection leads to evolution of structures used in combat with other males
Deer’s antlers or ram’s horns, bright colors, etc.
selective pressure is for ability to compete successfully with members of same sex
Intersexual selection- Females select amongst winners
agents of selection
Selection based on various cues – appearance and behavior
goal (evolutionary sense) select mate with best fitness

14. Female peahens prefer to mate with males with greater number of eyespots in their tail feathers

15. Geographic Range and Evolution
The geographic range of a species can result in phenotypic variation of neighboring populations
A wider geographic range includes a broader range of environmental conditions
In general, the greater the distance between populations, the more pronounced the phenotypic differences

16. Geographic Variation in Species: clines and ecotypes
Ecotype: Subpopulation of a species adapted to a specific environment
Cline: measurable change in a species characteristics over an environmental gradient
Variation in white tailed deer – larger animals at higher latitudes in NA

17. Natural Selection Can Result in Genetic Differentiation
Geographic isolates result when gene flow among subpopulations is prevented
The isolation is rarely complete, and so these isolated subpopulations are often classified as subspecies because of a set of unique characteristics
Appalachian salamanders (Plethodon jordani)

19. Genetic Variation of Ecotypes
An ecotype is a population adapted to its unique (often abrupt) local environmental conditions
Yarrow (Achillea millefolium) response to altitude
How much variation observable in the field is due to genetic variation and how much is due to impact of environment (phenotypic plasticity)?
Common Garden Experiment
Seed collected from plants of same species growing in different environments grow in same location(s)

20. Evolution without natural selection - Genetic drift:
Genetic drift refers to fluctuations in allele frequencies in a population arising from chance events.
Most pronounced in small populations resulting from
Natural disasters
Founding events
High levels of predation
Random: population not selected based on adaptations

21. Figure 4
Genetic drift:
Changes in allele frequencies across generations due to chance events:
Random disturbances
Small populations
Founder effects
Effect of genetic drift on a population. Because of chance events in the population, allele frequencies in later generations differ significantly from allele frequencies in the initial generation.

22. Several Processes Can Function to Alter Patterns of Genetic Variation
Inbreeding is the mating of closely related individuals
Increases homozygosity at all loci
Offspring of closely related individuals are more likely to inherit rare, recessive, or deleterious genes that lower overall fitness
These consequences are referred to as inbreeding depression

23. How do new species arise?
Allopatric speciation: spatial isolation –geographic isolation
Spread of species into new areas
Founding population in isolated location
Geologic changes – gradual process
Sympatric speciation: different niches/same habitat or geographic area
Genetic changes (as ploidy levels in plants)
Disruptive selection: divergent genotypes favored

24. Species have become specialists feeding on seed providing optimal energy efficiency

25. Adaptive Radiation: Single ancestral from give rise to multiple ‘specialist’ forms

26. Summary:
Evolution involves heritable differences amongst individuals in a population which ultimately impact fitness
Organisms become ‘tailor made’ for their niche within an environment by processes of evolution
Characteristics of individuals making up current populations are a product of natural selection in ancestral populations