1. Lecture 2 Evolution and Ecology
‘Nothing in biology makes sense, except in the light of evolution.’ Dobzhansky

2. Ecosystem Structure and Evolution
Niche: functional role an organism plays in an ecosystem….
‘the sum total of all activities and relationships in which individuals of a species engage as they secure resources necessary to survive and reproduce’… job or role in an ecosystem
Adaptation: ‘a genetically determined characteristic that … improves fitness
Fitness: the ability to leave reproductively viable offspring

3. Why is it important for species to evolve?
What is evolution?
Why is it important for species to evolve?
‘Evolution is a population thing’:
Individuals do not evolve
occurs over generations in populations.
‘descent with modification’ (see page 645)

4. Evolutionary thought: Charles Darwin/Alfred Russell Wallace Thomas Malthus: social philosopher - Charles Lyell: geologist/natural philosopher

5. Natural selection: Selection in a genetically variable population for those with traits which favor survival and reproduction:
1. Over reproduction
2. Struggle for existence – competition for resources
3. Differential survival and reproduction
Fitness: ‘the ability to survive and leave reproductively viable offspring
4. Genetic variation
Adaptation: a trait that is possessed by an individual enables an enhanced ability to survive and reproduce

6. Evolution has a genetic basis:
Genes are heritable units that determine traits
Variation in genes occurs naturally: due to mutation  Alleles
Gene Pool: the total collection of gene/alleles within a population
Changes in the gene pool  changes in a population

7. Shaping of Populations
Selection for one extreme
Selection for the ‘average’
Selection for two extremes (bimodal)

8. An example: Directional selection

9. Fig. 5-8

10. Fig. 5-9

11. Stabilizing Selection: average is favored
Goldenrod gall size – formed by fly
Small: attacked by wasp
Large: attacked by birds

12. Disruptive Selection: Specialist forms develop
Species have become specialists feeding on seed providing optimal energy efficiency
Disruptive Selection: Specialist forms develop

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

14. Sexual selection: an aspect of natural selection
A process in which (usually) females select amongst potential mates
Example - Peacocks: female selects males with large, bright, colorful tails- why?
Handicap hypothesis
Operates in plants too – competition for pollinators  various mechanisms to lure pollen transferring animals

15. 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

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

17. 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
Founding events
Natural disasters
High levels of predation - bottlenecks
Random: population not selected based on adaptations

18. Other processes leading to evolution:
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.

19. Natural Selection Can Result in Genetic Differentiation
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

20. Geographic Variation in Species
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

21. 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) (p 85)

22. Example: Ensatina spp. of California – a ‘ring species‘
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
Example: Ensatina spp. of California – a ‘ring species‘

23. 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