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Lecture 2

Evolution and Ecology

‘Nothing in biology makes sense, except in the light of evolution.’ Dobzhansky

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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

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• 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)

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Evolutionary thought:

Charles Darwin/Alfred Russell Wallace

Thomas Malthus: social philosopher -

Charles Lyell: geologist/natural philosopher

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Natural selection: Selection in a genetically variable population for those with traits which favor survival and reproduction:

1. Over reproduction2. Struggle for existence – competition for

resources3. Differential survival and reproduction

Fitness: ‘the ability to survive and leave reproductively viable offspring

4. Genetic variationAdaptation: a trait that is possessed by an individual enables an enhanced ability to survive and reproduce

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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

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Shaping of Populations

• Selection for one extreme

• Selection for the ‘average’

• Selection for two extremes (bimodal)

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An example: Directional selection

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Fig. 5-8

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Fig. 5-9

Stabilizing Selection: average is favored

Goldenrod gall size – formed by fly

Small: attacked by wasp

Large: attacked by birds

Species have become specialists feeding on seed providing optimal energy efficiency

Disruptive Selection: Specialist forms develop

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

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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

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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

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

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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

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Other processes leading to evolution:

Genetic drift:Changes in allele

frequencies across generations due to chance events:

• Random disturbances

• Small populations• Founder effects

• 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

Natural Selection Can Result in Genetic Differentiation

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

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)

• 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‘

http://evolution.berkeley.edu/evolibrary/article/0_0_0/devitt_02

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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