Transcript Lect 2 Evolution

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
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Niche: functional role an organism plays in an
ecosystem….
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‘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?
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Why is it important for species to evolve?
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‘Evolution is a population thing’:
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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 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
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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
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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
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
Copyright © 2009 Pearson Education, Inc.
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
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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
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
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Genetic Variation of Ecotypes
• An ecotype is a population adapted to its unique (often abrupt) local environmental
conditions
– Yarrow (Achillea millefolium) response to altitude
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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
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
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