Transcript Evolution - Napa Valley College

Some Useful Terms
• Species – Discrete
groups differing in
appearance, behavior,
ecology, genetics, etc.
• Population – a localized
group of individuals
• Gene – a sequence of
nucleotides producing a
protein
• Allele – alternate form
of a gene
• Gene pool – all alleles
at all gene loci in all
individuals in a
population
Species, population, gene pool, allele, gene
Why is genetic variation important?
Darwin and the ingredients for evolution in response to
natural selection
1. Individuals within species vary (phenotypic variation)
2. Some of this variation is heritable (genetic variation)
3. Survival and/or reproduction are non-random (natural selection)
The individuals that survive & reproduce the most are MORE LIKELY
TO BE those with variations most suited to their environment
Genetic variation is essential for evolution to occur;
and population dynamics is about likelyhood (normal
distribution)
Microevolution vs. Macroevolution
Microevolution: change in allele frequencies within a
population
Macroevolution: evolutionary change at the species level
or higher
Microevolution
• A generation to generation change a
populations allele or genotype frequency
• Causes include:
– Genetic drift
– Gene flow
– Mutations
– Non-random mating
– Natural selection
Genetic Drift
Change in gene frequencies of a population due to chance
Genetic Drift
• Bottleneck Effect
– Disaster reduces
population size so
gene pool is reduced
and unlikely to
represent the original
population
– By chance, some
alleles will be over- or
under-represented and
in some cases
eliminated
Genetic Drift
• Founder Effect
– A few individuals colonize
an isolated island, lake
etc.
– Small gene pool unlikely to
reflect variety of original
population
Gene Flow
• Genetic exchange between populations
Compare & Contrast
• Gene flow
– Less likely when
populations are isolated
– Movement of genes across
the landscape
– Increases genetic diversity
within populations
– Populations become similar
• Genetic drift
– More likely when
populations are small and
isolated
– Random change in allele
frequencies = direction is
unpredictable
– Decreases genetic diversity
within populations
– Populations can become
different
Mutations
• A change in DNA
– Point – single base
pair
– Chromosomal
– Polyploidy - The
doubling of the whole
sets of chromosmes
in the nucleus of a
developing egg or
seed
Non-Random
Mating
Natural selection
• Differential
reproductive success
– Alleles are passed on
to the next generation
in numbers
disproportionate to
current generation
• Only mechanism
likely to adapt a
population to its
environment
Natural selection
• Heritable Variation –
idea is central to
Darwin’s theory
– Within a population
• Polymorphism –
variation in a discrete
characteristic
– Among populations
• Geographic clines –
graded change in
some trait along a
geographic axis
Natural Selection
• Evolutionary
(Darwinian) Fitness –
the contribution an
individual makes to
the gene pool of the
next generation
• Selection acts on
phenotypes –
indirectly on
genotypes
Prerequisites for natural selection
• Trait variation
• Difference in fitness
among traits
• Inheritance
• Sources of variation
– Polymorphism –
occurrence of more
than one form of a
species
– Heterozygosity – the
fraction of loci that
have multiple alleles
Hamlet
Modes of natural
selection
Stabilizing
– Average
individuals
have highest
fitness
Directional
– Shift in mean
value or
frequency
Disruptive
– Average
individuals
have lowest
fitness
Effect of Selection: Stabilizing
Effect of Selection
Directional
• Most common during
periods of
environmental change
• Shifts the frequency
curve in one direction
or the other by
favoring what are
initially rare types
– E.g. size of black
bears during glacial
and interglacial
periods
Effect of Selection
Disruptive
• Selection favors
individuals on both
extremes
• Size of prey species
– Small fit in smaller
holes than predator
– Large – too big to
swallow
Constraints to adaptations
• Genetic forces prevent “perfect”
adaptations
– Mutation
– Gene flow
• Environments are constantly
changing
Constraints to adaptations
• Historical constraints
• Adaptations involve tradeoffs
Macroevolution - A Basic
Observation
• The living world is wildly diverse.
– Bacteria to whales (size)
– Antarctic ice to oceanic thermal
vents (habitat)
• All organisms now alive shared
ancestors 3500 mya.
• Basic unit of classification for life
forms is the species.
– Discrete groups differing in
appearance, behavior, ecology,
genetics, etc.
• Bridge between micro- and
macroevolution is speciation.
Result = diversity of life.
Mechanisms of Speciation
• Barriers that prevent
interbreeding
– Prezygotic – impede
mating or prevent
fertilization
– Postzygotic – mating
and fertilization occur,
but hybrid has low
viability and fertility
Mechanisms of Speciation
Prezygotic
• Habitat Isolation
– Species in
different habitats
in the same area
may not
encounter each
other
• Garter snakes –
thamnophis –
same area but 1
aquatic, 1
terrestrial
Mechanisms of Speciation
Prezygotic
• Behavioral Isolation
– Fireflies blinking pattern
– Bird songs
– Courtship rituals
Mechanisms of Speciation
Prezygotic
• Temporal Isolation
– Breed at different times of day,
seasons
– Frogs
Mechanisms of Speciation
Prezygotic
• Mechanical
– Anatomically
incompatible
• Pollination of
flowering plants
Species Separation by Pollinator Recognition
Columbines
Hawkmothpollinated
Hummingbirdpollinated
Character
A. formosa
A. pubescens
Flower position
nodding
erect
Spur and sepal
red
Pale yellow or white
Petal blade
Deep yellow
Pale yellow or white
Spur length
10-17 mm
29-37 mm
Petal blade length
2-4 mm
9-12 mm
Mechanisms of Speciation
Prezygotic
• Gametic Isolation
– Gametes may meet
but fail to form a
zygote
• External fertilization of
many marine species
Mechanisms of Speciation
• Reduced hybrid
viability
– a zygote is formed, but
is never born
• Reduced hybrid fertility
– hybrid dies before
reaching reproductive
age, or is infertile
• Hybrid breakdown
– hybrids can mate and
produce offspring, but
offspring are weak
and/or sterile
Postzygotic
Geographic Modes of Speciation
Allopatry vicariance
Ratites
Ratites = rhea, ostrich, emu
Geographic Modes of Speciation
Allopatrydispersal
Galapagos
finches
Geographic patterns
• Parapatric
• Evolution of
reproductive
isolation between
geographically
contiguous
(adjacent)
populations
Geographic patterns
Sympatry
Sympatric
speciation in
animals can work
when disruptive
selection coevolves
with assortative
mating.
3 spine sticklebacks