Natural Selection v Evolution
• Evolution = observed change in organisms
over historic and geologic time
• Natural selection = one hypothesized
mechanism for change
– Has enormous body of supporting evidence
What is natural selection?
• Because organisms with greater reproductive success leave
more offspring, they make a larger contribution to the gene
pool. Any heritable characteristics that contribute to
reproductive success will come to dominate the gene pool.
The species changes in the direction of those characteristics.
• In other words, the currency of natural selection is BABIES.
Survival is only important in promoting more babies.
• Natural selection occurs at the level of the INDIVIDUAL, not
the species. Selection is driven by what is good for the
individual, not what is good for the species.
Necessary conditions for natural
selection to occur
1. There must be variation within the
population.
– Variation from mutation, and from errors in
sexual reproduction
2. The variation must be heritable.
3. The variation must change the likelihood of
successful reproduction (including survival).
Elements in Reproductive Success
• Survival of parent: better defense, better
resource use, better adapted to physical
environment
• Health and fertility adaptations: maximizing
the number of viable offspring
• Ability to attract mates: sexual selection
– Can include characteristics that threaten survival,
as long as they enhance the probability of
attracting a mate
– Characteristic is a proxy for health or other
positive characteristic
Elements in Reproductive Success
• Caring for young: altruism and self-sacrifice
– Organisms may sacrifice themselves for relatives with
whom they share a significant proportion of their
genes
• Survival of young to reproductive age: 2
strategies
– Maternal care – have a few offspring and invest a lot
in caring for them (mammals)
– Independent offspring – have a zillion of them and let
them fend for themselves (plants, invertebrates)
The Evolution Wars
Separating
the
scientific
questions
from the
cultural
scuffle
What is in dispute?
• In the cultural battle, “evolution” means many
things:
– Origin of the universe – the Big Bang
– Origin of life
– Age of the earth
– Organic change within a species
– Origin of new species
What is in dispute?
• In science, “evolution” refers to specific
things:
– Origin of the universe – the Big Bang
– Origin of life
– Age of the earth
– Organic change within a species
• Mechanisms: natural selection, genetic drift
– Origin of new species
• Process: speciation
• Mechanism: reproductive isolation
Time out to review what we
already learned and to add a
couple new things
Mechanisms for change
• Natural selection: we already talked about
• Genetic drift: when populations are separated
from each other and their gene pools don’t
mix, variation that develops in each
population will tend to make the populations
more different
– E.g, variations in bird plumage, or lizard
coloration. Or blonde scandanavians
Origin of new species
• Species: set of populations that are actually or
potentially interbreeding to produce fertile
offspring
• Species are reproductively isolated from each
other; e.g., cannot successfully interbreed
• Speciation occurs when populations become
reproductively isolated:
– Geographically
– Other reason: structural, chromosomal, chemical,
behavioral
OK. THAT’S what scientists mean
when they talk about evolution.
None of that other stuff.
Now on to other issues in the
culture wars over evolution.
Redefining science
• Science is based on predictability
• Since everything follows physical laws, we can
predict what will happen, or explain what has
happened
• No physical laws, no predictability – no science
• Invoking supernatural explanations (creation
“science”, intelligent design) inserts
unpredictability – you can’t predict what a
supernatural agent might do
From a purely pragmatic viewpoint…
• Redefining science to include the supernatural removes
the useful part of science
• For example, suppose we conjecture that demons
cause disease.
• If it’s not testable, or we can’t make predictions, we’re
stopped dead – we can’t figure out what causes
disease.
• And that’s where humanity stalled out for several
millennia – until we took the supernatural out of our
explanations of the natural world.
Dealing with popular misconceptions
1. Evolution violates the 2nd law of
thermodynamics, and is therefore impossible
Here’s the three laws of
thermodynamics
1. Energy is neither created nor destroyed, just
transferred
2. Spontaneous natural systems cannot
decrease in entropy (entropy = disorder)
3. Entropy decreases with temperature, and
reaches a minimum at absolute zero.
Here’s the three laws of
thermodynamics
1. Energy is neither created nor destroyed, just
transferred
2. Spontaneous natural systems cannot
decrease in entropy (entropy = disorder)
3. Entropy decreases with temperature, and
reaches a minimum at absolute zero.
But…
• These laws do not say that systems cannot be
self-organizing.
– Crystals are self-organizing systems
– Rivers are self-organizing systems
– The Earth is a self-organizing system (think plate
tectonics)
• The overall trend in the entire system is toward
increasing entropy (e.g., the entire Earth system
tends toward entropy) but parts of the system
may become more ordered
More misconceptions
2. There are no intermediate forms in the fossil
record. Therefore different kinds of
organisms must represent separate creation
events
• No, no, no. There are many spectacular
examples of intermediate forms:
– Fish to amphibians to reptiles
– Reptiles to birds
– Pre-humans to humans
Yet more misconceptions
3. There are no examples of the gradual evolution
of complex structures. If the structure only
works when it is complicated, how could it
possibly evolve from something simple?
• But the usual example given – eyes – is in fact a
fabulous example of the evolution of a complex
structure from simple light-sensing cells to a
variety of independently evolved complex eyes
(e.g., arthropod v. cephalopod v. vertebrate)
And anyway…
• Evolutionary theory does not require the
gradual evolution of different morphologies
(forms or shapes)
• Two kinds of genes:
– Structural genes: code for proteins that build the
structures
– Regulatory genes: code for proteins that turn
other genes on and off
Regulatory genes determine…
• Fundamental body architecture (Hox genes):
– Bilateral v. radial is determined by 2 genes that either
act in tandem or they don’t
– E.g., starfish is bilateral as a baby and radial as an
adult
• When features develop
– Babies and adults often look nothing like each other
because of the sequencing of how genes turn on and
off
Pisaster, the orange sea star
Baby
Regulatory genes determine…
• Fundamental body architecture:
– Bilateral v. radial is determined by 2 genes that
either act in tandem or they don’t
– E.g., starfish is bilateral as a baby and radial as an
adult
• When features develop
– Babies and adults often look nothing like each
other because of the sequencing of how genes
turn on and off
Tunicates
Consider…
The girl is 8 years old. She has primordial dwarfism, a genetic endocrine disorder.
Or…
Size in dogs can
be controlled by
a single gene.
Small genetic
change, large
morphological
change.
So changes in regulatory genes…
• Can change body shape dramatically in one
generation
• Can create adults that look like juveniles
Consider…
Consider…
Neoteny
• Retention of juvenile physical traits into sexual
maturity
• Dogs from wolves
• Humans from ape-like ancestor
• Vertebrates from tunicates