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 starBaby

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