A critical phrase: variation precedes adaptation

selection only works on genetic variation that is already present in a population

acquired traits are not passed on to offspring, so do not evolve in response to selection

Make sure you can explain the difference between what Darwin proposed and what Lamarck proposed for how species adapt over time to their environment

Females

Males

streams withpredatory fish

streams withoutpredators

Selection on color in guppy fish

What kinds of selection are acting on guppies? Which is stronger?

Controversies: the politics of knowledge

According to a poll by the New Scientist, U.S. citizens lag way behind the rest of the western world in their acceptance of evolution

Only 15% of US adults believe humans evolved over millions of years

Controversies: the politics of knowledge

1. Evolution is “just a theory”

2. You should “teach the controversy”

3. Intelligent Design

Controversies..?

1. Evolution is “just a theory”

In science, theory = a general expression of a hypothesis that is well substantiated by lots of experimental data

- the best explanation for all the data

- a hypothesis that has withstood testing in thousands of experiments over many decades

This is not how “theory” is used in common parlance; that is what we call a hypothesis, a notion that needs to be tested

Controversies..?

1. Evolution is “just a theory”

2. You should “teach the controversy”

There isn’t one.

Evolution is the central principle of biology; nothing in biology makes sense except in light of evolution via natural and sexual selection.

There is no evidence that contradicts the theory of evolution

There is no controversy in biology about this!

Controversies..?

1. Evolution is “just a theory”

2. You should “teach the controversy”

3. Intelligent Design

Says it is “too unlikely” life could evolve by descent with modification, so organisms must have been created by God, as a product of deliberate design

This is untestable, therefore it is not science

Only testable ideas are subject to the scientific method; other ideas are a matter of opinion or belief, but are not science

“Evolution Has Failed” - oh really

Intelligent Design proponents frequently claim:

“Evolutionists have had 150 years to explain everything that has ever happened in the history of life on earth, but they still cannot explain everything, so toss evolution out”

In science, we work by disproving hypotheses with evidence

- ideas are not rejected because they cannot explain the entire universe in one sentence

Example: Newton’s physics vs. Einstein’s Theory of Relativity

Adaptation versus Design

Constraints of Ancestry:

Natural selection modifies traits that were present in an ancestor; radically new features rarely appear

There are no examples of clear “design” in nature, but endless examples of imperfect adaptation

The human body is adapted to walk upright, but all of physiology and medicine demonstrates it is not well designed for upright movement

Reconstructing Evolutionary Relationships: Taxonomy and Phylogeny

If all living things are descended from a common ancestor, how do we differentiate among the different kinds of life?

-

A phylogeny is a hypothesis of how organisms are related to each other by descent from a common ancestor

- often presented as a branching tree (ancestors not shown)

- shared, derived characteristics are called synapomorphies

- at each branch point, a new characteristic appeared in a common ancestor of all later branches

Showing Evolutionary Relationships on a Cladogram

Showing Evolutionary Relationships on a Cladogram

- every group that is decended from that common ancestor inherits this trait, unless it was later lost

- shared, derived characteristics are called synapomorphies

Scales (ancestral fish)

Scales (early reptile)

Scales (bony fish)

Scales (snakes)

Feathers (turkeys)

Feathers (dinosaur)

Feathers (parrots)

Feathers: a synapomorphy uniting all birds

Taxonomy and Phylogeny

Taxonomy & Phylogeny

Taxonomy is how we group related organisms together, using a system of hierarchical classification

- smaller groups are nested within larger groups

The binomial name combines the genus and species name of the organism

Panthera pardusorP. pardus

Monophyletic group: includes ALL descendants of an ancestor

Paraphyletic: leaves out some descendants

Most biologists think only monophyletic groups should get names, because only they are “true” groups

ParaphyleticMonophyletic

Clade = monophyletic group

Reptiles: paraphyletic group (not a clade)

Birds = clade

Mammals = clade

Taxonomists used to name groups that were morphologically distinctive, such as birds and cetaceans (whales & dolphins) - does not reflect their evolutionary status: both are nested within other groups, which they no longer resemble What do you think: should scientists consider birds and reptiles classes of vertebrates, if birds are a subset of reptiles?

Another reason is, we are often fooled by convergent evolution

- natural selection can make unrelated organisms look similar, if they are adapted to the same environment

Why are paraphyletic groups given names?

Convergent Evolution: Homoplasy

Both birds and bats have wings; does this mean they are related?

- similar traits can stem from a common ancestor, OR from convergent evolution when the two organisms live in similar habitats, or fill a similar ecological role

Homoplasy is when a trait makes us think two organisms are related, when in fact they are just similar because of convergent evolution

Homologous traits: structurally similar, but functionally different

- example: human arm, dolphin flipper, bat wing

- reflects descent with modification

Homoplasy: structurally different (unrelated), but functionally similar

- example: streamlined shape of sharks + killer whales

-

Recognizing the difference between homology and homoplasy allows us to sort out the phylogeny, or evolutionary relationship, among living things based on their morphology (= physical traits)

What do biologists mean when they refer to a “species” ?

Many different definitions have been proposed to explain what a “species” is - in different kinds of organisms, different criteria seem useful for defining what constitutes a species

Species concepts

Note – “species” is both singular (one species) and plural (two species)

there is no such word as “specie” !!

Big question: what can cause one population to split into 2 new populations that do not reproduce with each other?

MigrationDispersal

Gene flow = movement of alleles between populations

Reproductive isolation = two groups of organisms do not produce offspring together

- either they won’t mate, or their hybrid offspring are infertile/dead

Definitions for discussing speciation

movement of individuals between populations

Species are defined as groups of actually or potentially interbreeding individuals

If two organisms cannot produce fertile offspring together, then they are different species

- if a horse + a donkey mate, their offspring is a mule, which is sterile

- therefore, horses and donkeys are 2 different species under this definition (they are not interbreeding) The boundaries between species are defined by reproductive isolation, the inability of two organisms to successfully produce offspring together

Biological Species Concept (Mayr, 1942)

Reproductive isolation initially results from: - mate preferences (what’s sexy to you?) - when do you spawn (season, or time of day)

- where do you mate

pre-mating isolation: sperm never meets egg Natural selection favors pre-mating isolation, because it stops closely related organisms from trying to interbreed

- hybrid offspring are usually less fit than their parents

-

Reproductive isolation initially results from: - mate preferences (what’s sexy to you?) - when do you spawn (season, or time of day)

- where do you mate

pre-mating isolation: sperm never meets egg After a long period of separation, different populations become too genetically different to hybridize anymore

(3) developmental incompatibilities produce infertility or death in hybrid offspring....

post-mating isolation: genetic incompatibility results in dead or infertile offspring....

Step 1 – gene flow between 2 populations is interrupted

Step 2 – populations start becoming different, by random mutation or by adapting to different environments

Step 3 – mating preferences change (they don’t make babies)

Step 4 – post-mating isolation eventually results (they can’t make babies)

The populations have now speciated, or become distinct species

What can initially interrupt gene flow between 2 populations?

How do species form?

Edges of a species’ range are often defined by a geographical boundary (a river, a mountain range)

- species distributions can be determined by geological features

This is the result of allopatric speciation – physical barriers prevent migration between two populations, leading to speciation

Allopatric speciation

Consider a species found in a desert and a neighboring forest

Allopatric speciation

Hot, dry desert

Cool, rainy forest

Selection will favor different alleles in the desert and forest

Allopatric speciation

Hot, dry desert

Cool, rainy forest

Natural selection will favor forest-adapted individuals

Natural selection will favor desert-adapted individuals

After selection, the two populations will be genetically different

Allopatric speciation

Hot, dry desert

Cool, rainy forest

forest-adapted individualshave survived here

desert-adapted individualshave survived here

But, migration will keep mixing alleles between the populations

Allopatric speciation

Hot, dry desert

Cool, rainy forest

But, migration will keep mixing alleles between the populations

Allopatric speciation

Hot, dry desert

Cool, rainy forest

Natural selection won’t result in desert-adapted & forest-adapted populations when gene flow is high; remains one population

Now: say a barrier to gene flow arises between the 2 habitats

Allopatric speciation

Hot, dry desert

Cool, rainy forest

mountains

Now: say a barrier to gene flow arises between the 2 populations

Allopatric speciation

Hot, dry desert

Cool, rainy forest

Natural selection will favor forest-adapted individuals

Natural selection will favor desert-adapted individuals

mountains

Each population may evolve into a distinct, well-adapted species

Allopatric speciation

Hot, dry desert

Cool, rainy forest

Forest-adapted population Desert-adapted population

mountains

Species #1 Species #2