Transcript Lecture 04 - Phylogeny + speciation - Cal State LA

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
Selection on color in guppy fish
streams with
predatory fish
streams without
predators
Females
Males
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)
Showing Evolutionary Relationships on a Cladogram
- at each branch point,
a new characteristic
appeared in a common
ancestor of all later branches
- shared, derived characteristics are called synapomorphies
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
Feathers: a synapomorphy uniting all birds
Feathers
(turkeys)
Feathers
(parrots)
Feathers (dinosaur)
Scales (early reptile)
Scales (ancestral fish)
Scales
(snakes)
Scales
(bony fish)
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
Panthera pardus
or
P. pardus
The binomial name
combines the genus
and species name of
the organism
Monophyletic group: includes ALL descendants of an ancestor
Paraphyletic: leaves out some descendants
Monophyletic
Paraphyletic
Most biologists think only monophyletic groups should get
names, because only they are “true” groups
Clade = monophyletic group
Birds = clade
Reptiles: paraphyletic group
(not a clade)
Mammals = clade
Why are paraphyletic groups given names?
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
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)
Species concepts
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
Note – “species” is both singular (one species) and
plural (two species)
there is no such word as “specie” !!
Definitions for discussing speciation
Big question: what can cause one population to split into 2 new
populations that do not reproduce with each other?
Migration
Dispersal
movement of individuals between populations
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
Biological Species Concept (Mayr, 1942)
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
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....
How do species form?
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?
Allopatric speciation
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
Cool,
rainy
forest
Hot, dry
desert
Allopatric speciation
Selection will favor different alleles in the desert and forest
Cool,
rainy
forest
Natural selection will favor
forest-adapted individuals
Hot, dry
desert
Natural selection will favor
desert-adapted individuals
Allopatric speciation
After selection, the two populations will be genetically different
Cool,
rainy
forest
forest-adapted individuals
have survived here
Hot, dry
desert
desert-adapted individuals
have survived here
Allopatric speciation
But, migration will keep mixing alleles between the populations
Cool,
rainy
forest
Hot, dry
desert
Allopatric speciation
But, migration will keep mixing alleles between the populations
Cool,
rainy
forest
Hot, dry
desert
Natural selection won’t result in desert-adapted & forest-adapted
populations when gene flow is high; remains one population
Allopatric speciation
Now: say a barrier to gene flow arises between the 2 habitats
mountains
Cool,
rainy
forest
Hot, dry
desert
Allopatric speciation
Now: say a barrier to gene flow arises between the 2 populations
mountains
Cool,
rainy
forest
Natural selection will favor
forest-adapted individuals
Hot, dry
desert
Natural selection will favor
desert-adapted individuals
Allopatric speciation
Each population may evolve into a distinct, well-adapted species
mountains
Cool,
rainy
forest
Forest-adapted population
Species #1
Hot, dry
desert
Desert-adapted population
Species #2