Lecture 4 Evolution and Life Histories Principles of Ecology Eben Goodale College of Forestry, Guangxi University

Lecture 4Evolution and Life Histories

Principles of EcologyEben Goodale

College of Forestry, Guangxi University

Readings

• Up until now I have made reading notes for each lecture.

• I will make one more of these for Tuesday lecture (my wife presenting).

• It will be available Sunday night.• After that, we will have 2 chapters of textbook

(ocassionally 1) per lecture.

Readings

• The textbook chapters will be available as .xps files (10 pages per file).

• I will give out today pendrives with Chapters 9-12 (Apr 4, Apr 7 lectures).

• Please DO NOT give copies of these files to anyone outside of class (technically illegal).

• I do not like distributing like this, but do so because textbook not distributed in China, and very expensive to buy privately.

Readings

• Please do readings before class, because it will serve as a preparation for class.

• The exam material will, however, be directly take from lectures, not the textbook.

• Before the exam, I will make a summary for you on the concepts in the lectures that I will test you on.

• Lecture PPTs will continue to be available before class on Thursdays and Sundays.

Schedule Lecture times:

Review I• We have been talking about how organisms

interact with their environment, obtaining water, maintaining their temperature, and obtaining energy.

• We have described features of organisms which are “adapted （适应）” to their environment (fit their environment due to evolution).

• We have described some examples of “convergent evolution （趋同进化）” , where different organisms in similar environments look alike.

Today we talk more in depth about evolution

Today’s class

• Evolution– Evolution as change– Natural selection as mechanism of evolution– Other mechanisms of evolution– Speciation and evolutionary patterns

• Life histories

Evolution （进化） is change

• At the large time scale evolution is modification (change) in living things over time.

• Contrasted with creationism, which says that:1)species were developed independently of

each other2)species are fixed entities, they do not change3)creation was recent

Evolution is changeThings that led Darwin to believe in evolution:•Fossils•Vestigial （退化的） features•Relations between species

Vestigial features have lost their function, like eyes on blind salamanders, or the human tail bone.

Evolution is change

Things that led Darwin to believe in evolution:•Fossils•Vestigial features•Relations between species

Galapagos “Darwin” finches…The birds on different islands resembleeach other but also have differences.

Evolution is change• Contrasted with creationism, which says that:1)species were developed independently of

each other. Clear relationships among species.2)species are fixed entities, they do not change.

Fossil animals, vestigial features.3)creation was recent. Evidence of ancient

earth.

Question: was Charles Darwin the first to postulate evolution?

Evolution is change

• At a small time scale, evolution is the change in alleles （等位基因） (or change in gene frequencies) in a population over generations.

• Example of allele controlling coat color, or beak size.

An allele is an alternative version of a gene.

Natural selection is a mechanism of evolution

• So how to explain evolution as the small time scale in a population?

• Darwin postulated natural selection （自然选择） as a mechanism of evolution (this was his novel contribution).

• Let’s review the basic steps in the process.

Natural selection

What are the basic steps in this process:1. Variation between individuals in traits2. This variation is the trait is heritable （可继

承的）3. In every generation, there are more

offspring produced than can survive. 4. Individuals with traits that fit the

environment well survive and reproduce.5. More individuals in the next generation will

have the favored trait.

An important distinction:phenotype vs. genotype

• We see that the first ingredient for natural selection is variation, and we note that if variation is hereditary, natural selection will occur.

• BUT not all variation is genetic. Some of how an organism appears (its phenotype （表现型） ) is environmental. Think of a large, spreading tree in a good sunny place, compared to one in the middle of a forest.

• We should keep in mind that phenotypic plasticity （表现型可塑性） can explain variation among individuals.

• What human examples can you think of?

This booktells storyof identicaltwins separatedat birth

Natural selection in action

Images from: Freeman and Herron (2001)

Darwin’s Finches


Daphne Major

Peter and Rosemary Grant

Beak of the Finch1995By J. Weiner


1. Variation between individuals in traits



2. This variation is heritable


Natural selection in action3. In every generation, there are more offspring

produced than can survive.


# Birds

1975 1979

# Seeds

4. Individuals with traits that fit the environment well survive and reproduce.

5. More individuals in the next generation will have the favored trait.



Characteristicof seeds Small seeds

Large seeds

4. Individuals with traits （特质） that fit the environment well survive and reproduce.

5. More individuals in the next generation will have the favored trait.



1976

1978

Review: using another example, explain how evolution occurred

Year

1975 2005

Weight of sheep

Horn length

What Darwin didn’t know: genetics and mutations.

• We now know that genetic information is encoded in DNA.

• Mistakes in DNA copying make mutations （突变） , which create variation.

James Watson and Francis Crick, 1953

Wild type

A U G A A G U U U G G C U A AmRNA

5Protein Met Lys Phe Gly

Stop

Carboxyl endAmino end

3

A U G A A G U U U G G U U A A

Met Lys Phe Gly

Base-pair substitution

No effect on amino acid sequenceU instead of C

Stop

A U G A A G U U U A G U U A A

Met Lys Phe Ser Stop

A U G U A G U U U G G C U A A

MetStop

Missense A instead of G

NonsenseU instead of A

The simplest kind of mutation:a point mutation where onenucleotide replaced by another.

What Darwin didn’t know: genetics and mutations.

• Mutations can also be larger: like the deletion of entire parts of a chromosome.

• In addition to mutation, recombination （重组） , which occurs during sexual reproduction, also produces variation

Mutation as an evolutionary force

• Mutation and recombination produce variation among individuals, the first ingredients for natural selection.

• Mutation can also be thought of as a mechanism of evolution because it creates new alleles.

Another evolutionary force: genetic drift

Genetic drift （基因漂变） is the change in the frequency of alleles in a population over time through random events

Before After

Genetic drift

Genetic drift is more important factor in smallpopulations than large ones.

Cheetahsare an exampleof a small population

One example of genetic drift is when a species gets very rare

“Bottleneck”

Demonstration of Genetic drift in simulation:http://sites.sinauer.com/ecology3e/problem06.html

Or when a small group of individuals starts a new colony

Another evolutionary force: gene flow

• Gene flow （基因流） is when alleles move between populations.

• Because it tend to make populations more similar to each other, it acts to slow evolution.

Summary of evolutionary forces

• Natural selection: increases frequency of advantageous alleles.

• Mutation: creates new alleles.• Genetic drift: change in frequency of alleles,

can lose to loss of alleles in small populations.• Gene flow: movement of alleles between

populations. Tends to decrease changes in alleles.

Only one of these mechanisms of evolution leads to adaptation.Which is it?

Only natural selection creates adaptations

• Adaptation is the match of an organism to its environment.

• A great example is the story of the Soapberry Bugs.

The length of beak of soapberrybugs in different populations

There are limit to adaptations• There needs to be the variation

for natural selection to work with.

• The history of animals limits their possibilities. For example, mammals that return to the sea (dolphins) can’t just evolve the ability to breathe under water.

• Gene flow often acts against adaptation.

• The environment is itself changing… so there’s a “moving target”.

Nor can a panda immediately evolve an opposable thumb.

What does natural selection lead to?

What kind of selection was our bird example? Our sheep example?

Stabilizing （稳定性） selection

• Idea of trade-offs

Size of babies

Low birth weightoften higher riskof infection

High birth weightcomplications at birth

www.kellymannophotogr

aphy.com/

Disruptive selection

Back to Darwin’s finches …what would happen if twotypes of seeds were available…very large and very small?

www.blackwellpublishing.com

This kind of natural selection is rare and maylead to speciation…

T. B. Smith, Nature, 1987

Speciation

• Speciation （物种形成） can occur when:– Two populations become

isolated– Each population

undergoes evolution, usually through natural selection.

– When they meet again they do not reproduce.

Speciation

• Often occurs because these two populations are in different places (allopatric speciation（区域种化） ), but can occur in same place (sympatric speciation（同域物种形成） ) in some circumstances.

Famous example ofsympatric evolution, whereone population of applefly adapted to new invasivetree species

Speciation

How does Benkman (2003) hypothesize that speciation is occuring?

Assortative mating

Each type of cross-bill has itsown call type (so they canRecognize each other)

A phylogenetic (= evolutionary) treeshows history of speciation and extinction

This recent example of whale evolution similar to ideas Darwin proposed in his notebooks in the 1850’s

Story of life includes periods of mass extinction, followed by adaptive radiations• In history of earth, 5

mass extinction （大量消亡） events may have removed many species.

• Adaptive radiations（适应辐射） are when a group of organisms give rise to many species in a short time

Adaptive radiations occur when a new niche opens up after extinction (mammals afterdinosaurs), or a new place is colonized (Hawaii is formed by volcanos)

Evolution and ecology and linked together

• Take for example, the case of predator and prey, an ecological relationship.

• Over time predators become more efficient at catching prey through natural selection.

• But at the same time, prey become more efficient at escaping predators!

• Ecology at one instance a product of ever-changing “coevolutionary （共同进化） armsrace”.

Evolution and ecology and linked together

• Ecology also influences evolution:

• Take care for instance the soapberry bugs.

• New environments (different trees) make different kinds of beaks adaptive.

• Leads to two different kinds of populations and eventually speciation.

• Ecological change has lead to evolution.

Today’s class

• Evolution– Evolution as change– Natural selection as mechanism of evolution– Other mechanisms of evolution– Speciation and evolutionary patterns

• Life histories （生活史）

The case study of NemoWe all know that movie’s are not realistic… but did you know that in real life Nemo’s father probably would have changed sex and turned into a female?

Sequential （连续地） hermaphroditism（雌雄同体）

The diversity and complexity of lifecycles

Polyp reproducesasexually Small larva floating at sea

Metamorphosis into attached polyp

Colony of identical polyps

Release egg and spermNote :•Sexual / Asexual life phases•Metamorphosis （变形） Estimated 80% of animal species undergo

The diversity and complexity of lifecycles

Even more strange (to humans) becauseHaploid （单一的） life stages aremulticellular.

Basic question: to be sexual or not?

• Advantages asexual

Basic question: to be sexual or not?

Advantages sexual•Sexual reproduction increases genetic variability (new combinations of genes, recombination)•Evidence from C. elegans worms

Life history

• The study of life history looks at the timing of how organisms grow, develop, reproduce and survive until death.

• Stress on trade-offs（物质交换） . For example, early reproduction often means low survival.

When firstreproduce?

How manyoffspring?

Size of offspring?

How long survive?

Trade-off I: size of offspringTrade-off: the bigger your offspring, the less you can have of them.

For plants: For animals:

Advantages of being BIG

• Bigger organisms have fewer predators bigger than they are.

• Big seeds, eggs have more resources to sustain when very young

Bigger seed, better chance of surviving, growing

Seed size

Seedlingheight

An extreme in large offspring: the kiwi

Advantages of being small

• Small things can disperse farther– Think small wind-blown

seeds of weedy plant.– Or young fish that is so

small it has little resistance to waterflow and disperses widely

• Small things can also undergo dormancy （休眠） , and wait until conditions are good to grow.

Trade-off II: age when first reproduce

Trade-off: the more investment in reproduction early,the less survival.

It takes a lot of energy just to find mates (fly example)In addition, any that puts a lot of energy into reproduction has lessresources for its own growth and survival (tree example)


Trade-off: the more investment in reproduction early,the less survival.

In birds, individuals with increased clutches Survive the winter less well.

From Santos and Nakagawa 2012

• Extreme lifecycle called semelparous （只生一胎的） (reproduce only once): annual plants, insects…also bamboo, salmon (“big-bang”).

• Only once but big numbers.

• In contrast humans, perrenial plants are iteroparous （多繁殖的） .

• Reproduce repeatedly but not large numbers at any one time.


Advantages of short, long life

Advantages short life Advantages long life

Get in, get out of new area Stick around, but faceThe growing competition

Think weedy plants

r vs. K selection• Putting together these

trade-offs, we see two different strategies as extremes on a continuum:

• “r species （ R- 选择物种）” : short-lived species that make many, highly dispersible young. Think of a weed.

• These species go to a disturbed environment (plowed ground), grow quickly, reproduce, and leave (are out-competed).

r vs. K selection• “K species （ K- 选择物

种）” : long-lived species that have only a few, large offspring at a time, invest in each of them.

• These species live in mature (undisturbed) environments which are competitive (like a high canopy forest).

• Hence, life histories are influenced by ecological stability

Note: we will understand why these species are called ‘r’ and ‘K’ whenwe talk about population growth.

Explaining the clownfish strange system

• Clownfish live in anemones, stinging stationary animals, and are immune to the sting.

• Tiny young are planktonic and disperse widely

• Larger fish find anemones, which can support several fish; the group of fish at one anemone are not related.

• There’s a strong hierarchy among these fish: the biggest fish is the breeding female, second biggest breeding male, others are sexually immature.

• If breeding female dies, male changes sex and becomes female.

Explaining the clownfish strange system

What could explain this strange system?•Predation on reef very high, so small fish leave.•Large fish able to invest more in eggs (the larger gamete), so the largest fish are female.•Why do smaller fish wait their turn?

Homework

• Go over lecture notes again (Lecture 4).• By Sunday night, lecture 5 with notes available

on internet; read over.• http://sites.sinauer.com/ecology3e/

problem06.html This is the problem we looked at in class today.

• Reading for lecture 5: Petrie 4 on sexual selection in peacocks.

Key concepts• Evolution is the modification

of living things over time, and change in gene frequencies in a population.

• Natural selection is the only mechanism of evolution that produces adaptation, a fit between organism and environment. But there are other mechanisms of evolution: mutations, genetic drift and gene flow.

• Speciation and extinction describe patterns of life over long timescales.

• Life histories describe the timing of birth, reproduction and death; in many organisms they involve transitions between different life stages.

• The environment where an organism lives influence its life histories: stable environments favor long-lived organisms that invest heavily in a few offspring, whereas quickly-changing environments favor short-lived organisms with many small offspring.

Documents

Lecture 4 Evolution and Life Histories Principles of Ecology Eben Goodale College of Forestry, Guangxi University