Natural selection

Outline of the day

1. Turn in your lab reports at the front– Both the tidepool trip report and the horticulture

tour worksheet– More than 10 minutes late = bad

2. Any questions on last week’s lab?3. Quiz4. Introduction to the lab5. Lab!6. Check out

• Get a stamp• Make sure I mark you down for attendance

Quiz

• Ends 8 minutes after it’s started

– Ends at: ____

Lab this week!

• Looking at evolution by natural selection– Brief introduction– Modeling natural selection with tools– Selecting on alleles

A question

How might we be able to explain the diversity of life around us?

--OR--

How do you think all the species on earth came to be?

Evolution

• “Descent with modification” - Darwin

• “The change in a lineage of populations between generations” - Ridley 1993

• “Any genetic and resulting phenotypic change in organisms from generation to generation” - Purves et al. 1995

Evolution is change in a population over time …

• But how does this change over time occur?

Natural selection• Selection on traits that have heritable variation

– Those individuals that produce the most surviving offspring pass on their traits to more individuals than those individuals producing fewer offspring

• Thus certain traits increase in prominence in a population

• In other words:– The organisms that are best able to survive and reproduce

• Have the most offspring – And thus more of the next generation of that population is made

of up organisms with the genes of the best surviving and reproducing individuals

» So the population has changed over time.» Lather, rinse, repeat (and combine with a heterogeneous and

changing environment) to cause speciation, etc.

A catch-phrase we’ve all heard …

• “Survival of the fittest”• More accurately, “Reproduction of the fittest”

– Survival is NOT the key, reproduction is

• Individuals that have the highest reproductive output have the highest fitness– Fitness ranges from 0 to 1

• 0 = no offspring• 1 = tons of surviving (and reproducing) offspring

A key point

• Evolution occurs at a population level, not an individual level– An individual’s genetic makeup is fixed; it’s not going

to change.– But the genes contained in a population can change

over time (as less fit individuals die / reproduce less, and more fit individuals survive and reproduce quickly)

• So a population of squid can evolve in response to a new predator being introduced …– But a single squid can not.– Let’s watch squid respond to a predator that

selectively kills only small squid :)Images CC from: population - http://flickr.com/photos/bluesmoon/290785871/l; small - http://flickr.com/photos/maynard/842182839/

Squid!

Population with variation; Lots of small and big squid

(all of them happy)

New predators start eating small squid

Small squid get eatenLarge squid survive and reproduce

Over time all small squid are killed;only large squid are left in the population.

The population of squid has evolved!

(A quick, hypothetical example)

Images CC from: population - http://flickr.com/photos/bluesmoon/290785871/l; small - http://flickr.com/photos/maynard/842182839/; giant http://flickr.com/photos/checco/328347060/

For natural selection to occur three components are needed

1. Selection – some sort of selective force that alters organisms’ fitness

2. Heritable traits – the selective force needs to select on some trait that is heritable

3. Variation – there needs to be variation in heritable traits on which the selective force can differentially act

Natural selection

• “The process by which individuals with certain heritable traits tend to produce more surviving offspring than individuals without those traits, resulting in a change in the genetic makeup of the population.” – Freeman 2002

What might cause natural selection?

• Anything that can cause a change in the reproductive output (fitness) of an organism!

• Environmental pressures– Droughts, flooding, fires– Heat waves / changing temperatures– Amount / type of food available– Number of mates available– Predation

• Internal pressures– Circulatory / respiratory system’s ability to provide O2 /

nutrients– Balance of reproduction with other body maintenance tasks

(do you grow or reproduce?)– Ability of an organism’s skeleton to support its body

What are we doing in lab today?

• Modeling evolution by natural selection

• We’re going to become predators– We’ll be competing for prey– Our population will start out with a variety of

mechanisms of capturing prey– The more prey you eat, the more you’re going to be

able to reproduce

• We’ll go through three generations of selection

How will this work?1. Each person will start out with a different mouthpart

– Represented by tools: forceps, pliers, rules, screwdrivers, spoons, etc.

2. We’ll go outside to capture “prey” (toothpicks)– Must pick up prey using your mouthpart (tool) and move

it into your stomach (beaker)– Cannot steal food from another person’s stomach

(beaker), but can steal food at any other time– Do not harm your classmates!

3. We’ll come back in and count how much food everyone captured

• Those mouthparts that captured the most food will reproduce the best; those that captured the least will die out

Let’s go do this!

What we want to do now:

1. Count the number of toothpicks you captured

2. Add up the total number of toothpicks that were captured by those with your mouthpart type

• Give me this number!

3. Reassign mouthpart types and do it again!

Questions:

• Were the three components of natural selection (selection, heritability, and variation) present in our model?

• How would our results have changed if the food-type changed?– (e.g., if we changed eating to turning a screw

or filling up a cup with water)

Next: Do the marble selection• This models selection at a population genetics level

– Black and white marbles represent different alleles

• An individual has two alleles– Those two alleles will be drawn at random from a population of

alleles• This is much like how real reproduction work – you randomly

combine two alleles from your parents!

– The chart indicates which individuals live, and which have lethal mutations

• Carry out this selection for 5 generations for both scenarios (recessive lethal gene or dominant lethal gene)

• Remove any individuals who have lethal mutations from the population before doing the next generation

Before you leave

• Clean up your work area

• Show me your lab report so I can stamp it– Need to have all data fields filled in– Complete at home and then turn in at the beginning of next lab

• Remember that we’ll have a quiz at the beginning AND end of the next class– First quiz:

• 6-7 questions on today’s lab• 3-4 questions on the lab we’ll do next week

– Second quiz:• On next week’s lab

Notes for the instructor:

• I add quite a bit of introduction to this lab, as well as discussion afterwards, as I feel that our simulation isn’t intuitively linked to evolution by natural selection, especially if the students haven’t thought about evolution for many weeks. Thus, I think the intro and wrapup discussion help clarify that link, as well as help provide additional clarification on the topic. I may be going overboard, though.

• Add any relevant cleanup instructions to the final slide (that slide is a generic one I’m adding to each presentation).

License information

• This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/us/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.

• The slides in this presentation were originally created by Marc C. Perkins (http://faculty.orangecoastcollege.edu/mperkins).

• You are free to use, modify, and distribute these slides according to the terms of the Creative Commons license (e.g., you must attribute the slides, no commercial uses are allowed, and future distributions must be licensed under a similar license).

• Attribution should be given to Marc C. Perkins (and any later editors), including a link back to Marc’s current website. This applies both while distributing the slides and during use of the slides; attribution during use can be satisfied by, for instance, placing small text on at least one of the slides that has been shown (see below for an example).

Slides in this presentation based on those created by Marc C. Perkins. http://faculty.orangecoastcollege.edu/mperkins

History

• August 2007: Marc Perkins released first version. http://faculty.orangecoastcollege.edu/mperkins

(If you modify these slides and redistribute them, add your information to the list)