Evolution Lecture 17

Chapter 11

Topics for today

• Examples of adaptations
• Cautionary notes about adaptation
• How to study adaptation
• EvoBeaker: Sickle cell alleles
• “On your own”

Evolution can’t be totally random!

What about adaptations?

• Characteristics that enhance survival and reproduction
• Plants grow toward light
• Or not!

Adaptation of the growth and development process

• Young tropical vine grows toward the densest shade until it finds a trunk to climb
• Produces smaller oppressed leaves in shade
• Large leaves in light

Monstera tenuis

Fig. 11.11

Adaptation that exploit other organisms

• Orchids flowers mimic female insects
• Emit sex pheromone
• Males insects “mate” and pollinate

Some animals have features & behavior that only benefit the group

• Ants and other social insects
• Only one reproductive female
• Sterile workers form chains to pull leaves together for nest
• Other workers carry larvae that emit silk and weave leaves together

Fig. 11.14

Natural selection is the only evolutionary process that explains adaptation

• Organisms that possess variations that consistently enhance their survival or reproduction replace those less suitable that have lower survival or reproduction
• Natural selection occurs whenever there is statistically significant variation in fitness
• Fitness is the average per capita rate of increase in numbers and includes:
• Probability of survival to various reproductive ages
• Average number of offspring via female function
• Average number of offspring produced via male function

How do we define adaptation?

• Trait that enhances fitness compared to some other alternative
• Is a historical perspective necessary?

· Emphasis on today

“An adaptation is a phenotypic variant that results in the highest fitness among a specified set of variants in a given environment” Reeve and Sherman 1993

· Emphasis on past

“For a character to be regarded as an adaptation, it must be a derived character that evolved in response to a specifc selective agent” Harvey and Pagel 1991

Cautionary notes

• Useful functions can be acquired for traits that arose by natural selection for other purposes
• The opposable thumb:
• Selection for greater digital dexterity?
• Selection for piano playing ability?
• Sutures of infant skull are an excellent “adaptation” to aid in birth
• But reptiles and birds also have them
• Structures may be co-opted to serve a different function
• Guillemot uses it wings to fly and swim
• Penguins wings have been completely co-copted for swimming and have been further modified for that purpose

Fig. 11.17

Selection on some traits may be incidental

o Hypothetical example

• Sieves select for progressively smaller balls
• Ball size is under direct selection
• Small balls also also red
• “red” is not under direct selection

Fig. 11.5

• Real example
• Under selection to bind oxygen via an iron-containing heme group
• Iron is red. Hemoglobin is red. But red coloration is not under direct selection

Cooperative behavior may only appear to altruistic

• Reduces fitness of the individual
• Increases “inclusive fitness” via the success of its relative with whom it shares genes
• Selfishness with kin can evolve similarly

Fig. 11.14

Not all adaptations directly benefit the individual

• Some genetic elements may propagate themselves
• Increases the fitness of the gene
• Decreases the fitness of the individual that carries it
• An example: self genetic elements
• Inherited through the sperm
• Destroys paternal set of chromosomes
• All offspring are haploid
• Thus, all offspring are male
• Selfish genetic element guarantees its transmission to the next generation

How can we study natural selection and adaptation?

· Experimental studies

o Growth rate of short lived organisms

o Experimental manipulation of adaptive trait

· Comparative method

o Trait convergence under similar selection

Examples of experimental approaches

Experimental study of growth rates

· b-galactosidase breaks down lactase

· Different mutant strains grown in competition with wild type

· Lactose was sole energy source

· Ratio of mutant vs. wildtype strains monitored over time

· his+ codes for an enzyme that synthesizes histidine

· his- defective

· his+ and his- grown in competition with histidine

· Surprising fluctuation in alleles

· his alleles hitchhiking with advantageous mutations at other loci

Fig. 11.7

Experimental manipulation

· Do long tails enhance male reproductive success?

· Conflicting selection pressures result in tradeoffs

Fig. 11.9

Comparative method

Observe unrelated organisms in similar environments

• Similar structures that repeatedly evolve are adaptive

Develop hypotheses about traits that should repeatedly evolve if they are adaptive

• Breeding systems select for different sexual attributes
• Polygamous
• é sperm
• é matings
• é Sperm competition
• Monogamous
• ê sperm
• Single mating
• No sperm competition

Fig. 11.20