336-3 1

An overview of lines of evidence for evolution(or evolution in a nutshell)

Learning objectives:To assess types of evidence for evolution,

including:1. Evidence of change through time2. Evidence of shared ancestry3. Evidence for the action of natural selection

Background reading: Chapters 2-3 in FreemanUp next: Population Genetics (review Hardy-Weinberg)

336-3 2

Major contributions of Darwin’s work:

1. Populations and species change overtime = evolution happens.

2. All species are related through commonancestry = the tree of life connectsliving and extinct species.

3. Evolution happens through the actionof natural selection = differences insurvival and reproduction amongindividuals lead to changes in thecharacteristics of populations.

336-3 3

Evolution Defined

Evolution: Change in allele frequencies throughtime, OR changes in the traits of a speciesthrough time, OR descent with modification

A few things to keep in mind (or they willhaunt you):

• Evolution is not directed or directional - changedoes not imply improvement.

• Natural selection is not the only force that canbring about change in allele frequencies.Sometimes, change happens by chance.

• Example:336-3 4

1. Evidence of change through time

The Fossil RecordFossils are traces of organisms that lived in the past.Where do fossils form?

The fossil record provides evidence of change in the formof extinctions, and evidence of transitions in structuresthrough time.

336-3 5

Patterns of Change Through Time:Extinction

Extinction in the Fossil RecordFossils provide evidence for the existence of forms of lifethat have not existed in historic times.

The simple existence of fossils indicates that thedistribution, abundance and compositionof the fauna and flora has changedthrough time.

336-3 6

Patterns of Change Through Time:Extinction

Extinction and the ‘law ofsuccession’

The ‘law of succession’, first notedby 18th century paleontologists,describes a pattern in which fossilstend to be found in the samegeographical areas as their extantrelatives.Example:

Reconstruction of an extinctgiant kangaroo from fossilevidence from Australia

336-3 7

Patterns of Change Through Time:Transitional Forms

In some groups, the sequence offossil forms provides clear signsof change through time.

Most compelling is the existenceof transitional forms, fossils thathave traits intermediate betweenolder and more recent species.

Fabulous examples exist in thefossil records of horses andwhales.

Horse feet 336-3 8

TransitionalForms in Whales

While the closest modern dayterrestrial relatives of whalesbelong to the group of mammalsthat include cows and hippos,the fossils record provides aclear picture of the transitionfrom terrestrial to aquaticliving.Note the gradual loss ofterrestrial limbs, general changein form, including skull shape.

336-3 9

Evidence of change through time:Vestigial Traits

Some fossil and living organisms show evidence that theirancestors once possessed functional traits that no longerfunction (or partially function) in living representatives.

These vestigial traits include such things as the reducedwings of ostriches and kiwis, the human tailbone,‘goosebumps’.

336-3 10

2. Evidence of shared ancestry

Similarity often reflects commonancestry:Given that organisms changethrough time, the similaritiesbetween some groups oforganisms is often found toreflect shared recent commonancestry. When we studypatterns of DNA variation, wefind evidence of commonancestry between similar formsthat our intuition often suggestsare closely related.

DNA-basedphylogeny of thecarnivores

336-3 11

Evidence of Relatedness:Homology

Homology

Homology is a powerful concept that describes sharedsimilarities in structures, DNA sequences, behaviours,developmental sequences, due to shared common ancestry.

Evidence of shared homology is pervasive in nature: e.g.the fact that life forms share the same genetic code(with a few exceptions), that eukaryotes have DNAorganized into chromosomes, that eukaryotes share basiccell structures, that vertebrates with limbs all have fourlimbs, etc. are all examples of homologies.

336-3 12

Evidence of Relatedness:Structural Homology

Structural HomologiesStructural homologies can be seen in a host of structuresacross the tree of life. For example, in the limbs ofvertebrates.

336-3 13

Molecular evidence of homology:The genetic code

336-3 14see figures 3.23, 3.25, 4.6

Molecular evidence of (non-)homology: developmental biology

336-3 15

Pax6 controls eye development inanimals from flat worms to humans!

The human genetic condition aniridia (in which the iris is poorlydeveloped), as well as the mutant phenotypes small eye in mice andeyeless in Drosophila are all caused by mutations in Pax6.

336-3 16

Evidence of Relatedness:Genetic Homology

Genetic HomologiesGenetic homologies exist for a number of genetic regions.

(You can read moreabout the effects ofvariation in Hox andHOM genes and theirrole in thediversification ofanimals in section 19.2and Fig. 19.3 ofFreeman)

336-3 17

3. Evidence for natural selection

Natural Selection: The process whereby someindividuals contribute more offspring to the nextgeneration as a consequence of possessing some traitor traits that enhance their ability to survive orreproduce.

Artificial Selection: The process of selection wherebycertain traits in an organisms are considered favorableand are selectively bred by humans (humans decidewho survives and reproduces).

We’ll next go through an example of each from recentliterature.

336-3 18

Adaptation(noun)

Traits that enhancean organismsfitness are calledadaptations. Manytraits in nature areclearly adaptive,for example, thereis a dead leafpraying mantis inthis image. head thorax

336-3 19

Evolution in action

When we see the spines on a cactus, we understand thatthese structures would help deter herbivores. The crypticcoloration and shape of a katydid (below) is also clearlyadaptive.

A evolutionary biologists, we ask what evidence we cangather to show the steps involved in producing theseadaptations?

Evolutionary change canbe seen in humanlifetimes in nature andin artificial experiments.

336-3 20

• Studied weedy, wind-dispersedplants located on the islands offthe west coast of VancouverIsland

• Censused the plant populationsof 200 islands and a region ofthe mainland over a 10-yearperiod

• Extinction and recolonizationevents occurred frequently onthe islands.

Evolution in Natural PopulationsDispersal in island plant populations – Cody & Overton 1996

336-3 21

Evolution in Natural PopulationsDispersal in island plant populations – Cody & Overton

1996

Studied two species:Hypochaeris radicataLactuca muralis.

William S. Justice @

USDA-NRCS PLANTS Database

© Markku Savela 336-3 22

A BC example: change in dispersalability of plants on islands

The two species under study have fruits that areadapted for wind dispersal.

Individuals with a relatively higher pappusvolume/ seed weight) are better dispersers(their seeds will travel farther), and may havean advantage in colonizing islands, butmay be at a disadvantage once a populationis established on a small island.

pappus

Seed

336-3 23

A BC example: change in dispersalability of plants on islands

They found that old islandpopulations had decreaseddispersal ability relative tothe mainland populations.

They also saw thatnewly foundedpopulations had higherdispersal ability thanboth mainland and olderisland populations.

336-3 24

Speciation and selection?(An example of artificial selection in the lab)

Provisional definition of species (until we revisit the topiclater in the term): A species is an interbreeding group oforganisms that is reproductively isolated from (does notinterbreed with) other groups of organisms.

While there have been numerous demonstrations of theaction of natural selection in nature and in the lab, thetheory of evolution by natural selection also implies thatnatural selection plays a role in the birth of new species(speciation). Contemporary evidence (natural orexperimental) for this is more challenging to acquire.Speciation is harder to catch ‘in action’, because it oftenhappens over much longer time scales.

336-3 25

• Two papers by Rice and Salt (1988, 1990) impose artificial selectionon populations of fruit flies (Drosophila melanogaster) to test thehypothesis that selection on habitat preferences can lead to thedevelopment of reproductive isolation, a key step in speciation.

• The researchers set up an artificial habitat in which flies werefree to choose a habitat. Flies were found to take at least 8-12hours to make three choices about the habitat where they wouldreproduce. The choices involved light (light or dark), direction (upversus down), and smell (ethanol or acetaldehyde). They couldchoose any combination of these three features of theenvironment. In addition, researchers selected flies based onwhether they developed early (10 days) or late (14 days).

• They asked whether over successive generations, flies would evolvea preference for a given habitat type, and thus show evidence ofthe beginning of reproductive isolation.

Selection and speciation?Habitat selection in Drosophila

(Rice & Salt 1988, 1990)

336-3 26

• Flies were attached to the maze aspupae, and upon emergence as adultsthey entered the maze and chose

– lightness or darkness (leftvs. right) = selection forphototaxis

– up or down = selection forgeotaxis

– acetaldehyde (white vial) orethanol (black vial) =selection for chemotaxis

• Experimenters choose time period:early (E), middle (M), and late (L) –selection for development time

Selection and speciation?Habitat selection in Drosophila

(Rice & Salt 1988, 1990)

Up/ down choice

Acetaldehyde/ Ethanol choice

336-3 27

• Flies mated within the maze

• Control lines: 120 females chosenrandomly

• Selected lines – 60 females each from:

• dark, up, acetaldehde, early (5E)• light, down, ethanol, late (4L)

Habitat selection in Drosophila

336-3 28

• Experimental larvae were mixedand placed together in the mazeto start the next generation.

• Controls were run through themaze separately.

• Offspring of mothers collectedfrom 5E and half of the controlswere raised on a chemical thatturned their eyes brown. Thisallowed the researchers to trackany change in habitat preferenceover successive generations).

Habitat selection in Drosophila

336-3 29

5E

5E

4L

Dotted line: 5E

Solid line: 4L

1 Generations 35

% o

f flies

with

mot

hers

fro

m 5

E(a

s mea

sure

d by

the

% o

f flies

at 5

E an

d 4L

with

brow

n ey

es)

Control lines

Selected lines

5E

4L

Results of Habitat selection inDrosophila

The flies haveaccumulateddifferences inhabitatpreferencethat lead toreproductiveisolation = thestart ofspeciation! 336-3 30

5E

5E

4L

Control lines

5E

4L

Results of Habitat selection inDrosophila

These results show that over time, the flies in the experimental linesevolved stronger habitat preferences, that led to reproductiveisolation. Because mating occurred near the food vials (after theflies had chosen a habitat), flies from 5E tended to mate with otherflies from 5E, just because they shared the same habitat choice.Over time, this lead to a stronger tendency for the offspring of fliescollected at 5E to repeat their mother’s habitat choices.Note that when flies from 5E and 4L were mixed together for matingtrials outside the maze, they showed no mating preferences - themating patterns was entirely driven by habitat choice.