EvolutionMechanisms and Examples

Northwestern Senior High SchoolAccelerated Biology

What is evolution?• In the broadest sense evolution is change over

time.• If two parent species of Australopithecus afarensis have offspring, those

offspring will look very similar to their parents (however not identical). Later, those offspring will reproduce and have offspring of their own. These “grandchildren” of the original Australopithecus will be more like their parents than their grandparents (although still very similar to both). Continue this process for millions of years. Now the current offspring (after millions of years worth of reproducing new generations, each slightly different than the last) may appear considerably different than the original parents. One might argue that they are a new species (Homo habilis).

• This would mean that two parents of one species had offspring that could be considered members of a different species. Sound weird? Sure! But that’s a topic for another time.

What makes this change occur? • Various factors can influence change. • In this class we’ll look at several– Selection

• Artificial• Natural

– Includes sexual selection (peacocks good example)

– Isolation mechanisms– Genetic Drift– Mutation– Founder Effect– Migration

Selection

• Artificial selection:– Artificial selection is the process of changing the

characteristics of animals by artificial means. For example, animal breeders, are often able to change the characteristics of domestic animals by selecting for reproduction those individuals with the most desirable qualities such as peed in racehorses, milk production in cows, trail scenting in dogs.

– Dogs are a great example of artificial selection– All dog breeds have been shown to be descended

from wolves• How?

– Think back to the story on slide 2. Slowly, over long periods of time people “selected” the characteristics they wanted in dogs. More favorable characteristics were passed along and less favorable ones were not.

Selection

• Natural selection: – The process by which heritable traits that make it

more likely for an organism to survive and successfully reproduce become more common in a population over successive generations.

For example, the peppered moth exists in both light and dark colors in the United Kingdom, but during the industrial revolution many of the trees on which the moths rested became blackened by soot, giving the dark-colored moths an advantage in hiding from predators. This gave dark-colored moths a better chance of surviving to produce dark-colored offspring, and in just a few generations the majority of the moths were dark.

Geographic Isolation:

When part of a population of the same species becomes geographically isolated from the remainder, it may over time evolve characteristics different from the parent population.

Members of the two populations will be unable to successfully mate with each other. At this point, a new species has emerged.

Geographical isolation is thus a key factor in speciation, the formation of new species

Spotted owl subspecies living in different geographic locations show some genetic and morphological differences. This observation is consistent with the idea that new species form through geographic isolation.

Mutation• Important in evolutionary change• Most mutations occur in non-coding DNA• Other mutations are not beneficial• Occasionally though, a mutation occurs that is

beneficial and therefore is passed along (lactose tolerance, sickle cell anemia – both explained in great detail within The 10,000 Year Explosion).

Genetic Drift

• The change in the frequency of a gene (allele) over time

• Ex. Hh x HH could yield offspring HH. The recessive allele could be lost. Or Hh x Hh could yield hh. The recessive allele could increase.

To recap:

Evolutionary change in a species can be caused by:

1. Selection (natural, artificial)2. Genetic Drift3. Isolation mechanisms4. Mutation5. Genetic drift6. Founder Effects (we’ll get to these last two later)7. Migration

Evolution evidence within the geologic strata

• It is vital to gain an appreciation for deep time, not hundreds or thousands of years, but millions of years.

• NOT always an easy concept to grasp:– Check out Turney’s Bones, Rocks, and Stars • Excellent book, helps convey a sense of deep time

• The Earth is old!– Approx. 4.6 billion years old• Carbon dating, tree rings, K Ar dating, Uranium decay,

universe’s rate of expansion, etc.

The Fossil Record

How do fossils form?

• Fossils form when layers of sediment build up over a specimen

• Minerals from the surrounding rock seep into and fill the space between cells. As organic material decays the minerals crystallize and remain behind, thus preserving the former bone as rock

• Soft materials may also leave impressions in soft sediments. When those sediments harden the impressions may remain as fossils.

Fossils are found in layers of sedimentary rock

• Deeper layers generally contain older materials

The history of the Grand Canyon can be seen by studying its rock layers. Each layer contains information about a different chapter in the canyon’s history.

Hominid fossils can be arranged based on the age of the rock layers they are found in

Horses, provide us with one of the most complete fossil records known today. The record clearly illustrates how horses have changed (evolved) over the past 3 epochs (50 mya)

Fossils also allow us to study the origins of life itself. All mammals are believed to all share a common ancestor. As more fossils are unearthed the commonalities between species becomes more apparent.

Meet the fish with shoulders, a neck capable of looking side to side, gills, primitive lungs, eyes on top the head, and fins…

complete with “wrist” bones.

What about us?

• “Human” is a very general term (probably shouldn’t use)

• We are Homo sapiens – (our Genus and species name)

• Our evolution likely started in Africa, probably somewhere around the Afar depression and Great Rift Valley– Earliest hominid fossils are found here– Driven by environmental change, competition,

selection• Possibly accelerated by:

– Bipedal motion– Development of thumbs to manipulate the environment

» Thumbs, Toes, and Tears– Brain growth

» Frontal lobe development» Self awareness

Ardipithecus ramidus• Our earliest hominid ancestor• Approx. 4 - 5 millions years old• Ape-like, but bipedal (upright walking)

Australopithecus afarensis• Lucy (most famous specimen)• 3 – 4 million years old• Biped• Laetoli footprints provide evidence• Made in volcanic ash, which hardened and

preserved impressions

Homo habilis• Toolmaker (came after previous two species)• Biped• Showed initial signs of manipulating environment• More “human-like” (questionable word choice)

Homo erectus• Much more like modern humans• Similar in size• Language not fully formed, but starting• Hunter-gatherer

Homo neaderthalensis• Existed throughout Europe• Existed during the time of Homo sapiens• Might have interbred with Homo sapiens?• Heavy built, thick brow line, culture evident

Homo florensis• The Hobbit from Flores•Evolved form of Homo erectus? Debatable•Island syndrom• Reduced resources – reduced size• Interesting, but doesn’t really change the story much

Ardipithecus ramidus

Australopithecus afarensis

Homo habilis

Homo erectus (left)

Homo neaderthalensis (right)

Homo sapiens (left)

Becoming Human

http://www.becominghuman.org/node/interactive-documentary

Ardipithecus ramidus

• Hominid evolution hasn’t stopped. Agriculture might be responsible for accelerating it.• Lactose tolerance (not intolerance) is the “odd ball”• Skin color (melanin amounts have evolved over time)• Sickle Cell vs. Malaria???

– Beneficial mutation…sort of

• Check out the 10,000 Year Explosion

(very cool)

http://www.pbs.org/wgbh/evolution/educators/teachstuds/svideos.html

How do we know evolution happens? video clipHow does evolution work? video clip

How does evolution “look?”How do new species form? (speciation)

New species can arrive in through two pathways• Divergent Speciation– Species A gives rise to both Species B and C• Humans and apes share a common ancestor

• Phyletic Speciation– Species A gives rise to Species B which gives rise to

Species C• Australopithecus afarensis evolving to Homo habilis (we

believe)

The process of speciation generally occurs two ways

• Punctuated Equilibrium– Species A stays species A for a long time, then

“quickly” changes to species B• Quickly in terms of geologic time

• Gradualism– Species A slowly and methodically changes to

species B. There appears to be a continual evolution from one form to the next