Evolution

Evolution

• Change in a population of organisms over time.

• Cladogram – a chart that shows an organism’s evolutionary history

Theories of Evolution

• Jean-Baptiste Lamarck – 1809 published his theory of evolution – Mechanism of

Evolution = Inheritance of Acquired Characteristics

Theories of Evolution

• Charles Darwin – 1859 published his theory of evolution in the major work, On the Origin of Species– Mechanism of Evolution

= Natural Selection

Lamarck’s Ideas

• Organisms change over time• Organisms are adapted to their

environment• All organisms are continually changing

and acquiring features more suitable for their environment.

Larmarck’s Ideas • During an organism's lifetime, there is

selective use or disuse of its organs and traits. (Example: Lizard living in burrow…)

• These acquired traits can then be passed on to their offspring. Over time, this process leads evolution of the species.

Legless lizard: Note ear hole. Snakes do not have ears.

Comparison of Lamarck’s and Darwin’s Ideas on Evolution

Darwin• Same

• Same

• Same

Lamarck • Short necked

herbivore lives in savanna

• Grasses are available to the animals. Trees leaves are out of reach of most of them

• Animals are adapted to their environment.


Lamarck• The animals stretch

their necks to reach the leaves on the trees. Continued stretching causes an elongation of necks.

• The animals reproduce. The young have long necks.

Darwin• Due to variation in the

population there is a range of neck lengths. Some animals naturally have longer necks than others.

• Those with longer necks can reach the tree leaves.


DarwinIn the event of a shortage of grasses (drought), the animals are able to feed on leaves. The long necked animals are the most fit! These are the animals that survive and reproduce.

LamarckIn the event of a shortage of grasses (drought), the animals are able to feed on leaves. The long necked animals are the most fit! These are the animals that survive and reproduce.


Darwin• Over the next generations,

(due to mutations, new gene combinations, crossing over) there will again be variation in neck length

• Those with the longest necks will be be most fit--with the greatest chance of survival.

Lamarck• Over the next

generations, the longer necked animals will continue to stretch and elongate their necks.

• Each generation will pass on their longneck to their young

Microevolution according to

Darwin’s Ideas on Natural Selection

Macroevolution according to Darwin’s Ideas on Natural SelectionSpeciation = formation of new species

Variation and Natural Selection• Most

phenotypes (traits) are polygenic.

• A graph of these traits often shows a bell curve.

Types of Natural Selection

Stabilizing

Directional

Disruptive

Stabilizing Selection• If individuals near the

center of the bell curve have higher fitness (= leave a greater amount of fertile offspring) then stabilizing selection occurs.

Directional Selection

• If individuals at one end of the bell curve have higher fitness then directional selection occurs.

• Example: long necks

Disruptive Selection• If individuals at

both ends of the curve have higher fitness than those in the middle then disruptive selection occurs.

Genetic Drift

• If a small population is isolated from others of the same species, it is possible that just be “chance” one allele may become more common in a population.

Genetic Drift

• For example: The Bronx, NY, becomes built up and trees are cut down. Few squirrels are able to survive. However, one section of the Bronx is set aside for the development of the Bronx Zoo. Trees are NOT cut down in this area. By chance, several of the gray squirrels in the area of the zoo have black fur (a variation in fur color). Over time, the allele for black fur becomes common in this population.

I’ I’m an Eastern

Gray Squirrel

How do new species form?Example: Galapagos Finches

• A small group of a species of finch arrives on one of the Galapagos Islands

• These finches do not usually fly across open water—they may have gotten lost or were blown there by the wind.

Speciation continued!

• Over time these original finches may have ended up on other of the Galapagos Islands through some chance occurrence.

• USUALLY the finches do NOT fly from one island to another.


• The Galapagos Islands had different environments. – Some had a low elevation and were dry with

little plant material.


• Others had a higher elevation with greater rainfall and many plants.


• Directional selection occurs:– One island may have seeds that are large and

difficult to open. – Birds with a larger, thicker beak will have the

highest survival rate on this island


• Directional selection occurs:– Another island may have seeds that are small

and easy to open – Birds with a small, thinner beak will have the

highest survival rate on this island


• Reproductive Isolation: if the populations remain separate, and their gene pools continue to change, eventually they can no longer mate with each other—they are then TWO NEW SPECIES!!!

• On the Galapagos Islands this processes resulted in the evolution of 13 different finch species.

Darwin’s Finches

Cladograms – a diagram of evolutionary relationships

Cladograms – a diagram of evolutionary relationships

• At the beginning of a cladogram is a single branch. This branch then splits several times into the several animals you have on a cladogram.

• When you follow along a branch of the cladogram, it sometimes splits at nodes into two or more branches.

• You can also tell how closely related animals are on the cladogram. If you want to know the closest relative of one animal you trace its branch back to the most recent node.

Which are more closely related: roundworms and annelids (earthworms) or arthropods and

annelids?

Cladograms

• Each node represents a new trait that has evolved and the common ancestor that evolved that trait.

• As you follow an animal’s path in the cladogram from the beginning to the end, you can find out the animal’s adaptations through time. Each node it goes through is a trait that it evolved, in that order.

Cladograms

• 100 million years go by …

Cladograms: Complete the table below!

• Use the information in the table to construct a cladogram of these animals.

Organism Derived characteristics Backbone Legs Hair

EarthwormTroutLizardHuman

Absent AbsentAbsent

Present PresentAbsentPresent PresentPresent

Present AbsentAbsent

Making a Cladogram

• Which organism is least like the others?



Absent AbsentAbsent



Earthworm

Making a Cladogram

• Use your worksheet on cladograms as your guide.

• Develop a cladogram showing the evolutionary relationships between all 4 animals.



Absent AbsentAbsent



Making a Cladogram

Backbone

Legs

Hair

Earthworm Trout Lizard Human

Backbone

Legs

Hair

Earthworm Trout Lizard Human X

Which animal is species X most closely related to on the cladogram?

Evolution

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Transcript of Evolution