CharlesDarwin

Beagle Galapagos Islands

Galapagos Islands

Adaptive RadiationThe process in which one species gives rise to multiple species

that exploits different niches.

Adaptive RadiationThe process in which one species gives rise to multiple species

that exploits different niches.

The ecological niches exert the selection pressures that push the populations in various directions.

Selective Pressures: Environmental forces such as scarcity of food or extreme temperatures that result in the survival of only certain organisms with characteristics

that fits the change.

Factors that causes Adaptive Radiation: Evolution of New Physical Traits

Factors that causes Adaptive Radiation:

Availability of New Niches

Factors that causes Adaptive Radiation: Aftermath of Mass Extinctions

The amount of melanin in our skin. Shows our adaptations in living in areas of different climates. Dark skinned people will be found in hotter climates.

While light skinned people are found in areas with cold climates.

Biological SimilaritiesHomology

traits inherited by two different organisms from a common ancestor

Analogytraits serve similar functions but

are not evolutionary related

Convergent Evolution The same biological traits in unrelated

lineages

Divergent EvolutionThe same species

adapting to different environment

Tetrapods animals with four legs.

Notice how these tetrapod limbs are similar to one another: •They are all built from many individual bones.•They are all spin-offs of the same basic bone layout: one long bone (the humerus) attached to two other long bones (the radius and ulna), with a branching series of smaller bones (carpals, metacarpals and phalanges) on the end.

Here you can see the same bones labeled in these different limbs:

Even though these limbs are similar to one another, the animals they belong to are quite different from one another.

What animals possess these homologous limbs?

Homologytraits inherited by different

organisms from a common ancestor

Divergent EvolutionThe same species adapting to different

environment

The result of:

Divergent EvolutionThe same species adapting to

different environment

Homologytraits inherited by two different

organisms from a common ancestor

Biological SimilaritiesHomology

traits inherited by two different organisms from a common ancestor

Analogytraits serve similar functions but

are not evolutionary related

Convergent Evolution The same biological traits in unrelated

lineages

Divergent EvolutionThe same species

adapting to different environment

Sharks Dolphins

You have probably noticed that dolphins and sharks both have a streamlined body shape with a triangular fin on the back and two side fins. However, the two animals

also have many differences.

skeleton made of cartilage

use gills to get oxygen from the water in which they swim

don't nurse their young

don't have hair

skeleton made of bone

go to the surface and breathe atmospheric air in through their blowholes

do nurse their young

do have hair —around their "noses"

The following tree shows the relationship between various groups, including cartilaginous fishes (sharks) and mammals (dolphins).

They are not very closely related to one another. So how did they end up looking so much alike?

•If two species face a similar problem, challenge or opportunity, evolution may end up shaping them both in similar ways.•Both dolphins and sharks swim after prey in the ocean. Streamlined bodies and fins provide a big advantage for them, allowing them to swim faster.•We know that dolphins and sharks are not closely related, and they didn't inherit their similar body shapes from a common ancestor. •Their streamlined bodies, dorsal fins and flippers are the result of convergent evolution. •Since dolphins and sharks occupy similar niches and face similar challenges, similar adaptations have been advantageous to them, resulting in their analogous structures.

Test your understandingYou learned that homologies are traits that different lineages inherited from their common ancestor.Homologies are evidence that different species shared a common ancestor.Analogies, on the other hand, are similar traits that were not inherited from a common ancestor but that evolved separately. Analogies often exist because two different lineages became adapted for similar lifestyles.

Sugar gliders and flying squirrels look amazingly similar.They are both furry animals of about the same size, with big eyes and a white belly. And they both glide from treetops using a thin piece of skin that is stretched between their legs. This piece of skin helps keep them stable while gliding.

Flying squirrel

Sugar glider

Test your understanding

Flying squirrel

Sugar glider

However, these animals also have some key differences:

•Sugar gliders live in Australia, and flying squirrels live in North America.Sugar gliders have a pouch (like a kangaroo does), which provides shelter and safety for their tiny babies — at birth, a baby sugar glider is smaller than a peanut!

•Flying squirrels, on the other hand, have much larger babies and no pouch.

By studying their genes and other traits, biologists have figured out that sugar gliders and flying squirrels are probably not very closely related.Sugar gliders are marsupial mammals and flying squirrels are placental mammals.

Considering all of the evidence, are the "wings" (actually flaps of skin stretched between the legs)

of sugar gliders and flying squirrels homologous or analogous structures?

Homologous Analogous

The "wings" of sugar gliders and flying squirrels are analogous, not homologous.

Homologies are traits inherited from a common ancestor, and analogies are similar traits that evolved independently of one another.

Since sugar gliders and flying squirrels are very distantly related, it seems very unlikely that their common ancestor had flaps of skin stretched between its legs and that both modern animals inherited the trait from this animal.

Since sugar gliders and flying squirrels are very distantly related, it seems very unlikely that their common ancestor had flaps of skin stretched between its legs and that both modern animals inherited the trait from this animal.Instead, each lineage probably evolved the trait independently as adaptations for gliding and tree-living.

AnalogyHomology

In biology, the independent evolution of similar structures in species (or other taxonomic groups) that are not closely related, as a result of living in a similar way. Thus, birds and bees have wings, not because they are descended from a common winged ancestor, but because their respective ancestors independently evolved flight.

In such cases, the structures often differ in their anatomical origins and are only superficially similar. Such structures are said to be ‘analogous’, in contrast to the homologous organs of related groups.

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When Charles Darwin stepped ashore on the Galapagos Islands in September 1835, it was the start of five weeks that would change the world of science, although he did not know it at the time. Among other finds, he observed and collected the variety of small birds that inhabited the islands, but he did not realize their significance, and failed to keep good records of his specimens and where they were collected. It was not until he was back in London, puzzling over the birds, that the realization that they were all different, but closely related, species of finch led him toward formulating the principle of natural selection.In his memoir, The Voyage of the Beagle, Darwin noted, almost as if in awe, "One might really fancy that, from an original paucity of birds in this archipelago, one species had been taken and modified for different ends."Indeed, the Galapagos have been called a living laboratory where speciation can be seen at work. A few million years ago, one species of finch migrated to the rocky Galapagos from the mainland of Central or South America. From this one migrant species would come many -- at least 13 species of finch evolving from the single ancestor. This process in which one species gives rise to multiple species that exploit different niches is called adaptive radiation. The ecological niches exert the selection pressures that push the populations in various directions. On various islands, finch species have become adapted for different diets: seeds, insects, flowers, the blood of seabirds, and leaves.The ancestral finch was a ground-dwelling, seed-eating finch. After the burst of speciation in the Galapagos, a total of 14 species would exist: three species of ground-dwelling seed-eaters; three others living on cactuses and eating seeds; one living in trees and eating seeds; and 7 species of tree-dwelling insect-eaters.Scientists long after Darwin spent years trying to understand the process that had created so many types of finches that differed mainly in the size and shape of their beaks. Most recently, Peter and Rosemary Grant have spent many years in the Galapagos, seeing changing climatic conditions from year to year dramatically altering the food supply. As a result, certain of the finches have lived or died depending on which species' beak structure was best adapted for the most abundant food -- just as Darwin would have predicted.

Biological Similarities