Section 17.4Evidence of Evolution

Standards

• LS 4.1 Evaluate scientific data collected from analysis of molecular sequences, fossil records, biogeography, and embryology. Identify chronological patterns of change and communicate that biological evolution is supported by multiple lines of empirical evidence that identifies similarities inherited from a common ancestor (homologies).

• LS 4.2 Using a model that demonstrates the change in allele frequencies resulting in evolution of a population over many generations, identify causative agents of change.

Key Questions

1. How does geographic distribution of species today relate to their evolutionary history?

2. How do fossils help document descent of modern species?

3. What do homologous structures and similarities in development suggest about the process of evolution?

4. How can molecular biology be used to trace the process of evolution?

5. What does recent research of the Galapagos finches show about natural selection?

Vocabulary

• Biogeography

• Homologous structure

• Analogous structure

• Vestigial structure

Biogeography

• Biogeography- the study of where organisms live now and where they and their ancestors lived in the past

• Patterns in the distribution of living and fossil species, combined with information from geology, tell us how modern organisms evolved from their ancestors.

1. Closely related species can evolve diverse adaptations in different environments.

2. Very distantly related species can develop similar adaptations if they are living in similar environments.

1. Closely Related but Different

• Darwin suggested that the finch bird species on the Galapagos Islands had all evolved from a single mainland species.

• Over time, natural selection on different islands caused populations to evolve into different, but closely related, species.

2. Distantly Related but Similar

• In contrast, Darwin noted that birds in ecologically similar environments resembled each other, even though they were not closely related.

• Differences in their body structures provided evidence that they evolved from different ancestors.

• But, natural selection in similar habitats led to similar adaptations (like long legs and toes adapted for running).

The Age of Earth

• Darwin, Hutton, and Lyell all argued that Earth was a lot older than people tended to think, but could not determine just how old.

• The discovery of radioactivity and radioactive dating enabled geologists to determine the age of certain rocks and fossils.

• Radioactive dating indicates that Earth is about 4.5 billion years old.• Plenty of time for evolution by natural section to take place.

Recent Fossil Finds

• Many recently discovered fossils now show clearly how modern species evolved from extinct ancestors.

• The history of life is incomplete.

• Examples:• Evolution of whales from ancient land mammals

• Connections between dinosaurs and birds

• Connections between fishes and four-legged land animals

Evidence of Whale Evolution

Ambulocetus could both swim in shallow water and walk on land.

AmbulocetusRodhocetusPakicetus

Ancient artiodactyl

Rodhocetus

probably spent

most of its time in

water.

Evidence of Whale Evolution

Basilosaurus

Dorudon

Mysticetes

Odontocetes

Modern whales

Basilosaurus

only swims.

Modern whales have ancient structures

Homologous Structures

• Example: the bones of vertebrate limbs • The bone structures in vertebrate limbs are very similar (radius, ulna, etc.)

• But, the limbs have different functions (crawling, climbing, running, etc.)

• Why would the same basic structures appear over and over with such different purposes? (they must share a common ancestor)

• Homologous structures- similar structures that are shared by related species and have been inherited from a common ancestor

Homologous Structures

Notice the similarities in

bones

Homologous Structures

• Homologous structures can help determine how recently species shared a common ancestor.

• Example:• The bones of reptiles are birds are more similar to one another in structure than they

are to mammals• The common ancestor of reptiles and birds must have lived more recently than the

common ancestor of reptiles, birds, and mammals• Birds are more closely related to a crocodile (a reptile) than they are to a bat (a

mammal)… even though they both have wings

• The key to identify homology is to look for common structure and origin during development, not common function.

Homologous Structures

• Evolutionary theory explains the existence of homologous structures adapted to different purposes as the result of descent with modification from a common ancestor.

• Looking at homologous structures helped scientists determine that birds and reptiles are more closely related than birds and mammals.

Homologous Structures

• Some homologous structures do not serve important functions.

• Vestigial structures- inherited from ancestors, but have lost much of their original size and function

• Examples:• Human appendix• Pelvic bones in snakes• Hipbones in whales and dolphins• Wings of flightless birds

Analogous Structures

• Analogous structures- body parts that serve similar functions, but do not share structure and development

• Flying- wings

• Swimming- streamlined appendages

Development

• Similar patterns of embryological development provide further evidence that organisms have descended from a common ancestor.

• Example:• Early developmental stages of many

vertebrates look similar • The same groups of embryonic cells

develop in the same order and in similar patterns (to produce homologous structures)

Genetics and Molecular Biology

• At the molecular level, overwhelming similarities in the genetic code of all organisms, along with homologous genes and molecules, provide evidence of common descent.

Life’s Common Genetic Code

• All living cells use information coded in DNA and RNA to carry information from one generation to the next and to direct protein synthesis.

• The genetic code is nearly identical in almost all organisms.

Molecular Homology

• Hox genes- determine the identities of body parts • Help determine which part of an embryo becomes the head, tail, etc.

• In all vertebrates, sets of homologous Hox genes direct the growth of front and hind limbs

• Small changes in these genes can produce dramatic changes in size and shape of structures they control

• Some homologous Hox genes are found in almost all multicellular animals

• The widespread distribution of these genes indicate that they have been inherited from ancient common ancestors

Testing Natural Selection

• Darwin did not think it was possible to observe natural selection in action because he though evolutionary change always happened very slowly.

• In recent years, however, biologists have designed experiments to study natural selection in the wild.

Back to Galapagos

• Peter and Rosemary Grant of Princeton University• Have been studying finch

populations on one of the islands, Daphne Major for more than 40 years

• Have captured, measured, and identified every finch on the island

• Also have measured the different kinds of seeds that the birds depend on for food

A Testable Hypothesis

• Darwin noted that different Galapagos finch species have beaks of different shapes and sizes.

• Darwin hypothesized that natural selection had shaped the beaks of different bird populations as they became adapted to eat different foods.

• The Grants realized this hypothesis was testable:1. For beak size and shape to evolve, there must be enough heritable

variation to provide raw materials for natural selection.2. If those differences were involved in natural selection, birds with different

beak sizes and shapes should show differential survival and reproduction.

Natural Selection

• While the Grants were working on the island, a drought hit:• The total number of seeds dropped steadily.• The finches ate the smaller, softer seeds first.• As the drought continued, only the largest,

hardest seeds remained. • Many of the finches starved and died. • Birds with the largest beaks were more likely

to survive (they had higher evolutionary fitness)

• The drought caused the average beak size of this finch population to change dramatically.

Natural Selection

• The Grants documented that natural selection takes place in wild finch populations frequently, and sometimes rapidly.• It might take no more than 12-20 such droughts to change the beak size

enough to transform one species into a new species.

• Variation within a species increases a population’s ability to adapt to, and survive, environmental change. • If the population had not had enough variation in beak size when the drought

started, they would not have been able to adapt and change.

Evolutionary Theory Evolves

• Evolutionary theory is vital to all branches of biology, from research on infectious diseases to ecology.

• It is often called the “grand unifying theory of the life sciences”.

• Theory is constantly reviewed as new data is gathered.

• Any questions that remain are about how evolution works—not whether evolution occurs.

Section 17.4 Exit Ticket

1. How does the fossil record show the evolution of organisms over time?

2. How does the study of embryos of various organisms show evidence of common ancestry?

3. What are the differences between analogous structures, homologous structures, and vestigial structures? How do these structures show evidence of common ancestry between organisms of different species?

The End