Chapter 24: Speciation and Macroevolution (Part 2, Week 2)

Question 1

[Start 24.3 The Pace of Speciation] What is the concept of the rate of evolutionary change suggesting that species evolve continously over long spans of time?

Answer 1

Gradulism

The principal idea is that large phenotypic differences that produce new species are due to the gradual accumulation of many small genetic changes.

Question 2

What is another concept of the rate of evolutionary change that suggests that the tempo of evolution is more sporadic than gradual?

Answer 2

Punctuated equilibrium

Species rapidly evolve into new species followed by long periods of equilibrium with little evolutionary change.

Question 3

[Start 24.4 Evo-Devo: Evolutionary Developmental Biology]

Overview: The origin of new species involves genetic changes that lead to adaptations to environmental niches and/or to reproduction isolating mechanisms that prevent closely related species from interbreeding.

Answer 3

None.

Question 4

What is the field of biology that compares the development of different organisms in an attempt to understand relationships between organisms and the mechanisms that bring about evolutionary change?

Answer 4

Evolutionary developmental biology (aka evo-devo)

Question 5

In the past few decades, developmental geneticists have gained a better understanding of what?

Answer 5

Biological development at the molecular level.

Much of this work has involved the discovery of genes that control development in model organisms.

As the genomes of more organisms have been analyzed, researchers have become interested in the similarities and differences that occur between closely related and distantly related species.

The field of evolutionary developmental biology arose out of this interest.

Question 6

Genes that play a role in development influence what?

Answer 6

Cell division, cell migration, cell differentiation, and cell death.

The interplay among these four processes produces an organism with a specific body pattern, a process called PATTERN FORMATION.

Question 7

What are the key players in the evolution of many types of traits?

Answer 7

Genes

Question 8

What are the morphological differences between a non webbed (chicken foot) and webbed (duck foot) due to?

Answer 8

They are due to the differential expression of two different cell-signaling proteins called bone mophogenetic protein 4 (BMP4) and gremlin.

The BMP4 gene is expressed throughout the developing limb of both the chicken and the duck.

Question 9

What is the function of BMP4 and gremlin proteins in duck and chicken feet?

Answer 9

• BMP4 causes cells to undergo apoptosis and die.
• Gremlin protein inhibits the function of BMP4, thereby allowing cells to survive.

In the developing chicken limb, the Gremlin gene is expressed throughout the limb, except in the regions between each digit or toe. Therefore, in these regions, the cells die, and the chicken develops a nonwebbed foot.

By comparison, in the duck, Gremlin is expressed throughout the entire limb, including the interdigit regions, and the duck develops a webbed foot.

Scientists have been able to introduce gremlin protein into the interdigit regions of developing chicken limbs. This produces a chicken with webbed feet!

Question 10

What is found in nearly all animals, indicating that they originated very in animal evolution?

Answer 10

Hox genes.

These are homeotic genes, which specify the fate of a particular segment or region of the body.

Question 11

What have developmental evolutionary biologists have hypothesized that variation in WHAT has spawned the formation of many new body patterns?

Answer 11

Hox genes!

Question 12

What is unique about hox genes in different types of animals?

Answer 12

The number and arrangement of Hox geens vary considerably.

Question 13

What is the simplest of animals, that have at least one gene that is homologous to Hox genes?

Answer 13

Sponges.

Their bodies are not organized along a body axis.

Question 14

How many Hox genes do insects typically have?

Answer 14

9 or more

Question 15

In most cases, how do Hox genes occur along a chromosome?

Answer 15

In a cluster, lying close to each other.

Question 16

In mammals, how many times are Hox gene clusters duplicated over the course of evolution? And how many genes do they contain?

Answer 16

Twice to form four clusters

They contain a total of 39 genes.

Question 17

What are the four groups that Hox genes are divided into?

Answer 17

Anterior, group 3, central, and posterior based on their relative similarities.

Question 18

What is bilateral symmetry?

Answer 18

The body of an organism is organized along a well-defined anteroposterior axis, which right and left sides that show a mirror symmetry. These organisms are called bilaterians.

Question 19

What do Hox genes encode and what do they do?

Answer 19

Transcription (DNA into RNA) factors that act as master control proteins for directing the formation of particular regions of the body.

Each Hox gene controls a hierarchy of many regulatory genes that control the expression of genes encoding proteins that ultimately affect the morphology of the organism.

The evolution of the complex body patterns is associated with an increase not only in the number of regulatory genes–as evidenced by the increase in Hox gene complexity during evolution–but also in genes that encode proteins that directly affect an organism’s form and function.

Question 20

How would an increase in Hox genes enable more complex body patterns to evolve?

Answer 20

Answer lies in the spatial expression of the Hox genes.

Different Hox genes are expressed in different regions of the body along the anteroposterior axis.

Therefore, an increase in the number of Hox genes allows each of these master control genes to become more specialized in the region that it controls.

With fruit flys, one segment in the middle of the body can be controlled by a particular Hox gene to form wings and legs., whereas a segment in the head region can be controlled by a different Hox gene and develop antennae. Therefore, research sugges that one way for new, more complex body patterns to evolve is by increasing the number of Hox genes, thereby making it possible to form many specialized parts of the body that are organized along a body axis.

Question 21

What are the three lines of evidence that support the idea that increases in Hox gene number have been instrumental in the evolution and speciation of animals with more complex body patterns?

Answer 21

• Hox genes are known to control the fate of regions along the anteroposterior axis
• A general trend is observed in which animals with a more complex body structure tend to have more Hox genes and Hox clusters in their genomes than do simpler animals
• A comparison of Hox gene evolution and animal evolution reveals strinking parallels. Researchers have analyzed Hox gene sequneces among modern species and made estimates regarding the timing of past events.

Though the date is difficult to precisely pinpoin, the first Hox gene arose well over 600 mya. In addition, gene duplications of this primordial gene produced clusters of Hox genes in other species. Clusters such as those found in modern insects were like to be present approximately 600 mya. A duplication of a Hox cluster is estimated to have occured around 520 mya.

Question 22

What do the estimates of Hox gene correlate with?

Answer 22

Major diversification events in the history of animals.

Question 23

What period, stretching from 543 to 490 mya, saw a great diversification of animal species and occured AFTER the hox cluster was formed and possibly undergoing its first duplication to produce two hox clusters?

Answer 23

The Cambrian period

Question 24

When did a second duplication produce species with four hox clusters?

Answer 24

420 mya

This even preceded the proliferation of tetrapods–vertebrates with four limbs–that occurred during the Devonian period, approximately 417-354 mya.

This second duplication may have been a critical event that led to the evolution of complex terristrial vertebrates with four limbs, such as amphibians, reptiles, and mammals.

The striking correlation between the number of Hox genes and body complexity is thought have been instrumental in the evolution of animals.

Question 25

How many Hox clusters do modern tetrapods have today?

Answer 25

4

Question 26

What research suggests that the number of hox genes and body complexity might not be entirely true, or not just this sole purpose?

Answer 26

The number of Hox clusters in tetrapods is four, while fish , who do not have more complex bodies, actually have 7 to 8 Hox clusters. Additionally, researchers have discovered that specialized body structures can be formed by influencing the regulation of Hox geens and other genes that are controlled by Hox genes.

Question 27

What is the differences among species in the rate or timing of developmental events?

This is another way that genetic variation can influence morphology is by controlling the relative growth rates of different parts of the body during development.

Answer 27

Heterochrony

The speeding up or slowing down of growth appears to be a common occurence in evolution and leads to different species with striking morphological differences. With regard to the pace of evolution, such changes may rapidly lead to the formation of new species. (See image of chimp and human skull development)

Question 28

What is the master control gene that encodes a protein as a transcription factor that controls the expression of other genes within eye development, including those involved in the development of the eye in both rodents and humans?

Answer 28

Pax6 gene

A mutation in the Pax6 gene results in small eyes for mice and rats.

Question 29

What type of eye disorder is created when there is a mutation in the Pax6 gene for humans?

Answer 29

Aniridia (Ann-er-rid-e-yah) in which the iris and other structures of the eye do not develop properly.

Drosophila has a gene names eyeless that also creates a defect in eye development when mutant.

These two genes are homolgous genes; they are derived from the same ancestral gene.

Question 30

What experiment was conducted by a Swiss geneticist in regards to the expression of the eyeless gene in parts of Drosophila where it is normally inactive could promote the formation of additional eyes?

Answer 30

They were able to express the eyeless gene in the region where antennae should form. This resulted in the formation of an eye where the antennae are normally found.

Remarkably, the expression of the mouse Pax6 gene in Drosophila can also cause the formation of eyes in unusual places, like the formation of an eye on the leg of Drosophila.

Question 31

Why does the Pax6 gene, a mouse gene that controls eye formation, when introduced in Drosophila not produce a mouse eye, but a Drosophila eye?

Answer 31

This happens because the Pax6 gene activates genes from the Drosophila genome. In Drosophila, the Pax6 homolog called eyeless switches on a cascade involing several hundred genes required for eye morphogenesis.

Question 32

What did Darwin propose about the common ancestral form of the eye?

Answer 32

It is made of one photoreceptor cell and one pigment cell.