Chapter 24: Speciation and Macroevolution (Part 2, Week 2) Flashcards
(32 cards)
[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?
Gradulism
The principal idea is that large phenotypic differences that produce new species are due to the gradual accumulation of many small genetic changes.
What is another concept of the rate of evolutionary change that suggests that the tempo of evolution is more sporadic than gradual?
Punctuated equilibrium
Species rapidly evolve into new species followed by long periods of equilibrium with little evolutionary change.
[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.
None.
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?
Evolutionary developmental biology (aka evo-devo)
In the past few decades, developmental geneticists have gained a better understanding of what?
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.
Genes that play a role in development influence what?
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.
What are the key players in the evolution of many types of traits?
Genes
What are the morphological differences between a non webbed (chicken foot) and webbed (duck foot) due to?
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.
What is the function of BMP4 and gremlin proteins in duck and chicken feet?
- 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!
What is found in nearly all animals, indicating that they originated very in animal evolution?
Hox genes.
These are homeotic genes, which specify the fate of a particular segment or region of the body.
What have developmental evolutionary biologists have hypothesized that variation in WHAT has spawned the formation of many new body patterns?
Hox genes!
What is unique about hox genes in different types of animals?
The number and arrangement of Hox geens vary considerably.
What is the simplest of animals, that have at least one gene that is homologous to Hox genes?
Sponges.
Their bodies are not organized along a body axis.
How many Hox genes do insects typically have?
9 or more
In most cases, how do Hox genes occur along a chromosome?
In a cluster, lying close to each other.
In mammals, how many times are Hox gene clusters duplicated over the course of evolution? And how many genes do they contain?
Twice to form four clusters
They contain a total of 39 genes.
What are the four groups that Hox genes are divided into?
Anterior, group 3, central, and posterior based on their relative similarities.
What is bilateral symmetry?
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.
What do Hox genes encode and what do they do?
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.
How would an increase in Hox genes enable more complex body patterns to evolve?
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.
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?
- 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.
What do the estimates of Hox gene correlate with?
Major diversification events in the history of animals.
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?
The Cambrian period
When did a second duplication produce species with four hox clusters?
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.
