Evolution and Development Flashcards
What are the key concepts of evolution and development?
Selective pressures applies at the level of the adult (reproductive success) but differences in body structure between species must come from differences in embryonic development.
Developmental control genes govern developmental processes.
Most evolutionary changes in the body form are due to changes that affect developmental control genes.
Comparing the genomes of different animals allows us to identify differences affecting developmental control genes, these differences represent heritable changes that enabled evolution.
What are the key Darwin concepts?
- All organisms share a common ancestry and have diverged from that ancestor.
- The reproductive success of a phenotype within a population determines the prevalence of corresponding genotype, this is known as the survival of the fittest (natural selection).
- Certain organs in the individual, when mature become widely different and severe for different purposes, but in the embryo are exactly alike.
- The embryos, of distinct animals within the same class are often strikingly similar.
- Haeckel’s drew embryos of various animals to show how similar they were, stating that the embryos pass though all the developmental stages of earlier ancestral forms which is NOT correct. He also exaggerated the similarities and scaled embryos to minimise the differences.
What is the phylotypic stage?
Is a point in development where organisms are maximally similar and are represented by a pinch in the hourglass (look at notes for the diagram). After that point, they begin to diverge into their adult forms.
How are we able to use the phylotypic stage?
In the phylotypic stage, we can recognise all the common components so all vertebrates have got this basic arrangement (look at notes for the diagram):
- they have a head end and a tail end.
- there is an opening for the mouth -> the pharynx all the way from the gut to the anus.
- a central nervous system with the notochord beneath it (that’s a mesodermal structure what makes the animals chordates).
- either side of the notochord are the pair of structures called smites which are going to give rise to ribs, trunk muscle vertebrae etc.
- if you now look at genes being expressed in the organisms, you will notice that not only they look the same morphologically (discussed above) they express similar genes (orthologous genes).
What is urbilateria?
It is the name of the hypothetical common ancestor for all bilateria?
How is the urbilateria hypothesised to be the common ancestor?
- The diagram shows the hypothetical common ancestor of all animal called Urbilateria
- At the head end it expresses a group of genes (transcription factor) the trunk region and tail end also express a group of genes
- The circles represent a transverse section of the embryo and shows the dorsal (represented with a D) and ventral (represented with a V) axis.
- Chordin and BMP4 (bone morphogenetic protein 4) are signalling molecules which are expressed in the dorsal ventral aspect of vertebrates, they move from one cell to another and transfer a signal.
- We can find orthologues of those signalling molecules in insect (fruit flies). ‘Sog’ and ‘chordin’ are homologues.
- You will also notice that the D and V are flipped for insects and vertebrae. So it’s thought that during evolution the common ancestor has underwent an inversion of the dorso-ventral axis.
What does Bilateria mean?
-It means that they are bilaterally symmetrical animals with a head, tail, back and two sides.
What do all animals share?
-All animals share common features of embryonic body plan development
-Share expression of key transcription factors – referred to as the “tools” (transcription factors
and signaling molecules) of the “developmental toolbox”
-Complexity does not correlate with gene number – complexity arises from regulation of gene
expression (when you turn genes on and off)
*For example humans have more genes than a
chicken but fewer genes than a grape.
-The genes have a common pattern of expression between different vertebrates.
What are the genes that have a common pattern of expression between vertebrates?
- Hox genes are crucial for cranial-caudal patterning, expressed along the head tail axis and they specify different tissues along the head tail axis.
- Otx & Emx genes for head
- Cdx & Evx genes for tail
- Nkx2.5 for heart (specifies the heart)
- Pax6 for eyes
How do the changes in development make morphological development?
- The genetic variability that arises provides variability on which selection can act on.
- Genetic changes that develop new adult forms better adapted to their environment and more successful in reproduction are selected for during evolution.
- Development as “the production of the fittest”
- Vertebrates have a high degree of homology (sequence conservation) in the coding regions of their genomes but the difference start to come out side of the coding regions.
- Development entails precise temporal (time) and spatial (space) control of gene expression, so differences are likely to be in regions of the genome that control gene expression.
What are cis-regulatory elements?
- CREs found on the same stretch of DNA as gene.
- Cis - means that they are on the same strand on DNA.
- Contain several elements that can be bound by many different proteins.
- Positive and negative regulation (turn genes on and off).
- Changes (mutation) of CREs might create new TF binding sites, or remove TF sites, or allow cooperative binding of other proteins, etc.
How are hox genes part of the gene expression?
- Paralogous genes have arisen by tandem duplication events.
- Clustered along the chromosome.
- Spatial and temporal colinearity of gene expression along cranial caudal axis.
- Vertebrates have 4 Hox gene clusters (from 2 duplication events of the cluster).
What are the structures in the snake that make it limbless?
- Snake evolved from a lizard kinda ancestor that had legs, except snakes have expanded their thoracic region so that they now they have up to 300 vertebrae
- Snakes don’t have forelimb (front legs).
- Python has a rudimentary hindlimb (pelvic girdle & truncated femur).
How did snakes evolve to lose its limbs?
- If look at expression of certain hox genes in the snake embryo and compare them to the expression of hox genes in something with limbs like chicken, you will find that hoxc6 and hoxc8 when expressed together, they specify the flank region of the embryo.
- Hoxc-6 and Hoxc-8 together specifies flank
- In snakes there is an expanded expression of hoxc6 and hoxc8, so the same transcription factors are present but they are expressed in a slightly different pattern, and whenever they overlap that going to make flank a no forelimb.
- Hindlimb forms at the posterior (caudal) boundary of Hoxc-8 expression.
- Snakes have expanded expression domains of Hoxc6 and Hoxc-8, therefore expanded flank.
- Change is not in the Hox genes, but how they are controlled.
How did the chicken evolve to get it’s beak?
- Bird beaks form from fused, elongated premaxillae.
- Expression of facial patterning genes in amniotes.
- Inhibition of Fgf8 and Wnt in the facial midline of chick embryos leads to more alligator-like snout.
