How can embryology show that organisms are related

An embryo goes through a number of stages before it develops into a fetus and eventually into a living offspring. Studying the structures that develop during an embryo's various stages of growth is called embryology and can be used to show the genetic similarities that suggest certain patterns of evolution.

Most embryos look similar in their early stages, but as they develop, the differences between species become more obvious. Embryos of organisms that have a closer genetic relationship to one another tend to look similar for a longer period of time since they share a more recent common ancestor. In early embryonic stages, embryos will often form structures that will not be present in the organism's final form.

For example, human embryos develop a tail in the beginning stages of our development. However, further along in the process, certain genes kick in that absorb the tail cells, leaving us with a nub referred to as a vestigial tail. The development of this vestigial tail suggests that we had a tail at some point in our evolutionary history, but adaptations and evolutionary shifts made tails unnecessary for our survival.

Early on, a major player in the progression of embryology was a German scientist named Ernst Haeckel. While studying vertebrate embryos, he came up with many exaggerated drawings to illustrate the biogenetic law recapitulation theory.

This was a theory stating that "ontogeny recapitulates phylogeny". In much simpler terms, this means embryos develop in a way that mirrors how their ancestors evolved over time. Although this hypothesis was proven to be incorrect , it was still a major influential idea of the 18th century. Species that share a recent common ancestor will develop in similar ways. Biology Evidence of Evolution. Explanations 5. Such changes include structural changes within a species and speciation the evolution of new species from existing species.

Evolutionary changes over geological time periods involve phylogenetic transitions at a higher level, i. Such long term changes cannot be directly observed, but can be inferred, not only from the fossil record, but also from evidence provided by Comparative Anatomy of embryos and adults, Comparative Cell Biology, and Comparative Molecular Biology of the DNA and DNA products mRNA, proteins, peptides of organisms from different phylogenetic groups.

Both these types of evolution are part of the same process, though they have often been "separated" from each other by the Creationist movement as "microevolution" for the sorts of changes seen over short time periods and "macroevolution" for the sorts of changes seen over geological time periods.

So, what does Comparative Embryology tell us about evolution over geological time periods? If we look at the gametes and embryos from different phylogenetic groups, regardless of how different these organisms are from one and other, we find similarities that suggest possible relationships. So, for example, if we look at the gametes and embryos of two "distantly" related species such as sea urchins and frogs, we see such things as.

The same cellular structure e. The same general game plan for development e. If we look at more "closely" related organisms such as frogs and cows, in addition to the above similarities we find similarities in development that are sometimes so striking that it would be difficult to conclude that the organisms are not somehow related.

For example,. The same general body plan for the embryo dorsal nerve cord, notochord, somites, etc. The question as to how this extensive morphological similarity -- the "waist" of the hourglass -- arises is one that has long preoccupied researchers.

The extent to which the individual development of an organism ontogeny and that of a phylum phylogeny are linked was also previously unclear. For the first time, scientists have now demonstrated that the hourglass motif arises in organisms as diverse as the fruit fly and zebrafish, not only at morphological level but also at molecular level -- a finding that suggests that parallels do, indeed, exist between ontogeny and phylogeny.

In a study carried out on six fruit fly species Drosophila sp. Moreover, the scientists also observed that the expression pattern of key genes reflects the hourglass model most faithfully.

The same conclusion was reached in comparative analyses carried out on Drosphila , mosquitoes of the genus Anopheles and threadworms. These two studies throw new light on an age-old biological conundrum: that of the link between ontogeny and phylogeny.

Kalinka, a researcher from the Dresden group. Their findings explain how the "waist" in the hourglass arises. Fruit flies are one of the most thoroughly researched model organisms and offer unique possibilities for the study of the molecular mechanisms that underlie embryonic development.

The discovery of the hourglass pattern in different species makes it possible for evolutionary biologists to travel back in time to the earliest days of evolution when the differences between organisms arose. This method enables the measurement of the phylogenetic age of active genes.

The scientists explain the observation that the phlogenetically oldest genes are active in older zebrafish with the fact that animals which have passed reproductive age are "overlooked" by selection.

These studies show that naturalists like Karl von Baer, Charles Darwin and Ernst Haeckel were basically correct in their hypothesis that embryonic development is a reflection of phylogeny.