Have you heard anyone ever talk about "evo-devo"? Does it sound like some sort of synthesizer heavy band from the 1980s? It is actually a relatively new field in the realm of evolutionary biology that explains how species, that start out so similarly, become so diverse as they develop.
Evo devo stands for evolutionary developmental biology and has just started being included in the Modern Synthesis of the Theory of Evolution within the past few decades. This field of study encompasses many different ideas and some scientists disagree on what all should be included. However, all who study evo devo agree that the foundation of the field is based on the gene level of inheritance which leads to microevolution.
As an embryo develops, certain genes need to be activated in order for the traits carried on that gene to be expressed. Most of the time, there are biological clues for these genes to turn on based on the age of the embryo. Sometimes, environmental conditions can trigger the expression of the developmental genes as well.
Not only do these "triggers" turn on the gene, they also direct the gene on how to be expressed. There are subtle differences between the arms of different animals that are determined by how the genes that carry the trait for limb development are expressed. The same gene that creates a human arm can also create a sparrow's wing or a grasshopper's leg. They are not different genes, as previously thought by scientists.
What does this mean for the Theory of Evolution? First and foremost, it lends credibility to the idea that all life on Earth came from a common ancestor. This common ancestor had the exact same genes we see today in all of our modern species. It is not the genes that have evolved over time. Instead, it is how and when (and if) those genes are expressed that has evolved. Also, it helps to give an explanation for how the beak shape of Darwin's finches on the Galapagos Islands could have evolved.
Natural Selection is the mechanism that chooses which of these ancient genes are expressed and ultimately how they are expressed. Over time, the differences in gene expression led to the great diversity and large numbers of different species we see in the world today.
The theory of evo devo also explains why so few genes can create so many complex organisms. It turns out that the same genes are used over and over again, but in different ways. The genes that are expressed to create arms in humans may also be used to create legs or even a human heart. Therefore, it is more important how the genes are expressed than how many genes are present. Developmental genes across species are the same and can be expressed in a nearly unlimited number of ways.
Embryos of many different species are nearly indistinguishable from each other at the early stages before these developmental genes are turned on. Early embryos of all species have gills or gill pouches and similar overall shapes. It is crucial for these developmental genes to be activated correctly at the right time and at the right place. Scientists have been able to manipulate genes in fruit flies and other species to make limbs and other body parts grow in different places on the body. This proved these genes control many different parts of embryo development.
The field of evo devo reaffirms the validity of using animals for medical research. An argument against animal research is the obvious difference in complexity and structure between humans and the research animals. However, with such similarities on a molecular and gene level, studying those animals can give insight into the human, and particularly the development and gene activation of humans.