[LS4-1] Evidence for Evolution
This standard focuses on helping students conceptualize and synthesize different arguments supporting biological evolution and the idea of common ancestors.
Resources for this Standard:
Here’s the Actual Standard:
Communicate scientific information that common ancestry and biological evolution are supported by multiple lines of empirical evidence.
There are many lines of evidence supporting common ancestry and biological evolution, which allows this standard to be approached from many different angles. In general, however, the idea is to build on skills of scientific literacy, hypothesis building, and scientific communication.
Some common “lines of empirical evidence” are shared characteristics between parents and offspring, Mendel’s Laws of Inheritance, DNA evidence (such as shared segments), fossil evidence, and even recorded evolutionary events (see Peter and Rosemary Grant’s famous Galapagos Finch experiments).
The theory of common ancestry and biological evolution have been around for quite some time, and have become more robust as more evidence is gained. The idea goes back beyond Aristotle, who even developed rudimentary “laws” of inheritance (that were later overturned by Darwin and geneticists). So, there is a lot of material to work with.
A little clarification:
The standard contains this clarification statement:
Emphasis is on a conceptual understanding of the role each line of evidence has relating to common ancestry and biological evolution. Examples of evidence could include similarities in DNA sequences, anatomical structures, and order of appearance of structures in embryological development.
Let’s look at some lines of evidence and their role in common ancestry and evolution:
DNA technology is the most advanced and accurate understanding of inheritance available to science. By genotyping a parent and their offspring, we can actually “read” the genetic code and see which genes each parent contributed to the offspring. Further, scientists have documented and watched the process of cellular division under a microscope, confirming Mendel’s Laws of Inheritance.
Because all organisms on Earth share the same basic format of DNA, as well as the basic mechanisms for replicating DNA and expressing proteins, it can also be assumed that all of life on Earth arose from a common ancestor (or at least a very small handful of primitive, bacteria-like organisms). We can actually measure how closely related two species are by comparing their DNA. Bananas and humans, for example, share 25% of their DNA. While this may seem like a lot, humans and chimpanzees share about 99% of our DNA. This shows that humans, chimpanzees, and bananas all share a common ancestor. But, it also tells us that humans and chimpanzees share a common ancestor much more recently than humans and bananas.
Before the days of DNA, scientists had to rely on more primitive methods to model inheritance. Mendel simply counted phenotypic ratios of offspring, because he had no way to actually analyze the genotypes of organisms. Punnett squares are also used as a way to visualize the alleles of different genes, and how they interact.
One common method of (fairly) accurately determining how closely two species are related is computational phylogenetics. In the simplest form of this method, scientists create a spreadsheet containing a number of individual traits and anatomical features present on three or more different animals. The animals that share the most traits are considered more closely related than animals that share fewer traits. (A more accurate version of this compares actual genes carried by different animals to see which sets of genes various animals share.)
The study of embryology is the study of embryos as they develop. Though this entire field was created after the theories of common ancestors and biological evolution, embryology has many empirical lines of supporting evidence.
For example, all vertebrate embryos go through an embryological phase in which they have gill slits. That suggests that all vertebrates are related to fish, though the actual formation of functional gills is replaced by lungs in terrestrial vertebrates. Vertebrate embryos have several other traits that are seen across vertebrate species, while adults may not retain these traits. Looking at the first few rounds of cellular division also shows that animals essentially employ two different methods of forming different tissue layers, allowing animals to broadly be classified into deuterostomes and protostomes.
The fossil record, though incomplete and hard to study, offers many insights into evolution and common ancestors. One of the main concepts from the fossil record that supports evolution is the presence of transitional forms – organisms that have traits from both extinct groups and extant groups. Great examples here include feathers on dinosaurs and aquatic adaptations on the (mostly) terrestrial ancestors of whales.