Isn’t evolution “just” a theory?
Isn’t evolution “just” a theory? Scientific theories are broadly accepted, combine multiple hypotheses and an abundance of evidence to explain a phenomenon about the natural world. Episode 42 recAPs the multitude of evidence supporting the theory of evolution. Zoom in to review biogeography (2:14), the fossil record (3:00), homologous structures (4:00) vestigial structures (4:55), and molecular evidence (5:38).
The Question of the Day asks (7:20) What was the superocean called from Wegener’s Continental Drift Hypothesis?
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Hi and welcome to the APsolute Recap: Biology Edition.
Today’s episode will recap Evidence of Evolution
Lets Zoom out:
Unit 7 - Natural Selection
Topic - 7.6
Big idea - Evolution
Evolution - that's just a theory. Contrary to popular opinion, theories carry a lot more weight to them than just a hunch or passing guess. Scientific theories are broadly accepted, combine multiple hypotheses and an abundance of evidence to explain a phenomenon about the natural world. So what evidence is there to support the theory of evolution by natural selection - and is any of it still around?
Lets Zoom in:
We need to address that last part, first. Too often, evolution is thought of as something from the past, studied long ago, to prove how everything got to be so awesome now. But that is a large misconception. Evolution is just a change in the gene pool over time - and since time hasn’t stopped the last time I checked -then evolution is still occurring. There isn’t a set goal, or end point, or perfect version of any organism. As the world and its environments continue to change, organisms either adapt or ...well… ya know. They don’t stick around to see what happens next.
Evolution is supported by scientific evidence from many disciplines including geographical, geological, physical, biochemical, and mathematical data. The topics we discuss here is not an exhaustive list and only represent the tip of the iceberg (and ironically even icebergs provide evidence of evolution). Don’t memorize this list, but rather focus on how each piece of evidence contributes to the greater understanding of how living things have changed over time.
Let’s start with the fossil record and biogeography evidence and end the episode with a question that might help you win Jeopardy one day! Most students learn in middle school that the earth’s crust is made of segments that shift around - at times causing earthquakes, volcanic eruptions etc. If they are moving now, they likely also moved previously and there is much evidence to suggest they were once all connected (called Pangea). Even looking at a modern map - you can see the shape of South America mimic the coast of Africa - like puzzle pieces. Fossil evidence from the Triassic land reptile Cynognathus can be found on South America and Africa. Fossils of Mesosaurus have been found on both sides of the south atlantic as well - and nowhere else in the world.
Fossils don’t just form in any kind of rock. For example, rivers carry sediment into aquatic habitats such as seas and swamps. Over time, sedimentary rock layers, called strata, from under water. As water levels change and geological activity pushes layers of rock upward, the strata and some of their fossils are exposed. Fossils can be dated by a variety of methods, including aging rocks, isotope decay like Carbon-14, and geographical data. Often, younger strata with more recent fossils are towards the top with older fossils in deeper layers. The grand canyon is a great example of exposed strata, although recent history has been eroded by wind and rain. Fossils may be found in the most unlikely of places - like how you can find whale fossils and shells in deserts, once ancient oceans. The fossil record can also provide clues about extinct transitional species to those that are extant, still living. The Amazonian Dolphin has nostrils at the top of its skull while its ancestry, the prozeuglodon from 36 mya has nostrils halfway down the snout.
There is a lot of similarity out there between the anatomy (or structure) and physiology (the function) of certain species. Might that mean they are related? Consider the forelimb design of mammals, yourself included. There is one upper arm bone (humerus), two lower arm bones (radius and ulna) and then a series of fanned structures (the carpels, metacarpals and phalanges). Look more closely, and you will see this exact same pattern with slight modifications for function across several organisms. For example, a whale has long thick phalanges to support their flipper as a paddle through the water whereas a cat has short phalanges for pouncing and a bat? Their phalanges are long and thin to support their membranous wing for flight. Each of these structures are homologous suggesting a common ancestor.
Why does the whale have a hip bones? No, this is not the start of a bad joke - but rather an amazing example of our next piece of evidence. Vestigial structures have no apparent function and are residual instructions from a past ancestor. These remnant pieces exist because DNA still codes for them, even if their function is obsolete. So if whales have hip bones, this suggests that their ancestors once had legs and walked on land. We could spend an entire episode talking about the evolution of whales - cetaceans are the coolest. Other examples of vestigial structures are the human appendix and wings of flightless birds. As for the age old question of “why do men have nipples? When it comes to evolution, if the trait doesn’t provide an evolutionary disadvantage - the alleles will remain in the population gene pool. A neutral trait.
Perhaps the most compelling, and the most recent evidence for evolution is what is happening inside of our cells. Molecular evidence and DNA comparisons. Organelles are similar, processes are similar and everybody’s using the same alphabet - A, T, C, G and sometimes U. When studying the human genome, scientists found that nearly 99.9% of the DNA sequences among all humans is exactly the same. Even with the diverse amount of traits there are in the human population, we are far more alike than we are different. The more closely related two organisms are evolutionarily, the more overlap there will be in their DNA sequences. And this makes sense when we consider the central dogma - DNA, to RNA - to protein. Humans share approximately 85% of their DNA with mice and 75% with chickens. Consider Cytochrome C - a protein that has an essential role in cellular respiration. Over billions of years, this gene has accumulated neutral mutations that can be compared among species and used to predict when organisms diverged from a common ancestor. The greater the differences in amino sequence, the more distantly related two organisms are.
To recap….
Evolution - it's not just a theory. Molecular, morphological, geographical and genetic evidence support the change of organisms over time. The tree of life shows our commonalities and distinguishes between the species that have evolved over time. What's next for evolution? Only time will tell.
Coming up next on the Apsolute RecAP Biology Edition: Energy in Ecosystems
Today’s Question of the day is about Earth Science.
Question: What was the superocean called from Wegener’s Continental Drift Hypothesis?