Lecture 1: Introduction to Evolutionary Biology
Contents
Lecture 1: Introduction to Evolutionary Biology¶
The major challenge that drives Evolutionary Biology is how do we account for the tremendous about of biological diversity that we observe on this planet? Where did that diversity develop from? How do we even begin to analyze biological diversity?
Evolutionary explanations often can give us sensible answers to questions such as this and more, however fundamentally humans tend to take and anthropocentric (human centered) view of our existence and place in the universe. One goal of this class is to break you out of that pattern of thought.
We are insignificant in space and time!¶
Think of the example of Carl Sagan’s Cosmic Calendar that I used in lecture– the calendar starts with Jan 1 at the Big Bang (origin of the universe) and we play it forward to the present day represented by December 31st. On that scale all of life on earth didn’t begin until Sept 14th and humans don’t appear until the final minutes of December 31st! Indeed humans have been around for nearly no time when the timescale of the universe is considered.
Just as our species is new formed on this scale, the entirety of our home is insignificant with respect to space. For instance our solar system, a system of 8 planets circling a star (the sun!), is one of many many in our galaxy (the Milky Way). How many stars are there in the Milky Way? Well somewhere between 100 and 400 billion. To make matters worse, our galaxy isn’t so special either– it’s one of approximately two trillion galaxies which occupy the observable universe according to astrophysicists. The universe is a very big place!
So this leads to some natural questions:
is there life beyond Earth? If so what is it like?
Do we share ancestors with alien life? Does it use DNA? Instead is it independently evolved?
We’re going to be talking about genetic variation and the only way to make sense of it is through the lens of evolutionary biology.
What is Evolutionary Biology?¶
Evolutionary biology is the study of the patterns of organic diversity and the processes that generate those patterns.
Fundamental Observations- Diversity: Notice diversity of form when you walk to class; Notice diversity of humans in this room; birds and butterflies have a common ancestor, but look very different. Think about the amazing diversity even within species: populations of humans, breeds of dogs. Where does all this diversity come from? How is it maintained?
Evolutionary biology is also the study of the “good” fit of organisms to their environments: the study of adaptation. This involves both the recognition of a pattern (good fit) and testing hypotheses of mechanism to account for the pattern. Good fit: leafy sea dragon (camouflage); leaf katydids (camouflage); marine organisms with fusiform bodies (whales, dolphins, tuna, etc.).
Evolution as Explanation¶
Hierarchy of organization in biology.
Evolutionary tree (“Cladogram”) of whale, seal, penguin, shark, sea snake: knowing the relationships of these organisms gives us one level of “appreciating the diversity” of the natural world. Seals and penguins are distantly related but have comparable body forms for life in water (see figure 1).
Fig. 1 A simple cladogram of vertebrates¶
Proximate and ultimate causation refer to the “how” and “why” of natural patterns. When observing these patterns, we aim to understand why things exist as they do and how they came to be. For instance, why do cacti have thorns? The answer is perhaps to reduce herbivory, and the “how” is that those cacti with spines experience less predation, which may lead to more offspring. Another example is warbler seasonal migration. The “why” is related to the availability of food, daylight, and habitat. The “how” is that those individuals who left their wintering sites had more offspring.
Evolution provides an explanation for many issues of general human concern and welfare: the population genetics used in DNA fingerprinting; natural selection that has resulted in extremely dangerous of antibiotic resistant microbes; a new view of medical issues known as Darwinian or Evolutionary medicine that addresses issues of why we get sick, whether to treat a fever and why our knees and lower backs are frequent sources of mechanical failure; and concern for the long-term genetic consequences of living and reproducing in our own waste products of pollution and global warming. Obtaining scientifically sound explanations for these observations is very difficult and is the task of evolutionary biology as a discipline. Easy to fall into “adaptationist” story telling. “Just-So Stories”
What is Evolution?¶
Using a broad brush, we might define evolution to be the change in a biological lineage over time. Such evolutionary change can be observed in the fossil record, for instance Archeopteryx was an early bird that showed clear bird-like traits such as wings and feathers, as well as more dinosaur-like traits such as teeth. The fossil record was perhaps the best early evidence for evolution so, it’s interesting to consider some contra-history where fossils didn’t exist– would that change our understanding of evolution?
We can also observe evolution occurring at much shorter timescales. For instance the evolution of antibiotic resistance to new classes of drugs can occur within years, leading to a tremendous and inevitable public health crisis. Indeed there is speculation that without coordinated investment we could see the end of the antibiotic era in our lifetimes.
Another case of evolution over short timescales has been the meteoric rise in frequency of the delta variant of SARs-CoV-2. This particular genetic lineage was first detected in India in 2020 and rapidly swept through the entire globe, such that as of September 2020 all infections in the United States are currently delta. This rise in frequency was an incredible case of natural selection that we could observe in real time acting on a population (the virus!).
Such thinking leads to our ultimate definition of evolution - genetic change in populations - using this definition we will write down equations that describe how the forces of evolution– natural selection, drift, mutation work together to change allele frequencies within populations, and thus cause evolution to occur.
We’ve already gone from Archeopteryx to SARs-CoV-2 – a huge difference in timescales – but both are illustrative of evolutionary processes. Why? Evolution is simply the playing out of genetic processes over long periods of time. Of course sometimes an asteroid hits the earth but lets not worry about that yet…
This genetic process along would lead to change, but the eventual splitting of populations (and even genetic incompatibility) is what will lead to diversity.
Why study evolution?¶
Evolution is the process that has given rise to biological systems. If we wish to ask questions of ultimate causation (e.g., how did this arise? why is this like this?) then for biology we must consider evolution. Everything in biology has gotten here by evolution: photosynthesis, mushrooms, nervous systems, even pugs (by artificial selection and domestication).
Also note that this means that we, humans, also are the products of evolution. We claim as our ancestors a long lineage of ape-like relatives that stretches millions of years back in history. Before those ape grandmothers and grandfathers were smaller, more squirrel-like mammalian ancestors, and so on – this is a simple fact about our human inheritance, our human condition. Understanding how evolution has shaped us and the world around us will fundamentally change the way you think about your place in the world. From disease to death to sex, evolutionary thinking is a powerful lens to apply to life.