The APsolute RecAP: Biology Edition

The APsolute Recap: Biology Edition - Natural Selection

Episode Summary

Episode 40 recAPs Chucky D and Natural Selection.

Episode Notes

Episode 40 recAPs Chucky D and Natural Selection. “Nothing in biology makes sense, except in the light of evolution” (1:30). Charles Darwin was not the first to propose the theory of natural selection (2:00). Competition for limited resources results in differential survival and reproduction (3:00). #thestruggleisreal. Pesticide resistance models natural selection action (4:30).

The Question of the Day asks (6:12) “Which finch phenotype did Darwin most study?”

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Episode Transcription

Hi and welcome to the APsolute Recap: Biology Edition. 

Today’s episode will recap Natural Selection 

Lets Zoom out: 

Unit 7 - Natural Selection     

Topic  - 7.1 and 7.2

Big idea - Evolution 


Mick Jagger said it best “You can’t always get what you want, but if you try sometimes, you just might find, you get what you need.” Wouldn’t it be nice if evolution worked this way? Try as we might, there is very little that an organism can do to influence evolutionary success. Genotype dictates phenotype, throw in a little epigenetics and a few million years of environmental pressure and tada - change over time. 

Lets Zoom in:   

Evolution by natural selection is a very big topic. So large in fact that the AP Bio curriculum designates evolution as one of the four big ideas (along with energetics, information storage and transmission & systems interactions.) Geneticist, Theodosius Dobzhansky agreed, quoted with “Nothing in Biology makes sense, except in the light of evolution.” The process of evolution drives the diversity and interconnectedness of living things. It is a change in the genetic makeup of a population over time. The primary process which drives evolution is natural selection. In other words, natural selection is a mechanism, or the how of evolution. 

We can’t talk about natural selection without our man, Chucky D. Spoiler alert - Charles Darwin was not the first to come up with the idea for natural selection - but he was one of the first to publish about it, and sometimes that’s what it takes to get the word out. He got more than a few words out in his very long, very dry, and excessively titled 1859 scientific literature.  The official title is “On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.” Yikes. Here’s the long and short of it: Chucky D. didn’t want to grow up to a be a doctor like his dad, so he went on a several year voyage aboard the H.M.S Beagle while it sailed around South America (allegedly mapping the coast). He collected many organisms and is best known for his observations of finches and tortoises on the Galapagos Islands off the coast of Ecuador.  Upon return to England, he made note of their similarities and several years later, started to connect the dots.  

According to Darwin’s theory of natural selection, competition for limited resources results in differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more offspring, and so, they pass their traits to future generations. Let’s break this down into four steps. First - there needs to be an overproduction of offspring. The further you go back on the tree of life, the more offspring are produced (think fish laying thousands of eggs versus birds laying dozens). This is really a numbers game of playing the odds. The more offspring produced, the greater the chances the species will continue. Step two - the offspring produced need to be a little bit different from each other. We can thank the process of meiosis and sexual reproduction for the genetic diversity. Step 3 - #thestruggleisreal. Step three is where the magic happens. There needs to be some sort of competition for one of life’s necessities. Be it food, shelter, water or even mates - some organisms are going to have more advantageous traits than others. Lastly, and the only step that matters in the end - successful reproduction. You don’t win the game of natural selection if you manage to outrun the lion and never pass on those genes.  

This is known as evolutionary fitness - the measurement of reproductive success. Linking back to Darwin’s theory, the most favorable phenotypes will not only survive but also produce the most offspring. And the cycle continues. But which phenotypes, or physical characteristics, are favorable varies greatly depending upon the environment.  The environmental pressures may be biotic (from living things - like predator/prey interactions) or abiotic (from non-living things - like droughts). Every organism is not getting faster, and stronger, and more camouflaged. Mr. Jagger again - “you get what you need.” Or at least what the environment dictates is necessary to keep!

Let’s talk about a practical example. A farmer has a large field of crops where insects are having field day, consuming every leafy green in sight. There are alot of insects (overproduction of offspring) and they come in two colors (genetic variation). The farmer is annoyed at losing so many crops each year so he decides to spray pesticides on the crops to get rid of the insects (struggle to survive). Most of the green insects die, but the red don’t. Those that survive successfully reproduce. The next generation of insects is mostly red with a few green. The following year the farmer sprays the crops again, but even less insects die. Why is that? When the farmer first caused environmental change with the pesticide, some of the insects had genes that gave them pesticide resistance. This variation would not have been selected for in the population if the pesticide has not been applied. The environmental change applied selective pressure to the insects. Some other great examples include sickle cell anemia, the peppered moth during the industrial revolution, and flowering time with global climate change. 

To recap….

How long does evolution take? Well that depends on many factors - but the primary mechanism of change over time is natural selection. Genetically distinct organisms in a population compete to survive and successfully reproduce in their environment. And if the environment changes? The phenotypes will too. 

Coming up next on the Apsolute RecAP Biology Edition: Population Genetics 

Today’s Question of the day is about birds

Question “Which finch phenotype did Darwin most study?”