This week the post is on evolution, the basics of biology! As a bonus you get a lesson on genetic inheritance!
A word most people even slightly educated know about and dreaded from school. It is the proposition made by the controversial 1859 theory of natural selection (well, other than natural selection itself). Charles Darwin was the man who we believe suggested this idea, doing many things in the process: the two main points being the revolution in biology that it sparked, and the torture school children must go through.
Darwin proposed that all species of life evolved from a common ancestor. He said this occurred through a process he dubbed as ‘natural selection’.
A population is “a community of organisms of the same species that is able to interbreed freely”. One of the fundamental facts to understand is that even in a population, the probability that the genome of two different organisms will be the same is infinitesimally small. When I mention genome, I mean the genetic makeup of an organism. This will be discussed in the future, but a basic overview is this: there is something called deoxyribonucleic acid or more commonly known as DNA that makes up the pattern for all living organisms.
The study of DNA is essentially genetics, and the makeup of the DNA varies from species to species. An organism defined as a different species from another will have different traits, whether observable or unobservable. These are passed down from mother to child, and in the process of child development, if a mutation occurs in the DNA, some small part of the offspring will be different. This is genetic variation and always occurs.
Over time, however, the genetic differences begin to drive a species further and further away from its original ancestor. This is where the theory of natural selection, or evolution, comes into play. Now, in a population, the “collective variation” is the gene pool. The gene pool is the sum of all of the genes in the species, all of the ‘allelic’ forms. Allelic is a term referring to alleles: alleles are the bits of data transferred from parent to offspring.
The alleles passed down have a specific probability based on the genes of the two parents. Though I won’t get in-depth on alleles right now, I will talk about the basic transfer: how you calculate the basic probability.
How alleles work is there are two main types: dominant and recessive. Dominant alleles are, well, dominant, and recessive ones are, well, recessive. Dominant alleles are denoted with a capital letter for allele transfer, and recessive ones are denoted with lowercase letters.
In the simplified version I will show you, each parent has two alleles. For example, two parents could have Aa and aA.
Parents
|
A
|
a
|
a
|
Aa
|
aa
|
A
|
AA
|
Aa
|
The table above is the passing of alleles. In this example, imagine that this is overly simplified, and that a capital A is for black hair, and lowercase a is for auburn hair. Now, looking at the chart, the letter is passed down into both columns. One thing people don’t understand much is that if there is even a single dominant allele, the trait will always be the dominant one. So in this example, there are 3 diagrams with capital A’s (As? What would it be?), and all three would have black hair. For a recessive trait to come forward, the recessive allele must be repeated twice. This happens once, so the aa means auburn hair.
But what does this mean? you ask me. This means that a child has a 25% chance of getting each square, and adding up the 3 out of 4 chance for black hair, means one has 75% chance for black hair and 25% chance for auburn hair.
In the real world, there are thousands of alleles that are responsible for one trait. All the versions that exist are pooled together in an allelic pool. I hope you enjoyed that genetics lesson. Moving on.
Now, the gene pool is important in identifying the evolutionary change over time. One thing you must understand about populations: there will be genetic variation. ALWAYS, within a population, will there be variation.
Remember the alleles we discussed literally a minute ago? Well, they are responsible for mutation. The alleles are those that pass down traits, and invariably are those that pass down mutations. Mutations are the accidental changes in the gene patterns of a species.
This mutation is the direct cause of the gaining of traits in a population. These traits are consecutively inherited by future generations, and soon the trait becomes intertwined, synonymous with the species.
Migration between two populations can lead to genetic changes as well, as most populations have varying gene pools. You may be quoting me from earlier: ‘A population is “a community of organisms of the same species that is able to interbreed freely”.’ Double quotation marks.
I said “of the same species”. Now, you must realize that populations are only communities, not the only surviving little fragment group of a dying species. The same species can have different populations.
Now, not only does freak mutation lead to species change, so does the environment. The most famous case that proves the theory of natural selection is that of the peppered moth. I know I haven’t discussed the theory itself yet, all in due time.
The peppered moth is a species of moth that, in the 1800s in England, underwent a massive change. During that time was the industrial revolution: a time of environmental collapse when humans sought to be the most advanced species yet.
Back in that time, peppered moths were light, and dark patterned ones were rare. Then came the smog and smoke, leading to a change in moths. Normally, the lighter moths were the ones with the camouflage, but the smoke led to the darker moths surviving and passing down their genes.
This meant that the lighter moths began to become rarer and rarer till the variant no longer existed. The dark moth, on the other hand, became common. This is referred to as a prime example of natural selection.
What is natural selection? Natural selection is the theory suggested by Darwin. It essentially states that out of two species, whatever species is more outfitted to survive in specific conditions will survive. This is said to be nature’s method of selecting survivors, hence the name natural selection.
A better explanation is this: if there was an incoming ice age, and two types of squirrels are both being observed to note their species change, whichever one is more outfitted to the cold will survive. So, if squirrel species A, with wooly, wiry coats is compared with squirrel species B, with thin fur and desert-like heat retardant coat, squirrel species A would probably survive out of the two. Now, this isn’t very hard to work out on your own, as logically, the colder it is, you want to retain heat.
I really don’t know if heat retardant is the right word, but retardant doesn’t mean retaining, so the example stands firm. Also, the reason heat retardant was the property given was due to eggs being fried at Death Valley. Like, the squirrel is a mammal, I get it, probably won’t lay eggs soon, but what if the individual organism gets set on fire?
Natural selection could also happen in another way, other than natural selection, but by coincidence. Say there are three types of insects with different coloured shells, black, white and grey, with an equal 33% chance for all the colours The insects live on volcanic rock, obsidian, and they are parasitic. Of course humans want to kill them, because everything bad other than humans must go, right?
Well, the white ones are easy to see, so literally the entire population is demolished. The grey one is quite easy to spot but not as easy, as grey is the single tone of silver, whilst gray is the palette between black and white. Being in the middle, most can see it while some can’t, and around 75% are extinguished from existence. The black ones are hard to spot, and only about 25% are stopped from their parasitic habits.
The genes for the white shell is almost demolished and is rarely seen from then on. The grey shelled insects have smaller numbers but still exist, and the black ones are the same. However, the black ones only lost 25%, and the grey ones lost 75% of their population.
Working out a ratio leads to a 50/50 ratio for black to grey after the white is removed, and then taking out 25% of 50 and 75% of 50 leads to a ratio of 37.5 to 12.5.
Essentially, we take a new sample size of 100, with 50 black and 50 grey, and take out 25% and 75% respectively, giving a rounded version of 37 and 12. This adds up to 49, and leads to a 75.51% to 24.49%. This means an equal 33% to 33% to 33% percent lead to a big difference.
(Please check the math, it may be wrong)
Note the insect bit isn’t definitive fact, so don’t say that it doesn’t exist. Something like it has probably happened in the past before, however. This type of evolution is known as gene drift.
Another name for natural selection is the survival of the fittest, coined by Herbert Spencer, for reasons probably not worth mentioning, but here. The term survival of the fittest refers to how the so-called fittest species or members of a species will survive. Interestingly, this term was coined referencing an economic principle.
But what does fittest mean? Fittest means the ‘capability to pass down genes’, and can be mathematically be described through the ratios of viability (the lack of high infant mortality rates) and fertility (offspring:parents ratio).
Well, it’s all well and good, but if everything happening is its own form of natural selection, then what’s the use? To define the process, Darwin listed four rules:
Variation within traits in a population must exist, and some variation results in fitness.
There should be reproductive differences between species.
You must assume that genetic traits are inherited, leading to fitness being inherited too. In this case, the fitness refers to genetic variations like the different coloured shells of the volcanic insects we made an example of earlier.
Fitter individuals mate more often* so that means the fitter trait is passed down more and the weaker ones die out.
He goes on to say that unless all four conditions are met, natural selection won’t have been the contributor to evolutionary change.
There are, even within the one-of-many-branches evolution separate types: micro- and macro-evolution.
If you know your root words, or even better, study an ancient language like Latin, you know micro is small and macro is large. It doesn’t take a genius either.
Using this root word process you probably deduced correctly that microevolution is change on a small scale and macroevolution is change on a large scale.
Microevolution is the change in a gene pool: the small mutation generated additions to the collective data of a population. These microevolutions could be caused by a variety of things ranging from the aforementioned gene drift to good old mutation to natural selections.
Now, macroevolutions are a step higher. These are evolutions leading to a whole new species: a branch off of the parent species. This is through a process cleverly named speciation. This is a pain to observe as it normally takes a long time, compared to the relatively quick microevolutions.
The process of macroevolution ties in with two different types: a type known as allopatric speciation: when a species branches off into two parts and speciation occurs; and a type known as sympatric: when the species evolves from a parent species while sharing a geographical area.
Perhaps the most famous example of natural selection and allopatric speciation is that of Darwin’s finches. No, not a bunch of birds Darwin owned and observed, a bunch of birds he probably wished he owned but did still observe.
Darwin, as a young man, sailed to the Galapagos, and observed a bunch of finches. They all were of similar genetic makeups, were defined as the same species, but had different beaks.
Look at the image to the left. It is the distribution of various finches throughout the visitor islands. You can probably notice that no finches are on all of the islands. This is due to the fact that the finch isn’t perfectly adapted to the island environment.
Now, the thing is, if it adapted to the other islands, a new species would still be created, so natural selection still works.
Back to allopatric speciation. The way this works is that two separate gene pools are established, each facing its own natural selection.
The things is, since the finches are still sharing the same area, the finch evolution falls under both allopatric and sympatric, but that’s beside the point.
What happened with the finches was that not all finches could find the same food on different islands, so some finches who had mutated to have stronger beaks to say crack nuts would do better on islands with nuts.
So, the finches who were better suited for life on a specific island have a higher chance of surviving on the island. Now, this is an example of allopatric speciation.
One thing that can happen is that the two populations created by allopatry? allopatric speciation haven’t yet become species, meaning that in the case that two populations meet up again, sympatric speciation can occur.
Now, if two allopatric species become sympatric, i.e. begin to share the land, three things can occur:
The two populations, if not diverged much, could still merge back into one,
The two populations could fight for resources and eventually lead to the extinction of one,
Or the two populations could develop side by side and specialize in different areas.
The finch evolution is both allopatric and sympatric as looking at the chart, you can observe that two similar species don’t live on the same island. The cactus finches and large cactus finches simply don’t coexist due to their similar diets, leading to competition that, with the survival of the fittest doctrine, led to the most suitable to adaption surviving.
On the island of Genovesa, large ground finches and large cactus finches coexist. The important thing is, the beaks of large cactus finches are different on Genovesa than on their other island, Espanola. This is due to the fact that they have competition on the island of Genovesa.
Their competition is the large ground finch: and mostly, they coexist well; in the wet season, when there is a ton of food there is no worry, but in the dry season, the species must compete for the limited food. For that purpose, the species must specialize to get food. The thing is, there isn’t much of a difference between beaks as any cactus finch with variance from a ground finch won’t be able to compete with even it’s own or opposite species.
So, the species are sympatric as they share the same land, and since they are assumed to have evolved from the same root, the evolution is also allopatric.
The story is completely different on Espanola: there is no large ground finch, and the cactus ground finch is virtually alone with little competition.
This allows it to demonstrate variability that the other island’s finches cannot, and effectively will settle between the two of Genovesa. This will let it feed equally well all year round. This phenomenon is known as character displacement.
Darwin’s finches aren’t just scientific proof, remember, they were the ones who put Galapagos on the scientific radar. Give credit, most people can’t say the same about putting their home on the map.
Darwin’s theory may be the most accepted, and definitely the most well known, but it isn’t the only one. A theory known as punctuated equilibrium was suggested in 1972, challenging Darwin’s theory’s core.
Proposed by Stephen Jay Gould and Niles Eldredge, it states that species evolved rapidly over a short period of time known as a punctuation, and then rest in stasis, hence the name punctuated equilibrium.
This theory is based on the gaps from the fossil record, and says that any instance of selective evolution is coincidental.
Scientists have studied patterns and found some proof for the theory: namely that of bryozoans. They were a group of coral like sea organisms that lived some 140 million years ago and remained unchanged for around 40 million years till a burst of evolution and diversification, and then another period of rest.
If this is proven, the entirety of biology will be overturned, but in the end, it boils down to the fact that speciation led to most species.
Human evolution will be discussed on this blog later.
All of Science A book whose chief consultant is Associate Professor Allan R. Glanville. The front cover has a picture of a nautilus on it. Or a golden spiral, but it looks way too realistic. Then again, who knows what people do with photoshop and an hour of free time.
