Now let's take a look at Darwin and his theory of evolution. This famous theory provides a really great example of how scientists develop hypotheses and a theory gains acceptance. Darwin was an English naturalist who spent 30 years studying and observing the world around him before publishing on the origin of the species by means of natural selection.
This publication was a sensation when it was published, and the ideas of Darwin expressed in it have played a central role in the development of human thought ever since. But it was not only Darwin who came up with the idea of evolution. These ideas came about through the studies of a number of earlier naturalists and philosophers who had presented their views that living things must have changed during the history of life on Earth. That is, that evolution had occurred.
Living things are now different from when they were some time ago. Darwin simply contributed the concept that we can now call natural selection. He first published his manuscript on natural selection in 1842, but it remained hidden for years until a man named Alfred Russell Wallace came along with the same sorts of ideas and dug up some of Darwin's work. And in 1858, Alfred Russell Wallace, as well as Darwin, presented their ideas in a seminar in London. Darwin then completed his own book, expanding on the 1842 manuscript.
So this material remained hidden for years before it was exposed. You can see a map of the stops of Darwin's voyage on the HMS Beagle. They were doing a lot of surveying. He was simply the naturalist on board. taking note of variants in different species types throughout the course of their journey what darwin noticed on the beagle is that characteristics of similar species varied from place to place he's famous specifically for the galapagos finches he noted that fourteen related species only differed very slightly particularly in the shapes and form of their beaks He also noticed that the shapes and forms of their beaks were well suited for how they found their food.
For example, here the bird needs to dig its beak into a knot in the tree. This bird needs a larger beak for cracking larger seeds. Whereas this bird needs a smaller beak for smaller seeds.
So he came up with the idea of descent with modification. That is, the individual that is more fit. or more able to survive in the environment could reproduce preferentially. This is the idea of evolution. Darwin also studied the essays of Thomas Malthus.
Malthus'essays were written on the principle of population, and he simply said that populations of plants and animals increased geometrically. That is, 2 times 2 times 2. We see this exponential growth curve. However, We can only increase our food supply arithmetically, that is 2 plus 2 plus 2. So there's really no way that we can keep up with food production at the rate that we could potentially reproduce. And thus, there are limits on population size because of the restriction of food. So the populations of species then remain constant because death limits the population number.
Darwin saw that although every organism has the potential to reproduce more offspring, only a limited number can survive and reproduce themselves. This is where he came up with the idea of natural selection. In the beginning of his book, he talks about breeding pigeons and artificial selection, and also notes that we've artificially selected things like dogs.
And with this artificial selection, we've produced a wide array of specific differences amongst the species. So then he figures that nature probably also has some pressures like this. For example, if you can run faster than your friend from a bear, you're more likely to survive. So those individuals that run faster are much more likely to survive and reproduce and thus pass on their genes.
The evidence supporting Darwin's theory has only grown since the publication of his book in 1858. For example, the fossil record is now well known. The transitional forms have been found at particular places. predicted positions in time as we go down through the fossil record.
Also, we now have new information on the Earth's age. The physicists of Darwin's time seem to think that the Earth was only a few thousand years old. However, there's overwhelming evidence now that the Earth is very, very much older. In fact, our estimate today is 4.5 billion years.
This is a lot of time for the forces of natural selection to occur over. and accounts for the great biological diversity we see today. In addition, we now know of mechanisms for the heredity that we see evidenced in natural selection.
Mendel was doing his research at the same time as Darwin. Mendel found out laws of inheritance by working on pea plants. We'll investigate his studies later in the semester.
However, they were unknown to Darwin. and there have been vast expansions on them. In addition to this, there's a study of comparative anatomy.
All vertebrate forelimbs seem to share the same basic array of bones. We call it the pentadactyl limb. You can see here a human hand has five fingers and two bones in the forearm and one bone in the upper arm.
When we look at a cat leg, we see the same arrangement, the same five digits, two in the forearm and an upper arm bone. When we look at a bat wing, we also see the same five digits. Now of course these bones have various adjustments in length and girth. However, they're essentially the same bones. When we look into the forelimb of a porpoise or a whale, we also see the same limb arrangement.
In addition, we see it in the horse. The horse simply walks on its middle finger. The other four digits have been mostly lost, although there are some remnants that you see here.
So what appears to be the front knee in a horse is actually the wrist bones. That brings us to the idea of homologous versus analogous structures. Homologous structures are those that have the same evolutionary origin but differ in their structure and function.
Each of these limbs are homologous limbs. Now, some animals have limbs that have the same purpose, like wings. A butterfly and a bat both have wings.
However, they came about from a different origin. A butterfly's wings are not based on the pentadactyl limb. In fact, there are no birds in them at all. We call these analogous structures. Finally, there's a plethora of molecular evidence.
Now that we have so many tex- to compare genomes or proteins of different organisms, we're able to construct what we call phylogenetic trees that are based on tracing the origin of particular nucleotides, or DNA sequences, and the changes in them to reconstruct an evolutionary history. So we see a human and a rhesus monkey are fairly similar to each other in the amino acid sequence of this particular hemoglobin polypeptide. And each of those are fairly similar to a dog, and so on and so forth. So we can reconstruct what we call this phylogenetic tree that you see on the right. If you dig further into biology, and if you choose to move further into biology than just this first course in the sequence, you'll investigate many more phylogenetic trees and come to a greater understanding.
So time again for a color card quiz. Birds, bees, and bats have all evolved wings, but have different evolutionary histories. The wings of these animals are examples of red, evolutionary structures, yellow, analogous structures, green, homologous structures, or blue, vestigial structures.
Write down the color. Next, in a population of snails, The individuals with the thickest shells survive and reproduce more than those with thinner shells. Assuming that shell thickness is a heritable trait, what should happen to the mean shell thickness in a population of snails over time?
Red is increase. Yellow, decrease. Blue, remain the same. Or green, there would be no pattern. Write down the color.
Here are your two answers. If you answered both of those correctly, you're ready to move on to the next section in this chapter.