Professor Dave again, let’s examine the evolution of populations.
With the Origin of Species, Darwin transformed science, and provided a mountain of evidence
for the evolution of life by natural selection. But there were also holes in the evidence,
which have since been filled in many times over. Let’s go over a quick summary of the
main types of data that illustrate how evolution happens. The first of these types involve
direct observation. As we discussed in the previous clip, even though we didn’t watch
animals evolve, we can easily watch simple unicellular organisms evolve. In fact, whenever
we try to use drugs to kill pathogens, like certain bacteria, it is inevitable that a
drug-resistant strain evolves and proliferates quickly, as it is immune to the drug. The
resistance is not a product of evolution, this comes about by blind chance, but the
proliferation of the resistance is indeed a product of natural selection, as the lone
resistant bacterium won’t be killed by the drug while the other bacteria will, so eventually
all the bacteria in that vicinity will be descendants of the initial mutant and thus
also resistant to the drug. We can even watch adaptation occur with short-lived animals
like bugs. When certain insects have their food sources modified, their appendages do
indeed change over a number of generations to better suit their surroundings. We have
even discovered strains of bacteria capable of metabolizing nylon, which was invented
by humans in the 20th century. Thus, evolution by natural selection is not relegated to conjecture,
it can be observed right before our very eyes. Another source of evidence for evolution is
in the homology that exists between species. Homology is a word that refers to structural
similarities in certain species as a result of common ancestry. We can look at the arms
and legs of humans and any other mammal, even whales and bats, and see that they have remarkably
similar bone arrangements, even though one is used to walk, one to swim, and one to fly.
These homologous structures are completely consistent with the idea of a common ancestry
for all mammals. We can look at embryos to find other examples of homology. All vertebrates,
including humans, have a small tail early in embryonic development. This is easily explained
by considering that all vertebrates have a common ancestor. Certain features like these
can be present in fully formed animals as well, and when there are anatomical features
that are not useful to the organism, we call these vestigial structures, which we now understand
are remnants of the features of ancestors. These evolutionary relics include pelvis and
leg bones in snakes, and the remnants of eyes in blind fish that live in pitch black caves.
We can examine homology on a molecular level to go back even further, and see that when
phenotypes don’t match, there are still genotypes that link even humans and bacteria,
showing how such incredibly dissimilar species must still have a distant common ancestor.
This is why we can place all life on a single evolutionary tree, the tree of life, which
we will discuss later. Then, there is the fossil record. From this,
we get an idea of what kind of organisms existed and when, which helps us fill in the gaps
between existing species, and we have used the fossil record to assign dates to the emergence
of all kinds of different species, including us, homo sapiens. Countless times, fossils
have cropped up that provide missing links between various classes of organisms. Archaeopteryx
demonstrated a link from dinosaurs to birds. There are other fossils found that act as
intermediates between land mammals and ocean mammals like whales and dolphins. With each
discovery, the tree of life grows more consistent with evolution by natural selection. Lastly
there is biogeography, the study of how different species are distributed around the globe.
The continents move slowly over millions of years, with certain areas connected in the
past, which aren’t any longer. We have used this notion to make predictions about what
kinds of fossils should be found in certain areas, and these predictions have been successful.
Once again, we now understand that genetic variation is what makes evolution possible.
Any novel trait that an organism can exhibit must be the result of a change in the products
of gene expression, which must be the result of an alteration somewhere in the DNA sequence.
When we look at tiny changes on this level, we are describing microevolution. Natural
selection guides this process, but let’s examine some of the other factors at work,
like genetic drift and gene flow, as these are other ways that genetic variation can
propagate. First let’s recall that many phenotypic traits are determined on the basis
of two alleles, which can be homozygous or heterozygous, and if mutations occur in the
introns of a gene, or in the exons in such a way that the mutation is silent, this will
not produce any change in the organism. But as we know, even point mutations, a change
in a single base pair, can indeed produce novel proteins, and if this mutation occurs
in cells that produce gametes, this change will be passed on to offspring. Typically,
this will result in a less effective protein and will therefore be harmful to the organism.
If this is the case, the new allele will be removed by natural selection, unless it is
recessive, in which case it may proliferate, which is why there are so many genetic diseases
that stem from recessive alleles. But some mutations result in neutral variation, where
the change doesn’t give the organism an advantage or disadvantage. This is one way
that differences can accumulate over time, because there is no mechanism in place to
weed out these benign mutations. Once in a while, however, a mutation will bestow the
organism with a survival advantage, and this is rewarded with a higher likelihood of survival
and reproduction. When we apply this model to a population of organisms, we can see how
a species as a whole can gradually change over time. We can refer to the genetic material
of the entire population as its gene pool, consisting of all of the alleles for all of
the possible traits. Genetic variation in the gene pool will always occur, but there
must be some external factors present in order for evolution to occur, as mutations will
only proliferate in a statistically significant way if the organism receives a higher probability
of survival and procreation. We can use the Hardy-Weinberg equation to determine whether
evolution is occurring in a population. When evolution is not occurring, all alleles and
genotypes will reoccur with the same frequency, a situation we call Hardy-Weinberg equilibrium.
For a particular trait with a dominant and recessive allele, we represent the frequency
of the dominant allele with a p, and the frequency of the recessive allele with a q, so p plus
q will equal 1. The three genotypes must also add up to one, so if we make a Punnett square,
we should expect that the frequency of homozygous dominant, or p squared, plus twice the frequency
of heterozygous, or pq, plus the frequency of homozygous recessive, or q squared, will
add up to one, as these are the only three possible genotypes, and we can plug in our
p and q values to get the probabilities for each genotype. These numbers will remain constant
if there are no mutations, mating is random, natural selection is not a factor, the population
size is large, and there is no gene flow, as these parameters are characteristic of
a system in Hardy-Weinberg equilibrium. In such a case, measuring the frequency of any
genotype allows us to calculate the others, as they must add up to 1. But when one of
these assumptions no longer applies, the population is indeed evolving, so we can measure the
deviation of p or q from the expected value when examining genetic data, and the direction
of the fluctuation can offer clues as to the mechanism at work.
As we said, natural selection guides evolution, as this can pertain strictly to variance in
a trait, like neck length for giraffes. But we must also examine things like genetic drift
and gene flow. Genetic drift highlights how chance events, like the random elimination
of organisms that are homozygous recessive for a particular trait, can cause the gene
pool of a population to gradually skew in a particular direction. This is magnified
when a few organisms become isolated from a larger population, as any deviation in this
smaller group will be more statistically significant than otherwise expected. This is called the
founder effect. Similarly, a sudden change in the environment, like a fire, or drought,
or flood, can produce a bottleneck effect, whereby the population is dramatically reduced.
Again, by chance, the frequency of certain alleles may change suddenly due to the random
nature regarding the alleles of the survivors. So genetic drift is significant in small populations,
it can lead to a random change in the frequency of certain alleles, and it can lead to substantial
loss in genetic variation within a population. Gene flow, on the other hand, occurs because
of the movement of fertile organisms. When looking at species with migratory habits,
like many types of birds, alleles are transferred in or out of the gene pool as a result of
this behavior. Gene flow even occurs in humans, as it has become increasingly common for people
to move across the globe, so mating between members of different populations is typical
whereas it was quite rare even just a couple hundred years ago. But as we said, natural
selection is the only guiding hand to evolution that is not random. It is predicated on the
notion that beneficial adaptations will be passed on, which slowly over time produces
brand new species. This can work in a variety of ways. Sexual selection has to do with an
adaptation that makes an organism more likely to find a suitable mate. Continual sexual
selection is what has given rise to sexual dimorphism, a difference in secondary sexual
characteristics between the males and females of a species. This is certainly evident in
humans, but it takes many other forms, like the brightly colored male peacock, and the
variety of mating calls and dances performed by males of other species. These are examples
of intersexual selection, which is essentially the choosing of mates on the basis of certain
traits that indicate healthy genes, like bright colors. There is also intrasexual selection,
typically among males, who in many species will fight over females in ritualized displays,
including humans. Apart from sexual selection, there are forms of balancing selection, whereby
variation in the genome is preferred, such as the heterozygous advantage. This is strictly
regarding the genotype and not any particular phenotype. There is selection related to avoiding
predators, matching climatic conditions, and all kinds of other factors.
But with all this we must recognize the limitations of natural selection. Nature is blind, and
it works with the traits at hand, it can’t build new features from scratch. When land-bound
creatures evolve into flying ones, they don’t just sprout wings, their arms slowly become
wings over many generations and many intermediate characteristics. Flaws in the design of structures
like the giraffe’s neck, with the completely illogical pathway of its laryngeal nerve,
and the human eye, with its blind spot and other flaws, show how nature built upon what
was already there to get to something that is workable though imperfect. There are so
many factors simultaneously at play, but the end result is a vast ecosystem of organisms
that are well suited for their environments. Natural selection has produced a wide variety
of life indeed, so how do we categorize all of these organisms? Let’s move forward and
discuss the tree of life.