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.
