Genetic variation, gene flow, and new species


(relaxing music snippet) – [Instructor] Natural
selection is Darwin’s central and most brilliant insight underlying the mechanism for evolution. But selection of what? What is the raw material
upon which the selective forces in nature are acting? Although he had some ideas about
it, what Darwin didn’t know was that variation stems from
differences in the generic information contained in
each cell of every organism. It’s a bit like an alphabet
made of only four letters. Letters that can be arranged into words of almost any length. The genes can be viewed as
words made of these letters. The sequences of nucleotides
spell out codes that give orders to the cellular
machinery to make the cell work. In fact, the genes help
to make cells themselves, ultimately providing the coded information that builds the entire organism. The sequences of nucleotides
are arranged in long molecules that have a long name. Deoxyribonucleic acid. I like to break down complex
words to their roots. So, deoxyribo is the
long molecule’s backbone of special sugar molecules call deoxyribos that are joined together to make a pair of twisted long parallel chains. The nucleic part of the word
means we’re talking about the nucleotides, those
four special molecules that connect the two
sugary backbone chains and represent the letters
of the genetic code. The specific order of the
nucleotides makes the words, or genes, of the genetic code. Lastly, the term acid is
the chemical classification for the entire huge molecule,
because this whole thing is actually, chemically speaking, an acid. So it’s deoxyribonucleic acid. And that’s how I spell DNA. You can imagine that, with
the complexity of organisms, there must be a lot of
DNA that needs to be read, a lot of words in the
genetic code that dictate those marching orders
to build the organism, and to make it do all
the things that it does. And there is! By some estimates, there are
about two to three meters of DNA in each cell of a human, and that’s just in a single cell. If you add up DNA lengths
from all the cells in a human, that’s roughly a long enough string of DNA to go back and forth, back and forth between the Earth and the Sun 70 times. So this is a very very long string, but it’s a very very long
thin string that, as I said, is one long molecule, and
with that much DNA packed into each cell, there’s
actually a way of organizing that potential mess,
of preventing tangles. The DNA in the cells of
each type of organism is arranged neatly into
a species-specific number of packets or structures
known as chromosomes. The number of chromosomes
varies from species to species. Humans have 46 chromosomes,
but dolphins have 44. A platypus has 52. A dove has 78. A mosquito has six bitey
little DNA molecules. A slime mold has 12. Peas have 14. Rice has 24. The adder’s-tongue fern has 1260. And there are some kinds
of single-celled microbes that are said to have more than 15,000 tiny little chromosomes in each cell. You can see that chromosome
isn’t directly related to the complexity of the
organism, and you can also see that species vary genetically,
but it’s important to note that individuals within a species do too. So why do individuals vary? Scientists studying
genes know that changes of various types happen in
the genetic code itself, which introduces variations
among individuals and among species. We call some of these changes mutations. Mutations are actual changes
in the sequence of letters in the words that make
up the genetic code. Changes in the nucleotides
that make up the genes, and therefore, changes in the instructions that come from the DNA. Mutations happen regularly
through mistakes in replicating or reproducing the DNA
during cell division from chemicals that can interfere with the structure of
DNA, and from radiation. Though many of these factors are natural, they can be human-driven as well. The bottom line is that mutations can delete or change nucleotides. They can even change
pieces of a chromosome, or even the whole chromosome. Mutations result in different
forms of the same gene. These different forms are called alleles. For example, eye color is
coded by different alleles of the same gene. When the DNA’s instructions are read by the cell’s machinery,
these differing alleles can cause variations in
the traits of organisms. In their body shape, their
metabolism, their behavior, and any other genetically-determined
feature or process. Therefore, it’s not surprising
that every individual in a population is unique. Every individual is
composed of a complex mix of many many traits,
and behind those traits, there can be many many different alleles. But how do the alleles get
distributed to the offspring? That’s what Darwin wondered too, and now we have to talk about sex. But unlike Darwin, our
discussion of sex can center on how variations in genetic information can get passed on to offspring. In the process of making the
sperm cells and egg cells used in sexual reproduction, a huge amount of genetic recombination occurs. A kind of reshuffling of the genetic deck. This results in chromosomes
with new combinations of alleles, and when
these genetically-varried sperm and eggs come
together at fertilization, the result is a bunch of offspring that are genetically unique individuals. Even bacteria, which don’t
have sex in the same way as organisms with males and females do, have similar processes
going on that continuously reshuffle the genetic
deck during reproduction, allowing lots of variation
in their offspring as well. And remember, it’s those
individual differences that are the focus of
the selection process, because some of these
recombinations are gonna make an individual more fit than
others, more able to survive, and more able to have
offspring off their own. And that’s where natural
selection comes in. Removing, selecting against the non-viable and less fit individuals. Or, on the flip side,
selecting for the individuals that are more viable, more fit. That’s the idea behind the
survival of the fitter. So we’ve seen how sexual
reproduction can lead to tons of individual variation
within a population, and how populations will change over time as a result of natural selection. But ironically, at the same
time, when there is widespread breeding among members of
a population, the resulting mixing of genetic information
within the whole population also means that individuals
within a population don’t diverge too much from each-other in form or behavior or physiology. This will also mean that one
population in a species doesn’t differ too much from another
in a particular species. This mixing of genetic
information among interbreeding members of a population or
species is known as gene flow. It maintains enough
consistency among individuals and populations of the
species that members can still reproduce with one another. What happens if gene flow is slowed down or somehow prevented? Imagine a population in
which sexual reproduction and variation is happening all the time, and then some barrier
occurs that separates this population into two parts? A really famous example is
when the oceanic water levels dropped enough millions
of years ago to allow the Isthmus of Panama to become
a complete strip of land, separating the waters
of the Eastern Pacific from the Caribbean Sea. Species of marine organisms
that had ranges extending to both sides of the isthmus
now encountered a barrier that kept some members from
being able to breed with others. However, individuals on
either side of the barrier continued to reproduce among themselves and continued to have variable offspring that were selected for or against. But each of the sub-populations
continued to do that without the influence of the
genes in the sub-population on the other side of the isthmus barrier. Therefore, we now have what
is called a restriction of gene flow between the
two separated groups. What had once been a single
interbreeding population has become two separate
populations without gene flow between them because of the barrier. Scientists know of many
examples of marine species on one side of the Isthmus
of Panama that have, as their closest relatives,
a second sister species on the other side of
this important barrier. These related pairs of species
even have a special name: Geminate species, from the
same ancient Latin root as in gemini, or twin. With time, and that’s the
crucial ingredient here, time, enough time to make
generations of reproduction, the two populations
diverge in their traits. This can happen by random
changes that occur on either side of the barrier, but sometimes, the environmental
conditions on either side of the barrier may be slightly different, creating different selection
forces for the two populations, serving to accentuate
the difference between the two populations over
time, and at some point, the two populations will
have diverged enough in their traits that they’re recognizable as two different species. It’s this divergence that’s really crucial in understanding how evolution happens, and how new species are formed. That’s what we’re talking about here, speciation. This, for me, is the stuff of evolution. Speciation is not the
accumulation of changes within a single species, so that, at some point, you say that particular
species has somehow transformed wholesale into a new species. Instead, it is the splitting
of a single species into two descendant species. All of this happens
randomly by recombination and by mutation. Evolution has no goal,
it has no direction. I like to tell my students, stuff happens. The stuff is just random events. Mutations and new combinations of alleles that produce variability
in the genetic code, and therefore, in the
traits of individuals. Environmental circumstances
select for or against the genetically-encoded traits. Traits that are selected for are passed on to succeeding generations. But the true wonder of
it all is the result. As populations diverge and
continue to diverge over time, a branching tree full of
ancestor and descendant species is formed and keeps growing. This is the tree of life,
full of the diverging species that make up the endless
forms most beautiful that Darwin talked about. In other words, you end
up with biodiversity.

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