Evolution, variation and inheritance
Everyone has heard of it, but what do you need to know about it?
Firstly, there are two main characters in the arena and they had
1) Jean-Baptiste Lamark believed that characteristics that you build up in your life-time would be passed on to your children. Basically, if you and your partner weight lifted every day and became very muscular, your children would be born muscular also. We know this to be impossible now as no matter what you do in your lifetime, it will not alter your chromosomes and these are what are passed onto your children.
2) Charles Darwin went to the Galapagos Islands and suggested that through "natural selection" animals that have special characteristics that help them to hunt, eat or survive are more likely to be passed onto offspring as the parents will survive longer and have more babies. These individual episodes of natural selection add up together over a very long period of time to become Evolution.
Basically, an animal is born with a natural variation that allows it to
avoid being eaten will live longer and have more babies so passing on
this characteristic to more of the next generation.
Darwin's theory was only accepted gradually over a long period of time. The main opposition was due to lack of firm evidence and the strongly held belief that God had made everything. Because Darwin did not know about genes, he could not explain about variation which we covered in the last topic. A variation is a mutation of a gene where a characteristic never before seen in that species' history.
The finches that Darwin saw all had common ancestors, however, they were all different. They had evolved through natural selection. Each finch had a beak best suited to the food it ate as seen in the picture on the left.
One day, a bird was born with a beak that made it easier to eat seeds. This variation made it easier for that bird to get enough food so it became very successful. It mated every year and its children also had this beak. Because it was so successful, it lived a long time and had more children than the other finches because when they were dead, it was still producing offspring. Also, other finches would have wanted to mate with this one because of its success. After many years and many generations, all of the finches in that area have that beak and are successful seed eaters, that is evolution through natural selection.
Other examples include camouflage, speed and strength and there are many more. This is also known as "survival of the fittest".
By putting organisms into groups together with similarities, we can see how they are related. These are called classifications. The Natural Classification System is used to study the evolution of organisms. This simplest are "The Kingdoms" comprised of Plant Kingdom, Animal Kingdom and Kingdoms of microorganisms. At the other end of the spectrum, the smallest group in a classification are the "species". Only members of the same species are closely enough related to produce offspring.
This leads us to this diagram on the right. It is an Evolutionary Tree showing us that the closest relative to the Ground Finches is the Cactus Finch and the Warber Finch has evolved least and that they all had one common ancestor from which they all evolved. As they all lead to present day, none of the lines stop before getting to the right, none of the species have gone extinct.
Firstly inheritance, this is what we get from our biological parents. In
the nuclei of cells are chromosomes. Chromosomes are made up of threads
called genes and genes are made up of DNA. In a gamete (sex cell) there
are only half of the chromosomes and after fertilisation, the two parts
create a new whole set of chromosomes and that is a new person/animal.
These genes control how you develop and ultimately what you will look like and how your body will work. We met earlier that metabolic rate is influenced by genetics, this is the stage at which your rate was determined along with eye colour.
Reproduction can occur two ways, the above is sexual reproduction, two
gametes fuse and the offspring is made of a mixture of parents, this
creates variation. It is impossible for the offspring to be identical to
a parent, they may be similar but never identical as they have a mixture
Alternatively, there is asexual reproduction which only involves one parent and no gametes fuse. Offspring show very little variation. Offspring are identical to the parent.
Mixing genes leads to variation. Variation is essential to survival as it generates new characteristics that make it easier to hunt, hide, defend or generally be more successful.
Differences in people is not always down to genetics, part may be down to the environment in which they were raised although genes have by far the greatest impact on the way something looks. However, if we take two identical plants and put one in the Sun and the other in the dark, the one in the dark will look weak, will become ill and die despite having the same genes as the other. Likewise in humans, irrespective of genes, if a mother smokes or drinks alcohol during pregnancy, the baby is likely to have a small birth weight among other complications, none of which were caused by genes.
Cloning is a process by which you obtain a genetically identical offspring from a single parent. It is much more difficult to clone an animal than a plant due to its genetic complexity. Cloning a very successful agricultural plant can be as easy as taking a cutting and planting it with some rooting hormone. Large numbers of rare plants can be generated by the much more expensive method of of tissue culture when small groups of cells and grown under special conditions.
For animals, the process of embryo transplanting is used. An embryo with
unspecialized cells is split up into a smaller group of cells and
transplanted into the host animal. Each are genetically identical. Some
may be genetically modified at this stage to alter the DNA while there
are only a few cells.
Please note that the cloned lamb is identical to the sheep whose udder cells were harvested to get the nucleus.
A similar process on the right shows how the embryo cluster of a very sought after cow can be split and grown in surrogates. Each offspring is identical to parent, they all look the same and look nothing like the animals that gave birth to them (apart from being a cow).
In genetic engineering, a specific part of DNA can be cut out using an enzyme and put into a new embryo cell to pass on a desired characteristic. Also, a gene can be put into another species, e.g. a gene is put into a bacteria that makes it produce insulin for diabetics.
This can be done in plants and is called genetically modified crops. Despite agricultural success, it has become very unpopular with environmentalist groups.
Advantages include: better productivity, curing genetic illnesses, produce medical treatments, make crops resistant to herbicides and insects.
The disadvantages include: Crops are infertile so new seeds are bought each year, wild plants my be affected or cross pollenate, they may kill insects that are not pests, may affect long term human health if eating them and the general ethical argument about playing God, should we create new species just because we can?
In its simplest terms, inheritance is the
process that allows us to have characteristics
that are the same as our parents
and also why
when we produce new cells as we grow or repair
ourselves, they are identical. Chromosomes
are found in pairs and are made up of
genes (or alleles which
contain the information about characteristics),
and the genes are made up of sequences of DNA.
Before we go into the steps of making new
cells, it is worth pointing out what
stem cells are.
These are the first or earliest cells after an egg has been fertilised and an embryo is formed. These cells are not differentiated yet so can become any type of cell such as a skin cell, a muscle cell, nerve cell ect. The only place that stem cells can be found in adult humans is in bone marrow where they can change into new types of cells like blood cells. The most common process is cellular Mitosis. In this process, an existing cell will begin to make an exact and complete copy of all of the the DNA in its nucleus. Once complete, there will be double the number of required chromosomes in the cell and it is ready to split. After replication there is cell division and it will split evenly to produce two identical cells.
This is happening in your skin right now and this is why if one of your skin cells was found at a crime scene, it would have an exact match to your DNA because each of the 1.6 trillion skin cells you have has identical chromosomes inside. Just mention that one more time...you have 1,600,000,000,000 skin cells. The second type of cell production is slightly more complicated but only occurs in the production of sex cells, also called gametes. As you are a mixture of your mother's and your father's DNA, it is logical that a sperm cell and an egg cell (ova) each have only half of a full set of chromosomes so that when they combine, they create a full set that is identical to the DNA that you are made of and is in your 1.6 trillion skin cells (I like that statistic and will use it more often).
The first three stages in the production of
cells here is exactly the same as in mitosis,
however, in meiosis,
a second stage occurs of cell division. Each of
the 4 gametes at the bottom of this diagram has
23 chromosomes rather than 23 pairs of
chromosomes. This makes it ready to join with
another gamete to create 23 pairs in the first
cell of a new life.
away from cell division, we can look at the work
of Gregor Mendel.
He was a monk, who in the 1800s, grew pea plants and noticed, then documented observations that shaped our understanding of genetics. His ideas were ignored for a long time because people then had no idea of chromosomes and DNA. He saw that factors were passed on from parent plants to seedlings. He also saw that some traits skipped a generation which is the basis of modern terms like dominant and recessive alleles.
His "factors" are now called "genes" and his
experiments have led to DNA fingerprinting and
our understanding that apart from identical
twins, we all have different DNA. The most basic of chromosome outcome is the
sex of a baby. Of the 23 pairs, one is the
"which sex" chromosome. Females have "XX" and
Males have "XY". There are some key terms that
are essential for the next part of the topic:
a characteristic's physical appearance
the alleles that have been inherited (capital
and lower case)
both alleles are the same: FF or ff
Heterozygous - both alleles are different: Ff.
These terms make more sense when we look at
examples of genetic diseases being passed on.
Polydactyly is a condition in which children are
born with extra fingers and or toes. It is
passed on if either of the parents have the
allele because it is
If the condition was recessive, then both parents must have the allele for it to be passed on. On the right are punnett squares which predict the outcome of births depending on parent's alleles. These squares are a way of predicting the chances of children inheriting illnesses, for example, if two parents are both carriers of Cystic Fibrosis and have 4 children, 2 should be carriers, 1 all clear and 1 be affected by the condition because it is a recessive allele. Because polydactyly is dominant, if only one parent has it and they have 4 children also, 2 of those will (statistically) have polydactyly. The last thing to be aware of in this section, is the debate on stem cells. New tissues and organs can be grown from stem cells and they can be used to treat leukemia. Cells can be harvested and frozen at birth from the umbilical chord of a baby and used later in life if they are needed. Another way to get stem cells involved the creating of an embryo, collecting the stem cells and in turn killing off the embryo.
This raises massive Social, Moral, Spiritual and Cultural questions which has been fiercely debated from its discovery and is still to this day. These questions are also raised about the genetic technique that allows parents to see in advance if they are likely to have healthy babies and in some cases, test if their embryo has a condition which gives them the opportunity to terminate a pregnancy that would produce an unhealthy child. These are some facts, I'll leave you to formulate your own opinion.
There is still debate about where the oldest
species came from with some believing that life
arrived from outer space and others believing
that it started here. As we were not there, our
evidence comes from fossils in rocks and they
are dated using radio isotopes.
can either be hard
shells that survive, soft parts that were
replaced by minerals,
flesh that has been
frozen in ice or
traces such as
burrows or footprints. Most early fossils have
been destroyed by volcanic activity since.
Some species evolve and change a lot and others change very little. Some become extinct which means that they all died out completely. Extinction is caused by: new predators, new diseases, loss of food, massive changes to the environment or catastrophic events like comet strikes or prolonged volcanic eruptions. Changes to the environment affect survival the most. Climactic changes like ice ages have massive impacts as food sources my die out or the animals struggle to shelter and keep warn or reproduce. As nobody was there to witness it, scientists still argue over the cause of the dinosaur extinction. Theories range from a comet strike that caused the world to go dark, plants to die and dinosaurs to starve, dinosaurs killed off by volcanic gases and even warmer seas killed off the plankton which starved the food chain from the source.
If a species is separated then not mixed, they can evolve independently. This is called Geographical Isolation, if an area is cut off by the sea or a river or trapped in a valley or crater. Those in one area may be exposed to different environmental factors to the rest and so through evolution, they will become different. In both areas, natural selection will continue to occur. This continues up to the point when they are not capable of interbreeding if they met. Once they cannot breed, they are classed as different species, this is called speciation.