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Evolution of Populations
Genes and Variation
When Darwin was coming up with his theory of evolution, he did not have the benefit of the knowledge as to
how traits were passed from one generation to the next. It wasn’t until the early 1900’s that scientists began to
learn how traits were inherited.
Biologists who study evolution today often focus on specific populations. These members of a population share
a gene pool, or combined genetic information of all of the members of the population. A gene pool typically
contains two or more alleles for each inheritable trait. The relative frequency of an allele is expressed in a
percent.
There are three main sources of genetic variation in a population:
1. mutations – or changes in the sequence of DNA due to radiation, chemicals, or inherited.
2. gene shuffling – because chromosomes separate independently during meiosis, genes are constantly
shuffled, producing much variation. Also includes crossing over.
3. random mating – the fact that all mating is random further increases genetic variability
Inheritable variation can be expressed in a variety of ways. The number of phenotypes produced for a given
trait depends on how many genes control the trait. There are two ways it can be expressed:
1. Single-gene trait – a trait controlled by a single gene that has two alleles. Ex: widow’s peak; either
you have it or you don’t
2. Polygenic trait – a trait controlled by two or more genes that have two or more alleles. Ex: human
height. Can have many different variations.
Evolution as Genetic Change
Key point: natural selection does NOT act directly on genes; rather, it acts on phenotypes. But it CAN change
the frequencies of alleles in a population over time. How does natural selection change the frequencies of
alleles in a population?
1. Single Gene Traits and Natural Selection
 This occurs in one way: consider a population of lizards. The majority of the population is brown,
but there are some red and black mutations. If the population lives in an area with dark soil, the red
one will be seen by predators easier, but the black and brown will live. The red allele may
eventually disappear. See chart below:
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.
2. Polygenic Traits and Natural Selection
 Since there are multiple alleles with polygenic traits, there are three ways that a population can be
disrupted. Look at the examples below:
i. Directional Selection: consider the situation where the average beak size of a finch
increases. Instead of your normal bell-shaped curve, the curve shifts to the right.
ii. Stabilizing Selection: suppose that certain babies are born really small. They will
probably not survive. Babies that are much larger than usual have a harder time too. The
trend here is toward the center of the curve. It will become shorter and fatter.
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iii. Disruptive Selection: suppose that seed size changes for finches. Average-sized seeds
become less common, and small and large seeds become more common. The bird
population will split into two subgroups, each specializing in large or small seeds.
3. Genetic Drift – when a population is small, random changes in allele frequencies are much more
common. Genetic drift is when, in a small population, individuals that carry a particular allele may
leave more descendants than other individuals, just by chance. Over time, a series of chance
occurrences of this type can cause an allele to become common in a population.
Evolution vs. Genetic Equilibrium
Scientists often ask: “Is there any conditions under which evolution will NOT occur?”, and the answer to that
question is provided by the Hardy-Weinberg principle. This principle states that allele frequencies in a
population will remain constant unless one or more factors cause those frequencies to change. This is also
known as genetic equilibrium. There are 5 conditions under which must occur before the Hardy-Weinberg
principle holds true:
i.
ii.
iii.
iv.
v.
Must be random mating
Population must be very large
There can be no movement into or out of the population
No mutations
No natural selection
The general equation for calculating gene frequencies is called the Hardy-Weinberg equation:
p2 + 2pq + q2 = 1
Where p2 = the frequency of homozygous dominants; 2pq = the frequency of heterozygotes, and q2 = the
frequency of homozygous recessives.
Suppose you have flowers: RR and Rr = red flowers, and rr = white flowers. If the probability of drawing an R
is 0.8, and the probability of drawing an r is 0.2, then plug into the equation to figure out the probability of
drawing a pq:
(.8)2 + 2(.8)(.2) + (.2)2 = 1
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The Process of Speciation
The definition of speciation is the formation of new species. As new species evolve, populations become
reproductively isolated from each other. But how can speciation occur in nature?
1. Behavioral isolation - occurs when two populations are capable of interbreeding but have
differences in courtship rituals or other types of behavior.
2. Geographic isolation – two populations are separated by geographic barriers such as rivers,
mountains, or bodies of water.
3. Temporal isolation – two or more species reproduce at different times, such as flowers releasing
pollen at different times.
In Darwin’s finches on the Galapagos Islands, speciation occurred by a number of different scenarios. Look at
the chart below of a representative example of different finches discovered by Darwin:
Darwin hypothesized that these finches all had a common ancestor, and that over time, natural selection shaped
the beaks of different populations as they adapted to eat different foods. After testing and observing and
classifying, this is the hypothesis researchers have come up with to explain the speciation of the finches:
 Founders arrive – some finches traveled from South America to one of the islands, and survive
and reproduce.
 Separation of populations – some birds from species A cross to a nearby island to form
population B, so that the two populations no longer share a gene pool.
 Changes in gene pool – seed sizes on the second island favor birds with larger beaks. The
population on the second island evolves into population, B, with larger beaks.
 Reproductive isolation – some population B birds come back to the first island, but will not mate
with the birds of population A. Populations A and B are now separate species.
 Ecological competition – as species A and B compete for seeds on the first island, they continue to
evolve. A new species, C, may evolve. Some members of original species B may travel to a new
island.
 Continued evolution – the process continues, leading to the formation of all 13 finch species on
the Galapagos Islands.
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