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Without physically counting bacteria or viruses, scientists estimate that there are more than 8.7 million species of organisms living on Earth today. The origin and extinction of so many species has fascinated scientists for thousands of years, since the days of ancient Greece. The great diversity of living organisms on Earth is best explained by the evidence-based scientific concept of evolution by natural selection.
Evolution is the change in inherited characteristics or traits in a population of organisms over many generations. The mechanism that best explains evolution is a phenomenon known as natural selection. Natural selection is the process by which certain inherited traits—such as the color of a fish, height of a person, or shape of a leaf—are favored within a population. A population is a group of organisms that mate and reproduce with one another. In general,
Thousands of years of selective pressures have determined the shapes, colors, sizes, and behaviors that optimize the survival and reproductive success of organisms in the environment in which they evolved. In fact, it is often possible to tell a lot about where something lives by how it looks and behaves.
Evolutionary fitness and success refers to surviving long enough to pass genetic material on to offspring. Traits that are passed on to offspring because they contribute to success are “selected for continuation.” Traits that are eliminated from the population because they detract from success are “selected against continuation.”
Any force in the environment that favors or disfavors traits is a selective agent. A force can be biological, like a predator, or physical, like temperature. Over time, populations subjected to different selective agents may become so different that they are no longer able to breed with one another. The biological definition of species is a group of organisms that can successfully breed with each other. Under this definition, when populations can no longer breed with each other, they are considered to be different species.
In the 1800s, many people were trying determine why there were so many different kinds of plants and animals in the world. Charles Darwin wondered about the diversity of animals he saw while in the Galapagos Islands. This led him to develop the theory of natural selection, which is the best explanation we have for the diversity of life. Alfred Russell Wallace also hypothesized that the environment could help to shape the diversity of life by favoring certain traits over others. Wallace noticed that insects in the jungles of Africa and South America were very well adapted to unique environments. These two men, working independently of one another, developed the same basic explanation for the diversity of life: natural selection. These principles are supported by current scientific research.
For natural selection to occur, a population must have a wide variety of individuals with different traits. For example, natural selection would not influence fish body color if all individuals in a population were exactly the same color. The term phenotype is used to describe these physical traits. All phenotypes are the expression of genetic information in an individual’s DNA molecules. The term genotype describes the specific genetic code in the DNA molecular structure that produces a certain phenotype. Variations in genotypes can also produce variations in phenotypes.
New genotypes can be produced through the natural process of genetic mutation. A mutation is an error that is made during the DNA copying process. The mutation results in a change in the genetic code or genotype.
Fig. 1.7. A male Indian peacock (Pavo cristatus) with white leucistic color mutation displays his full plumage. Genetic mutations give rise to new genotypes. New genotypes may result in new phenotypes or traits such as body size or color patterning.
Image courtesy of Félix Potuit, Wikimedia Commons
Sometimes mutations errors occur within the sections of the DNA strand that do not code for any phenotype or trait. Similarly, some mutations are minor and do not cause any significant change in the appearance, physiology, or behavior of the organism. Other times, mutations can cause changes in the phenotype (Fig. 1.7).
Over millions of generations of copying, even little errors in that influence phenotype can add up to big changes. Sometimes a single change in a regulating gene that controls other parts of an organism’s DNA can also have a big effect. To picture how this could work, think of the game “telephone.” In this game, one person whispers a phrase to their neighbor, who whispers it to their neighbor, and so on. The last person who receives the message compares the phase they think they heard to the original phrase. Often mistakes in repeating or understanding the sample phrase lead to changes in the meaning. The same thing happens when DNA is replicated again and again. The changes to the message in the game are similar to what happens when there are mutations. Changes in the meaning of the “message” of the DNA can cause changes in the appearance, physiology, or behavior of an organism.
A mutation can be an especially powerful force for change if it has a significant impact on the survival of an organism. Some mutations, for example, have provided resistance to antibiotics in bacteria. Because bacteria reproduce very rapidly, these mutations can quickly take effect in changing the population of bacteria.
It is also important to remember that mutations are random. An organism cannot pick or choose its mutation. For example, some species of animals that live their whole lives in caves without light have no pigment, or coloration. Because the caves are dark, there is no benefit to being camouflaged to avoid a predator or having coloration to attract a mate. However, not all animals that live in the dark lack color. In order for a species to lose its coloration, mutations must occur that allow the elimination of pigment. If the mutation never arises, the animals will stay pigmented.
Sexual reproduction can also increase genetic variation in a population. Many single-celled microorganisms reproduce simply by duplicating their genetic material (i.e., DNA) and dividing themselves in half. This process is called asexual reproduction. The new cells produced by asexual reproduction are genetically identical to the original parent cell unless some mutation occurs. Sexual reproduction is the production of offspring through the combination of specialized sex cells. These sex cells (also called gametes) contain only a half-copy of an individual organism’s genetic material. When the two halves—one from the male and one from the female—are combined, the offspring ends up with a brand new combination of genes.
Fig. 1.8. Diagram of genetic mutation and natural selection
Image courtesy of Elembis, Wikimedia Commons
Mutations and sexual reproduction increase genetic variation in a population. Individual organisms with unfavorable traits (e.g. malformed wings in fruit flies or bright white color patterning in male peacocks) are less likely to survive and reproduce. These individuals and the genes they carry are “selected against” or disfavored by natural selection (Fig. 1.8). In contrast, individuals with beneficial genes are more likely to survive and reproduce.
In summary:
First, let's see what it looks like when a population is not evolving. If a population is in a state called, the frequencies of, or gene versions, and, or sets of alleles, in that population will stay the same over generations (and will also satisfy the Hardy-Weinberg equation ). Formally, evolution is a change in allele frequencies in a population over time, so a population in Hardy-Weinberg equilibrium is not evolving.
That's a little bit abstract, so let's break it down using an example. Imagine we have a large population of beetles. In fact, just for the heck of it, let's say this population is infinitely large. The beetles of our infinitely large population come in two colors, dark gray and light gray, and their color is determined by the A gene. AA and Aa beetles are dark gray, and aa beetles are light gray.
In our population, let's say that the A allele has a frequency of
0.3
, while the a allele has a frequency of0.7
. If a population is in Hardy-Weinberg equilibrium, allele frequencies will be related to genotype frequencies by a specific mathematical relationship, the Hardy-Weinberg equation. So, we can predict the genotype frequencies we'd expect to see (if the population is in Hardy-Weinberg equilibrium) by plugging in allele frequencies as shown below:p
2
+ 2pq + q2=1
p = frequency of A, q = frequency of a
Frequency of AA = p
2
= 0.72
= 0.49Frequency of Aa = 2pq = 2 (0.7)(0.3) = 0.42
Frequency of aa = 0.3
2
= 0.09Let's imagine that these are, in fact, the genotype frequencies we see in our beetle population (
9%
AA,42%
Aa,49%
aa). Excellent—our beetles appear to be in Hardy-Weinberg equilibrium! Now, let's imagine that the beetles reproduce to make a next generation. What will the allele and genotype frequencies will be in that generation?To predict this, we need to make a few assumptions:
First, let's assume that none of the genotypes is any better than the others at surviving or getting mates. If this is the case, the frequency of A and a alleles in the pool of gametes (sperm and eggs) that meet to make the next generation will be the same as the overall frequency of each allele in the present generation.
Second, let's assume that the beetles mate randomly (as opposed to, say, black beetles preferring other black beetles). If this is the case, we can think of reproduction as the result of two random events: selection of a sperm from the population's gene pool and selection of an egg from the same gene pool. The probability of getting any offspring genotype is just the probability of getting the egg and sperm combo(s) that produce that genotype.
We can use a modified Punnett square to represent the likelihood of getting different offspring genotypes. Here, we multiply the frequencies of the gametes on the axes to get the probability of the fertilization events in the squares:
A 2-part diagram. The first part, labeled Big pool o' gametes (gametes equals eggs and sperm), shows an oval containing 6 circles, each containing an uppercase A and 14 circles, each containing a lowercase a. The text below reads Frequency of uppercase A equals 0.3. Frequency of lowercase A equals 0.7. An arrow labeled Gametes combine at random points from the first part of diagram to the second part of the diagram. The second part of the diagram shows a Punnett square with two columns and two rows. The diagram shows that sperm uppercase A (0.3) combines with egg uppercase A (0.3) to produce black beetle uppercase A uppercase A (0.09). Sperm upper case A combines with egg lowercase a (0.7) to produce black beetle uppercase A lowercase a (0.21). Sperm lowercase a (0.7) combines with egg uppercase A (0.3) to produce black beetle uppercase A lowercase a (0.21). Sperm lowercase a combines with egg lowercase a (0.7) to produce white beetle lowercase a lowercase a (0.49). Text below reads 9 percent uppercase A uppercase A, 42 percent uppercase A lowercase a, 49 percent lowercase a lowercase a. Allele and genotype frequencies stay the same over generations.
As shown above, we'd predict an offspring generation with the exact same genotype frequencies as the parent generation:
9%
AA,42%
Aa, and49%
aa. If genotype frequencies have not changed, we also must have the same allele frequencies as in the parent generation:0.3
for A and0.7
for a.What we've just seen is the essence of Hardy-Weinberg equilibrium. If alleles in the gamete pool exactly mirror those in the parent generation, and if they meet up randomly (in an infinitely large number of events), there is no reason—in fact, no way—for allele and genotype frequencies to change from one generation to the next.
In the absence of other factors, you can imagine this process repeating over and over, generation after generation, keeping allele and genotype frequencies the same. Since evolution is a change in allele frequencies in a population over generations, a population in Hardy-Weinberg equilibrium is, by definition, not evolving.
As we mentioned at the beginning of the article, populations are usually not in Hardy-Weinberg equilibrium (at least, not for all of the genes in their genome). Instead, populations tend to evolve: the allele frequencies of at least some of their genes change from one generation to the next.
In fact, population geneticists often check to see if a population is in Hardy-Weinberg equilibrium because they suspect other forces may be at work. If the population’s allele and genotype frequencies are changing over generations (or if the allele and genotype frequencies don't match the predictions of the Hardy-Weinberg equation), the race is on to find out why.
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