the scale of evolution

coevolution

Two species may often interact with each other and have a close relationship. Examples include flowers and the animals that pollinate them. When one of the two species evolves new traits, the other species may evolve matching traits. This is called coevolution. You can see an example of this in Figure 7.16. The very long beak of this hummingbird co-evolved with the tubular flowers it pollinates. Only this species of hummingbird can reach nectar deep in the flowers.

textbook_image

convergent evolution

Sometimes two species evolve the same traits. It happens because they live in similar habitats. This is called convergent evolution. Caribbean Anoles demonstrate this as well. On each Caribbean island, anoles in similar habitats evolved the same traits. For example, anoles that lived on the forest floor evolved long legs for leaping and running on the ground. Anoles that lived on tree branches evolved short legs that helped them cling to small branches and twigs. Anoles that lived at the tops of trees evolved large toe pads that allowed them to walk on leaves without falling. On each of the islands, there were anole species that evolved in each of these same ways.

textbook_image

speciation

The evolution of a new species is called speciation. A species is a group of organisms that can mate and produce fertile offspring together but not with members of other such groups. What must happen for a new species to arise? Some members of an existing species must change so they can no produce fertile offspring with the rest of the species. Speciation often occurs when some members of a species break off from the rest. The splinter group evolves in isolation from the original species. The original species also continues to evolve. Sooner or later, the splinter group becomes too different to breed with members of the original species. At that point, a new species has formed. A good example of speciation involves anole lizards, like the one pictured in Figure 7.15. There are about 150 different species of anole lizards in the Caribbean Islands. Scientists think that a single species of lizard first colonized one of the islands about 50 million years ago. A few lizards from this original species eventually reached each of the other islands, where they evolved in isolation. Anoles in different habitats evolved traits that affected mating. For example, they evolved skin flaps of different colors. Females didnt respond to male anoles with the wrong color skin flap. This prevented them from mating. Eventually, all the different species of anoles known today evolved. Watch this interesting video to learn more about anole speciation in the Caribbean:

two scales of evolution

We now know how variation in traits is inherited. Variation in traits is controlled by different alleles for genes. Alleles, in turn, are passed to gametes and then to offspring. Evolution occurs because of changes in alleles over time. How long a time? That depends on the time scale of evolution you consider. Evolution that occurs over a short period of time is known as microevolution. It might take place in just a couple of generations. This scale of evolution occurs at the level of the population. The Grants observed evolution at this scale in populations of Darwins finches. Beak size in finch populations changed in just two years because of a serious drought. Evolution that occurs over a long period of time is called macroevolution. It might take place over millions of years. This scale of evolution occurs above the level of the species. Fossils provide evidence for evolution at this scale. The evolution of the horse family, shown in Figure 7.13, is an example of macroevolution.

textbook_image

rate of evolution

Darwin thought that evolution occurs very slowly. This is likely if conditions are stable. But what if conditions are changing rapidly? Evolution is likely to occur more rapidly as well. For example, the Grants showed that evolution occurred in just a couple of years in Darwins finches. This happened when a severe drought killed off a lot of the plants that the birds needed for food. Millions of fossils have been found since Darwins time. They show that evolution may occur in fits and starts. Long period of little or gradual change may be interrupted by bursts of rapid change. The rate of evolution is influenced by how the environment is changing. Today, Earths climate is changing rapidly. How do you think this might affect the rate of evolution?

genes in populations

A population is a group of organisms of the same species that live in the same area. All the genes in all the members of a population make up the populations gene pool. For each gene, the gene pool includes all the different alleles in the population. The gene pool can be described by its allele frequencies for specific genes. The frequency of an allele is the number of copies of that allele divided by the total number of alleles for the gene in the gene pool. A simple example will help you understand these concepts. The data in Table 7.2 represent a population of 100 individuals. For each gene, the gene pool has a total of 200 alleles (2 per individual x 100 individuals). The gene in question exists as two different alleles, A and a. The number of A alleles in the gene pool is 140. Of these, 100 are in the 50 AA homozygotes. Another 40 are in the 40 Aa heterozygotes. The number of a alleles in the gene pool is 60. Of these, 40 are in the 40 Aa heterozygotes. Another 20 are in the 10 aa homozygotes. The frequency of the A allele is 140/200 = 0.7. The frequency of the a allele is 60/200 = 0.3. Genotype AA Aa aa Totals Number of Individuals 50 40 10 100 Number of A Alleles 100 (50 x 2) 40 (40 x 1) 0 (10 x 0) 140 Number of a Alleles 0 (50 x 0) 40 (40 x 1) 20 (10 x 2) 60 Evolution occurs in a population when its allele frequencies change over time. For example, the frequency of the A allele might change from 0.7 to 0.8. If that happens, evolution has occurred. What causes allele frequencies to change? The answer is forces of evolution.

microevolution

Individuals dont evolve. Their alleles dont change over time. The unit of microevolution is the population.

macroevolution

What happens when forces of evolution work over a long period of time? The answer is macroevolution. An example is the evolution of a new species.

textbook_image

forces of evolution

There are four major forces of evolution that cause allele frequencies to change. They are mutation, gene flow, genetic drift, and natural selection. Mutation creates new genetic variation in a gene pool This is how all new alleles first arise. Its the ultimate source of new genetic variation, so it is essential for evolution. However, for any given gene, the chance of a mutation occurring is very small. Therefore, mutation alone does not have much effect on allele frequencies. Gene flow is the movement of genes into or out of a gene pool It occurs when individuals migrate into or out of the population. How much gene flow changes allele frequencies depends on how many migrants there are and their genotypes. Genetic drift is a random change in allele frequencies. It occurs in small populations. Allele frequencies in the offspring may differ by chance from those in the parents. This is like tossing a coin just a few times. You may, by chance, get more or less than the expected 50 percent heads or tails. In the same way, you may get more or less than the expected allele frequencies in the small number of individuals in the next generation. The smaller the population is, the more allele frequencies may drift. Natural selection is a change in allele frequencies that occurs because some genotypes are more fit than others. Genotypes with greater fitness produce more offspring and pass more copies of their alleles to the next generation. This is the force of evolution that Darwin identified. Figure 23.12 shows how Darwin thought natural selection led to variation in finches on the Galpagos Islands.

instructional diagrams

No diagram descriptions associated with this lesson

questions

Darwin thought that evolution occurs

-->  a. very slowly

b. very rarely

c. at a varying rate

d. only on islands

___all the genes in all the members of a population

a. population

b. allele frequency

c. gene flow

d. convergent evolution

e. natural selection

f. genetic drift

-->  g. gene pool

___random change in a small populations allele frequencies

a. population

b. allele frequency

c. gene flow

d. convergent evolution

e. natural selection

-->  f. genetic drift

g. gene pool

A gene pool consists of

a. all the alleles for a given gene in an individual

b. all the alleles for all the genes in a given species

c. all the alleles for a given gene in a population

-->  d. none of the above

All of the following are considered to be forces of evolution except

a. mutation

b. natural selection

c. genetic flow

-->  d. climate change

___process in which two species evolve the same traits because they live in similar habitats

a. population

b. allele frequency

c. gene flow

-->  d. convergent evolution

e. natural selection

f. genetic drift

g. gene pool

___change in allele frequencies that occurs because some genotypes are more fit than others

a. population

b. allele frequency

c. gene flow

d. convergent evolution

-->  e. natural selection

f. genetic drift

g. gene pool

In a population of 200 people, 20 people have the genotype BB and 180 people have the genotype bb. What is the frequency of the B allele in this population?

a. 0.0

-->  b. 0.1

c. 0.2

d. 0.5

___number of copies of an allele divided by the total number of alleles for the gene in a gene pool

a. population

-->  b. allele frequency

c. gene flow

d. convergent evolution

e. natural selection

f. genetic drift

g. gene pool

The ultimate source of new genetic variation is

-->  a. mutation

b. artificial selection

c. evolution

d. genetic drift

___group of organisms of the same species that live in the same area

-->  a. population

b. allele frequency

c. gene flow

d. convergent evolution

e. natural selection

f. genetic drift

g. gene pool

___movement of genes into or out of a population

a. population

b. allele frequency

-->  c. gene flow

d. convergent evolution

e. natural selection

f. genetic drift

g. gene pool

Darwin knew Mendels laws, so he understood how traits are inherited.

a. true

-->  b. false

Horses became smaller as they evolved over the past 50 million years.

a. true

-->  b. false

Microevolution can be measured by changes in allele frequencies.

-->  a. true

b. false

A total of 300 fossils have been discovered since Darwins time.

a. true

-->  b. false

Once a new species forms, it stops evolving.

a. true

-->  b. false

_It takes millions of years for microevolution to occur.

a. true

-->  b. false

_Individuals can evolve if their allele frequencies change.

a. true

-->  b. false

_The evolution the Grants observed in finches was macroevolution.

a. true

-->  b. false

_Population size determines how quickly allele frequencies change by genetic drift.

-->  a. true

b. false

_A gene pool is described by its allele frequencies.

-->  a. true

b. false

_Darwin thought that evolution occurs very quickly.

a. true

-->  b. false

_Mutation alone can cause rapid evolution.

a. true

-->  b. false

Forces of evolution include

a. gene flow

b. genetic drift

c. mutation

-->  d. all of the above

How did horses change as they evolved over the past 50 million years?

-->  a. Their body size increased

b. Their number of toes increased

c. Their number of legs decreased

d. all of the above

In a population of 100 individuals, there are 50 AA individuals and 50 aa individuals. What is the frequency of the A allele in this population?

a. 0.0

-->  b. 0.5

c. 0.7

d. 1.0

Darwin thought that evolution occurs by

a. genetic drift

-->  b. natural selection

c. mutation

d. gene flow

A group of organisms that can mate and produce fertile offspring together is called a(n)

a. gene pool

b. population

-->  c. species

d. splinter group

Anole lizards in similar habitats on different Caribbean Islands evolved the same traits. This is an example of

a. coevolution

b. speciation

-->  c. convergent evolution

d. genetic drift

Plants and the animals that pollinate them may evolve matching traits. This is an example of

a. gene flow

-->  b. coevolution

c. convergent evolution

d. none of the above

diagram questions

No diagram questions associated with this lesson