Question

Explain how each of the following are evidence for biological evolution. Provide an example for each.

A. Comparative anatomy

B. Embryology

C. Fossils

D. Biogeography

E. Artificial selection

F. Vestigial structures

G. Molecular biology

H. Direct observation

I. Experimental evolution

Answer 1

Answer :

Evidence from Fossils

Evolution of the Horse. The fossil record reveals how horses evolved. The lineage that led to modern horses (Equus) grew taller over time (from the 0.4 m Hyracotherium to the 1.6 m Equus). This lineage also developed longer molar teeth and the degeneration of the outer phalanges on the feet.

Fossils are a window into the past. They provide clear evidence that evolution has occurred. Scientists who find and study fossils are called paleontologists. How do they use fossils to understand the past? Consider the example of the horse, outlined in Fossils spanning a period of more than 50 million years show how the horse evolved.

The oldest horse fossils show what the earliest horses were like. They were only 0.4 m tall, or about the size of a fox, and they had four long toes. Other evidence shows they lived in wooded marshlands, where they probably ate soft leaves. Through time, the climate became drier, and grasslands slowly replaced the marshes. Later fossils show that horses changed as well.

• They became taller, which would help them see predators while they fed in tall grasses. Eventually, they reached a height of about 1.6 m.
• They evolved a single large toe that eventually became a hoof. This would help them run swiftly and escape predators.
• Their molars (back teeth) became longer and covered with hard cement. This would allow them to grind tough grasses and grass seeds without wearing out their teeth.

Evidence from Living Species

Scientists can learn a great deal about evolution by studying living species. They can compare the anatomy, embryos, and DNA of modern organisms to help understand how they evolved.

Hands of Different Mammals. The forelimbs of all mammals have the same basic bone structure.

Comparative Anatomy

Comparative anatomy is the study of the similarities and differences in the structures of different species. Similar body parts may be homologous structures or analogous structures. Both provide evidence for evolution.

Homologous structures are structures that are similar in related organisms because they were inherited from a common ancestor. These structures may or may not have the same function in the descendants. Figure shows the upper appendages of several different mammals. They all have the same basic pattern of bones, although they now have different functions. All of these mammals inherited this basic bone pattern from a common ancestor.

Analogous structures are structures that are similar in unrelated organisms. The structures are similar because they evolved to do the same job, not because they were inherited from a common ancestor. For example, the wings of bats and birds, shown in the figure that follows, look similar on the outside and have the same function. However, wings evolved independently in the two groups of animals. This is apparent when you compare the pattern of bones inside the wings.

Comparative Embryology

Comparative embryology is the study of the similarities and differences in the embryos of different species. Similarities in embryos are likely to be evidence of common ancestry. All vertebrate embryos, for example, have gill slits and tails. All of the embryos in except for fish, lose their gill slits by adulthood, and some of them also lose their tail. In humans, the tail is reduced to the tail bone. Thus, similarities organisms share as embryos may no longer be present by adulthood. This is why it is valuable to compare organisms in the embryonic stage.

Embryos of different vertebrates look much more similar than the animals do at later stages of life. Rows A through C illustrate the development of each embryo, from earliest to latest stages.

Vestigial Structures

Structures like the human tail bone are called vestigial structures. Evolution has reduced their size because the structures are no longer used. The human appendix is another example of a vestigial structure. It is a tiny remnant of a once-larger organ. In a distant ancestor, it was needed to digest food, but it serves no purpose in the human body today. Why do you think structures that are no longer used shrink in size? Why might a full-sized, unused structure reduce an organism’s fitness?

Comparing DNA

Darwin could compare only the anatomy and embryos of living things. Today, scientists can compare their DNA. Similar DNA sequences are the strongest evidence for evolution from a common ancestor. Look at the diagram in Figure 9.3.59.3.5. The diagram is a cladogram, a branching diagram showing related organisms. Each branch represents the emergence of new traits that separate one group of organisms from the rest. The cladogram in the figure shows how humans and apes are related based on their DNA sequences.

Cladogram of Humans and Apes. This cladogram is based on DNA comparisons. It shows how humans are related to apes by descent from common ancestors. Humans are most closely related to chimpanzees (our common ancestor existed most recently). We are less closely related to gorillas, and even less closely related to baboons.

Evidence from Biogeography

Biogeography is the study of how and why organisms live where they do. It provides more evidence for evolution. Let’s consider the camel family as an example.

Biogeography of Camels: An Example

Today, the camel family includes different types of camels All of today’s camels are descended from the same camel ancestors. These ancestors lived in North America about a million years ago.

Early North American camels migrated to other places. Some went to East Asia via a land bridge during the last ice age. A few of them made it all the way to Africa. Others went to South America by crossing the Isthmus of Panama. Once camels reached these different places, they evolved independently. They evolved adaptations that suited them for the particular environment where they lived. Through natural selection, descendants of the original camel ancestors evolved the diversity they have today.

Camel Migrations and Present-Day Variation. Members of the camel family now live in different parts of the world. Dromedary camels are found in Africa, Bactrian camels in Asia, and Llamas in South America. They differ from one another in a number of traits. However, they share basic similarities. This is because they all evolved from a common ancestor.

Island Biogeography

The biogeography of islands yields some of the best evidence for evolution. Consider the birds called finches that Darwin studied on the Galápagos Islands All of the finches probably descended from one bird that arrived on the islands from South America. Until the first bird arrived, there had never been birds on the islands. The first bird was a seed eater. It evolved into many finch species, each adapted for a different type of food. This is an example of adaptive radiation. This is the process by which a single species evolves into many new species to fill available ecological niches.

Galápagos finches differ in beak size and shape, depending on the type of food they eat. Those eating buds and fruits have the largest beaks. Insect and grub eaters have narrower beaks.

EYEWITNESSES TO EVOLUTION

In the 1970s, biologists Peter and Rosemary Grant went to the Galápagos Islands to re-study Darwin’s finches. They spent more than 30 years on the project, but their efforts paid off. They were able to observe evolution by natural selection actually taking place.

en Español      print Artificial selection Long before Darwin and Wallace, farmers and breeders were using the idea of selection to cause major changes in the features of their plants and animals over the course of decades. Farmers and breeders allowed only the plants and animals with desirable characteristics to reproduce, causing the evolution of farm stock. This process is called artificial selection because people (instead of nature) select which organisms get to reproduce. As shown below, farmers have cultivated numerous popular crops from the wild mustard, by artificially selecting for certain attributes. These common vegetables were cultivated from forms of wild mustard. This is evolution through artificial selection. Evidence for evolution: Molecular biology Like structural homologies, similarities between biological molecules can reflect shared evolutionary ancestry. At the most basic level, all living organisms share: The same genetic material (DNA) The same, or highly similar, genetic codes The same basic process of gene expression (transcription and translation) These shared features suggest that all living things are descended from a common ancestor, and that this ancestor had DNA as its genetic material, used the genetic code, and expressed its genes by transcription and translation. Present-day organisms all share these features because they were "inherited" from the ancestor (and because any big changes in this basic machinery would have broken the basic functionality of cells). Although they're great for establishing the common origins of life, features like having DNA or carrying out transcription and translation are not so useful for figuring out how related particular organisms are. If we want to determine which organisms in a group are most closely related, we need to use different types of molecular features, such as the nucleotide sequences of genes. Homologous genes Biologists often compare the sequences of related genes found in different species (often called homologous or orthologous genes) to figure out how those species are evolutionarily related to one another. The basic idea behind this approach is that two species have the "same" gene because they inherited it from a common ancestor. For instance, humans, cows, chickens, and chimpanzees all have a gene that encodes the hormone insulin, because this gene was already present in their last common ancestor. In general, the more DNA differences in homologous genes between two species, the more distantly the species are related. For instance, human and chimpanzee insulin genes are much more similar (about 98% identical) than human and chicken insulin genes (about 64% identical), reflecting that humans and chimpanzees are more closely related than humans and chickens^ccstart superscript, c, end superscript. Direct observation of microevolution In some cases, the evidence for evolution is that we can see it taking place around us! Important modern-day examples of evolution include the emergence of drug-resistant bacteria and pesticide-resistant insects. For example, in the 1950s, there was a worldwide effort to eradicate malaria by eliminating its carriers (certain types of mosquitos). The pesticide DDT was sprayed broadly in areas where the mosquitoes lived, and at first, the DDT was highly effective at killing the mosquitos. However, over time, the DDT became less and less effective, and more and more mosquitoes survived. This was because the mosquito population evolved resistance to the pesticide. The evolution of DDT resistance in mosquito populations was observed directly in the 1950s as a result of a campaign to eradicate malaria. Resistance to the pesticide evolved over a few years through natural selection: 1) Within mosquito populations, a few individuals had alleles that made them resistant to the pesticide, DDT. The majority of individuals had alleles that did not confer resistance. 2) When DDT was sprayed, individuals carrying the resistance allele survived, while those carrying the non-resistant allele died. 3) Over several generations, more resistant offspring were born and the population evolved. The population now contained more resistant than non-resistant individuals. Emergence of DDT resistance is an example of evolution by natural selection^77start superscript, 7, end superscript. How would natural selection have worked in this case? More on natural selection Before DDT was applied, a tiny fraction of mosquitos in the population would have had naturally occurring gene versions (alleles) that made them resistant to DDT. These versions would have appeared through random mutation, or changes in DNA sequence. Without DDT around, the resistant alleles would not have helped mosquitoes survive or reproduce (and might even have been harmful), so they would have remained rare. When DDT spraying began, most of the mosquitos would have been killed by the pesticide. Which mosquitos would have survived? For the most part, only the rare individuals that happened to have DDT resistance alleles (and thus survived being sprayed with DDT). These surviving mosquitoes would have been able to reproduce and leave offspring. Over generations, more and more DDT-resistant mosquitoes would have been born into the population. That's because resistant parents would have been consistently more likely to survive and reproduce than non-resistant parents, and would have passed their DDT resistance alleles (and thus, the capacity to survive DDT) on to their offspring. Eventually, the mosquito populations would have bounced back to high numbers, but would have been composed largely of DDT-resistant individuals. In parts of the world where DDT has been used extensively in the past, many of the mosquitoes are now resistant. DDT can no longer be used to control the mosquito populations (and reduce malaria) in these regions. Why are mosquito populations able to evolve rapid resistance to DDT? Two important factors are large population size (making it more likely that some individuals in the population will, by random chance, have mutations that provide resistance) and short lifecycle. Bacteria and viruses, which have even larger population sizes and shorter lifecycles, can evolve resistance to drugs very rapidly, as in antibiotic-resistant bacteria and drug-resistant HIV. Experimental evolution is the use of laboratory or controlled field manipulations to investigate evolutionary processes. It usually makes use of organisms with rapid generation times and small physical size, often microbes, to observe phenomena that in large multicellular organisms occur too slowly.