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T6 - Microevolution and Macroevolution

I can present evidence for the processes and patterns of microevolution and macroevolution

Introduction

In this lesson, you can explore and evaluate for yourself the many kinds of evidence which support the theory of evolution by natural selection. You will also have the opportunity to appreciate the power of evolution to explain observations in every branch of biology.

The Fossil Record: Structural Changes Through Time

Few would argue that dinosaurs roamed Earth in the past, but no longer exist. The fossil record is a revealing window into species that lived long ago. Paleontologists have carefully analyzed the preserved remains and traces of animals, plants, and even microorganisms to reconstruct the history of life on Earth (see the History of Life chapter for more detail). Relative (rock layer position) and absolute (radioisotope) dating techniques allow geologists to sequence the fossils chronologically and provide a time scale. Geology also reveals the environmental conditions of past species.

For many reasons, the fossil record is not complete. Most organisms decomposed or were eaten by scavengers after death. Many species lacked hard parts, which are much more likely to fossilize. Some rocks and the fossils they contained have eroded and disappeared. Moreover, much of evolution happens in the small populations that survive changes in environmental conditions, so the chance that intermediates will fossilize is low. Nevertheless, the current record includes billions of fossils – over 300 million from Los Angeles’ LaBrea Tar Pits alone, and an estimated 800 billion in South Africa’s Beaufort Formation. Analysts have identified 250,000 species among these remains.

Although the fossil record is far more detailed today than in Darwin’s time, Darwin was able to use it as powerful evidence for natural selection and common descent. Throughout geological history, species that appear in an early rock layer disappear in a more recent layer. Darwin argued that a species’ appearance recorded its origin, and that its disappearance showed extinction. Moreover, he noted remarkable similarities among structures in differing species, supporting common ancestry. Finally, he could often correlate environmental conditions with structures, supporting his idea that natural selection led to adaptations which improved survival within certain habitats.

The fossil record for relatives of the modern horse is unusually complete, allowing us to select a few which show major change over time. These changes can be correlated with environmental changes, supporting the ideas of evolution and natural selection. However, the linear arrangement is misleading; addition of all known fossils would show a branching, bushy path of descent and common ancestry.

As an example, let’s analyze a relatively complete set of fossils which record the evolution of the modern horse. Figure above sequences five species which show major evolutionary changes. The oldest fossil shows a fox-sized animal with slender legs and nearly vertical digits: Hyracotherium bit and chewed soft leaves in wooded marshlands. Geology and paleontology suggest that the climate gradually dried, and grasslands slowly replaced the marshes. Mesohippus was taller, with fewer, stronger digits – better able to spot and run from predators, and thus more likely to survive and reproduce in the new grasslands. Merychippus was taller still, and kept only one, enlarged digit – a hoof to run fast on the hard ground. By Pliohippus time, molar teeth had widened and elongated to grind the tough grasses. These fossils show gradual structural changes which correspond to changes in the environment. They appear to show a smooth, linear path directed toward the “goal” of the modern horse, but this is deceiving. These five fossils are merely “snapshots” of a bushy family tree containing as many as 12 genera and several hundred species. Some transitions are smooth progressions; others are abrupt. Together, they support natural selection and descent with modification from common ancestors.

Does The Fossil Record Support Evolution? (8e) This video can be seen at http://www.youtube.com/watch?v=QWVoXZPOCGk (9:20).

Comparative Anatomy and Embryology

The evidence Darwin presented in The Origin of Species included not only fossils but also detailed comparisons of living species at all life stages. Naturalists in Darwin’s time were experts in comparative anatomy – the study of the similarities and differences in organisms’ structures (body parts). At different times during his life, Darwin studied the comparative anatomy of closely related species of marine mammals, barnacles, orchids, insectivorous plants, and earthworms.

Species which share many similarities are closely related by a relatively recent common ancestor. For example, all orchids share parallel-veined leaves, two-sided flowers with a “lip,” and small seeds (Figures A and B below). Species which share fewer similarities, sharing only basic features, are related by relatively distant ancestor. The sundew, one of the insectivorous plants Darwin studied, shares leaves and petals with orchids, but the leaves are wide with branching veins and the flowers are radially symmetrical rather than two-sided (Figure C below). The many species of orchids, then, share a recent common ancestor, but they also share a more distant ancestor with the sundew.

Darwin

Homologous and Analogous Structures

are similarities throughout a group of closely related species. The similar bone patterns in bat

Similarities can show two different kinds of relationships, both of which support evolution and natural selection.

(1) Similarities shared by closely related species (species who share many characteristics) are homologous, because the species have descended from a common ancestor which had that trait. Homologous structures may or may not serve the same function. Figure above shows the forelimbs of mammals, considered homologous because all mammals show the same basic pattern: a single proximal bone joins a pair of more distal bones, which connect to bones of the wrist, “hand,” and digits. With this basic pattern, bats build wings for their lives in the air, whales form fins for their lives in the sea, and horses, as we have seen, construct long, hoofed legs for speed on land. Therefore, homologous structures support common ancestry.

The wings of pterosaurs, bats, and birds illustrate both homologous and analogous structures. Similarities in the patterns of bones are due to descent from a common vertebrate (reptilian) ancestor, so they are homologous. However, the wings of each evolved independently, in response to similar environments, so they are analogous, and provide evidence for natural selection.

(2) Similarities shared by distantly related species may have evolved separately because they live in similar habitats. These structures are analogous because they serve similar functions, but evolved independently. Figure above compares the wings of bats, bird, and pterosaurs. Bats evolved wings as mammals, pterosaurs as dinosaurs, and birds from a separate line of reptiles. Their wings are analogous structures, each of which evolved independently, but all of which suit a lifestyle in the air. Note that although the wings are analogous, their bones are homologous: all three share a common but more distant vertebrate ancestor, in which the basic forelimb pattern evolved. Because analogous structures are independent adaptations to a common environment, they support natural selection.

Embryology

Embryology is a branch of comparative anatomy which studies the development of vertebrate animals before birth or hatching. Like adults, embryos show similarities which can support common ancestry. For example, all vertebrate embryos have gill slits and tails, shown in Figure below. The “gill slits” are not gills, however. They connect the throat to the outside early in development, but in many species, later close; only in fish and larval amphibians do they contribute to the development of gills. In mammals, the tissue between the first gill slits forms part of the lower jaw and the bones of the inner ear. The embryonic tail does not develop into a tail in all species; in humans, it is reduced during development to the coccyx, or tailbone. Similar structures during development support common ancestry.

reveals homologies which form during development but may later disappear. All vertebrate embryos develop tails, though adult humans retain only the coccyx. All vertebrate embryos show gill slits, though these develop into gill openings only in fish and larval amphibians. In humans, gills slits form the lower jaw and Eustachian tube. Many scientists consider developmental homologies evidence for ancestry, although some embryologists believe that these particular drawings exaggerate the similarities.

Vestigial Structures

Structures which are reduced and perhaps even nonfunctional, such as the human tail and the human appendix, are considered vestigial structures. The tail, of course, functions for balance in many mammals, and the human appendix may have served digestive functions in herbivorous ancestors. Whales, which evolved from land mammals, do not have legs or hair as adults; both begin to develop in embryos, but then recede. Vestigial leg bones remain, buried deep in their bodies, shown in Figure A below.

Vestigial structures show evolutionary reduction or loss of unneeded structures which were useful to ancestors. A: Whales retain remnants of their mammalian ancestors

True flies have reduced the second pair of wings found in most insects to halteres for balance shown in Figure B above. Cavefish lose both eyes and pigment, because both would require energy to build and are useless in the lightless habitat they have adopted shown in Figure C above. You are probably very familiar with a fine example of a vestigial behavior: goosebumps raise the sparse hairs on your arms even though they are no longer sufficiently dense to insulate you from the cold by trapping warm air next to your skin; in most mammals, this reflex is still quite functional shown in Figure D above. Most vestigial structures are homologous to similar, functioning structures in closely related species, and as such, support both common ancestry and (incomplete!) natural selection.

Molecular Biology

Did you know that your genes may be 50% the same as those of a banana?

Unknown in Darwin’s time, the “comparative anatomy” of the molecules which make up life has added an even more convincing set of homologies to the evidence for evolution. All living organisms have genes made of DNA. The order of nucleotides – As, Ts, Cs, and Gs - in each gene codes for a protein, which does the work or builds the structures of life. Proteins govern the traits chosen (or not) in natural selection. For all organisms, a single Genetic Code translates the sequence of nucleotides in a gene into a corresponding chain of 20 amino acids. By itself, the universality of DNA genes and their code for proteins is strong evidence for common ancestry. Yet there is more.

If we compare the sequence of nucleotides in the DNA of one organism to the sequence in another, we see remarkable similarities. For example, human DNA sequences are 98-99% the same as those of chimpanzees, and 50% the same as a banana’s! These similarities reflect similar metabolism. All organisms have genes for DNA replication, protein synthesis, and processes such as cellular respiration. Although metabolic processes do not leave fossils, similar DNA sequences among existing organisms provide excellent evidence for common ancestry.

The Scale of Evolution

We now know that variations of traits are heritable. These variations are determined by different alleles. We also know that evolution is due to a change in alleles over time. How long a time? That depends on the scale of evolution.

  • Microevolution occurs over a relatively short period of time within a population or species. The Grants observed this level of evolution in Darwin’s finches.
  • Macroevolution occurs over geologic time above the level of the species. The fossil record reflects this level of evolution. It results from microevolution taking place over many generations.

Genes in Populations

Individuals do not evolve. Their genes do not change over time. The unit of evolution is the population. A population consists of organisms of the same species that live in the same area. In terms of evolution, the population is assumed to be a relatively closed group. This means that most mating takes place within the population. The science that focuses on evolution within populations is population genetics. It is a combination of evolutionary theory and Mendelian genetics.

Gene Pool

The genetic makeup of an individual is the individual’s genotype. A population consists of many genotypes. Altogether, they make up the population’s gene pool. The gene pool consists of all the genes of all the members of the population. For each gene, the gene pool includes all the different alleles for the gene that exist in the population. For a given gene, the population is characterized by the frequency of the different alleles in the gene pool.

Forces of Evolution

Mutation

Mutation creates new genetic variation in a gene pool. It is how all new alleles first arise. In sexually reproducing species, the mutations that matter for evolution are those that occur in gametes. Only these mutations can be passed to offspring. For any given gene, the chance of a mutation occurring in a given gamete is very low. Thus, mutations alone do not have much effect on allele frequencies. However, mutations provide the genetic variation needed for other forces of evolution to act.

Migration or Gene Flow

Gene flow occurs when individuals move into or out of a population. If the rate of migration is high, this can have a significant effect on allele frequencies. Both the population they leave and the population they enter may change.

During the Vietnam War in the 1960s and 1970s, many American servicemen had children with Vietnamese women. Most of the servicemen returned to the United States after the war. However, they left copies of their genes behind in their offspring. In this way, they changed the allele frequencies in the Vietnamese gene pool. Was the gene pool of the American population also affected? Why or why not?

Genetic Drift

Genetic drift is a random change in allele frequencies that occurs in a small population. When a small number of parents produce just a few offspring, allele frequencies in the offspring may differ, by chance, from allele frequencies in the parents. This is like tossing a coin. If you toss a coin just a few times, you may, by chance, get more or less than the expected 50 percent heads or tails. In a small population, you may also, by chance, get different allele frequencies than expected in the next generation. In this way, allele frequencies may drift over time. There are two special conditions under which genetic drift occurs. They are called bottleneck effect and founder effect.

  1. Bottleneck effect occurs when a population suddenly gets much smaller. This might happen because of a natural disaster such as a forest fire. By chance, allele frequencies of the survivors may be different from those of the original population.
  2. Founder effect occurs when a few individuals start, or found, a new population. By chance, allele frequencies of the founders may be different from allele frequencies of the population they left. An example is described in Figure below.

Founder Effect in the Amish Population. The Amish population in the U.S. and Canada had a small number of founders. How has this affected the Amish gene pool?

Natural Selection

Natural selection occurs when there are differences in fitness among members of a population. As a result, some individuals pass more genes to the next generation. This causes allele frequencies to change. The example of sickle-cell anemia is described in Figure below and Table below. It shows how natural selection can keep a harmful allele in a gene pool. You can also watch a video about natural selection and sickle-cell anemia at this link: http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html.

  1. Stabilizing selection occurs when phenotypes at both extremes of the phenotypic distribution are selected against. This narrows the range of variation. An example is human birth weight. Babies that are very large or very small at birth are less likely to survive. This keeps birth weight within a relatively narrow range.
  2. Directional selection occurs when one of two extreme phenotypes is selected for. This shifts the distribution toward that extreme. This is the type of natural selection that the Grants observed in the beak size of Galápagos finches.
  3. Disruptive selection occurs when phenotypes in the middle of the range are selected against. This results in two overlapping phenotypes, one at each end of the distribution. An example is sexual dimorphism. This refers to differences between the phenotypes of males and females of the same species. In humans, for example, males and females have different heights and body shapes.

Natural Selection for a Polygenic Trait. Natural selection may affect the distribution of a polygenic trait. These graphs show three ways this can happen.


Macroevolution is evolution over geologic time above the level of the species. One of the main topics in macroevolution is how new species arise. The process by which a new species evolves is called speciation. How does speciation occur? How does one species evolve into two or more new species?

Origin of Species

To understand how a new species forms, it’s important to review what a species is. A species is a group of organisms that can breed and produce fertile offspring together in nature. For a new species to arise, some members of a species must become reproductively isolated from the rest of the species. This means they can no longer interbreed with other members of the species. How does this happen? Usually they become geographically isolated first.

Geographic Isolation

Assume that some members of a species become geographically separated from the rest of the species. If they remain separated long enough, they may evolve genetic differences. If the differences prevent them from interbreeding with members of the original species, they have evolved into a new species. Speciation that occurs in this way is called allopatric speciation. An example is described in Figure below.

Allopatric Speciation in the Kaibab Squirrel. The Kaibab squirrel is in the process of becoming a new species.

Reproductive Isolation

Less often, a new species arises without geographic separation. This is called reproductive isolation. The following example shows one way this can occur.

  1. Hawthorn flies lay eggs in hawthorn trees (see Figure below). The eggs hatch into larvae that feed on hawthorn fruits. Both the flies and trees are native to the U.S.
  2. Apple trees were introduced to the U.S. and often grow near hawthorn trees. Some hawthorn flies started to lay eggs in nearby apple trees. When the eggs hatched, the larvae fed on apples.
  3. Over time, the two fly populations—those that fed on hawthorn trees and those that preferred apple trees—evolved reproductive isolation. Now they are reproductively isolated because they breed at different times. Their breeding season matches the season when the apple or hawthorn fruits mature.
  4. Because they rarely interbreed, the two populations of flies are evolving other genetic differences. They appear to be in the process of becoming separate species.

Sympatric Speciation in Hawthorn Flies. Hawthorn flies are diverging from one species into two. As this example shows, behaviors as well as physical traits may evolve and lead to speciation.

Isolating mechanisms are discussed in the following video http://www.youtube.com/watch?v=-e64TfKeAXU (2:57).

Coevolution

Evolution occurs in response to a change in the environment. Environmental change often involves other species of organisms. In fact, species in symbiotic relationships tend to evolve together. This is called coevolution. As one species changes, the other species must also change in order to adapt.

Coevolution occurs in flowering plants and the species that pollinate them. The flower and bird in Figure below are a good example. They have evolved matching structures.

Results of Coevolution in a Flower and Its Pollinator. The very long mouth part of this hummingbird has coevolved with the tubular flower it pollinates. Only this species of bird can reach the nectar deep in the flower. What might happen to the flower if the bird species went extinct?

Timing of Macroevolution

Is evolution slow and steady? Or does it occur in fits and starts? It may depend on what else is going on, such as changes in climate and geologic conditions.

  • When geologic and climatic conditions are stable, evolution may occur gradually. This is how Darwin thought evolution occurred. This model of the timing of evolution is called gradualism.
  • When geologic and climatic conditions are changing, evolution may occur more quickly. Thus, long periods of little change may be interrupted by bursts of rapid change. This model of the timing of evolution is called punctuated equilibrium. It is better supported by the fossil record than is gradualism.