An overview of lines of evidence for evolution (or evolution in a nutshell) Major contributions of Darwin s work: Learning objectives: To assess types of evidence for evolution, including: 1. Evidence of change through time 2. Evidence of shared ancestry 3. Evidence for the action of natural selection Background reading: Chapters 2-3 in Freeman Up next: Population Genetics (review Hardy-Weinberg) 1 1. Populations and species change over time = evolution happens. 2. All species are related through common ancestry = the tree of life connects living and extinct species. 3. Evolution happens through the action of natural selection = differences in survival and reproduction among individuals lead to changes in the characteristics of populations. 2 Evolution Defined 1. Evidence of change through time Evolution: Change in allele frequencies through time, OR changes in the traits of a species through time, OR descent with modification A few things to keep in mind (or they will haunt you): Evolution is not directed or directional - change does not imply improvement. Natural selection is not the only force that can bring about change in allele frequencies. Sometimes, change happens by chance. Example: 3 The Fossil Record Fossils are traces of organisms that lived in the past. Where do fossils form? The fossil record provides evidence of change in the form of extinctions, and evidence of transitions in structures through time. 4
Patterns of Change Through Time: Extinction Extinction in the Fossil Record Fossils provide evidence for the existence of forms of life that have not existed in historic times. The simple existence of fossils indicates that the distribution, abundance and composition of the fauna and flora has changed through time. 5 Patterns of Change Through Time: Extinction Extinction and the law of succession The law of succession, first noted by 18th century paleontologists, describes a pattern in which fossils tend to be found in the same geographical areas as their extant relatives. Example: Reconstruction of an extinct giant kangaroo from fossil evidence from Australia 6 Patterns of Change Through Time: Transitional Forms In some groups, the sequence of fossil forms provides clear signs of change through time. Most compelling is the existence of transitional forms, fossils that have traits intermediate between older and more recent species. Fabulous examples exist in the fossil records of horses and whales. 7 Transitional Forms in Whales While the closest modern day terrestrial relatives of whales belong to the group of mammals that include cows and hippos, the fossils record provides a clear picture of the transition from terrestrial to aquatic living. Note the gradual loss of terrestrial limbs, general change in form, including skull shape. Horse feet 8
Evidence of change through time: Vestigial Traits Some fossil and living organisms show evidence that their ancestors once possessed functional traits that no longer function (or partially function) in living representatives. These vestigial traits include such things as the reduced wings of ostriches and kiwis, the human tailbone, goosebumps. 9 2. Evidence of shared ancestry Similarity often reflects common ancestry: Given that organisms change through time, the similarities between some groups of organisms is often found to reflect shared recent common ancestry. When we study patterns of DNA variation, we find evidence of common ancestry between similar forms that our intuition often suggests are closely related. DNA-based phylogeny of the carnivores 10 Evidence of Relatedness: Homology Homology Homology is a powerful concept that describes shared similarities in structures, DNA sequences, behaviours, developmental sequences, due to shared common ancestry. Evidence of Relatedness: Structural Homology Structural Homologies Structural homologies can be seen in a host of structures across the tree of life. For example, in the limbs of vertebrates. Evidence of shared homology is pervasive in nature: e.g. the fact that life forms share the same genetic code (with a few exceptions), that eukaryotes have DNA organized into chromosomes, that eukaryotes share basic cell structures, that vertebrates with limbs all have four limbs, etc. are all examples of homologies. 11 12
Molecular evidence of homology: The genetic code 13 Molecular evidence of (non-) homology: developmental biology Pax6 controls eye development in animals from flat worms to humans! see figures 3.23, 3.25, 4.6 14 Evidence of Relatedness: Genetic Homology Genetic Homologies Genetic homologies exist for a number of genetic regions. The human genetic condition aniridia (in which the iris is poorly developed), as well as the mutant phenotypes small eye in mice and eyeless in Drosophila are all caused by mutations in Pax6. 15 (You can read more about the effects of variation in Hox and HOM genes and their role in the diversification of animals in section 19.2 and Fig. 19.3 of Freeman) 16
Adaptation (noun) 3. Evidence for natural selection Natural Selection: The process whereby some individuals contribute more offspring to the next generation as a consequence of possessing some trait or traits that enhance their ability to survive or reproduce. Artificial Selection: The process of selection whereby certain traits in an organisms are considered favorable and are selectively bred by humans (humans decide who survives and reproduces). We ll next go through an example of each from recent literature. 17 head thorax 18 Dispersal in island plant populations Cody & Overton 1996 When we see the spines on a cactus, we understand that these structures would help deter herbivores. The cryptic coloration and shape of a katydid (below) is also clearly adaptive. A evolutionary biologists, we ask what evidence we can gather to show the steps involved in producing these adaptations? Evolution in Natural Populations Evolution in action Evolutionary change can be seen in human lifetimes in nature and in artificial experiments. Traits that enhance an organisms fitness are called adaptations. Many traits in nature are clearly adaptive, for example, there is a dead leaf praying mantis in this image. Studied weedy, wind-dispersed plants located on the islands off the west coast of Vancouver Island Censused the plant populations of 200 islands and a region of the mainland over a 10-year period Extinction and recolonization events occurred frequently on the islands. 19 20
Evolution in Natural Populations Dispersal in island plant populations Cody & Overton 1996 Studied two species: A BC example: change in dispersal ability of plants on islands The two species under study have fruits that are adapted for wind dispersal. Hypochaeris radicata Lactuca muralis. Individuals with a relatively higher pappus pappus volume/ seed weight) are better dispersers (their seeds will travel farther), and may have an advantage in colonizing islands, but may be at a disadvantage once a population is established on a small island. William S. Justice @ USDA-NRCS PLANTS Database Markku Savela 21 A BC example: change in dispersal ability of plants on islands They found that old island populations had decreased dispersal ability relative to the mainland populations. They also saw that newly founded populations had higher dispersal ability than both mainland and older 23 island populations. Seed22 Speciation and selection? (An example of artificial selection in the lab) While there have been numerous demonstrations of the action of natural selection in nature and in the lab, the theory of evolution by natural selection also implies that natural selection plays a role in the birth of new species (speciation). Contemporary evidence (natural or experimental) for this is more challenging to acquire. Speciation is harder to catch in action, because it often happens over much longer time scales. Provisional definition of species (until we revisit the topic later in the term): A species is an interbreeding group of organisms that is reproductively isolated from (does not interbreed with) other groups of organisms. 24
Selection and speciation? Habitat selection in Drosophila (Rice & Salt 1988, 1990) Two papers by Rice and Salt (1988, 1990) impose artificial selection on populations of fruit flies (Drosophila melanogaster) to test the hypothesis that selection on habitat preferences can lead to the development of reproductive isolation, a key step in speciation. The researchers set up an artificial habitat in which flies were free to choose a habitat. Flies were found to take at least 8-12 hours to make three choices about the habitat where they would reproduce. The choices involved light (light or dark), direction (up versus down), and smell (ethanol or acetaldehyde). They could choose any combination of these three features of the environment. In addition, researchers selected flies based on whether they developed early (10 days) or late (14 days). They asked whether over successive generations, flies would evolve a preference for a given habitat type, and thus show evidence of the beginning of reproductive isolation. 25 Up/ down choice Selection and speciation? Habitat selection in Drosophila (Rice & Salt 1988, 1990) Flies were attached to the maze as pupae, and upon emergence as adults they entered the maze and chose lightness or darkness (left vs. right) = selection for phototaxis up or down = selection for geotaxis acetaldehyde (white vial) or ethanol (black vial) = selection for chemotaxis Experimenters choose time period: early (E), middle (M), and late (L) selection for development time Acetaldehyde/ Ethanol choice 26 Habitat selection in Drosophila Habitat selection in Drosophila Experimental larvae were mixed and placed together in the maze to start the next generation. Flies mated within the maze Control lines: 120 females chosen randomly Selected lines 60 females each from: dark, up, acetaldehde, early (5E) light, down, ethanol, late (4L) Controls were run through the maze separately. Offspring of mothers collected from 5E and half of the controls were raised on a chemical that turned their eyes brown. This allowed the researchers to track any change in habitat preference over successive generations). 27 28
Results of Habitat selection in Drosophila Results of Habitat selection in Drosophila % of flies with mothers from 5E (as measured by the % of flies at 5E and 4L with brown eyes) Control lines 29 5E 4L 5E Selected lines 1 Generations 35 5E 4L These results show that over time, the flies in the experimental lines evolved stronger habitat preferences, that led to reproductive Dotted line: 5E Solid line: 4L isolation. Because mating occurred near the food vials (after the flies had chosen a habitat), flies from 5E tended to mate with other flies from 5E, just because they shared Control the lines same habitat choice. The flies have Over time, this lead to a stronger tendency for the offspring of flies 5E accumulated collected at 5E to repeat their mother s habitat choices. Note that when flies from 5E and 4L were mixed together for mating differences in trials outside the maze, they showed no mating preferences - the habitat mating patterns was entirely driven by habitat choice. preference 4L that lead to 5E 5E reproductive isolation = the start of 4L speciation! 30