Evolution and its processes Chapter 11

Evidence of Evolution Evolution of a population is observable in the field and in the laboratory Guppies Adult male guppies exhibit genetically determined spots Juvenile and female guppies lack spots Male and female guppies from genetically diverse populations were collected from different mountain streams and placed together in an isolated environment containing no predators

Evidence of Evolution Evolution of a population is observable in the field and in the laboratory Guppies The population was maintained for several generations Separated into two groups: one with a predator and one without a predator Each group was maintained through several generations Throughout the experiment, the mean number of spots per adult male guppies was determined

What happens to the mean number of spots during the first 6 months? What do the bars going through each point represent? What happens to the genetic variation during the first 6 months? What is a possible explanation for the changes?

What happens to the mean number of spots in the group with the predator? What happens to the mean number of spots in the group without the predator? What is a possible explanation for the differences?

Species and Speciation A species is a distinct group Morphologically different from other groups Do not interbreed with other groups Reproductively isolated

Species and Speciation Speciation is the formation of new species A population is divided into subpopulations Subpopulations are reproductively isolated Natural selection, genetic drift, and mutations can occur differently in each subpopulation Evolution takes a different course in each subpopulation After a long period of time this may result in two different species

Species and Speciation Reproductive Isolation The end of gene flow between populations Always a part of speciation

Species and Speciation Reproductive Isolation Mechanisms Temporal isolation Individuals reproduce at different times Cicadas: different reproductive cycles (13 verses 17 year)

Species and Speciation Reproductive Isolation Mechanisms Mechanical isolation Physical incompatibilities prevent interbreeding Black sage and white sage: specialized for different pollinators Black sage flowers are small and delicate. It is pollinated mainly by honeybees and other small insects White sage pollen coated anthers are too far away from the petals to be brushed by smaller insects. Larger bees and hawkmoths are the pollinators.

Species and Speciation Reproductive Isolation Mechanisms Ecological isolation Adapted to different microenvironments in the same region Two species of manzanita: One is drought-adapted, the other is not

Species and Speciation Reproductive Isolation Mechanisms Behavioral Isolation Bird courtship displays

Species and Speciation Reproductive Isolation Mechanisms If mating occurs… Gamete incompatibility Molecular signals are species specific Hybrid inviability Chromosome and gene differences Hybrid sterility Horse X donkey = sterile mule

Species and Speciation Speciation can occur in three different ways Each requires some mechanism to ensure reproductive isolation

Species and Speciation Allopatric Speciation (with geographic isolation) Subpopulations become geographically isolated, thus reproductively isolated

Figure 17. 20 Allopatric speciation in snapping shrimp Figure 17.20 Allopatric speciation in snapping shrimp. The Isthmus of Panama (above) cut off gene flow among populations of these aquatic shrimp when it formed 4 million years ago. Today, individuals from opposite sides of the isthmus are so similar that they might interbreed, but they are behaviorally isolated: Instead of mating when they are brought together, they snap their claws at one another aggressively. The photos on the right show two of the many closely related species that live on opposite sides of the isthmus. Fig. 17.20, p. 272

Species and Speciation Sympatric Speciation (without geographic isolation) Within the same population some individuals are reproductively isolated Plants Change in chromosome number Here we see two species of anemone flower and their chromosomes. Changes in number of chromosomes, as has occurred in this genus, can cause speciation.

Species and Speciation Sympatric Speciation (without geographic isolation) Within the same population some individuals are reproductively isolated Indigobirds are brood parasites Young learn the song of the host species More likely to mate with birds that learned the same song Ciclids Females choose mate based on colors she perceives

Species and Speciation Parapatric Speciation (without geographic isolation) Small local populations are more likely to breed within their immediate population than with another local population (new niche formed) Sea urchins in a tidepool Velvet walking worms Although continuously distributed, different flowering times have begun to reduce gene flow between metal-tolerant plants and metal-intolerant plants.

Summary Observations of nature changed our thinking Change over time /natural selection/evolution Hardy-Weinberg Equation Populations evolve Mechanisms Genetic drift, gene flow, mutation, natural selection Species and Speciation

Early Evolutionary Concepts Count de Buffon (1707-1788) Described all known plants and animals (44 volumes) Observed structural variations in particular organs of related animals Believed in a special creation and considered change as being degenerate Apes = degenerate humans Provided no theories on how change/degeneration might take place

Change over Time Charles Lyell Gradual geological processes sculpted Earth’s current landscape over great spans of time Challenged the prevailing belief of the age of Earth Thomas Malthus Economist that correlated increases in the size of human populations with episodes of famine, disease, and war Humans reproduce beyond the capacity of their environment Must compete for limited resources or develop technology to increase productivity