Speciation 17

Concept 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life Speciation—divergence of biological lineages and emergence of reproductive isolation between lineages. Species—groups of organisms that mate with one another.

Concept 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life Linnaeus described species based on their appearance—the morphological species concept. Members of species look alike because they share many alleles. He originated the binomial system of nomenclature. Kingdom, Phylum, Class, Order, Family, Genus, Species

Classification

Figure 17.1 Members of the Same Species Look Alike—or Not (Part 1) Limitations: members of the same species don’t always look alike (e.g., male and female wood ducks).

Concept 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life Biological species concept: “Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” Ernst Mayr Reproductive isolation: two groups of organisms can no longer exchange genes. Reproductive isolation is necessary for lineages to remain distinct over evolutionary time. It is also responsible for morphological differences—mutations that result in morphological changes cannot spread between reproductively isolated species.

Concept 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life Lineage species concept: species as branches on the tree of life. A lineage is an ancestor–descendant series of populations followed over time. How can one lineage ever split into two reproductively isolated species?

Concept 17.2 Speciation Is a Natural Consequence of Population Subdivision Not all evolutionary changes result in new species. Speciation requires interruption of gene flow.

Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry Allopatric speciation—when populations are separated by a physical or geographic barrier. Barriers can form when continents drift, sea levels rise and fall, glaciers advance and retreat, or climates change. The populations evolve through genetic drift and adaptation to different environments in the two areas.

Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry Some members of a population may cross an existing barrier and establish an isolated population. Finch species in the Galápagos Islands evolved from a single South American species that colonized the islands. The islands are far apart, and have different environmental conditions.

Figure 17.7 Allopatric Speciation among Darwin’s Finches

Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry Sympatric speciation— speciation without physical isolation. May occur with disruptive selection—individuals with certain genotypes prefer distinct microhabitats where mating takes place. This appears to be taking place with apple maggot flies. One group prefers to lay eggs on hawthorne fruits, the other group lays eggs on apples. They are partially reproductively isolated.

Concept 17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact Mechanical isolation—differences in sizes and shapes of reproductive organs.

Temporal isolation—species may breed at different times of year or different times of day.

Behavioral isolation—individuals reject or fail to recognize mating behaviors of other species

Concept 17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact Habitat isolation—when two closely related species evolve preferences for living or mating in different habitats.

Gametic isolation—sperm and eggs of different species will not fuse. Important for aquatic animals that release gametes into the water.