1. Evolution and Speciation
Effects on the Flora and Fauna of the Galápagos

2. Charles R. Darwin
1809 – 1882
On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life
Published in 1859
Title is NOT “Origin of the Species”
The word “species” is used in the plural sense
Publication spurred by development of similar theories by Alfred Russell Wallace, a biologist who had traveled and studied in the islands of Malaysia.

3. Main components of The Origin of Species
Evidence that evolution has occurred
Mechanism that could cause evolution (mechanism = natural selection)

4. Evidence for evolution
Fossil record
Biogeography – patterns of geographical distribution of living things on earth
Structural and anatomical similarity
Change in domesticated plants and animals

6. Some (rather obvious) patterns from the fossil record.
There are many extinct groups that once were common and now are gone.
Many modern groups were rare or non-existent in the past.
The farther back in time you look, the more unfamiliar the flora and fauna were.
Suggests a process of continual change (maybe gradual, maybe not).

7. Biogeography patterns of geographical distribution of living things on earth
One species is replaced by another as you move from one location to another. The farther you move, the more unfamiliar the biota becomes.
Patterns are especially clear on islands.
Many animals and plants of the Galapagos are unique, endemic species. The most similar species are those of South America, but they are not the same.
Within the Galapagos, each island has its own unique species or subspecies of animals and plants. Again, islands that are close to each other tend to be similar.

8. “Darwin’s finches”
Galapagos tortoises
Above: Saddleback shell
Below: Dome shell
Every island has a slightly
different shape of shell.

9. Similarity of structures
Analogous structures – similar because they serve a common function.
e.g. wings must be flat and positioned on either side of the body in order to allow flight.
Homologous structures – similarity that goes beyond function; may even go against function.
e.g. bone structures similar in legs, wings, flippers, and even internal vestigial bones in legless animals

11. Homologous structures
Homology arises from common ancestry.
Analogous structures are similar because of function only, not because of common ancestry.

12. Variation under domestication
Humans have caused new varieties of domesticated animals and plants to come into existence.

15. Variation under domestication
Humans have caused new varieties of domesticated animals and plants to come into existence.
This shows that species can change.
It also hints at why species change.
We alter and improve domesticated animals and plants, by deciding which individuals are used as breeding stock, and who they mate with.
Could something similar happen in nature?

16. Natural Selection: Supporting Observations
More are born than can survive (Malthus)
Variation exists within populations
Offspring resemble parents
The characters which vary among members of a species may affect survival and reproductive success.

17. Natural Selection
In a variable population, characteristics that increase survival and reproductive success of the organisms that possess them will be passed down to future generations more often than less beneficial traits. Thus, the population will change over time.

18. Variation within populations
Darwin was the first to realize the true importance of variation in populations.
Unlike Lamarck, Darwin did not propose that useful characteristics arise because they are needed.
Instead, he realized that variation is just a natural fact of life; all species are variable.
Natural selection works on this variability. Beneficial traits become more common over time.
If a population is not variable, it cannot evolve.
If characters are not heritable (genetic) they cannot evolve.

19. Studying Evolution in Action
The techniques of Population Genetics allow us to study how gene frequencies change in populations, i.e. how populations evolve.
This can be modeled mathematically, but we won’t go into the details here.
Natural selection can cause a specific gene to become more or less common, depending on whether it is beneficial or harmful.
But there are factors other than natural selection that can cause evolution.

20. Genetic Drift
In a small, isolated population, the genetic makeup of the population can change randomly.
One subtype of this is Founder Effect, where a few individuals colonize a new area and start a new population.
The new population will reflect the unique genetic makeup of the founder group, which may by chance be different from that in the population from which they came.

22. What about continuous variation?
Traits like height, weight, etc. are controlled by many genes (polygenic inheritance).
The result is a continuous spectrum of phenotypes, often with a normal distribution.
Instead, we can simply look at the results of selection on the phenotype. This approach is called “quantitative genetics”.

23. 3 Patterns of Natural Selection
Directional selection:
One extreme more fit than the other.
Causes population mean to shift over time.
Stabilizing selection:
Both extremes are less fit than the intermediates.
Causes population to become less variable.
Disruptive selection:
Both extremes are more fit than the intermediates.
Causes population to become more variable, perhaps to have distinct polymorphisms (different forms).

25. The formation of new species
Speciation
The formation of new species

26. What is a “species”
A basic kind of living thing. But how to define scientifically??
Morphological species concept: similarities and differences in overall body form.
“Biological Species Concept”
A species is a group of actually or potentially interbreeding natural populations, reproductively isolated from other such groups.

27. How do new species form?
Over time, members of one species might evolve so much that that they are quite different from how they used to be. But that does not explain the number of species on earth and patterns of common ancestry.
How does one species diverge to become two different species?

29. Models of speciation
Allopatric model: Speciation through geographic isolation
Considered most important, and also most relevant to the Galápagos
Sympatric model: Speciation in a single place.
Parapatric model: Speciation in neighboring populations

30. Allopatric speciation
Populations become geographically isolated.
Selection causes populations to diverge. (Because they are isolated, there is no movement of genes between populations, which might prevent them from diverging.)
Populations come back together. 3 outcomes:
Differences blend away. (They were never species.)
They can’t interbreed. (They have speciated.)
They interbreed, but hybrids between the populations are less fit, and selection favors the evolution of isolating mechanisms (reproductive character displacement).

33. Causes of geographic isolation
Geologic changes
Formation of mountain ranges or canyons
Changes in river course, water level; formation of lakes.
Population fragmentation
Population reduced into isolated sub-groups
Long-distance dispersal and colonization
Notes:
Genetic drift and founder effect can be important along with selection when small groups become isolated.

34. Why so many endemic species in the Galapagos?
Rare colonization. Little contact with mainland.
Exceptions: mangroves, some sea birds
Small populations: genetic drift and founder effect.
Island-hopping and further divergence within the archipelago.
Diversity of different habitats leads to diversity of form.
Evolution in situ (“neo-endemics”)