Chapter 21: The Evidence for Evolution
The Beaks of Darwin's Finches: Evidence of Natural Selection
Galápagos Islands, 1835: Charles Darwin collected specimens.
He collected 31 specimens of finches from 3 islands.
Ornithologist John Gould confirmed their identification.
There are now 14 recognized species of these finches.
These finch species exhibit significant differences in their beaks and feeding habits.
This diversity originated from an ancestral finch that colonized the islands and subsequently diversified to fill various habitats that lacked other types of small birds.
The beaks of several finch species bear resemblances to those of different bird families found on the mainland (e.g., the warbler finch's beak is very similar to that of warblers, despite not being closely related).
Variations Related to Food Gathering:
Some species feed on seeds, which they crush with powerful beaks.
Other species possess smaller and narrower beaks, specialized for different food sources.
Some finches are fruit and bud eaters.
The sharp-beaked ground finch (a subspecies) normally consumes seeds and insects but occasionally supplements its diet by drinking blood from seabirds, such as blue-footed boobies.
The woodpecker finch is unique in its tool use; it shapes twigs, cactus spines, or leaf stalks and pokes them into dead branches to extract grubs.
Darwin's Observations and Hypothesis: Darwin noted that differences in beak size and shape among species had evolved as each species adapted to utilize distinct food resources, forming the basis of his selection hypothesis.
Artificial Selection: Human-Induced Change
Definition: Artificial selection is the change in the genetic structure of populations resulting from selective breeding by humans.
Humans have been imposing selection on living organisms since the dawn of civilization.
This process is evident in laboratory experiments, agriculture, and the domestication of animals.
Mechanism: Artificial selection favors individuals with certain desirable phenotypic traits, allowing them to reproduce and pass on their genes to the next generation, ultimately leading to substantial evolutionary change.
Experimental Selection Produces Changes (Example: Fruit Flies):
With the rise of genetics (1920-1930s), researchers began experiments on fruit flies.
Hypothesis: Selection can produce evolutionary change.
Selection was imposed on numerous traits, including body size, eye color, growth rate, behavior, and life span.
Consistent Result: Selecting for a specific trait consistently leads to a strong and predictable evolutionary response.
Scientific Thinking: Can Artificial Selection Lead to Substantial Evolutionary Change?
Question: Can artificial selection lead to substantial evolutionary change?
Hypothesis: Strong directional selection will quickly lead to a large shift in the mean value of the population.
Experiment (Drosophila Bristle Number):
In one population, the top 20\% of individuals with the most bristles were chosen for reproduction each generation.
In another population, the top 20\% of individuals with the fewest bristles were chosen.
Result: After 35 generations, the mean number of bristles had significantly changed in both populations.
Interpretation: The range of variation at the end of the experiment extended beyond the range observed in the initial population. This demonstrates that selection can move a population beyond its original range because mutation and recombination continuously introduce new genetic variation.
Agricultural Selection
Agricultural selection has profoundly modified crops and livestock.
Many familiar livestock (e.g., cattle, pigs) and crops (e.g., corn, strawberries) are essentially human inventions developed through generations of human selection for desirable traits (e.g., greater milk production, larger corn ear size).
Example: Corn Oil Content:
In 1896, scientists began selecting for the oil content of corn kernels.
Initially, the average oil content was 4.5\%.
The top 20\% of individuals with the highest oil content were allowed to reproduce.
By 1986, after 90 generations, the average oil content had increased fivefold.
Domesticated Breeds
Human-imposed selection has produced a vast array of domesticated animals, including dogs, cats, and pigeons.
Breeds developed for particular purposes:
Greyhound dogs: Selected for maximal running ability, resulting in long legs, an arched back to increase stride length, and great muscle mass.
Dachshunds: Selected for their ability to enter narrow holes to pursue badgers.
Breeds selected for appearance: In other cases, varieties, such as many cat breeds, have been selected primarily for aesthetic qualities.
Fossil Evidence of Evolution
The fossil record provides the most direct evidence of evolution.
Fossils: Preserved remains of once-living organisms, found in amber, Siberian permafrost, dry caves, and most commonly as rock fossils.
Formation of Rock Fossils: This process typically involves three events:
The organism becomes buried in sediment.
Calcium in the bone or other hard tissues mineralizes.
The surrounding sediment hardens to form rock.
Limitations of the Fossil Record:
Only a tiny fraction of all species that have ever existed are known from fossils.
Fossilization is a rare process; remains often decay or are scavenged before it can occur.
Many fossils are located in rocks inaccessible to scientists.
Even when discovered, fossils are frequently destroyed by erosion and other natural processes.
Significance: Despite these limitations, the discovered fossils are sufficient to provide detailed information on the course of evolution through time.
Estimating the Age of Fossils
In Darwin's Day:
Rocks were dated by their relative position to one another, with rocks in lower strata generally considered older.
19^{th} century geologists used this understanding along with rates of erosion to gain a fairly accurate idea of rock ages.
Today (Isotopic Dating):
When a rock forms, some of its constituent elements exist as different isotopes.
Isotopes: Forms of the same element that have the same number of protons but different numbers of neutrons.
Over time, less stable isotopes decay and convert into more stable isotopes, changing their ratio.
Isotopic Decay and Half-life: Isotopes decay at a known rate, called their half-life. After one half-life, half of the original parent isotope has transformed into a daughter isotope. This process continues, with half of the remaining parent isotope transforming after each successive half-life.
Fossils Document Evolutionary Change and Transitions
History of Evolutionary Change: The fossil record presents a grand history of life on Earth, documenting:
The origin of prokaryotes.
The origin of eukaryotes.
The first multicellular organisms.
The rise of land-dwelling organisms.
The reign of the dinosaurs.
The eventual origin of humans.
It also illustrates the waxing and waning of biodiversity over geological time.
Documenting Evolutionary Transitions: Despite gaps in the fossil record (due to low preservation and recovery rates), intermediate forms are often available.
These intermediate forms illustrate how major transitions in life have occurred.
Example: Archaeopteryx, the oldest known bird:
Lived approximately 165 million years ago.
Remarkably preserved, it represents an intermediate form between birds and dinosaurs.
It possessed ancestral dinosaur traits alongside feathers and other characteristics shared with modern birds.
The first fossil was discovered in 1859, and since then, paleontologists have continued to fill gaps.
Fossils have now been found linking all major groups of vertebrates.
Anatomical Evidence for Evolution
Homologous Structures: These structures suggest a common evolutionary derivation.
They are features with different appearances and functions but are all derived from the same body part in a common ancestor.
Example: Vertebrate Forelimb Bones: As vertebrates diversified, the same fundamental bones have been adapted for different uses (e.g., wings, arms, flippers), yet the underlying bone structure remains recognizable.
Similarities in Early Embryonic Development:
Embryos of different vertebrates often show striking similarities early in development.
All vertebrate embryos possess pharyngeal gill pouches initially.
Later in development, these become different structures: in humans, they differentiate into various glands and ducts, while in fishes, they develop into gill slits.
All vertebrate embryos also possess a postanal tail.
This tail is lost later in development in certain groups, such as apes and humans.
These similarities strongly suggest that vertebrate development has evolved, with new instructions modifying ancestral developmental patterns.
Imperfectly Suited Structures
Organisms are not perfectly adapted to their environment.
Natural selection can only act on the variation already present in a population and must work with available materials to craft workable solutions.
Example: Neck Vertebrae in Long-Necked Animals:
Most animals with long necks (e.g., geese with up to 25 neck vertebrae, plesiosaurs with as many as 76) have many cervical vertebrae for enhanced flexibility.
In contrast, giraffes have only 7 very long neck vertebrae, the same number found in all other mammals.
Mutations that would result in the development of more vertebral bones are unlikely, as many genes are involved in bone development.
Instead, elongation of the bones already present is a more common evolutionary outcome.
Vestigial Structures
Definition: These are morphological features that have no apparent current function and are considered an evolutionary relic.
They resemble structures that ancestors possessed and were functional in those ancestors.
Examples:
Boas and pythons possess hip bones and rudimentary hindlimbs.
Manatees have fingernails on their flippers.
Humans also have several vestigial structures, including the coccyx (tailbone), the appendix, and wisdom teeth.
Pseudogenes: These are traces of previously functioning genes.
When a functional trait disappears, the underlying gene may still exist but has been rendered inactive by mutations.
Over time, other mutations can accumulate in these inactive genes, further obscuring their original function.
Convergent Evolution and the Biogeographical Record
Biogeography: This is the study of the geographic distribution of species.
Different geographical areas can exhibit groups of organisms with strikingly similar appearances, even if they are only distantly related.
Mechanism: Natural selection favors parallel evolutionary adaptations in similar environments, leading to changes that make these groups more alike.
This process is known as convergent evolution.
Example: Marsupials and Placentals Demonstrate Convergence:
These are two major groups of mammals that have evolved in remarkably similar ways in different parts of the world.
Marsupials: Their young are born very premature, latch onto the mother's nipple (often within a pouch), and continue to grow and develop with a constant source of nutrients until they emerge from the pouch.
Placentals: Their young are not born until they are more developed and can safely survive in the external environment, with varying degrees of parental care.
Australia's Isolation: Australia separated from other continents approximately 80 million years ago.
Both marsupials and placental mammals had evolved by this time but in different geographical locations.
Due to this separation, very few placental mammals (primarily rodents and a few colonizers that arrived relatively recently, starting 5 million years ago) are native to Australia.
The Australian marsupials closely resemble the placental mammals found on other continents, filling similar ecological niches.
These similarities strongly indicate convergent evolution, where similar forms evolved in different, isolated areas because they were subjected to similar selective pressures in similar environments.
Convergent Evolution is a Widespread Phenomenon
When species interact with their environment in similar ways, they are often exposed to similar selective pressures, frequently resulting in the development of the same evolutionary adaptations.
Example: Streamlined Body Form: Fast movement through water necessitates a streamlined body shape to reduce friction, an adaptation that has evolved numerous times independently across diverse aquatic species (e.g., dolphins, sharks, ichthyosaurs).
Example: Lactase Production in Humans:
Most human populations cease producing lactase (the enzyme that digests milk) after weaning, leading to lactose intolerance.
However, African and European populations that traditionally practice cattle raising have independently evolved the ability to produce lactase throughout their lives.
The retention of lactase production in these populations is a result of different mutations arising in Africa and Europe, demonstrating that these populations have independently (convergently) acquired this adaptation.