Evolution and Natural Selection

The Core Theme: Evolution “Nothing in biology makes sense except in the light of evolution” - Theodosius Dobzhansky The evolutionary changes seen in the fossil record are observable facts. Evolutionary mechanisms account for the unity and diversity of all species on Earth.

Classifying the Diversity of LIfe Biologists have so far identified and named about 1.8 million species Including at least 100,000 species of fungi, 290,000 plant species, 57,000 vertebrate species, and 1 million insect species Researchers identify thousands of additional species each year. Biologists face a major challenge in attempting to make sense of this variety.

Grouping species Taxonomy is the branch of biology that names and classifies species based on the degree to which they share characteristics (more in chapter 26) Figure 1.14 To help make sense of the diversity of life, biologists classify species into groups that are combined into even broader groups: species, genus, family, order, class, phylum, kingdom, domain

The Three Domains of Life Bacteria, Archaea, and Eukarya Bacteria and Archaea are prokaryotic All eukaryotes are in group Eukarya Eukarya includes three kingdoms of multicellular eukaryotes: plantae, fungi, and animalia which are distinguished by their modes of nutrition

Eukarya’s three kingdoms Plantae- plants produce their own sugars and other foods by photosynthesis Fungi- absorb dissolved nutrients from their surroundings; many decompose dead organisms and organic wastes Animalia- animals obtain food by ingestion, which is the eating and digesting of other organisms.

Evolution Evolution – includes all of the changes in the characteristics and diversity of life that occur throughout time. Evolution can occur on both large and small scales. Microevolution Macroevolution

Introduction The concept of evolution, that organisms may change over time, was not new in Darwin’s time. However, it was not a widely accepted concept because no one understood how it could work. A mechanism was missing.

Natural Selection Darwin provided that mechanism with his theory of Natural Selection. He formed his theory during his time on the HMS Beagle, in which he surveyed the Galapagos Islands

Darwin & Natural Selection The history of life, as documented by fossils and other evidence, is a saga of changing Earth billions of years old, inhabited by an evolving cast of living forms. This evolutionary view of life came into sharp focus when Charles Darwin published “On the Origin of Species by Means of Natural Selection” in 1859.

Origin of Species The Origin of Species articulated two main points: First, Darwin presented evidence to support his view that contemporary species arose from a succession of ancestors. He called this evolutionary history of species “descent with modification”. Darwin’s second main point was to propose a mechanism for descent with modification and he called this evolutionary mechanism natural selection.

Darwin’s observations Individuals in a population vary in their traits, many of which seem to be heritable. Also, a population can produce far more offspring than can survive to produce offspring of their own. With more individuals than the environment can support, competition is inevitable. Lastly, species generally suit their environments.

The Tree of Life The unity of mammalian limb anatomy reflects the inheritance of that structure from a common ancestor. The diversity of mammalian forelimbs results from modification by natural selection operating over millions of generations in different environmental contexts.

The Tree of Life Darwin’s finches are an example of adaptive radiation of a new species from a common ancestor. The finches had different beaks, which are adapted to food sources on the different islands of the Galapagos.

Natural Selection In any population of organisms there is natural variation. Some of these variations will allow the organisms possessing them to survive and reproduce better than those without these particular traits.

Natural Selection The successful traits will spread through the population. This change in the frequency of alleles in the population is evolution.

Natural Selection – High Reproductive Potential Darwin observed that organisms have the potential for very high fertility. Organisms have the potential to produce, and often do produce large numbers of offspring. Population size would quickly become unmanageable if all of the offspring survived.

Natural Selection – Population Size Remains Constant Despite this high potential fertility, natural populations usually remain constant in size, except for small fluctuations. Not all of the potential offspring survive. Resources that organisms need to survive are limited. Food, water, shelter, nesting sites, etc.

Natural Selection – Competition If there are not enough resources for all of the individuals, there will be competition for those resources. Survivors represent a small part of the individuals produced each generation. Which individuals will survive is often not a matter of luck. Populations show variation – individuals are not identical. They differ in many different traits.

Natural Selection – Some traits Enhance Survival Some of the traits found in the population enhance the survival and reproduction of the organisms possessing them. The favored traits will spread through the population. Over many generations, the species will become adapted to its environment. Over time, these changes can lead to the formation of a new species.

Adaptation A species may become adapted to its environment in response to environmental pressures. A trait may be favored due to enhanced survival or reproduction when faced with a particular aspect of the environment.

Adaptation When an environment changes, or when individuals move to a new environment, natural selection may result in adaptation to the new conditions. Sometimes this results in a new species.

Populations Evolve Individuals do not evolve; populations evolve. Evolution is measured as changes in relative proportions of heritable variations in a population over several generations.

Natural Selection – Important Points Natural selection can only work on heritable traits. Acquired traits are not heritable and are not subject to natural selection. Environmental factors are variable. A trait that is beneficial in one place or time may be detrimental in another place or time.

Natural Selection – Important Points Natural selection is not random. It occurs in response to environmental pressures and results in adaptation.

Natural Selection – Important Points When natural selection is occurring, some individuals are having better reproductive success than others. Alleles are being passed to the next generation in frequencies that are different from the current generation.

Upsetting Genetic Equilibrium Natural selection is not the only way that allele frequencies can change from one generation to the next. Genetic Drift – a random loss of alleles. Mutation – a new mutation can add alleles. Nonrandom mating – inbreeding increases the number of homozygous traits. Migration – shuffles alleles between populations; can prevent speciation.

Genetic Drift The smaller the sample, the greater the chance of deviation from expected results. These random deviations from expected frequencies are called genetic drift. Allele frequencies are more likely to deviate from the expected in small populations.

Genetic Drift Which allele gets lost is due to random chance. Over time, drift tends to reduce genetic variation through random loss of alleles. Frequency CR = 0.7 Frequency CW = 0.3 Frequency CR = 0.4 Frequency CW = 0.6 Frequency CR = 1.0 Frequency CW = 0 CRCR = red CRCW = pink CWCW = white

The Bottleneck Effect Sometimes a catastrophic event can severely reduce the size of a population. The random assortment of survivors may have different allele frequencies. This is a type of genetic drift called the bottleneck effect.

The Bottleneck Effect The actions of people sometimes cause bottlenecks in other species. N. California elephant seal population reduced to 20-100 individuals in the 1890s. Current population > 30,000. Variation drastically reduced – 24 genes with 1 allele.

The Founder Effect Founder effect – Another type of genetic drift occurs when a small group of individuals becomes separated from the population and form a new population. The allele frequencies in their gene pool may be different than the original population.

A researcher studying the evolution of flight in birds is focusing on: Question 1 A researcher studying the evolution of flight in birds is focusing on: Microevolution Macroevolution The bottleneck effect

Question 1 Sorry! That is incorrect. Try again!

Question 1 Congratulations! You are correct!

What was the mechanism of evolution that Darwin proposed? Question 2 What was the mechanism of evolution that Darwin proposed? Natural Selection Macroevolution Genetic drift Chromosomal basis of inheritance

Question 2 Sorry! That is incorrect. Try again!

Question 2 Congratulations! You are correct!

Question 3 In every population there is variation. It is important that this variation Involves a variety of colors Is heritable Is not noticeable Is acquired during an organisms lifetime

Question 3 Sorry! That is incorrect. Try again!

Question 3 Congratulations! You are correct!

Question 4 During natural selection, some organisms will survive & reproduce better than others. This is due to: Random chance Humans choosing which animals to breed Environmental pressures resulting in organisms with certain traits having the best reproductive success Luck

Question 4 Sorry! That is incorrect. Try again!

Question 4 Congratulations! You are correct!

Which statement about adaptation is NOT true? Question 5 Which statement about adaptation is NOT true? A species may become adapted to its environment in response to environmental pressures. A species is perfectly adapted to its environment from the beginning. As favored traits spread through the population, a species will become adapted to its environment. When an environment changes, or when individuals move to a new environment, natural selection may result in adaptation to the new conditions, sometimes this results in a new species.

Question 5 Sorry! That is incorrect. Try again!

Question 5 Congratulations! You are correct!

How can allele frequencies change from one generation to the next? Question 6 How can allele frequencies change from one generation to the next? Genetic drift Natural selection Mutation Migration All of the above

Question 6 Sorry! That is incorrect. Try again!

Question 6 Congratulations! You are correct!

Which of the following is NOT due to random chance? Question 7 Which of the following is NOT due to random chance? Genetic drift The bottleneck effect Natural selection The founder effect

Question 7 Sorry! That is incorrect. Try again!

Question 7 Congratulations! You are correct!

Question 8 After a catastrophe reduces the size of a population, the survivors may have a different set of allele frequencies. This is called The bottleneck effect Natural selection The founder effect All of the above

Question 8 Sorry! That is incorrect. Try again!

Question 8 Congratulations! You are correct!