Chapter 24 Evolution by Natural Selection
- Populations and species evolve, meaning that their heritable characteristics change through time. More precisely,
evolution is defined as changes in allele frequencies over time.
- Natural selection occurs when individuals with certain alleles produce the most surviving offspring in a population.
An adaptation is a genetically based trait that increases an individual’s ability to produce offspring in a particular
- Evolution by natural selection is not progressive, and it does not change the characteristics of the individuals that are
selected- it changes only the characteristics of the population. Animals do not do things for the good of the species,
and not all traits are adaptive.All adaptations are constrained by trade-offs and genetic and historical factors.
24.1 The Evolution of Evolutionary Thought
-Typological thinking: ideas that species are unchanging types.
-Lamarck was the first to propose the theory of evolution- idea that species change through time.
Species change through time via the inheritance of acquired characteristics (giraffe necks).
-Darwin claimed that variation among individuals in a population was the key to understanding
the nature of species. Biologists refer to this view as population thinking.
-Population- individuals of same species living in the same area at the same time.
-Darwin and Wallace proposed that evolution occurs because traits vary among the individuals in
a population, and because individuals with certain traits leave more offspring.
24.2 The Pattern of Evolution: Have Species Changed Through Time?
Descent with modification: Species existing today have descended from pre-existing species,
and that species are modified, or change through time. Theory of natural selection: 1) They change through time.
2) Related by common ancestry
Evidence for Change through Time
Fossil- trace of an organism that lived in the past. Fossil record consists of all the fossils that
have been found and described in the scientific literature.
-Most fossils are found in sedimentary rocks- which form from sand or mud or other materials
deposited at locations such as beaches or river mouths. Known to form in layers.
-geological time scale: different periods of geologic times, creating the sequence of eons, epochs,
Extinction- species that no longer exists. Ex. Irish elk
Law of succession: extinct species in the fossil record were succeeded, in the same region, by
Transitional form: Fossil species with traits that are intermediate between those of older and
Vestigial traits: Reduced or completely developed structure that has no function or reduced
Evidence That SpeciesAre Related
-Geographic relationships: Galapagos mockingbirds- Instead of being created independently,
mockingbird populations that colonized different islands had changed through time and formed
new species. Similar because descended from same ancestor.
-Homology: Study of likeness
1. Genetic homology- Similarity In the DNAsequences of different species.
2. Developmental homology: similarity in embryonic traits. Observed in the overall
morphology- form of embryos and on the fate of particular embryonic tissues. In humans, gill
pouches and tails. Embryonic tails are vestigial in humans
3. Structural homology: similarities in adult morphology. Developmental homologies are due to
homologous genes and give rise to structural homology. In many cases, traits are similar in different species because the species in question are related to
each other by common descent. If species were created independently of one another, these types
of similarities would not occur.
Evidence for Evolution
Prediction 1: Species are not static, but change through time
- Most species have gone extinct
- Fossil (extinct) species frequently resemble living specie