17.08 Evolution and Speciation
Evolution
Definition
- Evolution is the cumulative process through which populations of organisms change over successive generations. It involves alterations in the genetic composition (allele frequencies) of populations, leading to the development of new species and the diversity of life observed today.
Result of Evolution
- Speciation: The emergence of new species from existing ones as a result of significant changes in gene pools over extended periods.
- Adaptation: Populations become better suited to their environments through advantageous traits becoming more common.
- Diversity of Life: The vast variety of organisms arises from evolutionary processes acting over millions of years.
Mechanisms Influencing Evolution
- Natural Selection
- Definition: The differential survival and reproduction of individuals due to differences in phenotype.
- Process:
- Variation: Within a population, individuals exhibit variations in traits.
- Inheritance: Traits are heritable and can be passed to offspring.
- Differential Survival and Reproduction: Individuals with advantageous traits are more likely to survive and reproduce.
- Increase in Favorable Alleles: Over time, beneficial alleles become more prevalent in the gene pool.
- Example: The peppered moth (Biston betularia) during the Industrial Revolution; darker moths became more common due to better camouflage on soot-covered trees.
- Genetic Drift
- Definition: Random fluctuations in allele frequencies within a population, especially pronounced in small populations.
- Types:
- Bottleneck Effect: A significant reduction in population size due to an external event (e.g., natural disaster), leading to a loss of genetic diversity.
- Founder Effect: A new population is established by a small number of individuals, resulting in limited genetic variation and potentially different allele frequencies compared to the original population.
- Example: The high frequency of the sickle cell allele in certain populations due to a historical bottleneck.
- Mutation
- Definition: Changes in the DNA sequence of an organism’s genome, introducing new genetic variations.
- Types:
- Point Mutations: Substitutions, insertions, or deletions of single nucleotides.
- Chromosomal Mutations: Large-scale changes affecting entire genes or chromosomes.
- Role in Evolution: Mutations provide the raw material for natural selection by creating new alleles.
- Gene Flow (Migration)
- Definition: The transfer of alleles or genes from one population to another through interbreeding.
- Impact: Increases genetic diversity within populations and reduces genetic differences between populations.
- Example: Individuals migrating to a new habitat and breeding with the local population, introducing new alleles.
- Sexual Selection
- Definition: A form of natural selection where certain traits increase an individual’s chances of mating and reproducing.
- Types:
- Intrasexual Selection: Competition among the same sex (e.g., male-male competition for mates).
- Intersexual Selection: Preferences by one sex for certain traits in the other (e.g., peacock’s tail feathers).
- Impact: Can lead to the development of elaborate traits that may not directly contribute to survival but enhance mating success.
Other Influences on Evolution
- Environmental Changes: Shifts in climate, habitat, or availability of resources can drive evolutionary adaptations.
- Coevolution: Reciprocal evolutionary changes between interacting species, such as predators and prey or pollinators and plants.
Species and Speciation
Species Definition
A species is traditionally defined as a group of organisms that:
- Interbreed: Can mate with each other and produce fertile offspring.
- Reproductive Isolation: Are reproductively separated from other such groups, preventing gene flow between species.
- Shared Traits: Exhibit similar morphological, physiological, biochemical, and behavioral characteristics.
Key Characteristics of Species
- Morphological Characteristics
- Definition: Structural features such as body shape, size, coloration, and anatomical structures.
- Example: The distinct beak shapes of Darwin’s finches adapted to different food sources.
- Physiological Characteristics
- Definition: Functional aspects like metabolic pathways, enzyme activities, and hormonal processes.
- Example: Different metabolic rates in desert vs. rainforest species adapting to their environments.
- Biochemical Characteristics
- Definition: Molecular-level traits, including DNA sequences and protein structures.
- Example: Differences in mitochondrial DNA sequences used to distinguish closely related species.
- Behavioral Characteristics
- Definition: Patterns of behavior such as mating rituals, feeding habits, and social interactions.
- Example: Distinct courtship dances in bird species that prevent interbreeding.
Determining Species Boundaries
- Reproductive Isolation
- Prezygotic Barriers: Prevent mating or fertilization between species (e.g., temporal isolation, habitat isolation, behavioral differences, mechanical incompatibility, gametic isolation).
- Postzygotic Barriers: Occur after fertilization, leading to hybrid inviability or sterility (e.g., mule infertility from horse and donkey crossbreeding).
- Challenges in Defining Species
- Asexual Reproduction: Difficulty applying reproductive isolation to organisms that reproduce without mating.
- Hybridization: Some species can interbreed and produce fertile offspring, blurring species lines.
- Cryptic Species: Morphologically similar but genetically distinct species.
- Practical Limitations: Constraints in observing mating behaviors or conducting breeding experiments.
- Alternative Methods for Species Identification
- Morphological Analysis: Comparing physical structures and forms.
- Biochemical Analysis: Examining DNA, RNA, and protein sequences.
- Physiological Analysis: Studying metabolic processes and other functional traits.
- Behavioral Analysis: Observing mating rituals and other behaviors.
- DNA Sequence Analysis: Utilizing genetic data to clarify relationships and distinctions between species, often through techniques like DNA barcoding or genome sequencing.
Speciation Processes
- Allopatric Speciation
- Definition: Speciation occurring when populations become geographically isolated.
- Mechanism: Geographic barriers (e.g., mountains, rivers) prevent gene flow, allowing genetic divergence through mutation, natural selection, and genetic drift.
- Example: Darwin’s finches on different Galápagos Islands evolving distinct beak shapes.
- Sympatric Speciation
- Definition: Speciation occurring within a single geographic area without physical barriers.
- Mechanism: Can result from factors like polyploidy (especially in plants), sexual selection, or ecological niche differentiation.
- Example: Apple maggot flies (Rhagoletis pomonella) shifting from hawthorn to apple trees, leading to reproductive isolation.
- Peripatric Speciation
- Definition: A form of allopatric speciation where a small peripheral population becomes isolated from the main population.
- Mechanism: Similar to allopatric speciation but involves smaller populations, making genetic drift more significant.
- Example: A few individuals colonizing a new island and evolving separately from the mainland population.
- Parapatric Speciation
- Definition: Speciation occurring when populations are partially separated and have a narrow zone of overlap.
- Mechanism: Limited gene flow and different selective pressures across the range lead to divergence.
- Example: Grass species growing on different soil types along a gradient.
Key Terms and Concepts
- Allele Frequency: The proportion of a specific allele within a population’s gene pool.
- Gene Pool: The complete set of genetic information within a population.
- Reproductive Isolation: Mechanisms that prevent species from mating with others, maintaining species boundaries.
- Hybrid: Offspring resulting from the crossbreeding of two different species or populations.
- Genetic Variation: Diversity in gene frequencies within a population, crucial for evolution.
- Adaptive Radiation: Rapid evolution of diversely adapted species from a common ancestor upon introduction to new environments.
- Phylogenetics: The study of evolutionary relationships among species.
- Cladistics: A method of classifying species based on shared derived characteristics.